BOAC
- I'm still intrigued by these thoughts - how do you envisage the motion of 587 itself causing the co-pilots legs to push on the pedals - do you see him sliding around in his seat? Why did the first wt encounter not do the same?
587 to me exhibits a simple, classic PIO. The only query I have is why did it start.
Why did it start? First lets look at what Hess said in his report regarding the subject of Triggering events:
Triggering Events
PIO incidents are almost invariably preceeded by a "triggering" event [3]. A trigger is a
stimulus that can cause a pilot to change his/her control behavior. Triggers have been categorized as (a) Environmental Triggers (such as those that involve changes in the vehicle dynamics that cause a mismatch between pilot control strategy and the aircraft dynamics), and (c) System Failures, (such as the failure of an actuator or hydraulic system).
Triggers can cause a pilot to move from non-tracking or low-gain tracking behavior to high-gain tracking behavior. For example, a sudden and large turbulence encounter can cause a pilot to actively begin high-gain, compensatory attitude tracking when previous to the encounter he/she was only monitoring aircraft trim or making low-gain corrections to vehicle attitude.
Of and by itself, a triggering event may not be a catalyst for a PIO. Typically, some flight control system property conducive to a PIO is revealed when high-gain behavior begins. A good example of this is the PIO that occurred in the Shuttle Orbiter Enterprise in October of 1977 (Alt-5). This flight involved the Enterprise being carried aloft on a Boeing 747, then released to make a landing at specific touch-down point on a concrete runway at Edwards AFB (previous landings took place on a dry lake bed). The triggering event here could be described as an Environmental Trigger associated with the stress of attempting what amounted to a spot landing. High gain pilot tracking activity then began that involved large and rapid pilot inputs in the final segment of the approach. The combination of large time delays in the flight control system coupled with the aforementioned control inputs caused the vehicle's elevon actuators to rate saturate or rate limit. This means that the actuators were moving the elevons as rapidly as their designs permit. The intrinsic time delays constituted the control system property conducive to a PIO. The rate saturation dramatically changed the vehicle dynamics by introducing even larger time delays into the control loop. A PIO in both lateral and longitudinal axes ensued. The PIO was terminated when the pilot-in -command released the control stick, i.e. completely "backed out of the loop".
It has been hypothesized that a true PIO will involve the pilot adopting a "regressive" form of tracking behavior marked by the control of error-rate rather than an error itself. For example, if a PIO in the roll-control axis has begun, the pilot will regress to control of roll-rate rather than roll-attitude. Once this regressive behavior has been adopted by the pilot, it is difficult for the pilot to "back out of the loop". A sustained PIO is likely. Often at this point the pilot believes that something is wrong with the aircraft, i.e., that a failure has occurred. As far as the pilot is concerned, the aircraft is behaving strangely. Some pilots who have survived serious PIO encounters have said that they simply "no longer recognized the aircraft".
As I mentioned before Hess seems to be looking at AA587 from a control rate limiting perspective, There may be another way to look at the APC forcing function.
But first, a short story about my attempts at driving my brother’s 1967 Pontiac GTO.
I attempted to accelerate the vehicle in a normal manner by letting out the clutch and applying a little accelerator only to be embarrassed by a jerky heel toe type acceleration profile. When this happened a second time immediately afterward, I began to realize that there was more at play than just a clumsy operator.
A few experiments revealed that while starting in first gear, the seat back yielded enough to pull me off the clutch and the accelerator together. As I my weight left the clutch pedal the clutch engaged smartly which further increased the acceleration and completed pulling me off the accelerator. But now the deceleration caused by no throttle moved me forward and the throttle was applied again and the clutch disengaged. As long as I attempted to accelerate this jerky charade continued. The fix was simple…Start in second gear.
This is a simple (and I hope easily understood) linear single axis DIO (Driver Induced Oscillation) that resulted from accelerations upon the “crew”. The enabling factor for this oscillation was seat back rigidity incompatible with the accelerations created on the crew member and by a clutch that engaged over a very short pedal travel distance. The initiating event was starting the vehicle in first gear.
I have a few experiments anyone can try in the safety of your own car.
Consider the case of cornering in a vehicle. As long as the turn is in one direction, you settle into whatever support you can find and ride out the turn. If the turn rapidly reverses, you (and your passengers) are now accelerated in the opposite direction. If the forces (speed) is low, friction holds you in place and nothing much happens. With higher forces (speed) your upper body displaces to the opposite side of the seat to find whatever purchase it can and particularly if you are a passenger, you use your legs to help support your body. At still higher turn rates and forces, your posterior slides to the opposite side of the seat until it finds support. The key event as far as your body is concerned is the reversal of force from one direction to another. If this occurs rapidly and unexpectedly it is difficult to steady your body and you use all your appendages to steady yourself.
The following is a generic pilot seat support assessment based on various seats I have ridden in: Your favorite aircraft seat may be different.
Subjective support levels:
1. Good support against positive g events,
2. Fair support against negative g events,
3. Good support against linear acceleration,
4. Fair support against linear deceleration,
5. Fair to poor support on lateral acceleration depending upon whether there are arm rests, location of anchoring straps, and how much the seat width exceeds your actual width.
My expectation is that lateral support in aircrew seats is going to have the worst level of performance.
I suggest you do the car experiment yourself and observe what your turn coping strategies are. The driver in the vehicle has forewarning of the maneuvers and can hold onto the wheel so the best results would be with you as passenger. Observe how you use your legs. Do you use your right leg to brace yourself in a left turn (and vice versa)?
I am going to have to stop at this point. You won’t hurt my feelings if you tell me I am all wet after having run this experiment. I don’t expect 100 % consistent results, but you should be able to see where I am going with this in the context of AA587.