Hey barit1:
Yes, I completely agree with you that the AF B737 had sub-par acceleration. And, I also agree that the sub-par acceleration was due to having bad EPR readings. There is very little doubt about that. While there was no way to confirm that ice was the culprit (any ice would have melted in the water and the probes were clear when the engines were recovered), I do believe that ice was present and was the source of the erroneous EPR readings. The primary source of confirmation for me is the sound spectrum analysis done on the sound of the engines as recorded on the CVR. The frequencies registered approximately 75% of the expected RPM.
I think you and I exchanged opinions on ground acceleration checks and the methods by which that may be done in an earlier thread – and I still support having some kind of acceleration measurement, particularly if it is independent of airplane systems. [I flew KC-135s for quite a while during my “military life.” We used a minimum acceleration check time between 80 and 120 knots – as called out by the pilot monitoring and timed by the navigator. It was simple and effective.]
Having said all that, however, the AF B737 did accelerate (slowly, yes) but enough to get to V1. They also got to V2, but that call was made just as the stick shaker started and the stall buffet was entered. They did get airborne. What I’m trying to say is that while they got airborne, they did so thorough no effort on their part. The F/O (who was flying) specifically said he was going to “takeoff the nose gear and let her fly off.” Going from 3-on-the-ground to being in the stall buffet in approximately 2 seconds (requiring a rotation rate of something like 4 – 6 times normal) is a whole lot more than merely “taking off the nose gear and letting her fly off.” What they were concerned about at that very instant – and they were very concerned – was the pitch attitude. “Come on, forward. Forward. Just barely climb. We only want 500 hundred. Forward.”
I’m looking at straightforward aerodynamics, and what role aerodynamics played in what happened. To do so, I’ll ask that you allow me the luxury of postulating for a moment. We know that during the initial portion of any acceleration for takeoff, the wing is beginning to develop lift. But it is not until the pilot rotates the airplane, getting the wing to an AoA that generates enough lift, that the airplane gets into the air. I’m sure that it isn’t any super revelation to state that the wing does not generate lift uniformly and the entire wing doesn’t generate lift simultaneously. You and I know that you can get an airplane into the air, in ground effect, before it is really ready to fly outside of ground effect. We know that Vmu tests are done where the controls are essentially held back from early in the acceleration run, forcing the tail onto the ground (or very close to it) at a speed well below what is necessary to fly – to see what the minimum lift off speed will be. The reason pilots don’t rotate the airplane prior to reaching “rotate speed” is that they don’t want the airplane getting into the air until the wings support the airplane properly and controllably. This is what they learn. This is what they expect. This is what they do.
But, what would happen if we changed the equation a bit – right at that critical moment – when the pilot moves the control column to a “neutral position, or slightly aft of the neutral position, in preparation to rotate.” (A quote from the Boeing manual.) What if, at that moment, the pilot realized that pulling further back on the controls would not get the airplane rotated? The pilot, pulling like crazy on the controls, gets no rotation. Well, in my KC-135 days, instructors and evaluators were trained on the unique use of the spoiler panels. Under the glare shield were 2 guarded switches that controlled 2 valves to open or shut off hydraulic pressure to either the inboard spoilers (L) or the outboard spoilers (R) on each wing. In this case (no rotation – and you had to know that the KC-135 had p*ss poor brakes and no reverse thrust – quick stops were, well, rare.) and you wanted to get into the air, you would turn off the inboard spoilers, grab the speed brake lever, and gradually, very gradually, raise the speed brakes. With the inboard panels shut off, the only spoiler panels being raised were the outboard panels, creating differential lift – lift on the inboard portion (forward) and no lift on the outboard portion (aft). The airplane would rotate just like “normal” and when you got the pitch attitude you needed, you lowered the speed brakes, just as gradually. If you yanked the spoiler handle up quickly, you would very likely smack the tail on the ground. Differential lift at takeoff can be very interesting, to say the least.
In the AF B737 situation, a very similar circumstance was handed to an unsuspecting crew. When the F/O moved the control column to the neutral position, the inboard portion of the wing was generating lift – and outboard portion was not. Presto. Rotation. Quick rotation. Very quick rotation. Rotation all the way into the stall buffet in approximately 2 seconds. But here, the crew couldn’t get rid of the differential lift. It was “glued” onto the wing leading edges. Straightforward aerodynamics.
If the airplane had been kept on the ground long enough to accelerate to a speed that would have allowed the outboard portion of the wing to generate enough lift to counter act the rotational moment they probably would have recognized a “sluggish” airplane – with only 75% power. But they didn’t know to do that. They wouldn’t have been able to do that – even if they had shoved the throttles to the firewall at brake release. They planned to rotate at the computed speed. They got ready to rotate at the computed speed. They expected the airplane to fly at the computed speed. What they didn’t expect was a wing deiced with water; a wing that now had a thin coating of ice that deformed the leading edges; deformed just enough to cause this very unique aerodynamic problem – from which they were incapable of recovering.
Again, thanks for your comments, and for allowing me to praddle on.
_______
AirRabbit