STALL Update Procedure - Aerodynamic Review

Scrap from 17th Performance and Operations Conference in Dubai

Aerodynamic Review

STALL

•  A loss of speed can result in an aircraft reaching the stall AoA
•  BUT it remains an AoA issue

Stall is only an AoA problem [AoA = Angle of Attack]

• Low speed is a common contributing factor

AoA Control –Pitch control effect

The pitch control is a direct AoA command

• The elevators control DIRECTLY the AoA.
• A nose down command has an IMMEDIATE effect:

AoA Control –Thrust effect

• Aircraft with engines below the aircraft Center of Gravity
⇒ Thrust has a significant pitch effect

Stall Recovery

When Aircraft is stalled

•  FIRST: AoA MUST BE REDUCED
•  Release back pressure on stick or column
•  Nose down pitch input may be needed

 Note: Increasing thrust has an adverse effect on AoA reduction for Aircraft with engines below aircraft CG

•  SECOND: If speed needs to be recovered
•  When stall indications cease, increase thrust with care due to possible pitch up effect

  AoA comes first, speed  second

The change from approach to stall versus actual stall is not easy to determine, even for a specialist.

Approach to Stall versus Stall recovery procedure

• The classic recovery procedure associated with “Approach to stall” was characterized by a recovery focusing on:
-  Maximum thrust application
-  Minimum loss of altitude

• Whereas the recovery procedure from an “Actual stall” has always focused on:
-  AoA reduction as first action, followed by a speed recovery if needed

 Need for procedure change

• Drawbacks associated with this dual recovery approach are:
- TOGA application may lead to an increaseof the AoA
- Reluctance to apply nose down input
- Challenge to identify the change from “Approach to stall” to “Actual stall”

• Numerous accidents where the “Approach to Stall” recovery procedure
- was applied whereas the aircraft was actually stalled
- or generated a stall through thrust effect

A single procedure focusing on AoA reduction,           

• as a first action, required to cover both
• the “Approach to Stall” and the “Actual Stall” recovery:

It is called the “Stall Recovery” procedure

New Procedure

The FAA Stall Recovery Working Group issued a generic “Stall Recovery” procedure

•  A generic procedure for ALL types of aircraft
•  One single procedure to cover ALL stall conditions
•  Prevent full thrust/TOGA from being first action
•  Focus on AoA reduction

 Generic Stall Recovery Procedure

STALL WARNING OR AERODYNAMIC STALL RECOVERY PROCEDURE

Immediately do the following at the first indication of stall (buffet, stick shaker, stick pusher, or aural or visual indication) during any flight phases except at lift off.

1. Autopilot and autothrottle…………………………….…… Disconnect

Rationale: While maintaining the attitude of the aircraft, disconnect the autopilot and autothrottle. Ensure the pitch attitude does not change adversely when disconnecting the autopilot. This may be very important in mis-trim situations. Manual control is essential to recovery in all situations. Leaving one or the other connected may result in in-advertent changes or adjustments that may not be easily recognized or appropriate, especially during high workload situations.

2.
a) Nose down pitch control…Apply until out of stall (no longer have stall indications)

b) Nose down pitch trim…….………………………………………..As needed

Rationale:

a) The priority is reducing the angle of attack. There have been numerous situations where flight crews did not prioritize this and instead prioritized power and maintaining altitude. This will also address autopilot induced full back trim.

b) If the control column does not provide the needed response, stabilizer trim may be necessary. However, excessive use of trim can aggravate the condition, or may result in loss of control or in high structural loads.

3. Bank………………………….…………………………………….….Wings Level

Rationale: This orientates the lift vector for recovery.

4. Thrust ……………………………………………………………….. As Needed

Rationale: During a stall recovery, many times maximum power is not needed. When stalling, the thrust can be at idle or at high thrust, typically at high altitude. Therefore, the thrust is to be adjusted accordingly during the recovery. For engines installed below the wing, applying maximum thrust can create a strong nose up pitching moment, if speed is low. For aircraft with engines mounted above the wings, thrust application creates a helpful pitch down tendency. For propeller driven aircraft, thrust application energizes the air flow around the wing, assisting in stall recovery.

5. Speed Brakes…………………….………………………………………. Retract

Rationale: This will improve lift and stall margin.

6. Return to the desired flight path.

Rationale: Apply gentle action for recovery to avoid secondary stalls then return to desired flight path.

Conclusion

• Working together with other aircraft manufacturers, we have:
-  Agreed the principle with the FAA Stall Recovery Working Group
- Issued a harmonized procedure focusing on AoA
   reduction as a first action

When Stalls Most Often Occur
 
Circumstantial evidence shows that most full or near-full stalls of transport aircraft occur in one of five situations, as for other paths to loss of control, often but not always when the aircraft is either in IMC or during ‘dark night’ conditions clear of cloud so that no natural horizon is available:

• During inappropriate response to an un-commanded autopilot disconnect at high altitudes. (Uncommanded AP Disconnect due to malfunction of other systems)
• at low altitudes when the indicated airspeed is unintentionally allowed to deviate significantly from the intended and necessary target (Airspeed Awareness)
• at low altitudes in the presence of frozen deposits on the wings (Airframe Icing)
• during a mishandled go around (Aircraft management and Flying Skills)
• because of insufficient understanding of automation as it affects flight envelope protection systems.
• improper slats/flaps configuration (Aircraft Configuration)

Uncommanded AP Disconnect due to malfunction of other systems is not only liable to create a significant ‘startle factor’ for both pilots but is also likely to remove some of the high level flight envelope protections commonly provided by Fly-By-Wire (FRW) flight control systems. Flying manually at high altitude is not a feature of normal operations and there is not always sufficient awareness of the different ‘feel’ of the flight controls in the high altitude case compared to the routinely- experienced low altitude case. The simultaneous removal of some or all automated flight envelope protections at the same time, often because of an automatic reversion to a lower FBW Control Law, creates a heightened imperative to retain control within that envelope in a situation where a full understanding of the different degree of protection provided by Control Laws, other than "Normal", may not be fully understood or appropriately recalled.