High AoA Stall and Behind the Power Curve

[louross]

My question being, are these two different terms for the same thing? Old vs. new? Or are they different?
lr.

[Tom Goodrick]

They are related but different. A high angle of attack stall can occur in a variety of situations. Being "behind the power curve" can lead to a high angle of attack stall when a pilot follows normal procedures.

The power curve is a curve showing how increasing power will give more airspeed under most conditions. But for aircraft with high-lift devices like blown boundary layers or complex Fowler flaps, at low airspeed the power curve can rise both to the right and to the left of a minimum airspeed that is close enough to stall speed (and high angle of attack) so that flight in this area is dangerous. The point is that proper procedures should keep the plane ahead of this area, at a faster airspeed where increasing power increases airspeed. Behind the power curve, a rise in power can accompany a reduction in airspeed giving you no way out.

The curve is actually a plot of power (or thrust) versus indicated airspeed for the equation showing a balance of thrust versus drag and lift versus weight which gives sustained and balanced level flight at the selected airspeed. Normally it shows an increase in power or thrust is required to fly faster (to the right of the minimum point).

This is generally a problem with airliners where the power curve varies with fuel and passenger load and for passenger load distribution. Only general policies are available to keep flying in the safe zone, well ahead of the minimum in the power curve.

One problem is that you can be safe with one flap configuration and then get unsafe fast with a change in flap setting on a long approach.

I have used FS to explore this area. I can plot the power curve for an aircraft with various loads and then set the aircraft in flight in a safe condition and work it back into a dangerous area of the curve.

[louross]

Okay. If the a/c stalls, the nose falls, the a/c quits flying. Descending behind the pwr crv, the a/c rate of descent will reamin constant, or increase some, but the a/c is still flyable. The stall recovery is nose down. Recovery from behind the pwr crv is also nose down.
lr.

[Tom Goodrick]

There are two types of stability - speed stability and angle of attack stability. Your position on the power curve determines speed stability. You are stable in speed if more power leads to more speed. But you may also be in a region of poor angle of attack stability where nose-down control does not produce an instant nose-down response. You may have to fall a bit before the nose drops enough to regain controlled flight. On approach the ground may get in the way of a recovery. this is where the passenger load distribution can become a factor.

Don't take for granted that the descent rate will remain constant or that the pitch response will be normal. In most flight conditions, the one control that remains fast and positive is throttle response. Shove the levers forward and you will get a good response toward regaining flyability which was lost in the stall. But on the back side of the power curve you lose this.

[Tom Goodrick]

I did some flying today with the Shrike in low-speed conditions. I did not find any problems. I think a bigger gap is need between flaps stall and clean stall to see this effect. But I need to review what makes this condition more pronounced and exactly what the forcing conditions are when you are behind the power curve. The mere fact that the curve shows more thrust is required to fly slower does not mean you are forced into a high angle of attack condition.

I did notice it years ago when I did a lot of modeling of low aspect ratio aircraft (high induced drag). I was looking at things NASA needed for landing from space.

[louross]

Behind the power curve is a "hazardous" situation for any a/c (I suppose).
There is a video, actually taken with a movie camera, back in about 1954 or there abouts of a an Air Force fighter landing, got behind the power curve, and resulted in the normal way- he bought the farm.
A UAL 727 landing one night at SLC, late 68 or early 69, same situation, same result. (There were also other contributing factors, but, bottom line, behind the curve.)
In my long and dubiuos flying career, I'd been on the Otter about 6 months and a co-pilot on approach to Podunk, Iowa, got behind the power curve, but either never realized it, or didn't know how to recover. He increased power up to about 25%, nothing changed. He brought power up to at least 50%, nothing changed. Then he realized there was a problem and added full power. Nothing changed- airspeed, pitch, sink rate remained constant. I simple said, "Dump the nose," which he did almost immediately. As we taxied in, he kind of gasped, "Jeeze". My response- "That approach would have killed you. I was going to wait about one second and dump the nose myself."
Now there is talk about a high AoA stall concerning the AF flight. Never hearing the term, I thought it could be a term for behind the power curve. Apparently not.
lr.

[Tom Goodrick]

The only way to make complete sense of that Air France crash is to see the flight data. But I am sure Airbus considers that proprietary and, with the NTSB not having jurisdiction we will probably never see the data.

The situation you describe in the Otter is exactly what I have heard about being behind the curve. Lowering the nose is the only way to get the speed back. It is very hard to figure that happening at cruise altitude.

Any pilot knows the most critical instrument is the airspeed indicator. Take that away and any pilot can get in serious trouble especially with other factors such as darkness and high turbulence.

MSNBC and NBC have been employing Capt Sullenburger as a consultant to comment on these Air France reports. He has some very good comments. He refuses to place blame without more data. His main suggestion is that pilots be kept more in the loop while flying long flights. Reduce the automation and increase the flight information shown to the pilots. For example on the Airbusses, AoA values are used by the system but not shown to the pilots.