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Vmcg Testing  
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3476 posts, RR: 67
Posted (10 months 3 weeks 5 days 14 hours ago) and read 6636 times:

This thread has been created due to a discussion on this thread:

A350 Flight Tests Official Thread Part 4 (by LipeGIG Aug 28 2013 in Civil Aviation)

In that thread, Zeke made this statement.

Quoting zeke (Reply 10):
I do think they have been looking at Vmca/Vmcg with the tail skid.

I disagreed with his statement since, in my certification experience, use of a tail skid is irrelevant to determining either Vmca (minimum control speed in the air) or Vmcg (minimum control speed on the ground).

This thread will discuss why I disagree. As part of the discussion, I’ll be quoting from FAR 25.149, Minimum Control Speeds and AC 25-7C FAA “Flight Test Guide For Certification Of Transport Category Airplanes”. Links to both these documents are provided below

http://www.ecfr.gov/cgi-bin/text-idx...ode=14:1.0.1.3.11.2.156.24&idno=14

http://www.faa.gov/regulations_polic...ent.information/documentID/1020494

The purpose of a tail skid is to protect the airplane aft body from contact with the ground due to over-rotation on either takeoff or landing. As such, it’s pretty easy to see why a tail skid is irrelevant when establishing Vmca since FAR 25.149 states as one of the conditions for determining this speed:

(6) The airplane airborne and the ground effect negligible….

For an airplane the size of an A359, ground effects do not become negligible until the the airplane is at least 200 ft AGL. If the tail skid protects against on the ground aft body contacts, it obviously won’t have any impact on Vmca testing.

As to Vmcg testing, my experience has been that this speed is determined with the airplane on the ground and no attempt is to become airborne during the test sequence. This being the case, the airplane is not rotated during Vmcg testing, eliminating the possibility of aft body contact and making a tail skid irrelevant.

Zeke did raise another point though:

Quoting zeke (Reply 49):
And no, Airbus and Boeing DO NOT follow the same procedures to certify an aircraft, they design and certify aircraft to the same regulations using different approved test plans.



Let’s examine the possibilities of this point. The purpose of Vmcg testing is to establish the minimum value of V1 when taking off with both engines at full power and a subsequent failure of the critical engine. AC 25-7C specifies the following conditions for Vmcg testing:

(c) During determination of VMCG, engine failure recognition should be provided by:

1) The pilot feeling a distinct change in the directional tracking characteristics of the airplane; or

2) The pilot seeing a directional divergence of the airplane with respect to the view outside the airplane.

(d) Directional control of the airplane should be accomplished by use of the rudder only. All other controls, such as ailerons and spoilers, should only be used to correct any alterations in the airplane attitude and to maintain a wings level condition. Pilot input to controls to supplement the rudder effectiveness should not be used. Care should also be taken not to inadvertently apply brake pressure during large rudder deflections, as this will invalidate the test data.

(e) VMCG testing should be conducted at the most critical weight in the range where VMCG may impact AFM V1 speeds.

(f) VMCG testing should be conducted at aft c.g. and with the nose wheel free to caster, to minimize the stabilizing effect of the nose gear. If the nose wheel does not caster freely, the test may be conducted with enough nose up elevator applied to lift the nose wheel off the runway.

(g) VMCG testing should not be conducted on runways with excessive crowning (i.e., cross-runway slope) unless the effects of such crowning are determined to be conservative.

The parameter of merit for determining Vmcg is:

(a) It must be demonstrated that, when the critical engine is suddenly made inoperative at VMCG during the takeoff ground roll, the airplane is safely controllable if the takeoff is continued. During the demonstration, the airplane must not deviate more than 30 ft…….. from the pre-engine-cut projected ground track.

An obvious question is question is if Vmcg should be determined during a Stop or a Go. Here are some Pros and Cons for each approach:

Vmcg Determined via Stop

Pro

Pilot technique constant during test, always Stop oriented
Reduces test risk as airplane doesn’t rotate and remains on runway
Runway deviation easily determined

Con

Need to be careful with brake temperatures
Requires separate test sequence
Takeoff verification test required after Vmcg determined

Vmcg Determined via Go

Pro

May be able to integrate Vmcg testing with other engine inoperative testing
Takeoff verification test may not be required
No brake cooling issues

Con

Pilot technique must change from Go to Stop during testing to gain full Vmcg performance credit
More risk of airplane ground contact during testing (tail skid an advantage?)
Can be more difficult to quantify runway deviation

Boeing has typically done Stop oriented Vmcg testing. Since the airplane doesn’t rotate, the aft body doesn’t need protection.

Here’s a public domain video of a 777-300ER Vmcg test. The narrator is Frank Santoni, 777 Chief Pilot at the time of 777-300ER certification. The video starts with a computer generated graphic of the test condition and then shows the airplane during the test. The graphic is interesting since it shows the runway deviation during the test condition.

http://www.youtube.com/watch?v=TKZjlvagrPo

Does Airbus do Go oriented Vmcg testing? Perhaps Zeke can answer.

Here’s a public domain video of an A340-600 during what appears to be a takeoff during a Vmcg type test. I don’t know if it’s a final verification test or one as part of the Vmcg determination. Please remember the video title is that of the video poster, not me.

http://www.youtube.com/watch?v=XWsnxJGgy1c

From the video, it appears that the part of the airplane with the highest risk of ground contact is the outboard nacelle, not the aft body.


Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
20 replies: All unread, jump to last
 
User currently offlinezeke From Hong Kong, joined Dec 2006, 8864 posts, RR: 75
Reply 1, posted (10 months 3 weeks 4 days 7 hours ago) and read 6171 times:

Quoting OldAeroGuy (Thread starter):
I disagreed with his statement since, in my certification experience, use of a tail skid is irrelevant to determining either Vmca (minimum control speed in the air) or Vmcg (minimum control speed on the ground).

Great that is your experience, it is not the only way. BTW I never said they were doing the Vmca or Vmcg tests, I said they were having a look at it.

Quoting OldAeroGuy (Thread starter):
As to Vmcg testing, my experience has been that this speed is determined with the airplane on the ground and no attempt is to become airborne during the test sequence

There is video on the net somewhere with a head on view of the A380 doing Vmcg testing, you can see the tail skid installed after it rotates. The A380 also had a tail skid that looked like a KA44B ADF antenna that was painted. It provided protection while the conducted various test, unless you were looking for it, or aware of it, it would go unnoticed.

A400M Vmcg flight test with takeoff

http://www.youtube.com/watch?v=tzM6k-JAlc4

C130 Vmcg flight test with takeoff

http://www.youtube.com/watch?v=XSAB0XKUjLY

Quoting OldAeroGuy (Thread starter):
This being the case, the airplane is not rotated during Vmcg testing, eliminating the possibility of aft body contact and making a tail skid irrelevant.

That might be the way Boeing does it, it does not mean it is the way other organisations do it as evidenced with the videos above. The A380 had extensive testing with configuration and flight control laws for the Vmcg testing, the flight controls laws were actually modified to improve Vmcg.

A description on how the conducted the tests on the A380 is contained in the Airbus Safety First Magazine

http://www.ukfsc.co.uk/files/Safety%...0First%20Mag%20-January%202011.pdf

Note the comments made about the limit of yaw control.

Also from the article

"Measurement of VMCs is not a key priority at the beginning of the development of a long range aircraft. The reason is that all these speeds are rather low and therefore do not affect takeoff and landing performances, except for operations at very low weights. This is not penalizing for an aircraft like the A380. However, it is always useful to perform some measurements at an early stage of the flight program to be sure that we will not have a bad surprise, which might have an impact on performances at higher weight than expected or could necessitate a modification of the design of the flight controls."

This is all I was saying, they were having a look at the minimum control speeds early on. All the takeoff and landing speeds have to have margins at or above the minimum control speeds, without know what these speeds are, or how the aircraft reacts can adversely impact takeoff and landing performance. It is also important part of testing the various flight control laws, they need to know how the aircraft reacts, and the limits of control input.

The flap/slat/and aileron droops settings on any Airbus FBW aircraft are arbitrary numbers, the settings are 0,1,2,3, and FULL. To the pilot they do not represent any "degree" of flap, they are just settings that are common on all FBW types. During flight testing they optimize the actual angles for these surfaces to provide the best performance, as well as common handling and operation. The way a pilot configures an A318 for an approach, is the same as an A380, and thus for commonality, the A350 is "tuned" for similar handling.

During the flight control law tuning process, the various transitions from ground to flight, and flight to ground laws needs to be looked at. Starting with the A340, Airbus brought in a rotation law (transit from ground to flight), they further developed this law again for the A380 and A350. During flight testing of the A380 where they were tuning the rotation law at low speed, they did have a ground contact with the bumper installed, analysis found that if the bumper was not installed they would have just cleared the ground without contact. For landing, the aircraft has a flare law with a de-rotation component to transition from flight to ground law. This is one of the most difficult ones to tune, as well as the flight control laws changing, the aircraft configuration and CG are changing. It is also very difficult to simulate ground effect with the law and configuration changes.

During the flight control law optimization process it is possible to get very close or even to get a tail scrape (as they did on the A380 during takeoff). This is more likely on landing than takeoff, it is not necessarily due to over rotating, different flap/slat combinations will provide different amount of ground clearance.

Quoting OldAeroGuy (Thread starter):
From the video, it appears that the part of the airplane with the highest risk of ground contact is the outboard nacelle, not the aft body.

With a pod strike, repairs could be a new cowl, or replacement of an engine, which can be under a day with minimal impact on the flight test schedule. With a tail strike, possible skin replacement, replacement/repair of stringers/frames, and even possibly the replacement of the aft pressure bulkhead. Considerable down time, large impact on the flight test schedule.

Quoting OldAeroGuy (Thread starter):
Boeing has typically done Stop oriented Vmcg testing. Since the airplane doesn’t rotate, the aft body doesn’t need protection
Quoting OldAeroGuy (Thread starter):
Does Airbus do Go oriented Vmcg testing? Perhaps Zeke can answer.

Yes, Airbus is go minded, as seen in the videos above. As you can see from the LM C130 test, they are also go minded. Gulfstream I am sure you can recall when they were doing their G650 engine out test that resulted in a fatal accident was also go minded.

I said in the other thread, "Airbus and Boeing DO NOT follow the same procedures to certify an aircraft, they design and certify aircraft to the same regulations using different approved test plans."

There are numerous organisational, industrial, and philosophical differences between manufacturers (and I am not just talking about Airbus and Boeing), how they design and certify their products is a parts of the way they do business. There is not requirement to follow the SAME method to achieve certification. The Advisory Circulars by the FAA have some words to the effect that these are guides, however not the only way to achieve certification. In the past few decades, every new aircraft certified I can recall had to have "equivalent levels of safety", where due to new technology outpacing the regulations, the manufacturers have basically had to come up with the ways certify this new technology to show it is just as safe as the legacy technology. This is where you see the large differences between manufacturers.

Take passenger evacuation for example, Boeing did not do one on the 787, they used computer simulation. Airbus did on the A380, A400M, and will also on the A350. They use different procedures to achieve the certification requirements.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlinetommytoyz From Tonga, joined Jan 2007, 1353 posts, RR: 6
Reply 2, posted (10 months 3 weeks 4 days 3 hours ago) and read 6145 times:

Quoting zeke (Reply 1):
BTW I never said they were doing the Vmca or Vmcg tests, I said they were having a look at it.
Quoting zeke (Reply 1):
This is all I was saying, they were having a look at the minimum control speeds early on

You made a clear connection between Vmca or Vmcg tests (or looking) and the tailskid:

From earlier Zeke post:
I do think they have been looking at Vmca/Vmcg with the tail skid.

However, the videos you linked to, show no skid on those planes during these tests. And even if there is a tail skid during Vmca or Vmcg tests on other aircraft tests, it is merely coincidental.

Your continued assertions that there is somehow a connection between the tail skid and Vmca or Vmcg testing or even just "looking" are interesting, especially since your own videos fail to show this.

Rather than a long winded meandering post, why don't you just answer the simple question posed by OldAeroGuy:

What does the tail skid have to do with Vmca or Vmcg?

[Edited 2013-09-03 22:04:27]

User currently offlinezeke From Hong Kong, joined Dec 2006, 8864 posts, RR: 75
Reply 3, posted (10 months 3 weeks 4 days 2 hours ago) and read 6127 times:

Quoting tommytoyz (Reply 2):
You made a clear connection between Vmca or Vmcg tests (or looking) and the tailskid:

Yes, they have an early look, as I underlined above. You might want to have a look at who wrote that article.

Quoting zeke (Reply 1):
it is always useful to perform some measurements at an early stage of the flight program to be sure that we will not have a bad surprise, which might have an impact on performances at higher weight than expected or could necessitate a modification of the design of the flight controls
Quoting tommytoyz (Reply 2):
However, the videos you linked to, show no skid on those planes during these tests.

Actually it it is clearly seen on the A400M video, IF you know what you are looking for.


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The A380 had it painted, it was hard to spot unless you were looking for it.


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Quoting tommytoyz (Reply 2):
And even if there is a tail skid during Vmca or Vmcg tests on other aircraft tests, it is merely
coincidental.

Not at all.

Quoting tommytoyz (Reply 2):
What does the tail skid have to do with Vmca or Vmcg?

Its got to do with protecting the aircraft. During the initial flight testing they do not know where Vmca and Vmcg really are, and the flight control laws are "unknown". On the A380 they calculated Vmcg was around 120 kts, during the early tests they could not even control it 15 kts above the calculated figure. This resulted in an early change to the fin and changes to the flight control software well before the certification tests to demonstrate the final numbers.

They need to have an early look at these minimum control speeds before they freeze the aerodynamic configuration.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 4, posted (10 months 3 weeks 3 days 10 hours ago) and read 5964 times:

Quoting zeke (Reply 3):
Its got to do with protecting the aircraft.

Vmcg and Vmca testing does not require any tail protection. If Airbus choose to take such tests into the air and there is a risk of over-rotation then that is a different matter. But the rotation is not part of the Vmcg test itself and so the skid is not needed for that purpose.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlinePihero From France, joined Jan 2005, 4389 posts, RR: 76
Reply 5, posted (10 months 3 weeks 3 days 9 hours ago) and read 5946 times:
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Quoting Jetlagged (Reply 4):
But the rotation is not part of the Vmcg test itself and so the skid is not needed for that purpose.

1/- See Oldaeroguy initial post:

Quoting OldAeroGuy (Thread starter):
Vmcg Determined via Go

2/- See fig 2, page 13 on the *Safety First* Magazine Zeke quoted : The Vmcg test profile does indeed include rotation for takeoff.
Moreover,
This is an old Youtube video on the A340 Vmcg tests, viewed from the extended runway centerline facing the test aircraft:A340 Vmcg test

I daresay the tailskid could be quite useful, for the reasons Zeke did mention.



Contrail designer
User currently offlinezeke From Hong Kong, joined Dec 2006, 8864 posts, RR: 75
Reply 6, posted (10 months 3 weeks 2 days 23 hours ago) and read 5852 times:

Quoting Jetlagged (Reply 4):
Vmcg and Vmca testing does not require any tail protection. If Airbus choose to take such tests into the air and there is a risk of over-rotation then that is a different matter. But the rotation is not part of the Vmcg test itself and so the skid is not needed for that purpose.

This is correct, however there is a long road between first flight and conducting those test, and it involves a lot of flying, and they may inadvertently reach the control limits. On the A380 they did have a tail strike well before doing the final tests, and this essentially was due to not knowing enough about the aircraft, that is what the flight testing is about. They needed to modify the airframe and control laws to get the desired performance. Also on the A380 the wings level Vmca is around 10 kts higher than the configuration allowed for under the test. Aircraft tend to land rather wings level, meaning that control authority during testing can be diminished to a point where it results in an undesired state very close to the ground even if maintaining a speed above Vmca.

Keep in mind Airbus does not install mechanical tail protection on production aircraft like many Boeing models.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3476 posts, RR: 67
Reply 7, posted (10 months 3 weeks 2 days 15 hours ago) and read 5752 times:

Thanks Zeke, you've convinced me that Airbus does do Go oriented Vmcg testing.

I'm amazed at the daring of the Airbus test pilots though. During the early Vmcg testing they made the decision to Go even though the airplane had seriously missed its predicted Vmcg performance.

If you read the article you provided here:

http://www.ukfsc.co.uk/files/Safety%...0First%20Mag%20-January%202011.pdf

you find that Airbus did a first evaluation of Vmcg with a Vef of 132 kts, 10 kts faster than the estimated value of 122 kts. The airplane lateral deviation from the initial heading was 15 m vs a planned value of 2 m. The certification allowable deviation is 9.1 m. At this point I'm sure that it was apparent that something was amiss and I'm surprised that they didn't decide to stop rather than continue the takeoff with an airplane that had some serious control issues.

As it turned out, there was a latent failure of a rudder actuator that reduced the rudder rate. While this was a contributing factor, Vmcg was not reduced to the planned 120 kt region until modifications had been made to the A380 flight controls to differentially deflect the ailerons and one of the spoilers on the wing in the direction of rudder deflection (ie the wing with all engines operating). This was a good use of the FBW system as this type of change wouldn't be possible with a cable system and pilot action to approximate it is specifically denied by FAR 25.149.

As to if other airplane companies are as Go oriented during Vmcg testing, I'm afraid the jury is still out.

The C-130 video is inconclusive. After Vmcg is determined, a takeoff verification test must be done with an engine cut at the developed Vef and subsequent Vmcg. It isn't clear if the video you provided shows a Vmcg development test or a takeoff verification.

As to Gulfstream's philosophy, I'm surprised you even mention it given the crash of the G650 during takeoff testing. In any case, the purpose of the test was to reduce V2 where it is necessary to perform a Go. It says nothing about Gulfstream Vmcg tests.

http://www.ntsb.gov/news/events/2012/gulfstream/index.html

While all this is very interesting, the central point of discussion is your statement that a tail skid is necessary to protect the aft body during Vmcg testing. While the skid is a nicety for aft body protection during the variety of takeoff and landing conditions encountered during flight testing, here's a final analysis of why one isn't necessary to establish Vmcg even if the tests are Go oriented.

The purpose of determining Vmcg is to establish the minimum value of V1. The limit condition for VR is when VR = V1. Consider how VR is determined per FAR 25.107:

(e) V R, in terms of calibrated airspeed, must be selected in accordance with the conditions of paragraphs (e)(1) through (4) of this section:
(1) V R may not be less than—
(i) V 1 ;
(ii) 105 percent of V MCA ;
(iii) The speed (determined in accordance with § 25.111(c)(2)) that allows reaching V 2 before reaching a height of 35 feet above the takeoff surface; or
(iv) A speed that, if the airplane is rotated at its maximum practicable rate, will result in a VLOF of not less than —
(A) 110 percent of VMU in the all-engines-operating condition, and 105 percent of VMU determined at the thrust-to-weight ratio corresponding to the one-engine-inoperative condition; or
(B) If the VMU attitude is limited by the geometry of the airplane ( i.e., tail contact with the runway), 108 percent of VMU in the all-engines-operating condition, and 104 percent of VMU determined at the thrust-to-weight ratio corresponding to the one-engine-inoperative condition.

By parts (iv) (A) & (B), VR must be chosen so VLOF is 4% to 5% higher than VMU with an engine inoperative, with the result that dynamic pressure at VLOF is 8% to 10% higher than at VMU. Since Airbus will have an estimate of VMU even if they haven't actually done the maneuver yet, the scheduled VR will reflect the increment in VLOF over VMU. The increase in "q" will be sufficient to prevent aft body contact during even an early look at Vmcg. I also doubt that Airbus is performing rotations at the "maximum practicable rate" during Go Vmcg testing or operating at the limit condition of V1 = VR. In fact, there is a considerable pause between re-establishing directional control and rotation in both Airbus Vmcg videos referenced above. In neither case does the aft body come close to the runway during the continued takeoff.

http://www.youtube.com/watch?v=XWsnxJGgy1c

http://www.youtube.com/watch?v=RbPVIqgOpvM

And there is no evidence of a tail skid in either one of these videos.

"Quoting OldAeroGuy (Thread starter):
From the video, it appears that the part of the airplane with the highest risk of ground contact is the outboard nacelle, not the aft body.

With a pod strike, repairs could be a new cowl, or replacement of an engine, which can be under a day with minimal impact on the flight test schedule." (Zeke quote from Reply 1)

This is a cavalier attitude to take with a flight test engine which typically has been calibrated and fitted with exhaust rakes. Replacement of a flight test engine (if a spare is available) takes considerably longer than an in-service line replacement.

Quoting zeke (Reply 6):
On the A380 they did have a tail strike well before doing the final tests, and this essentially was due to not knowing enough about the aircraft, that is what the flight testing is about.

Did the aft body contact occur during Vmcg testing? No aft body contact is mentioned in the excellent A380 Vmcg testing article you provided. Besides, the 132 kt Vef selected for the initial test vs the estimated 122 kt Vef would have provided an extra 17% "q" margin on top of the 8% to 10% margin discussed above, making an aft body contact very on likely.

Quoting zeke (Reply 6):
Quoting Jetlagged (Reply 4):
Vmcg and Vmca testing does not require any tail protection. If Airbus choose to take such tests into the air and there is a risk of over-rotation then that is a different matter. But the rotation is not part of the Vmcg test itself and so the skid is not needed for that purpose.

This is correct, however there is a long road between first flight and conducting those test

Thanks for agreeing with the point I have been making all along.

[Edited 2013-09-05 10:06:19]


Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlinezeke From Hong Kong, joined Dec 2006, 8864 posts, RR: 75
Reply 8, posted (10 months 3 weeks 2 days 10 hours ago) and read 5674 times:

Quoting OldAeroGuy (Reply 7):
Thanks Zeke, you've convinced me that Airbus does do Go oriented Vmcg testing.

That sounds like a half arsed apology admitting that Airbius and Boeing do not follow the same procedure to achieve the certification outcomes.

Quoting OldAeroGuy (Reply 7):
As it turned out, there was a latent failure of a rudder actuator that reduced the rudder rate.

I was of the understanding it was not a failure, from what I understand it was the variable control gain programmed into the FBW.

Quoting OldAeroGuy (Reply 7):
While this was a contributing factor, Vmcg was not reduced to the planned 120 kt region until modifications had been made to the A380 flight controls to differentially deflect the ailerons and one of the spoilers on the wing in the direction of rudder deflection (ie the wing with all engines operating).

Yes, just goes to show why they have a look at the minimum control speeds early. From what I understand, there was a physical change to the fin as well.

This was not the only modification to the FBW they made, and that test was not done early in the piece. They were flying around below the actual Vmcg for some time and they did have a tail scrape.

A sense a lot of "old school" thinking by you on this thread, these minimum control speed limits be it Vs1g, Vmca, Vmcg etc today are a function of the flight control system as much as they are of the actual airframe aerodynamics. During flight testing they have the ability to "tune" the FBW system to achieve better performance than what the airframe could achieve without it.

Quoting OldAeroGuy (Reply 7):
While all this is very interesting, the central point of discussion is your statement that a tail skid is necessary to protect the aft body during Vmcg testing. While the skid is a nicety for aft body protection during the variety of takeoff and landing conditions encountered during flight testing, here's a final analysis of why one isn't necessary to establish Vmcg even if the tests are Go oriented.

The purpose of determining Vmcg is to establish the minimum value of V1. The limit condition for VR is when VR = V1. Consider how VR is determined per FAR 25.107:

That is all very nice, however they have no real idea of the actual Vmcg, Vmu, V1, Vr, Vmca, Vlc etc. These numbers can change on the same day as the change the FBW control laws. It is a chicken and egg scenario, the FBW tuning needs to be completed before they determine those final numbers. That does not mean they can run out of control before then and modify the response with e FBW change.

Quoting OldAeroGuy (Reply 7):
The C-130 video is inconclusive. After Vmcg is determined, a takeoff verification test must be done with an engine cut at the developed Vef and subsequent Vmcg. It isn't clear if the video you provided shows a Vmcg development test or a takeoff verification.

Fair enough, you are being stubborn. It does not show what you are saying either.

Quoting OldAeroGuy (Reply 7):
As to Gulfstream's philosophy, I'm surprised you even mention it given the crash of the G650 during takeoff testing. In any case, the purpose of the test was to reduce V2 where it is necessary to perform a Go. It says nothing about Gulfstream Vmcg tests.

Not at all, the main reason for the crash was a change to the FBW which brought on the impending stall too late for the crew to recognize. They reached their minimum control speed, and entered into an undesired state with no chance of recovery.

You are so black and white with you interpretation on minimum control speeds it is a worry, a test pilot is only concerned with maintaining control, and during testing they will inadvertently not be in control at some stage. The aero guys calculate what they think various values should be, invariably they are wrong. There is a lot of interaction between the aircraft configuration, ground effect, flight control laws, and the dynamic maneuvers which means the "static" numbers often calculated are out by a long way as evidenced by the A380 numbers.

Quoting OldAeroGuy (Reply 7):
In fact, there is a considerable pause between re-establishing directional control and rotation in both Airbus Vmcg videos referenced above. In neither case does the aft body come close to the runway during the continued takeoff.

True, Airbus did not install tail bumpers on the A330 or A340, they changed the way they test since then. I would ask you to look at the VTP on the A346 before assuming it was Vmcg testing. As to how they control the aircraft for the test, they are aware from earlier testing how the aircraft behaves, and plan the test accordingly.

Quoting OldAeroGuy (Reply 7):
This is a cavalier attitude to take with a flight test engine which typically has been calibrated and fitted with exhaust rakes. Replacement of a flight test engine (if a spare is available) takes considerably longer than an in-service line replacement.

They have a spare,it it does not take that long to replace. Nothing like a fuselage repair.

Quoting OldAeroGuy (Reply 7):
Thanks for agreeing with the point I have been making all along.

While still ignoring what Airbus and myself said that they have an early look at the minimum control speeds so there are no surprises later on. All I said is that they were having a look at those speeds, and you turned it into they are conducting the final test. Anybody that has been involved with certification SHOULD know the final tests are not done that early.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3476 posts, RR: 67
Reply 9, posted (10 months 3 weeks 2 days 4 hours ago) and read 5612 times:

Quoting zeke (Reply 8):
All I said is that they were having a look at those speeds, and you turned it into they are conducting the final test. Anybody that has been involved with certification SHOULD know the final tests are not done that early.

Very good, you still don't understand (or chose not to) what I've been taking issue with and what Jetlagged has also said. At no time did I say they were conducting the final tests. Conducting the final tests would require a finalized configuration, a declaration of TIA (Test Inspection Authorization) and the presence of Certification Agency pilot.

If you're unfamiliar with the acronym TIA you can find the definition here:

http://www.acsnw.net/upload/conformity%20manual%20rev%20A.pdf

Quoting Jetlagged (Reply 4):
Quoting zeke (Reply 3):
Its got to do with protecting the aircraft.

Vmcg and Vmca testing does not require any tail protection. If Airbus choose to take such tests into the air and there is a risk of over-rotation then that is a different matter. But the rotation is not part of the Vmcg test itself and so the skid is not needed for that purpose.

Airbus may check and Vmca and Vmcg at any place in the test program and as often as they like. It's your insistence that a tail skid is necessary to protect the aft body when these tests are conducted that is incorrect.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3476 posts, RR: 67
Reply 10, posted (10 months 3 weeks 2 days 3 hours ago) and read 5601 times:

Quoting zeke (Reply 8):
Quoting OldAeroGuy (Reply 7):
As to Gulfstream's philosophy, I'm surprised you even mention it given the crash of the G650 during takeoff testing. In any case, the purpose of the test was to reduce V2 where it is necessary to perform a Go. It says nothing about Gulfstream Vmcg tests.

Not at all, the main reason for the crash was a change to the FBW which brought on the impending stall too late for the crew to recognize. They reached their minimum control speed, and entered into an undesired state with no chance of recovery.

Sorry I didn't comment on your above statement earlier. Based on what you've said either you didn't read the NTSB summary reference I provided for this acident or didn't understand it. Here's a direct quote from the reference:

"Rather than determining the root cause for the V2 exceedance problem, Gulfstream attempted to reduce the V2 speeds and the takeoff distances by modifying the piloting technique used to rotate the airplane for takeoff. Further, Gulfstream did not validate the speeds using a simulation or physics-based dynamic analysis before or during field performance testing. If the company had done so, then it could have recognized that the target V2 speeds could not be achieved even with the modified piloting technique. In addition, the difficulties in achieving the target V2 speeds were exacerbated in late March 2011 when the company reduced the target pitch angle for some takeoff tests without an accompanying increase in the takeoff speeds."

GA had modified the pilot technique for takeoff rotation, not the FBW system.

These are the Executive Summary Conclusions:

1. The test team's focus on achieving the takeoff safety speeds for the flight tests and the lack of guidance specifying precisely when the pitch angle target and pitch limit applied during the test maneuver contributed to the team's decision to exceed the initial pitch target and the pitch angle at which a takeoff test was to be discontinued.

2. A stall on the right outboard wing produced a right rolling moment that the flight crew was not able to control, which led to the right wingtip contacting the runway and the airplane departing the runway from the right side.

3. Given the airplane's low altitude, the time-critical nature of the situation, and the ambiguous stall cues presented in the cockpit, the flight crew's response to the stall event was understandable.

4. The impact forces from the accident were survivable, but the cabin environment deteriorated quickly and became unsurvivable because of the large amount of fuel, fuel vapor, smoke, and fire entering the cabin through the breaches in the fuselage.

5. The airplane stalled at an angle of attack (AOA) that was below the in ground effect stall AOA predicted by Gulfstream and the AOA threshold for the activation of the stick shaker stall warning.

6. If Gulfstream had performed an in-depth aerodynamic analysis of the cause of two previous G650 uncommanded roll events, similar to the analyses performed for roll events during previous company airplane programs, the company could have recognized that the actual in-ground-effect stall angle of attack for the accident flight test was significantly lower than the company predicted.

7. Gulfstream's decision to use a takeoff speed development method from a previous airplane program was inappropriate and resulted in target takeoff safety speed values that were too low to be achieved.

8. By not performing a rigorous analysis of the root cause for the ongoing difficulties in achieving the G650 takeoff safety speeds (V2), Gulfstream missed an opportunity to recognize and correct the low target V2 speeds.

9. Before the accident flight, Gulfstream had sufficient information from previous flight tests to determine that the target takeoff safety speeds (V2) could not be achieved with a certifiable takeoff rotation technique and that the V2 speeds needed to be increased.

10. Deficiencies in Gulfstream's technical planning and oversight contributed to the incorrect speeds used on the day of the accident.

11. Because Gulfstream did not clearly define the roles and responsibilities for on site test team members, critical safety-related parameters were not being adequately monitored and test results were not being sufficiently examined during flight testing on the day of the accident.

12. Gulfstream's focus on meeting the G650's planned certification date caused schedule related pressure that was not adequately counterbalanced by robust organizational processes to prevent, identify, and correct the company's key engineering and oversight errors.

13. Gulfstream's flight test safety program at the time of the accident was deficient because risk controls were insufficient and safety assurance activities were lacking.

14. The inherent risks associated with field performance flight testing, and minimum unstick speed testing in particular, could be reduced if airplane manufacturers considered the potential for a lower maximum lift coefficient in ground effect when estimating the stall angle of attack in ground effect.

15. Effective flight test standard operating policies and procedures that are fully implemented by manufacturers would help reduce the inherent risks associated with flight testing.

16. Flight test safety management system guidance specifically tailored to the needs of manufacturers would help promote the development of effective flight test safety programs.

17. External safety audits would help Gulfstream monitor the implementation of safety management principles and practices into its flight test operations and sustain long-term cultural change.

18. Flight test safety would be enhanced if manufacturers and flight test industry groups had knowledge of the lessons learned from Gulfstream's implementation of its flight test safety management system.

19. Advance coordination between flight test operators and airport operations and aircraft rescue and firefighting personnel for high-risk flight tests could reduce the response time to an accident site in the event of an emergency.

Probable Cause

The National Transportation Safety Board determines that the cause of this accident was an aerodynamic stall and subsequent uncommanded roll during a one engine-inoperative takeoff flight test, which were the result of (1) Gulfstream's failure to properly develop and validate takeoff speeds for the flight tests and recognize and correct the takeoff safety speed (V2) error during previous G650 flight tests, (2) the G650 flight test team's persistent and increasingly aggressive attempts to achieve V2 speeds that were erroneously low, and (3) Gulfstream's inadequate investigation of previous G650 uncommanded roll events, which indicated that the company's estimated stall angle of attack while the airplane was in ground effect was too high. Contributing to the accident was Gulfstream's failure to effectively manage the G650 flight test program by pursuing an aggressive program schedule without ensuring that the roles and responsibilities of team members had been appropriately defined and implemented, engineering processes had received sufficient technical planning and oversight, potential hazards had been fully identified, and appropriate risk controls had been implemented and were functioning as intended.

Recommendations

To the Federal Aviation Administration:

1. Inform domestic and foreign manufacturers of airplanes that are certified under 14 Code of Federal Regulations Parts 23 and 25 about the circumstances of this accident and advise them to consider, when estimating an airplane's stall angle of attack in ground effect, the possibility that the airplane's maximum lift coefficient in ground effect could be lower than its maximum lift coefficient in free air.

2. Work with the Flight Test Safety Committee to develop and issue detailed flight test operating guidance for manufacturers that addresses the deficiencies documented in this report regarding flight test operating policies and procedures and their implementation.

3. Work with the Flight Test Safety Committee to develop and issue flight test safety program guidelines based on best practices in aviation safety management.

4. After the Flight Test Safety Committee has issued flight test safety program guidelines, include these guidelines in the next revision of Federal Aviation Administration Order 4040.26, Aircraft Certification Service Flight Test Risk Management Program.

5. Inform 14 Code of Federal Regulations Part 139 airports that currently have (or may have in the future) flight test activity of the importance of advance coordination of high risk flight tests with flight test operators to ensure adequate aircraft rescue and firefighting resources are available to provide increased readiness during known high risk flight tests.

To the Flight Test Safety Committee:

6. In collaboration with the Federal Aviation Administration, develop and issue flight test operating guidance for manufacturers that addresses the deficiencies documented in this report regarding flight test operating policies and procedures and their implementation, and encourage manufacturers to conduct flight test operations in accordance with the guidance.

7. In collaboration with the Federal Aviation Administration, develop and issue flight test safety program guidelines based on best practices in aviation safety management, and encourage manufacturers to incorporate these guidelines into their flight test safety programs.

8. Encourage members to provide notice of and coordinate high-risk flight tests with airport operations and aircraft rescue and firefighting personnel.

To Gulfstream Aerospace Corporation:

9. Commission an audit by qualified independent safety experts, before the start of the next major certification flight test program, to evaluate the company's flight test safety management system, with special attention given to the areas of weakness identified in this report, and address all areas of concern identified by the audit.

10. Provide information about the lessons learned from the implementation of its flight test safety management system to interested manufacturers, flight test industry groups, and other appropriate parties.

At no point is the airplane's FBW flight control system or changes to it implicated.

Quoting zeke (Reply 8):
A sense a lot of "old school" thinking by you on this thread, these minimum control speed limits be it Vs1g, Vmca, Vmcg etc today are a function of the flight control system as much as they are of the actual airframe aerodynamics. During flight testing they have the ability to "tune" the FBW system to achieve better performance than what the airframe could achieve without it.

Yet even without exact knowledge of these terms or final tuning of the FBW, the A350 flies today using the estimates provided by the Airbus Aero Guys.

With regard to using FBW to improve performance, sometimes you can do it, sometimes you can't. There are hard aerodynamic limits on all configurations that no amount of FBW tuning will improve. Do you think that FBW tuning could have reduced the G650 stall speed in ground effect, allowing a reduction in V2 and meeting the takeoff field length requirement. I would call this evidence of "naive" thinking.

[Edited 2013-09-05 22:40:16]


Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4190 posts, RR: 37
Reply 11, posted (10 months 3 weeks 2 days 3 hours ago) and read 5593 times:

Quoting zeke (Reply 8):
Not at all, the main reason for the crash was a change to the FBW which brought on the impending stall too late for the crew to recognize. They reached their minimum control speed, and entered into an undesired state with no chance of recovery.

You missed something big on the G650 accident- the stall AOA can be considerably lower when in ground effect, which is the situation they were in. The stall speed in ground effect was quite a bit higher than predicted by Gulfstream engineers, which was a major contributing factor to the accident.



Chicks dig winglets.
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 12, posted (10 months 3 weeks 1 day 20 hours ago) and read 5540 times:

Quoting zeke (Reply 8):
That is all very nice, however they have no real idea of the actual Vmcg, Vmu, V1, Vr, Vmca, Vlc etc. These numbers can change on the same day as the change the FBW control laws. It is a chicken and egg scenario, the FBW tuning needs to be completed before they determine those final numbers. That does not mean they can run out of control before then and modify the response with e FBW change.

The FBW can increase Vmcg by reducing the pilot rudder input in response to engine failure. Ultimately old fashioned aerodynamics determines the true (i.e. minimum) Vmcg. Similarly Vmu and Vmca. I don't really see why Airbus would want to design the FBW in such a way as to reduce the maximum rudder input on ground or maximum nose up elevator. IIRC on ground the control laws are more or less direct law.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlinetommytoyz From Tonga, joined Jan 2007, 1353 posts, RR: 6
Reply 13, posted (10 months 3 weeks 11 hours ago) and read 5361 times:

Quoting zeke (Reply 8):
Fair enough, you are being stubborn.
Quoting zeke (Reply 8):
That sounds like a half arsed apology

I suggest you be a little more respectful with your language, especially since you are wrong on several counts and refuse to admit it.

Quoting zeke (Reply 8):
While still ignoring what Airbus and myself said that they have an early look

Aibus have never used the work "look" or that terminology as far as I have been able to determine. Only you use that terminology in Aibus testing regarding Vcmg and Vcma. And in none of the material you linked to and I have been able to find, does it support that a skid is necessary for these.

Quoting zeke (Reply 3):
Actually it it is clearly seen on the A400M video, IF you know what you are looking for.

A skid is not seen on the C-130 video. That means your own linked video doesn't support what you are saying. It shows you link and post stuff you do not understand.

Quoting zeke (Reply 8):
Not at all, the main reason for the crash was a change to the FBW

I suggest you reread the material your have been asked to read and actually comprehend it. You will then see you are wrong and perhaps give an apology yourself.


User currently offlinezeke From Hong Kong, joined Dec 2006, 8864 posts, RR: 75
Reply 14, posted (10 months 1 week 4 days ago) and read 4314 times:

Quoting OldAeroGuy (Reply 9):

I understand that you have tried to accuse me that they were doing the final tests, that no one is go-orientated for these tests, and everyone has to conduct tests either they way Boeing does, or the FAA publishes in the ACs. That is all nonsense.

The reason why, and I have explained this many times the tail skid is required, is they do not know where these minimum control speeds are, the margins are being pushed. The speeds with a FBW are not based upon aerodynamics alone, they are a function of the aerodynamics and the control system. When they look the low speed response in various configurations they can get in a in a situation where they cause a tail scrape. This did happen on the A380.

Quoting OldAeroGuy (Reply 10):

I understand the dynamics of the crash as the NTSB published. The motivation for the FBW change was to meet the 6000' +/- 8% takeoff distance performance guarantee made to customers. That is what was driving the change to the FBW to meet the schedule.

Quoting XFSUgimpLB41X (Reply 11):

I didn't miss that.

Quoting Jetlagged (Reply 12):

Minimum control speed are a function of the airframe aerodynamics and the control system. With the A380 for example they were able to program in spoiler deployment with the rudder to reduce the Vmcg to the desired level. It is in the article linked previously.

Quoting tommytoyz (Reply 13):
I suggest you be a little more respectful with your language, especially since you are wrong on several counts and refuse to admit it.

Nobody has shown me to be wrong, or better still to provide a counter reason for the installation. Airbus in the article I linked does say they take "early measurements", all I said is they were "having a look". These are minimum control speeds airborne and on the ground, they change with test conditions (e.g. configuration, weight etc), and as they refine the FBW. They need to protect the airframe while they do that.

In another article Airbus published on the development of the A380 flight control laws, they say "For all these tests: development of a rotation law and, later on, measurements of take-off distances, there is always a risk of tail strike because we are frequently on the limit of maneuverability of the aircraft. Therefore, the aircraft is equipped with a tail bumper, the same that is used for the VMU tests."

http://www.ukfsc.co.uk/files/Safety%...20First%20Mag%20-%20Jan%202012.pdf

I linked the C130 video as it was stated the test does even get airborne, I used that as an example as well as the A400M of where it is. BTW if you look for photos of N130X the C130 HTTB that crashed during Vmcg testing, it did have a tail bumper installed. The tail bumper itself not necessary for the final Vmcg or Vmca tests, however I did not suggest they were doing that on the A350, I was saying they were having a look at the lower speeds in various configurations. There is always a risk of a tailscrape, in the article linkdd in this reply Airbus state "Later on, in February 2006, another flight allowed the team to fine-tune the protection, which was designed to avoid getting a tail strike. It is to be noted that during these tests, we did experience a slight tail strike on the tail bumper, proof that we were looking for the minimum margin while keeping the safety level. The computations performed later on, demonstrated that the tail strike would not have happened on the fuselage without the installation of the bumper"

As for the G650 test, they changed the FBW to shortcut the development to try and meet the takeoff performance guarantee made to the customers, the cart was driving the horse.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 15, posted (10 months 6 days 13 hours ago) and read 3972 times:

Quoting zeke (Reply 14):
Minimum control speed are a function of the airframe aerodynamics and the control system. With the A380 for example they were able to program in spoiler deployment with the rudder to reduce the Vmcg to the desired level. It is in the article linked previously.

To be honest I'm surprised this is allowed to be included in a certified Vmcg, but given that it is I take your point.

Quoting zeke (Reply 14):

In another article Airbus published on the development of the A380 flight control laws, they say "For all these tests: development of a rotation law and, later on, measurements of take-off distances, there is always a risk of tail strike because we are frequently on the limit of maneuverability of the aircraft. Therefore, the aircraft is equipped with a tail bumper, the same that is used for the VMU tests."

But that article doesn't refer to Vmcg tests (they are dealt with in a separate issue which is listed). "All these tests" refers to developing a rotation law and takeoff distance measurements. Both may well necessitate a Vmu style skid. But Vmcg tests don't even if they do use spoilers to reduce the certified Vmcg, because no rotation needs to occur. If Airbus choose to plan their tests that way that's altogether different to the tests requiring the skid.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3476 posts, RR: 67
Reply 16, posted (10 months 6 days 9 hours ago) and read 3910 times:

Quoting Jetlagged (Reply 15):
But that article doesn't refer to Vmcg tests (they are dealt with in a separate issue which is listed). "All these tests" refers to developing a rotation law and takeoff distance measurements. Both may well necessitate a Vmu style skid. But Vmcg tests don't even if they do use spoilers to reduce the certified Vmcg, because no rotation needs to occur. If Airbus choose to plan their tests that way that's altogether different to the tests requiring the skid.

Well stated.

Quoting zeke (Reply 14):
I understand that you have tried to accuse me that they were doing the final tests

Now you're being disingenuous. Please provide proof that I ever said that.

Quoting zeke (Reply 14):
Nobody has shown me to be wrong, or better still to provide a counter reason for the installation

But you haven't been able to show you're right. When you state:

Quoting OldAeroGuy (Thread starter):
Quoting zeke (Reply 10): I do think they have been looking at Vmca/Vmcg with the tail skid.

You might as well say: "I do think they have been looking at Vmca/Vmcg with tufts on the left aft wing-body fairing". Both tufts and tail skid have equal impact on Vmcg.

You keep saying that the tail skid, a device used to provide protection for pitch axis issues, is needed for yaw axis testing.

If you had said: "I do think they are evaluating takeoff rotation pitch control laws/characteristics for various rotation speeds with the tail skid", I would have had no issue with your statement.

Quoting zeke (Reply 14):
I linked the C130 video as it was stated the test does even get airborne

Since the C130 had a crash during Vmcg testing as a result of getting airborne, it's not a good argument for Vmcg - Go testing.

Quoting zeke (Reply 14):
As for the G650 test, they changed the FBW to shortcut the development to try and meet the takeoff performance guarantee made to the customers, the cart was driving the horse.

Please provide some backup for this statement that a FBW change was responsible for the G650 crash. I find no mention of it in the acccident investigation Summary Conclusions (see Reply 10). You don't need a ptich axis FBW change to target higher V2 attitude and lower V2 speed.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineaerotech777 From United States of America, joined Aug 2009, 67 posts, RR: 0
Reply 17, posted (10 months 14 hours ago) and read 3556 times:

Quoting zeke (Reply 14):
Minimum control speed are a function of the airframe aerodynamics and the control system. With the A380 for example they were able to program in spoiler deployment with the rudder to reduce the Vmcg to the desired level. It is in the article linked previously.



FAR 25-149: (e) VMCG , the minimum control speed on the ground, is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), ...

AC 25-7C FAA: (d) Directional control of the airplane should be accomplished by use of the rudder only. All other controls, such as ailerons and spoilers, should only be used to correct any alterations in the airplane attitude and to maintain a wings level condition. Pilot input to controls to supplement the rudder effectiveness should not be used. Care should also be taken not to inadvertently apply brake pressure during large rudder deflections, as this will invalidate the test data.

Airbus used one spoiler and 3 ailerons with the rudder to reduce Vmcg to the desired level as indicated the article posted in reply 1. FAA recommends using the rudder only in Vmcg test for directional control of aircraft. It seems that EASA allows using other controls (spoilers and ailerons) for directional control of aircraft for Vmcg test.
What do you think about the difference between FAA and EASA regulation regarding Vmcg test ? (I am assuming here EASA allows using spoilers and ailerons in Vmcg test for directional control of aircraft).

It will be very helpful if someone can post EASA regulation regarding Vmcg test.

Feedback appreciated.


User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3476 posts, RR: 67
Reply 18, posted (10 months 3 hours ago) and read 3497 times:

Quoting aerotech777 (Reply 17):
FAR 25-149: (e) VMCG , the minimum control speed on the ground, is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), ...

The EASA regulatory language is the same as the FAA (FAR vs CS)

CS 25.149 Minimum control speed

(e) VMCG, the minimum control speed on the ground, is the calibrated airspeed during the take-off run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane using the rudder control alone (without the use of nose- wheel steering), as limited by 667 N of force (150 lbf), and the lateral control to the extent of keeping the wings level to enable the take-off to be safely continued using normal piloting skill.


FAR 25.149 Minimum control speed.

(e) VMCG , the minimum control speed on the ground, is the calibrated airspeed during the takeoff run at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane using the rudder control alone (without the use of nosewheel steering), as limited by 150 pounds of force, and the lateral control to the extent of keeping the wings level to enable the takeoff to be safely continued using normal piloting skill.

EASA has allowed the use of lateral controls (ailerons and spoiler) as commanded by the rudder pedals via FBW to establish Vmcg for the A380. The pilot is not required to make lateral control inputs through the sided stick to deflect the ailerons and spoiler to establish Vmcg.

See the article referenced by Zeke in Reply 1. Since the A380 has concurrent EASA/FAA Type Certification, the FAA has accepted this control arrangement.

[Edited 2013-09-28 21:56:28]


Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineaerotech777 From United States of America, joined Aug 2009, 67 posts, RR: 0
Reply 19, posted (9 months 4 weeks 1 day 13 hours ago) and read 3415 times:

Quoting OldAeroGuy (Reply 18):
EASA has allowed the use of lateral controls (ailerons and spoiler) as commanded by the rudder pedals via FBW to establish Vmcg for the A380. The pilot is not required to make lateral control inputs through the sided stick to deflect the ailerons and spoiler to establish Vmcg.

See the article referenced by Zeke in Reply 1. Since the A380 has concurrent EASA/FAA Type Certification, the FAA has accepted this control arrangement.



Thanks for the reply. I would like to know your opinion regarding my inquiry.
Even though the FAR and the advisory circular recommends to use only the rudder for the VMCG test, yet the EASA and FAA allowed Airbus to use spoilers and ailerons. So for me the FAR 25. 149 should be interpreted differently than it is written. I guess the reason for not allowing the use of spoilers and ailerons is the use control cables: too much workload for the crew if they use control cables in rudder, spoilers, and ailerons in the critical phase of takeoff compared to FBW.

Here my inquiry: How do you interpret this FAR ? Why EASA allowed to use spoiler and ailerons even though the FAR recommend to use only the rudder ?

Feedback appreciated.


User currently offlinezeke From Hong Kong, joined Dec 2006, 8864 posts, RR: 75
Reply 20, posted (9 months 4 weeks 1 day 9 hours ago) and read 3386 times:

Quoting OldAeroGuy (Reply 16):
Now you're being disingenuous. Please provide proof that I ever said that.

You quoted chapter and verse the various certification requirements for the final tests, and never at any stage admitted that Airbus do look at the low speed tests early in the certification program.

Quoting OldAeroGuy (Reply 16):
You keep saying that the tail skid, a device used to provide protection for pitch axis issues, is needed for yaw axis testing.

The minimum takeoff and landing speeds are a function of Vs, Vmca, Vmcg etc... when they were testing the A380 the yaw control which include spoilers in the FBW setup resulted in increased sideslip and sink in different configurations.

Quoting OldAeroGuy (Reply 16):
If you had said: "I do think they are evaluating takeoff rotation pitch control laws/characteristics for various rotation speeds with the tail skid", I would have had no issue with your statement.

I did say many times they are looking at various configurations, weights, speeds, Cg positions, flight control laws etc etc, you were the one who took one post in isolation and ignored everything else I have said. Keep in mind you had the mindset when you initially objected to my comment that no one ever continues Vmcg testing to a takeoff, I have demonstrated that Airbus does indeed do that. You even tried to tell us the only way an organisation to do their certification campaigns as per the FAA ACs, even Boeing deviates from them, the classic example was the 787 evacuation test.

Will you admit that the minimum takeoff and landing speeds are a function of the minimum control speeds ? and the experimental part of this flight testing is they will fly the aircraft beyond the certification minimums ? and the characteristics change with configuration, weight, and CG position etc ?

Quoting OldAeroGuy (Reply 16):
Since the C130 had a crash during Vmcg testing as a result of getting airborne, it's not a good argument for Vmcg - Go testing.

I disagree, the reason for the crash was the crew were not ready to control the yaw like you would be if you were go orientated. This was not a go-orientated test, the was supposed to be a ground run, however it demonstrates that during flight testing, you get surprises.

PROBABLE CAUSE: "Disengagement of the rudder fly-by-wire flight control system resulting in a total loss of rudder control capability while conducting ground minimum control speed tests. The disengagement was a result of the inadequate design of the rudder's integrated actuator package by its manufacturer; the operator's insufficient system safety review failed to consider the consequences of the inadequate design to all operating regimes. A factor which contributed to the accident was the flight crew's lack of engineering flight test training."

http://aviation-safety.net/database/record.php?id=19930203-1

Quoting OldAeroGuy (Reply 16):
Please provide some backup for this statement that a FBW change was responsible for the G650 crash. I find no mention of it in the acccident investigation Summary Conclusions (see Reply 10). You don't need a ptich axis FBW change to target higher V2 attitude and lower V2 speed.

The NTSB made various references to the performance requirements, and lists in one of the bullet point the FTE change to the FBW, from page 41 of the report.

The company‟s key engineering and oversight errors that likely resulted, in part, from schedule pressure and adversely affected the safety of G650 field performance flight testing included the following:
Gulfstream‟s willingness to proceed with takeoff performance testing before analysis of VMU test data was completed,
Gulfstream‟s development of takeoff speed schedules for Roswell II testing without adequate consideration of the VMU data from Roswell I testing,
Gulfstream‟s decision to experiment with pilot technique during high-risk testing rather than conduct a thorough analysis of the V2 overshoot problem,
FTE1‟s and the airplane performance principal engineer‟s decision to change the target pitch for flaps 10 OEI continued takeoff tests without determining the impact of the change on the takeoff speeds, and
FTE1‟s last-minute addition of a pitch limit without ensuring that this limit was adequately defined on applicable test cards.

It is this last minute pitch limit change in the FBW I was referring to, the NTSB said in their presentation the following

Flight Crew Response to Stall and Roll
• PIC’s column push after first stick shaker activation was appropriate
• Pitch was reduced below PLI, and stick shaker activation ceased
• Airplane remained in a stall that overpowered lateral controls
• PIC was likely confused by airplane’s response

The FTE change to the FBW pitch limit before the test, during the recovery the pilots reduced below the pitch limit stopping the stick shaker, however the aircraft remained in a stalled state. The FBW AOA limiter function disabled at the time as the software was incomplete. The NTSB said during a postaccident interview, the Gulfstream senior test pilot stated that he and other G650 test pilots relied on the PLI to prevent takeoff stalls during flight testing.

Quoting aerotech777 (Reply 19):
Even though the FAR and the advisory circular recommends to use only the rudder for the VMCG test, yet the EASA and FAA allowed Airbus to use spoilers and ailerons.

I think you are missing an important point, that FAR is talking about control inputs, not control surfaces. When it is taking about "rudder", it is talking about the rudder control input in the cockpit, limited to 150 lbf. It in entirely feasible to design an aircraft these days without a rudder control surface. In such an aircraft the rudder control input is telling the FBW they desire a yaw demand, the FBW then works out what it has available to generate that yaw demand.

Airbus only uses the rudder control input to control the aircraft laterally for the test, it is the FBW that activates the various control surfaces to generate the control surface movement based on the demand required by the pilots through the rudder pedals. The FAR does not say they are to use the the rudder control surface only, it is referring to the rudder control input. The only control input the pilots make is rudder, they are not making the spoiler inputs, the FBW system does this automatically in response to the rudder demand input.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
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