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
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
Pilot technique constant during test, always Stop oriented
Reduces test risk as airplane doesn’t rotate and remains on runway
Runway deviation easily determined
Need to be careful with brake temperatures
Requires separate test sequence
Takeoff verification test required after Vmcg determined
Vmcg Determined via Go
May be able to integrate Vmcg testing with other engine inoperative testing
Takeoff verification test may not be required
No brake cooling issues
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.
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.
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.