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High altitude stall recovery B737

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High altitude stall recovery B737

Old 15th Feb 2018, 01:05
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High altitude stall recovery B737

We know that if a high altitude stall occurs for whatever reason, the initial action is to level the wings and lower the nose; thus reducing the angle of attack. Simultaneously thrust is applied. For 20,000 lbs thrust CFM 56 engines at 37,000 ft the thrust available is about one quarter of that at sea level and the pitch up from thrust application is easily countered. Different of course during a go around at sea level

Once recovery from the stall is made (almost instantaneously if the angle of attack if reduced sufficiently and promptly) the question arises what minimum safe airspeed should be attained before the pilot eventually aims to level out? To quick a level out after stall recovery at high altitude invites secondary stalls as evidenced by continued buffeting. The Boeing FCTM does not address this clearly, especially as the pilot may be tempted to prematurely initiate levelling out for minimum height loss.

One clue may be found in the FCTM under Chapter 4, Climb, Cruise, Descent and Holding where in part, it states the following:

"Recommended holding speeds can be approximated by using the following guidance until more accurate speeds are obtained from the QRH.
Above FL250, use VREF 40 +100 knots to provide at least a 0.3 g margin to initial buffet (full maneuver ability)"

This suggests that once the airspeed reaches approximately 230 knots (depending on weight of course) during the course of recovery after lowering the nose, it would be safe to stop further descent and review the situation from there. It follows that if the stall was the result of mishandling due to severe turbulence and this severe turbulence was still present during the recovery process, the recovery to level flight should be delayed until reaching severe turbulence optimum penetration speed.

This recovery technique when used in the simulator results in a height loss of approximately 3500 feet. If using optimum severe turbulence penetration speed as the aiming point before recovery to level flight, then a height loss of approximately 5000 ft is needed.

Constructive comments welcomed.
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Old 15th Feb 2018, 01:52
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This isn't 737-specific (never flown the type) but if it's an EFIS one with an AOA-based stall cue on the airspeed tape, it's a perfect time to use it for live info in a dynamic situation where everything that matters is changing: airspeed, stall speed, elevator authority, actual G, and stall G.
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Old 15th Feb 2018, 10:54
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Forget the wings level bit first.


Disconnect ap
Disconnect at
Pitch down to -5 degrees
Trim
Full thrust
Roll wings level
Check soeedbrake lever down.

Wait for vref 30 plus 100 then recover
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Old 15th Feb 2018, 11:31
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the initial action is to level the wings and lower the nose
This from a highly experienced training captain? Gosh.
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Old 15th Feb 2018, 12:08
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Airbus also recommends pinhead method.
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Old 15th Feb 2018, 13:16
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This from a highly experienced training captain? Gosh.]
From the QRH:
Hold control column firmly.
Disconnect AP and AT.
Smoothly apply nose down elevator to reduce the angle of attack until buffet or stick shaker stops.
Nose down stabilizer trim may be needed. (See warning note re excessive use of pitch trim).
Continue recovery (See note)
Roll in the shortest direction to wings level if needed (see warning Note)
Advance thrust levers as needed.
Retract the speed brakes.

The critics have a point. Just testing you The order should be lower the nose while actioning all the other parameters asap.
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Old 15th Feb 2018, 13:29
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This is the danger of PPRuNe. Follow QRH / FCTM guidance. It is NOT full thrust initially due 737 thrust pitch characteristics and potential for secondary stall. Also swept wing jet aircraft have very different stall characteristics to a Cessna 150 exacerbated by high altitude / mach effect. AF447 changed the stall recovery philosophy and it is key that light aircraft and swept wing jets are different aerodynamically.
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Old 15th Feb 2018, 13:53
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Pitch down as necessary. Don’t be an numbers guy and pitch to X, pitch to what is needed.

Recovery? Read the language - 0.3G protection .... that is the lightest of back pressure. How many guys only pull at 0.3G at the minimum recovery speed? Very few, which is when the secondary stall, now an accelerated stall, occurs.
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Old 15th Feb 2018, 16:19
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Never heard -5 degrees before, only pitch to unstall the wing. Depending on how deeply the stall was developed that could be a very significant nose down attitude.

Iím sure someone will correct me but I have heard the A350 in one of its stall tests required almost 30 degrees nose down pitch to recover, in part due to some CofG issues, but still shows the point!
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Old 15th Feb 2018, 16:25
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At high altitude, with mach effect and less dense air coupled with swept wing aerodynamics the pitch attitude to unstall the wing is markedly below the horizon and will result in a considerable loss of height. This is one of the conclusions of the AF447 catastrophe, the fact that a stall condition was not recognised by an apparently well trained crew. The traditional 'cessna' style recoveries with minimum height loss are not applicable to jet transports. The height loss is a factor when related to terrain proximity which is part of general SA. QRH / FCTM applies and beware tips / techniques from unverifiable sources!
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Old 15th Feb 2018, 16:44
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Originally Posted by misd-agin View Post
Pitch down as necessary. Donít be an numbers guy and pitch to X, pitch to what is needed.
But then how could it be taught on a slideshow and tested by a multiple choice test, what are you out of your mind?!
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Old 15th Feb 2018, 19:03
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On a serious note, a big pet peeve of mine about this topic is when people call it a "high altitude, low energy" situation. It is not low energy, from 30,000 feet you have gobs of energy below you (namely, 1491 knots' worth) to use. That is, as long as you have the understanding and fortitude to use it.

Of course, I know they mean "low kinetic energy" and not "low total energy," but in that case they could have just said "low airspeed" with no loss of information. But no, they have to muddy the waters by using the fancy high tech sounding term. Unqualified "energy" is supposed to mean "total energy."

Last edited by Vessbot; 22nd Feb 2018 at 03:10.
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Old 15th Feb 2018, 20:25
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In my high altitude stall sessions, which notably just began a little over a decade ago during recurrent training, the recovery has never been immediate, particularly when the setup included plenty of NU trim by George prior to the stall.

Centaurus is quite right regarding the underslung engine pitch considerations at altitude - the onset of thrust is far slower than when down low and the sim modeling clearly shows this.

Speaking of sim modeling, according to the experience of NG test pilots, the buffet during the approach to a high altitude stall is so pronounced, one would have to be in a coma to miss it. This is an area where the simulator lacks fidelity.
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Old 15th Feb 2018, 20:51
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Originally Posted by RHS View Post
Iím sure someone will correct me but I have heard the A350 in one of its stall tests required almost 30 degrees nose down pitch to recover, in part due to some CofG issues, but still shows the point!
Are all modern type high altitude stall tested? I'd really be interested in seeing the video of a 380 or 777max stalled at fl400
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Old 16th Feb 2018, 01:45
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I must admit our -800 sim gives a very significant amount of buffet. Perhaps no where near that of an aircraft but I can't imagine not noticing..
In the simulator, that is a good reason for the instructor to follow up the high level stall recovery (37,000 in the case of the 737 CL) where heavy buffet occurs well before stick shaker actuation, with a stall recovery in the landing configuration at low altitude - typically ILS - where the stick shaker occurs and no buffet at all. The contrast between the two types of stall and recovery technique (clean high altitude and landing configuration on final), is quite astounding.

Replicating the Turkish Airlines B737 Amsterdam accident where the thrust levers closed due to a defective radio altimeter while flying the ILS is also most instructive.

There, the continuous back trimming of the stabiliser as the autopilot tries to maintain the aircraft on ILS glideslope at idle thrust means when the stall recovery is attempted when the stick shaker actuates while disconnecting the autopilot, considerable forward stab trim is needed for several seconds, coupled with a delicate balancing act in terms of elevator pitch control by the pilot to prevent critical height loss during the recovery phase from VREF minus 25 knots. This is where manual flying skills are absolutely vital; especially if the event happens in IMC or night.
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Old 16th Feb 2018, 01:57
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Speaking of sim modeling, according to the experience of NG test pilots, the buffet during the approach to a high altitude stall is so pronounced, one would have to be in a coma to miss it. This is an area where the simulator lacks fidelity
I agree with the NG test pilots comments. I don't think all simulators lack the fidelity. Certainly the buffet is very heavy in the 737 Classic simulator I am familiar with. Maybe it depends on the fidelity (serviceability) of each particular simulator?
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Old 16th Feb 2018, 04:00
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Low level recovery I target 5degrees NU with a mid thrust settings.

As mentioned target the pitch attitude, hold then possibly like Turkish, lots and lots of trim.
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Old 16th Feb 2018, 10:39
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Simulators are programmed to behave according to the data package which is given by the manufacturer based on test flight results and wind tunnel data. The buffet will not happen unless it is programmed. The device limitation may produce less but not more. Also about deep stall etc. Simulator behaviour is not authentic in the region outside the envelope where test pilot hasn't gone and no data is provided. So such exercises are misrepresentative of actual aircraft behaviour which no one knows.
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Old 16th Feb 2018, 13:53
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Dan, most manufacturers explore both the stall boundaries and after effects.
Depending on aircraft type, stall characteristics can vary widely. Small differences in build quality or maintenance, structural flexing, weight variation, control rigging, ... all add variability to many uncalculable aerodynamics effects.

See the NASA report relating to 737 stalling, including high altitude. There is reference to a simple stall model suitable for the task, but first define the task; awareness, recognition, avoidance, or recovery. Then provide the funding according to means and need, considering the ever present variability of the human. You could train everyone, but never be assured of the behaviour in operation.
And how many accidents have their been from unrecovered stall encounters in situations where recovery was feasible, i.e. not very low altitude, impaired controls, etc.

ďAll airline pilots agreed or strongly agreed that they were surprised by the surprise stall scenario. In that scenario, less than one quarter of the airline pilots strictly followed the proper stall recovery procedure on which they had been briefed. Less than half maintained a nose-down input until the stall warning stopped.
It may not matter what stall model you use if pilots are going to react inappropriately; better to avoid the situation altogether.

https://www.faa.gov/pilots/training/...r_Training.pdf
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Old 16th Feb 2018, 14:12
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This thread has gone from a simple discussion to the absurd! Deep stall discussion is similar to how low can you go. Simulators are toys programed to behave and react in certain ways that are predetermined. Stall should be recovered from with the least amount of altitude lost even if the buffet continues for several seconds longer. A buffet can occur at cruise when operating at almost maximum altitude and a bank of something around 20 degrees. Apply some thrust and minimum roll and the buffet is gone. I remember the days of training in light aircraft. Idle thrust hold altitude and wait of the buffet/horn. Keep the nose up and wings level in falls through. Don't change a thing, the nose falls, speed rebuild followed by another stall, again and again.

The discussion about don't roll wings level is really more airframe specific. Some use a spoilers for roll, that brings about a whole new topic.

Keep it simple and you will not kill yourself.
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