B737 High Altitude Stall Recovery Technique

[Centaurus]

When conducting high altitude stall recovery on stick-shaker in the B737 simulator (say 37,000 ft), it takes around 3000 feet of height loss to attain a comfortable minimum level flight speed.

This assumes idle thrust leading up to stick shaker. Rate of thrust increase is markedly slow at high altitude when increasing power as part of stall (stick shaker) recovery. Hence partly the reason for loss of 3000 feet while picking up to safe speed. The FCTM recommends VREF 40 plus 100 knots as a safe holding speed above 25,000 ft if no FMC but offers no guidance as to what minimum speed should be attained before recovery to level flight. By experimentation, Vref 40 plus 100 knots works out OK. Typically around 230-235 knots IAS.

Would appreciate your views on what is considered a minimum safe speed to attain before levelling out.

[Old Smokey]

Hi Centaurus,

I've done a lot of stall test flying at high altitudes.(Fully developed / Full back-stick stalls). None were on the B737 series.

The standard technique (as used by the manufacturer's test pilots) was to -

(1) Immediately apply Maximum Thrust,

(2) Gently lower the nose until the speed trend vector 10 knots per second increase, and

(3) Smoothly pitch up (to avoid a secondary stall) when speed reaches Minimum Manoeuvre Speed or VLS (1.3G) for glass cockpits.

These are the certification routines, and if it works for them.................

So there it is - Minimum Manoeuvre Speed (don't forget EAS effect at high altitude) for non-glass cockpits, or VLS for those with that facility.:ok: Whatever gives you 1.3G protection is the yard stick.:)

Regards,

Old Smokey

[CaptainSandL]

The only thing that I would add to Old Smokeys technique is to apply max thrust slowly and smoothly.

I am assuming that you are at a safe altitude and terrain is not a factor. Applying thrust slowly allows both engines to spool up symmetrically. We have all flown old aircraft in which either the engines (maybe intermixed) do not spool up together or there is a significant thrust lever stagger. The last thing you want is asymmetric thrust near VLS. Also slow & smooth application of thrust will reduce the chances of engine stall/surge.

S&L

Edit to add that when I do these from the stickshaker at approx 15,000ft the height loss is approx 1500ft so your 3,000ft at high altitude sounds reasonable. NB this is not a minimum height loss technique, it is a keep it safe technique!

[Old Smokey]

CaptainSandL,

I fully agree with you regarding the necessity to avoid thrust assymetry during the recovery.

During the (deliberate) stall testing, it was mandatory to ensure that (1) Engine Synch was ON to ensure symmetrical thrust, and (2) Feet were placed flat on the floor to avoid any inadvertant rudder deflection.

Regards,

Old Smokey

[John_Mc]

Old Smokey,

Just a PPL student myself but I find it interesting about the "feet flat on the floor" because when practising stalls in SEP's, any roll introduced by the stall should only be counter acted by rudder, to avoid entering a spin.

I was told this is because using roll to level the wings would further change the AOG of the wing. Is this not the same in Jets?

Thanks in advance,
John

[Sciolistes]

I was told this is because using roll to level the wings would further change the AOG of the wing. Is this not the same in Jets?
SEPs suffer from a helical prop wash around the empenage and so will roll once the ailerons loose authority. Swept wing jets however have an extremely strong roll effect in response to rudder initiating potentially considerable roll for which the only way to counter it maybe to apply opposite rudder. Loss of control is a distinct possibility.

Some Jets also utilise washout to encourage the tips to stall last (tapered wings stall tip first which is extremely problematic). They also utilise flight spoilers for roll that do not in themselves change the AOA like ailerons do.

[John_Mc]

Very interesting, thanks very much for the information :ok:

[Sunshine Express]

The rudder on big jets is purely used to counter asymmetric thrust. It is an extremely powerful flight control and aggressive use can over stress the tail itself. (eg. American Airlines A300 out of JFK).

Also big jets (eg. B777) have inboard flaperons near the wing root and outboard ailerons near the wing tip for roll control. At high speeds and altitude the outboard ailerons are locked out as they would be way more powerful than required. This will also prevent the wings going into a deeper stall when ailerons are used to control roll during a stall.

Finally, jets have a stick shaker to warn pilots when the aircraft is approaching a stall condition and a stick pusher to prevent the aircraft becoming fully stalled. Therefore the ailerons are still effective and safe to use as the aircraft is not fully stalled.

I hope this helps.

[kijangnim]

Greetings
I use to leave a little bit if thrust, no Idle, in order to have the CoT not too aft.
It takes longer but it is smoother :}

[Old Smokey]

John_Mc,

The reply given by Sciolistes is absolutely correct!:ok:

(1) When stalling in a STRAIGHT winged aircraft, actively apply rudder to keep the wings level, avoiding the use of roll control (Ailerons), and

(2) When stalling in a SWEPT winged aircraft, actively apply roll control to keep the wings level, avoiding the use of rudder.

Basically, it's one more opposite between props and jets, both intended to avoid uncontrolled wing drop and possible spin.

NOTE - If practicing "Approach to Stall" (Stick Shaker) in swept wing aircraft, you MAY get away with inadvertant rudder use. In the fully developed stall, such an action is LETHAL.:eek:

Regards,

Old Smokey

[bburks]

I just assisted in the design of a high altitude stall demonstration for a 737-800 at heavy weight (160,000 lbs.) at altitude (FL 370). We also invited test engineers as well as upset recovery experts to ensure we had constructed an accurate representation within the aerodynamic model of the level D full flight simulator we used.

As far as "learning points" as relates to the recovery:

1. At FL 370 the thrust available is approximately 10% of that available at Sea-level. Thrust will assist only marginally in the recovery, and then mostly to assure that once recovery has been effected, that airpseed can be maintained in level flight.

2. You will lose significant altitude (2500-4000 feet).

3. Once un-loaded, the jet (simulator) will continue to fly (nose-down, recommeded at least pitch to the horizon) but with very little margin (G) available. If a pilot tries to recover with a little back pressure prior to recommended recovery speed (we also use Vref 40 plus 100 knots), the jet will very quickly go back into stick shaker (very little AOA or G available for margin). The jet will "fly" un-loaded at .3 G's but pulling quickly brings it right back near approach to stall.

4. Keep the pitch attitude no higher than the horizon, assure airspeed is increasing, and do not recover for altitude until reaching nominal speed (as recommended by AFM).

5. Keep in mind that particularly with the 737-NG, you are thrust limited at altitude, and recovery to level flight below normal cruise speed could put the aircraft behind the power curve, creating more induced drag and a higher AOA than that required for normal cruise speed.

6. The primary flight control for recovery is the elevator. Most literature from in-flight upsets indicates over-aggressive inputs from pilots can induce more damage or injuries than the initial event. But the approach to stall must be recognized and recovered.

7. A full flight simulator cannot accurately replicate a full stall condition. If the simulator goes into "full stall" it is not representative of what the actual jet may do.

8. Remember, you are just "flying the simulator" not the real jet. However, such training is very beneificial to impart to pilots in training the importance of minding the jet and recognizing and avoiding these scenarios!

[John_Mc]

Very interesting stuff, thanks very much for your replies :)

[Pugilistic Animus]

just a touch of guilt for a long gone post that I hope that I have not misworded,..wrt to straight wing I said once,... stalling from banked attitudes with a straight wing keep wings level meaning ,...

.....not to apply the aileron in most cases [non-Frise type] meaning keep the attitude and maintain the ailerons neutral and use your rudder to overcome any gyroscopic or yaw effect,....


I think Old Smokey's admonition comes mainly because you would never want initiate a spin event through asymmetric stall:\ I guess then you may need the rudder 'cuz no one does it purposely:}
of course if not ,.... he will definitely expand:8

O_S great TP stuff ,...perfect for a wannabe like me:ok:

PA:)

[john_tullamarine]

The old master in such things (JF) has a number of posts in various threads relating to stalling. Recommend a search to review same.

Overall comment is along the lines that rudder should be used only to prevent yaw, unloading is essential, and then worry about recovering.

Game plan should be never to stall anywhere near terra firma.

The certification design standards require that aileron input can be used without too much concern.

As far as I see things, stalling is for test pilots to play with ... real world working fellows such as you and I should try to stay right away from such undesirable scenarios.