Is there a conflict in this NASA video

[mkusch]

After watching the NASA video, which re-surfaced with the Buffalo Q400 crash, a question from a friend arose about the possibility of conflicting information.

http://www.youtube.com/watch?v=_ifKd...om=PL&index=54

The NASA recommendation for tailplane stall recovery is to "pull back on yoke to reduce negative AOA at tail"

The clips I selected from the NASA video on tailplane icing show a pre-stall tail angle of attack (TAOA) of -15.57 at 15:48 into the video. During the stall recovery at 15:55, the TAOA is -19.77 with the yoke back. It appears that pulling back on the yoke actually increases the negative AOA.

Doesn't the video contradict the recovery procedure they recommend?

 

[mkusch]

Here's the animation of Colgan flight 3407 during the accident sequence.

http://www.youtube.com/watch?v=lxywEE1kK6I&feature=fvsr

Was Captain Renslow pulling back on the yoke in order to maintain his altitude during the stall recovery? Since they never did stick pusher training in the sim, I'm wondering if, based on company training, he was concerned about losing a substantial amount of altitude if he lowered the nose. His nose high attitude kept him right around 2,300 feet (towards the end it wandered to 2,500 feet).

For you airline pilots out there...what is the purpose of a company training to maintain altitude during stall recovery? My thinking is that if you have sufficient altitude, why not use some of it to regain flying speed?

Here's a section from one of the NTSB reports (pg 35):

http://www.ntsb.gov/dockets/aviation...027/417441.pdf

5.1.4.3 Stall Training Q400

"The Colgan Air DHC-8 Q400 Series Simulation Training is described in Appendix F in the Training Manual, pages F-1 to F-13. The training described is for Q400 initial, transition, upgrade, recurrent and requalification pilot simulator training. Flight training events are described in detail for eight flight simulator training modules, the last of which is a check ride.

Stall training in the Q400 simulator training is conducted in lessons 1, 4 and 7 and stalls are evaluated on the proficiency check. The following training for stalls is programmed:

• Lesson One - Approach to Stall – takeoff, enroute, and landing configuration
• Lesson Four – Approach to Stall – takeoff and landing configuration
• Lesson Seven – Approach to Stall – landing configuration

5.1.4.4 Stall Profiles

Illustrations of the three stall profiles to be flown are depicted in the CFM, section ten, revision 1, pages 7-9.

The clean stall, which refers to a stall performed with gear and flaps retracted, is entered from an airspeed of 180 kts, at a minimum altitude of 5000’ AGL, and with power at flight idle. The pilot flying (PF) calls out “stall,” advances power to the rating detent, and states “check power.” The profile says that during the stall the PF is to maintain heading and altitude. Power should be adjusted to maintain 180 kts to exit the maneuver.

The takeoff stall is entered at 180 kts, at a minimum altitude of 5000’ AGL, with flaps set to 15°, gear down, and power at flight idle. The PF is to maintain heading and altitude during the maneuver and begin a 20° bank turn at 120 kts. The PF calls out “stall,” advances power to the rating detent, rolls wings level, and states “check power.” The PM calls out “positive rate,” the PF calls “gear up,” the PM calls “Vfri,” and the PF calls “flaps FACTUAL REPORT DCA09MA027350.” Power should be adjusted to maintain 180 kts to exit the maneuver.

The landing stall is entered at 180 kts, at a minimum altitude of 5000’ AGL, with flaps set to 35°, gear down, and power at flight idle. The PF is to maintain heading and altitude during the maneuver. The PF calls out “stall,” advances power to the rating detent, and states “check power, flaps 15.” The pilot monitoring (PM) calls out “positive rate,” the PF calls “gear up,” the PM calls “Vfri,” and the PF calls “flaps 0.” Power should be adjusted to maintain 180 kts to exit the maneuver.

No reference is made to use of the autopilot.

It was stated in several interviews, that during the stall recovery exercises for initial simulator training, the candidates were instructed to maintain an assigned altitude and complete the recovery procedures while not deviating more than 100’ above or below the assigned altitude, as this was the practical test standards (PTS) for the check ride."

Here's a section from this NTSB report (pg 27):

http://www.ntsb.gov/dockets/aviation...027/431210.pdf

"The PTS standard is to recover from the stall with a minimal loss of altitude. See Operations Group Chairman Factual Report, pp. 36-37. During the stall recovery exercises for initial simulator training, the candidates were instructed to maintain an assigned altitude and complete the recovery procedures with minimal altitude loss."

 

[tgrayson]

The clips I selected from the NASA video on tailplane icing show a pre-stall tail angle of attack (TAOA) of -15.57 at 15:48 into the video. During the stall recovery at 15:55, the TAOA is -19.77 with the yoke back. It appears that pulling back on the yoke actually increases the negative AOA.

Looks to me that the increase in negative AoA occurs during the pitch down; after the pilot pulls back aggressively on the yoke, the AoA grows rapidly less negative.

Nose down pitching will increase the negative AoA on the tail, because it creates a relative wind that comes from the direction of rotation.

 

[mkusch]

Looks to me that the increase in negative AoA occurs during the pitch down; after the pilot pulls back aggressively on the yoke, the AoA grows rapidly less negative.

Nose down pitching will increase the negative AoA on the tail, because it creates a relative wind that comes from the direction of rotation.

How about this as a general question, if the relative wind remains constant over a horizontal stabilizer and up elevator is applied, doesn't that result in a change in angle of attack of the airfoil due to a chord line change?

 

[tgrayson]

How about this as a general question, if the relative wind remains constant over a horizontal stabilizer and up elevator is applied, doesn't that result in a change in angle of attack of the airfoil due to a chord line change?

No, but that's because of the definition of AoA that you're using. The normal definition of AoA is the angle between the relative wind and the chordline of the unaugmented airfoil, meaning without flaps (and the elevator is a flap.)

It would make the data rather confusing if they kept changing the implied chordline whenever the elevator position changed. If you look at airfoil lift curves, they measure the AoA as I describe, and adding flaps will increase lift at every AoA, but the AoA doesn't change.

Some branches of aerodynamics do use a different definition: the angle between the relative wind and the zero lift line of the airfoil. This is called the "absolute" AoA and it will change with flap setting. But it's mostly stability and control studies that use this definition, since it makes the math easier.

For any publication, you really have to read their definitions carefully so that you understand what's being measured.

 

[mkusch]

No, but that's because of the definition of AoA that you're using. The normal definition of AoA is the angle between the relative wind and the chordline of the unaugmented airfoil, meaning without flaps (and the elevator is a flap.)

It would make the data rather confusing if they kept changing the implied chordline whenever the elevator position changed. If you look at airfoil lift curves, they measure the AoA as I describe, and adding flaps will increase lift at every AoA, but the AoA doesn't change.

Some branches of aerodynamics do use a different definition: the angle between the relative wind and the zero lift line of the airfoil. This is called the "absolute" AoA and it will change with flap setting. But it's mostly stability and control studies that use this definition, since it makes the math easier.

For any publication, you really have to read their definitions carefully so that you understand what's being measured.

So you are saying this would be a faulty diagram (crude as it may be) of the change taking place in AOA when up elevator is applied based on an incorrect chord line application? Thanks for your help.

 

[SpiceWeasel]

No one seems to find it too remarkable that CA Renslow did exactly as we were trained. I have no idea how Colgan did its stall series, but, prior to the crash, and changed VERY quickly afterwards, our stall recovery was:

max power spoilers in
pull up aggressively respecting the stick shaker
don't lose altitude (100 ft is a bust)

I can't emphasize this enough - we were taught to power out of the stall.

 

[tgrayson]

So you are saying this would be a faulty diagram (crude as it may be) of the change taking place in AOA when up elevator is applied based on an incorrect chord line application? Thanks for your help.

Rather than "incorrect", can I say "non-standard", and not corresponding to the definition used by the studies under discussion? And, perhaps, "not useful."

Your drawing is one commonly used in pilot literature to describe how flaps work, but it's more to provide an intuitive understanding, rather than defining what AoA is. For pilots, it doesn't matter so much, but if you maintain that understanding when reading aerodynamic literature, you'll get really confused.

I have the printed document this NASA study came from and it's a wonderful report. If you take the time to read through it, you can learn more than you can from the video. On page 12, they define the tail AoA:

AoA_t = AoA_a/c - epsilon - i_tail.
​

epsilon is the downwash angle from the main wing onto the tail, and i_tail is the tailplane angle of incidence. AoA_a/c is, of course, the AoA of the main wing. Note what's missing? Any mention of the elevator angle. This formula can only be understood by the assumption that AoA as used in the document uses a fixed chordline.

 

[Roger, Roger]

No one seems to find it too remarkable that CA Renslow did exactly as we were trained. I have no idea how Colgan did its stall series, but, prior to the crash, and changed VERY quickly afterwards, our stall recovery was:

max power spoilers in
pull up aggressively respecting the stick shaker
don't lose altitude (100 ft is a bust)

I can't emphasize this enough - we were taught to power out of the stall.

I find it hard to believe that what CA Renslow did was "respecting the shaker"

Were you also taught to pull flaps to zero before full recovery, as FO Shaw did?

 

[mkusch]

No one seems to find it too remarkable that CA Renslow did exactly as we were trained. I have no idea how Colgan did its stall series, but, prior to the crash, and changed VERY quickly afterwards, our stall recovery was:

max power spoilers in
pull up aggressively respecting the stick shaker
don't lose altitude (100 ft is a bust)

I can't emphasize this enough - we were taught to power out of the stall.

After reading quite a few NTSB documents, I think that the stall training test criteria is insane, "don't lose altitude (100 ft is a bust)". I don't want to see CA Renslow made out to be a bumbling idiot in their findings while airlines are telling pilots to maintain altitude. It's sad that the air beneath them was used to descend to their death rather than for a stall recovery. I hope that future airline training includes, lowering the nose, altitude permitting, to increase airspeed.

 

[Roger, Roger]

After reading quite a few NTSB documents, I think that the stall training test criteria is insane, "don't lose altitude (100 ft is a bust)". I don't want to see CA Renslow made out to be a bumbling idiot in their findings while airlines are telling pilots to maintain altitude. It's sad that the air beneath them was used to descend to their death rather than for a stall recovery. I hope that future airline training includes, lowering the nose, altitude permitting, to increase airspeed.

Oh, we've been over this time and again.

"You don't know anything about big airplane aerodynamics, keep your mouth shut, you're just a CFI, blah blah blah".

All I can say is, watching that animation was painfully similar to things I've seen private students do in their power-on stalls.

 

[SpiceWeasel]

I find it hard to believe that what CA Renslow did was "respecting the shaker"

Were you also taught to pull flaps to zero before full recovery, as FO Shaw did?

Well, it isn't uncommon under our old technique to get the pusher... but you just let go of the pressure, wait for the pusher to cease, then pull back again. The shaker would be going off the entire time until you got the airplane flyable (you're simply pulling back as much as you can without getting the pusher).

Now, consider this: when we do it in the sim, we know our life isn't on the line, and we're fully prepared for the stall because we induce it by slowing down, adding flaps, etc.

Put a tired crew out on final approach on a dark night, everyone is tired, and try again... the CA pulls back too hard, gets the pusher, and thus begins the standard oscillation that I've seen throughout my newhire training. Then the FO panics, because she is expecting her part of the stall recovery (positive rate, flaps 8 gear up for us) and does it too early without prompting.

They screwed up - yes. The airline should be culpable in its training. You'll notice how they threw the crew under the bus. Well, if they were doing everything right on their end, we wouldn't have immediately ditched the stall profiles we had.

Back in 2004 when an RJ was lost due to a climb to 41000 feet, engine failure and core lock, one of the first things to change was that we're restricted to 37000 feet. Does this fix a symptom or a problem? "Instead of figuring out why they didn't understand high altitude aerodynamics, we'll just quietly revise the books and not let people get that high up."

We're back to 39000 feet approved, but the stars have to align for us to be able to accept that as an altitude.

 

[tgrayson]

I think that the stall training test criteria is insane, "don't lose altitude (100 ft is a bust)".

The airline stall recovery typically means recovering at the stick shaker, when the aircraft isn't really stalled. Adding thrust and adjusting the yoke to maintain level flight will reduce the AoA, because it will taking increasing forward pressure on the yoke to keep from climbing. Preserving altitude in this scenario is perfectly reasonable.

 

[SpiceWeasel]

The airline stall recovery typically means recovering at the stick shaker, when the aircraft isn't really stalled. Adding thrust and adjusting the yoke to maintain level flight will reduce the AoA, because it will taking increasing forward pressure on the yoke to keep from climbing. Preserving altitude in this scenario is perfectly reasonable.

But how to adjust the yoke to maintain level flight? Do you pull back? Push forward? What do you do?

 

[mkusch]

I find it hard to believe that what CA Renslow did was "respecting the shaker"

Were you also taught to pull flaps to zero before full recovery, as FO Shaw did?

No doubt many mistakes were made by the crew, but why should the company get a free pass on training when they didn't even provide full stall recoveries in the sim involving the stick pusher (were they afraid someone was going to crash their sim?). It reminds me of the saying...doctor it hurts when I do this and doctor replies...don't do that. I hope they had the placard posted which stated "don't stall our planes".

 

[tgrayson]

But how to adjust the yoke to maintain level flight? Do you pull back? Push forward? What do you do?

I think we're flipping back and forth between regular stalls and tail stalls.

 

[SpiceWeasel]

I think we're flipping back and forth between regular stalls and tail stalls.

Regular stall.... what do you do? Pull back? That's what we were taught...

 

[mkusch]

All I can say is, watching that animation was painfully similar to things I've seen private students do in their power-on stalls.

Did you teach them to maintain altitude during the recovery?

 

[tgrayson]

Regular stall.... what do you do? Pull back? That's what we were taught...

Does your airplane have a nose down pitching moment with increase thrust? (What airplane?)

 

[Roger, Roger]

No doubt many mistakes were made by the crew, but why should the company get a free pass on training when they didn't even provide full stall recoveries in the sim involving the stick pusher (were they afraid someone was going to crash their sim?). It reminds me of the saying...doctor it hurts when I do this and doctor replies...don't do that. I hope they had the placard posted which stated "don't stall our planes".

Oh, I absolutely agree with you. The stall recovery training they were given was somewhere between "unrealistic" and "dangerous".

Did you teach them to maintain altitude during the recovery?

No, I taught them approximately the same thing that everyone teaches in little airplanes:

Pitch to approximately level while verifying full power.

Stop yaw with rudder.

Keep ailerons neutral until the airplane is flying again.

However, no matter how many times you TELL a student something, the first ATTEMPT usually involves the instinctive response of yanking back on the yoke while trying to steer with the ailerons. Which, if you watch that animation, is EXACTLY what CA Renslow was doing.