Measured 
 
Comm and 

1.2
1.1 EPR
1.0
.9 500 520 540 560 580 600
Left 
Right  Ce nter 

Time, sec
970607
(a) Airspeed command increase from 247 to 270 kn.
Figure 26. Time histories of three-engine PCA speed control mode, gear and .aps up.

29,500
Altitude, 29,000 ft 28,500
.5
0 Flightpath angle, – .5 deg
–
1.0

–
1.5

20
Bank
angle, 10
deg

0
350
Magnetic track, 250 deg
150
325
Airspeed, 320 kn 315
1.4
1.2
EPR
1.0
.8

Time, sec
970608
(b) PCA operation at aft CG, high altitude, high speed, CG = 31 percent, speed control on.

Three-Engine Dynamic Pitch Control
Figure 27 shows the three-engine dynamic pitch control mode from .gure 14(b). This mode showed better pitch response with less overshoot than the two-engine pitch control. Note that the initial response of the center engine is opposite that of the wing engines, thus increasing the pitch-rate-generating moments due to thrust.

High-Airspeed Tests
PCA was tested at airspeeds to 360 kn and Mach numbers to 0.83 with no degradation in performance.

Recovery From Upsets
With the PCA system enabled but not engaged, the airplane was upset using the normal controls. When the desired conditions were achieved, the pilot released the controls and engaged PCA. All recoveries were successful, the most severe being from a 45° bank and a 7° dive (.g. 28). In this case, with gear and .aps up, two-engine pitch mode, and the center engine at idle, the PCA system logic commanded collective thrust for the .ightpath error and then differential thrust to reduce the bank error to less than 20° . A more suitable procedure for upset recovery would be to .rst level the wings, then correct the .ightpath angle error, but PCA system logic is programmed so that on initial engagement, it tries to simultaneously achieve the commanded .ightpath and track. In the upsets, the track deviated many degrees during the recovery; so after .ightpath was stabilized, there was still a track error, and the airplane returned back to the reference track command.
A recovery from a more severe upset was .own in the FDS (.g. 29). In this case, the bank angle was 80° with a .ightpath of +5° . The initial differential thrust input was overridden (still with a 60° bank) by both engines going to full thrust to reduce the FPA error. After 13 sec, the differential thrust input again was honored, with bank angle reduced to 20° , before the pitch command error drove both engines toward idle thrust. Recovery to level .ight was completed 57 sec after the upset.
In both the .ight and FDS cases, getting the wings level was delayed by pitch priority. A more suitable procedure for upset recovery might be to .rst test the bank angle upon engagement, and if excessive, level the wings .rst, then correct the .ightpath angle error, and .nally correct back to the commanded track.

PCA Operation With Two of Three Hydraulic Systems Off
Turning off hydraulic systems 2 and 3 allowed three of the four aileron surfaces to .oat. Those surfaces .oated up, reducing the lift and increasing trim speed. The offset was small at 300 kn but signi.cant at 200 kn. Figure 30 shows a portion of this test. Both outboard ailerons .oated to +12° , and the right inboard aileron .oated to +7° . The aileron offset required enough differential thrust that the right engine was near idle. Speed control was still on, so the center engine was well above idle. This situation made turning right dif.cult (note that the commanded heading was overshot by several degrees), but PCA did maintain gross control. Several solutions were available: turning off speed control, turning off the last hydraulic system, trimming the remaining inboard aileron to eliminate the asymmetry, or lowering the landing gear to raise the throttle settings.
　
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