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Base to Final Turning Stall

 



By Robert Reser, 25th August 2016

In a low indicated-airspeed, high drag configured turn, if the aircraft is overshooting the final approach, it seems to be human nature to concentrate on continuing the approach by maneuvering into a steep bank and pull the control wheel attempting to correct back to the runway extended centerline.

A steep banked attitude while inputting aft elevator control results in increasing the “g” loading with associated increase of the stalling indicated-airspeed. Added aft elevator control also increases angle-of-attack allowing more slowing.

The slow indicated-airspeed steep turning stall often is considered the result of restricting increased bank angle with aileron control while using rudder to attempt “steering” the turn.  When beyond 45-degree bank, rudder input with the turn primarily causes nose-up/down steering pitch, but the control combination results in a cross-controlled condition. Pulling the elevator causes increased aerodynamic loading with increased stalling indicated-airspeed and slowing by increased angle-of-attack that is always the cause of any stall.  Cross-controlling rudder input does contribute to a stall.

The high angle-of-attack required to stall is always relative the direction of motion.  The nose up attitude normally learned to be associated with approaching a stall will now be in a very different direction. The steep banked descending attitude has the direction of motion turning and descending.  There is no visual reference to indicate being pitched very high nose up. In the turn, this hides the real attitude from the pilot, especially the descending turn.

There must be a practiced and drilled discipline for pilots to always be aware, and know how to make this turn safely. If you have to pull the elevator control in a slow indicated-airspeed steep turn, it may be time to abort the approach.

What is really happening?

We know stall occurs when exceeding the wing critical angle-of-attack.  The FAA handbook and tests say exceeding the wing critical angle-of-attack CAUSES the aircraft to stall.  I have quizzed many professional Flight Instructors and Airline Pilots and all gave the FAA test answer; exceeding the critical angle-of-attack as the CAUSE of stall.

I have adapted a series of questions for these people.

Exceeding the wing critical angle-of-attack is not the CAUSE, but is WHEN stall occurs. 

What then CAUSES the aircraft to attain this extreme angle?

Everyone agrees, pitching the aircraft up.

What CAUSES the aircraft to pitch up?  Pulling the control wheel.

What pulls the control wheel?  The pilot!

Is there any other way to stall an aircraft?  No!

The pilot must input aft elevator control to CAUSE stall. In any maneuver, reducing aft elevator input is required for stall recovery.

Upset from wake turbulence or extreme autopilot input to the elevator is all maneuvering the pilot has done or allowed, but only with excessive aft control input during recovery will there be actual stall.

En-route complacency allowing the autopilot to do something dangerous is not an excuse. The pilot is responsible at all times for the conduct of the flight.

Pilot input controls pitch, but not just with elevator input.  “Engine power setting contributes to pitch”.

I have found no text that describes the part engine thrust plays in aircraft pitch control, but aircraft in slower flight have considerable nose-up pitched attitude above the direction of motion. Vy flight will have at least six to ten degrees wing angle-of-attack and any slower flight begins approaching the wing critical angle-of-attack, which for most wings will be 16-20 degrees pitch above the direction of motion.

These pitched up attitudes direct the engine thrust at some angle above that direction of motion and result in an added thrust-component vector of lift acting at the engine attachment along with the associated large sustaining thrust-component in the direction of motion.

Sine of six-degrees is one-tenth (.1). At Vy, one-tenth or more of the level flight sustaining thrust for constant indicated-airspeed is lifting at the engine attachment and acting over the fuselage as the moment arm to the center of pressure.

The sustained level flight engine lifting also contributes to the longitudinal balancing along with the elevator aerodynamic loading or lifting at the tail.  The engine lifting allows less nose-up elevator trim for setting angle-of-attack when coordinating at a desired indicated-airspeed.

Adding excess thrust, from a trimmed indicated-airspeed, increases the engine lifting causing pitching up to a climb angle and changed direction of motion with increasing altitude at that same indicated-airspeed. There is merely more lift at the engine attachment and the added excess thrust component in direction of motion sustains the climb. 

Descent with its reduction of power reduces the engine lifting contributing to the current angle-of-attack. This reduction of angle-of-attack allows some initial acceleration when beginning descent.

Now throughout all descent, being below the level flight sustaining thrust setting, to maintain a constant indicated-airspeed, it requires continuous coordination of elevator trim to compensate for the related thrust-component lifting caused by power changes.

So, you are in a landing configured descending turn trimmed for the slowed approach indicated-airspeed. If over-shooting the turn occurs, aft elevator input and increased bank considered necessary to cause turn results in additional slowing toward stall. There is no visual reference of extreme nose-up attitude in this turn.

The stall warning horn sounds and you add lots of power, you instantaneously add those few degrees of nose up trim effect from engine thrust-component lifting related to the newly trimmed slower indicated-airspeed from manual elevator aft input with its increased “g” loading.

You were already at a slowed indicated-airspeed set with elevator trim and with the additional angle-of-attack from increased aft elevator input, the power input instantaneously caused exceeding the wing critical angle-of-attack with immediate low altitude approach stall, and you will never know why.

We tend to rely on the FAA for guidance, but it is up to the Training Schools to teach instructors and pilots correct concepts.

It may seem counter intuitive that thrust component-lift is a basic flight control and with increased power can actually contribute to causing stall, yet is also required to recover from a high indicated-airspeed stall. However this is indeed the case if not exercising proper elevator control.

I appreciate comment.

Best Regards
Bob   

                                          

Robert Reser

http://safe-flight.net

The North is Calling
SOME AVIATION HUMOUR FOR THE DAY
 

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