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BEA Recommendations - AF447 Part 2

Mon Jul 30, 2012 1:25 am

Continuing the discussion from the following thread:

BEA Recommendations - AF447 (by tommytoyz Jul 5 2012 in Tech Ops)
 
flyingturtle
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RE: BEA Recommendations - AF447 Part 2

Mon Jul 30, 2012 10:36 am

Can't easily quote you words, Tommytoyz, as this is a new thread...

You are comparing two different aerodynamic configurations with each other. Same as if you compared an airliner with flaps extended and one without flaps. Critical AoA and target AoA change because of the aerodynamic changes to the plane, not because of the weight changes.

A clean F-18 Vs. an F-18 with a bunch of external stores and fuel tanks, are two very different birds aerodynamically.


Yes. But the aux tanks and the weapons do not contribute to the lift as flaps and slats do. Weapons and auxiliary tanks surely contribute more to the weight than to lift and drag.

But now consider a F-35 with nearly empty tanks and empty weapon stations, and a F-35 with a 2 ton concrete weight in its internal bay.

No different configuration there. No flaps, no extended gears, no slats...

And now you want to tell me that they can fly the same AoA while holding speed and altitude constant? As I explained to you, AoA depends on the lift you need to maintain constant speed and constant altitude, because lift must equal weight for that purpose.

I am pretty sure (without seeing it) that the F-18 AoA system does not compensate nor adjust for weight.

I'm pretty sure about that, too. 

Because AoA values are different for different weight configurations, if you hold all other factors equal.

But if memory serves me correctly, at least some Navy aircraft need to jettison bombs and auxiliary tanks before they land on the carrier. So they land at +/- the same weight, giving them the same target AoA to fly during the approach. But I can assure you that a F-18, loaded up to MTOW, is a pretty lame duck with a high pitch.

(Remembers me of a German Air Force Tornado that crashed some years ago in Switzerland. It had been refuelled just some 20 mins or so before, it was heavy and there was no easy way out of the valley: http://www.focus.de/politik/ausland/bundeswehr_did_15227.html )


David
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flyingturtle
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RE: BEA Recommendations - AF447 Part 2

Mon Jul 30, 2012 6:14 pm

...and just a little suggestion:

Rent a GA airplane, one that is equipped with a simple autopilot - one with ALT HOLD and HDG HOLD modes.

And do some flying in several weight configurations, but at the same altitude.

Repeat it with constant weight, but varying altitudes.

You will soon see how AoA depends on airspeed and air density. And you will see that in any UAS incident, you *will* need a table that gives you the right target AoA depending on:

- weight
- altitude
- flaps configuration

There is simply no "correct" AoA. Keep it on the 3 o'clock position? Dang! The NTSB report will be a fascinating read. I hope they'll publish this a.net thread as Annex 1.

So, to begin with, you would fly AoA and you would need a working AoA indicator - and a table giving you the right AoA.

While with pitch & power, you do not need that indicator, and you would still have a table giving the right thrust and pitch settings. So the latter technique is more robust.



David
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RE: BEA Recommendations - AF447 Part 2

Mon Jul 30, 2012 8:18 pm

Quoting flyingturtle (Reply 2):
You will soon see how AoA depends on airspeed and air density. And you will see that in any UAS incident, you *will* need a table that gives you the right target AoA depending on:

I meant weight, not airspeed.


Sorry for bumping the thread. 


David
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RE: BEA Recommendations - AF447 Part 2

Mon Jul 30, 2012 10:46 pm

Quoting flyingturtle (Reply 1):
And now you want to tell me that they can fly the same AoA while holding speed and altitude constant?

Not at all. The velocity varies, depending on weight and altitude, even though the target AoA remains the same for a particular configuration, regardless of weight and altitude. Critical AoA is also the same (excluding compressibility effects), regardless of weight and altitude. Basic AoA 101.
 
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 1:27 am

Quoting tommytoyz (Reply 4):
The velocity varies, depending on weight and altitude, even though the target AoA remains the same for a particular configuration, regardless of weight and altitude.

But we're talking about a situation with a dead air data system. If you just fly to an AoA value without adjusting for anything else (weight, altitude, config) then you don't know how fast you're going and, more crucially, you don't know if you're gaining or losing altitude.

If your target AoA is, say, 4 degrees and you capture that but you don't know what thrust to set (in other words, you don't know the pitch/power) you have no idea if you're climbing, level, or descending. You could happily fly into the ground or ocean doing that.

Tom.
 
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 2:52 am

Quoting tommytoyz (Reply 4):

You would still need a table to look up the correct AoA given your current airspeed/weight/altitude/aircraft configuration.

Which is no improvement upon pitch & power.

Quoting tdscanuck (Reply 5):

As a suggestion to Tommytoyz: FlightGear has an autopilot mode where you can select a constant AoA. Play around with it.   


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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 8:43 am

Quoting tdscanuck (Reply 5):
If your target AoA is, say, 4 degrees and you capture that but you don't know what thrust to set (in other words, you don't know the pitch/power) you have no idea if you're climbing, level, or descending. You could happily fly into the ground or ocean doing that.

These are extreme examples. Vertical Speed, Barometric Altitude and probably GPS Altitude were available to AF447 at all times, so descending into the ocean would not have been a problem if they had flown target AoA and set power to just about anything initially. Even if they had cut power to idle, how long would it have taken to descend 36,000 feet all the way down? Even in this extreme example, I bet over 40 minutes. No pilot in their right mind would fly 40 minutes at idle power.

The main thing is to fly the target AoA to avoid a stall and fly correct speed no matter the weight or configuration. Secondary is control the glide slope with the correct power setting. Power settings for glide slope can be verified against the charts and the VS, once the checklist is run.

What's interesting, is that even when the pitch angel was nose down on AF447, which was for about 20 seconds, the AoA still remained off the charts for that period. Once you get way off AoA wise, pitch/power alone is not adequate to get back it back to a safe AoA, IMHO.

Quoting flyingturtle (Reply 6):
You would still need a table to look up the correct AoA given your current airspeed/weight/altitude/aircraft configuration.

No. You need to rethink everything.

The target AoA does not change, no matter the weight or the altitude and adjusts automatically for configurations changes. This will put you at a safe speed. You fly the same target AoA under the same configuration, regardless of weight, altitude or power setting. If you fly the target AoA, the speed is always safe. You might climb or descend, but your speed will be correct and control your glide slope with power settings.

Power charts, Vertical Speed, altimeter or GPS altitude - any of these can be used to set power. But you fly the same target AoA under the same configuration - always. The green band is displayed on the AoA. No need to look up target AoA. It's right in front of you.
 
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 9:32 am

Quoting tommytoyz (Reply 7):
The main thing is to fly the target AoA [...] Secondary is control the glide slope with the correct power setting

So you're suggesting to fly AOA & power...
First stabilise by setting a gross target AOA, then refine your settings by looking up the proper values in tables given in the documentation

Wow, that's so much better than pitch & power
 
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 9:40 am

Quoting tommytoyz (Reply 7):
Once you get way off AoA wise, pitch/power alone is not adequate to get back it back to a safe AoA, IMHO.

They got themselves into that situation by apparently not formally recognising an "Unreliable Airspeed situation", by not flying pitch and power and by not responding appropriately to the aural stall warning. They missed all the cues from instruments with which they were familiar. They didn't know which instruments to trust.
 
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 9:52 am

Quoting tommytoyz (Reply 7):
The main thing is to fly the target AoA to avoid a stall and fly correct speed no matter the weight or configuration.

How do you propose to fly the correct speed without a valid speed indication?

Quoting tommytoyz (Reply 7):
Power settings for glide slope can be verified against the charts

One of the major 'benefits' you propose of using an AoA indicator is not having to consult charts, as you do in the pitch-power procedure. In fact, on all Airbus airraft you do not need to consult a chart for the initial pitch/power setting. It is a memory item, and in the case of the A330 calls for 5 degrees pitch and set the thrust levers into the CLB detent. You may actually gain some altitude during this maneuver, but it still offers a safe margin to the stall AoA limit as your Flight Path Vector will stabilise depending on your gross weight and configuration. Once stabilised at 5/CLB, you can consult the QRH and start troubleshooting.

It just seems, that setting 5 degrees nose up pitch and CLB is as easy as setting the AoA into the green band, with the added advantage that it is the procedure that is taught from your very first flying lesson.

I also think that potentially it could be very easy to 'chase' a raw data AoA needle when in a unusual situation such as these guys found themselves in, and end up with some PIO and possibly making a bad situation much worse.

Quoting tommytoyz (Reply 7):
Vertical Speed, Barometric Altitude and probably GPS Altitude were available to AF447 at all times,

So, once again, what makes you think that these guys having an AoA indicator would have made any difference? If they can't even acknowledge that their most basic instruments are telling them they are stalled then they sure as hell aren't going to turn to an unfamiliar and potentially still incorrect instrument reading.
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 12:03 pm

Quoting tommytoyz (Reply 7):
The main thing is to fly the target AoA to avoid a stall and fly correct speed no matter the weight or configuration.

If airspeed is out and you're not allowed to use a table, what is the correct speed?

Quoting tommytoyz (Reply 7):
The target AoA does not change, no matter the weight or the altitude and adjusts automatically for configurations changes. This will put you at a safe speed.

Not necessarily. Especially at altitude, it's trivially easy to overspeed at constant AoA because a phugoid is a constant AoA maneuver.

Quoting tommytoyz (Reply 7):
If you fly the target AoA, the speed is always safe.

No. This is flat out incorrect. Suppose you're at 4 degrees AoA...there are flight conditions for that AoA that are undersped and oversped.

Quoting tommytoyz (Reply 7):
The green band is displayed on the AoA. No need to look up target AoA. It's right in front of you.

It's a *band* because the airplane doesn't know. It's usually a pretty wide band too; it will put you in a safe range if you're in level stable flight (i.e. the opposite of AF447) but it won't give you a particular value.

Tom.
 
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 8:27 pm

Quoting tdscanuck (Reply 11):
it will put you in a safe range if you're in level stable flight (i.e. the opposite of AF447) but it won't give you a particular value.

No. AoA has the advantage over pitch/power that it shows 1) that your AoA is off and 2) in which direction and 3) by how much, and 4) the effects of your corrective action. Pitch/Power tells you zero on these.

You encounter a wind shear and are clueless. Or you realize your pitch/power was wrong and want to correct, but you don't know by how much you should correct or the effects of your corrections. Simply going back to the correct pitch/power may delay your recovery severely. With AoA you can correct immediately and appropriately and bring the AoA back to a safe value.

I know Tom likes to take a snapshot of when the AoA was well past 30 degrees on AF447 as "evidence". But that would imply the crew would have completely ignored the AoA, and only at that moment, started paying attention to it. Used that way, any instrument is useless or dangerous.

Quoting tom355uk (Reply 10):
on all Airbus airraft you do not need to consult a chart for the initial pitch/power setting.

Same could be done with AoA under UAS. Power setting a memory item. Then if you want to adjust your glide slope, you can use VS or GPS altimeter, say going from cruise to approach and landing. I am not pretending to write a SOP here. I am not and will not. Suffice it to say, using AoA under UAS is safer and no harder for pilots than pitch/power. That's all I am saying here.

BUSS displays AoA and works very similar as I have advocated under UAS and the AoA on American Airlines displays all the time. Maybe there is a good reason? The are speed independent AoAs mind you. BTW, I had a few conversations with BF Goodrich on this.

Quoting tom355uk (Reply 10):
So, once again, what makes you think that these guys having an AoA indicator would have made any difference? If they can't even acknowledge that their most basic instruments are telling them they are stalled then they sure as hell aren't going to turn to an unfamiliar and potentially still incorrect instrument reading.

That is a fair question and goes to the heart of this. I think yes, it would have helped and avoided the accident. They were not ignoring their instruments, IMHO. They acknowledged many of them verbally and their indications and tried to make sense of them. Being confused is not the same thing as ignoring the instruments. But we won't really know what was going on between their ears.

IMHO an AoA would have shown them why that the stall warning sounded and that it was due to reaching Critical AoA and that they were not over speeding. It took about 100 seconds for the AoA to travel from normal to 30 degrees, after which the AoA was useless. And if the AoA is always displayed as on some American Airlines planes now, it would not have skipped a beat and they would have seen it traveling off target, as it happened, if they had so much as glanced at it at any point of those 100 seconds.

If we assume the crew ignored instruments, then nothing would have helped. If we assume they were confused, but were paying attention, it would have helped, IMHO. I think the latter is the case. Maybe they were looking at the FD or were confused by it....it could have been anything. They were clearly confused for some reason. But looking at an AoA or BUSS would have told them they were doing something wrong and how to correct it.

Quoting tdscanuck (Reply 11):
If airspeed is out and you're not allowed to use a table, what is the correct speed?
Quoting tom355uk (Reply 10):
How do you propose to fly the correct speed without a valid speed indication?

I think this thread should stop here, because we're going in circles. If it is not yet fully understood, after all these posts, that the target AoA, as determined in testing, puts the plane at the correct air speed to maintain that target AoA - I throw my arms up. My work is done.

Happy flying.

[Edited 2012-07-31 13:28:35]

[Edited 2012-07-31 13:33:34]
 
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 8:58 pm

Quoting tommytoyz (Reply 12):
If it is not yet fully understood, after all these posts, that the target AoA, as determined in testing, puts the plane at the correct air speed to maintain that target AoA - I throw my arms up.

Two simple questions to clear up the misunderstanding:
-Do you know what a phugoid is?
-Do you know what happens to airspeed and AoA during a phugoid?

Tom.
 
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 9:27 pm

Quoting tdscanuck (Reply 11):

It's a *band* because the airplane doesn't know. It's usually a pretty wide band too; it will put you in a safe range if you're in level stable flight (i.e. the opposite of AF447) but it won't give you a particular value.

Out of curiosity I had a look at the corrected AoA values computed by the ADRIU in flight. Clean during climb the average was around 1.9 degrees, in cruise around 2.3 degrees. In light turbulence the AoA varied by up to 2 degrees without the CAS/Mach moving. During descent, the values crept up to 5 degrees, during approach over 8 degrees, on final approach back to around 2.3 degrees.

During turns the values increased by up to one degree. There was always a split between the measured values between the AoA sensors, this split averaged around .5 degree in cruise, it was worst in turbulence and and in turns.
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RE: BEA Recommendations - AF447 Part 2

Tue Jul 31, 2012 10:36 pm

Quoting zeke (Reply 14):
Clean during climb the average was around 1.9 degrees, in cruise around 2.3 degrees. In light turbulence the AoA varied by up to 2 degrees without the CAS/Mach moving. During descent, the values crept up to 5 degrees

So, supposing that you were trying to fly a target AoA of, say, 2.5 degrees, what would that flight path look like? I can't imaging it being stable.

Tom.
 
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RE: BEA Recommendations - AF447 Part 2

Wed Aug 01, 2012 11:20 am

Quoting zeke (Reply 14):

   That's the kind of first-hand information that makes Tech/Ops what it is. Thanks for that.
 
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RE: BEA Recommendations - AF447 Part 2

Wed Aug 01, 2012 11:48 am

Quoting tdscanuck (Reply 15):

So, supposing that you were trying to fly a target AoA of, say, 2.5 degrees, what would that flight path look like? I can't imaging it being stable.

IMHO I do not this it would not be possible on the raw data, the values I saw were continuously changing (atmospheric noise), and they were split between the actual vanes. I am sure the BUSS software uses some form of rate limiter and moving average to make sense of the raw input. BUSS is only for 1g flight as well, and adjusts based upon configuration.
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RE: BEA Recommendations - AF447 Part 2

Wed Aug 01, 2012 6:49 pm

Quoting tommytoyz (Reply 12):
No. AoA has the advantage over pitch/power that it shows 1) that your AoA is off and 2) in which direction and 3) by how much, and 4) the effects of your corrective action. Pitch/Power tells you zero on these.

I don't think you understand much about aerodynamics and stability. Assuming you recognise and recover from any stall condition first, if you set power and maintain pitch you will end up in a safe flight condition, which you can then easily maintain. With pitch and power you aren't correcting for anything, you are setting up stablised flight parameters. The aerodynamics and stability of the aircraft will take care of the rest.

Tom has told you several times that at the kind of very low airspeed AF447 got down to AoA indication would not tell you anything, certainly not which direction to recover in.
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RE: BEA Recommendations - AF447 Part 2

Fri Aug 03, 2012 3:43 pm

... and they all lived happily ever after.

The End.
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