970596
(a) Throttle increase, Flaps 28.
Figure 15. Response of MD-11 to open-loop step throttle inputs, PCA off, center engine idle, LSAS off, yaw dampers off, landing gear down.
Angle of attack,
pitch attitude
and FPA,
deg

Airspeed,
kn

Throttle
angle,
deg

EPR

8
6

4
2
0

–2
–4
–6 235 230 225 220 215 60 55
50
45

1.20
1.15
1.10
1.05

0 10 20 30 Time, sec
970597
(b) Throttle reduction, .aps/slats retracted.
Figure 15. Concluded.

PCA System Pitch Response
Pitch response tests with the initial gain set were performed. Figure 16 shows a .ight response to a series of .ightpath angle step commands and a comparison with the FDS for the two wing-engine PCA control mode. The pilot .rst selected a –2° .ightpath step. Both engine EPRs decreased sharply because of the .ightpath error, then almost immediately began to increase from the pitch rate feedback and decreasing .ightpath error. The –2° command was reached in about 7 sec, then overshot approximately 25 percent. Angle of attack followed EPR closely, and airspeed varied inversely as it did in the open-loop throttle steps. FDS data show more angle-of-attack change and less .ightpath overshoot than the .ight data. The overshoot was not judged objectionable by the pilots. Later pitch control law changes added velocity feedback that reduced the overshoot in the pitch response.
The speed changes (in two-engine PCA control) that result from .ightpath commands result from the pitching moment change caused by changes in engine thrust. For a descent, lower thrust is required, which reduces angle of attack and thus increases trim speed. The converse is true for a climb. Care had to be taken when selecting climbs to consider the speed loss that would occur; a 5° climb could reduce speed by 15 kn. Speed on a 3° glideslope was typically 10 kn higher than the level .ight trim speed. The pilot could manually advance the center-engine throttle during PCA climbouts to increase the trim speed.

PCA System Track Response
Lateral control with the initial gain set was evaluated at 10,000 ft with step inputs in track angle command. Track was controlled to within a degree, and track captures showed no overshoot; however, initial response was slow. Pitch control was also good during turns up to the bank angle limit of 20° . After completing the tests at 10,000 ft, more step responses were done at 5000 ft; performance was good in pitch but slow laterally. Following these tests, pilot A used the PCA system to make approaches to the runway. The August afternoon with surface temperatures in excess of 112° produced continuous moderate turbulence from thermal activity. Pitch control remained adequate; track control was very sluggish. On approaches it was dif.cult to anticipate the track commands required to maintain runway lineup because of the sluggish response.

Small Track Change
Based on linear analysis of the PCA system response, some gain changes were made (ref. 12) that improved the initial lateral control response; data in .gure 17 show the original and improved track response. The time for a 5° track change was reduced from 22 to 17 sec; maximum bank angle increased from 4° to 6° . With the higher gains, the pilots could feel a distinct sideforce in the cockpit as the turn was initiated.

Large Track Change
Figure 18 shows a time history of a large track command with the improved control gains, gear down, and Flaps 28. The pilot commanded a right 80° turn from 80° to 160° ; the engine differential thrust response resulted in a maximum roll rate of 3.5° /sec, and the 20° bank limit was reached in about 10 sec. Flightpath angle dropped about 0.5° but corrected back with a loss of only 30 ft. Once stabilized in the 20° bank, the EPRs increased from 1.20 to 1.25, and airspeed increased about 7 kn to maintain the .ightpath command. Turn rate averaged 2.5° /sec. In the rollout, .ightpath was again well controlled with an overshoot of 0.5° .
　
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