Development and Flight Test of an Emergency Flight Control System Using Only Engine Thrust on an MD-11 Transport Airplane(31)

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The initial test conditions .own were at a speed of 220 kn at 13,000 ft AGL with the landing gear down, .aps up, and slats extended. The gross weight and CG were ~361,000 lb and 24.7 percent, respectively. The roll thrust gain was zeroed with an MCDU input so that only aileron control was available for the lateral mode and the vertical mode was controlled through the PCA system. The gain set was the same con.guration used for most approaches and landing. Once the con.guration was achieved, all elevators and rudders .oated to the trail position, which was approximately the normal faired position. The PCA system FPA mode had no problem maintaining level .ight or a –1° FPA commanded descent and return back to level .ight. With the center engine (number 2) at idle, the wing engines achieved maximum authority at a commanded +1° FPA. Once engine 2 was advanced above idle, the wing engines could modulate and allow a steeper climb.
In the opinion of the .ight crew, a safe landing could easily be made in this con.guration. The aileron effectiveness was adequate for roll control, and aileron forces were signi.cantly lower than in the simulator. Pitch control was greatly improved using the PCA .ightpath mode. No evidence of a phugoid oscillation was seen, even when the stabilizer trim was used to change airspeed.

PCA System Operation With All Hydraulics Off
Figure 31 shows a time history of tests with all three hydraulic systems turned off. Before the time shown, all hydraulic systems had been off for several minutes. The outboard ailerons had .oated to +12° , the inboard ailerons .oated to +5° , while the elevators .oated near 0° . Contrary to simulation results, the .oating surfaces resulted in a nose-up pitch and caused a lower trim speed. Lowering the landing gear was planned, which was expected to lower the trim speed further. Because airspeed was already near the minimum speed for .aps-up .ight, hydraulic system 3 was turned on as shown, the ailerons returned to the trim position causing a pitchdown, and the stabilizer was retrimmed to a higher speed position. When hydraulic system 3 was again turned off, the ailerons again .oated up and the pitchup occurred again. The PCA system reduced the throttles to idle to reestablish the commanded .ightpath, and speed stabilized at 212 kn. After a small track change, the landing gear was lowered using the alternate gear extension system. With the gear doors down and the wheel wells remaining open, speed decreased to 195 kn. Note that lowering the gear moved the aileron and elevator slightly; the hydraulic pressure was zero, but of course .uid was still in the system. The PCA system maintained .ightpath well in this transient. Track and pitch control at this point were normal, identical to that with hydraulics on. A simulated landing approach was made using a –2.5° glideslope with a small track change. A large track change was commanded at 560 sec; bank angle stabilized at its 20° limit, and .ightpath was stepped back to level .ight. The center main landing gear was then extended, resulting in another small disturbance to the surface positions. PCA control remained normal. Hydraulics were then turned back on still under PCA control.

Hydraulics 1 and 2 off Hyd 3 on

Hyd 3

Center landing
Main landing
Retrim stabilizer
off

gear extended
gear extended
12
8
Surface
position, 4
deg
0

–4
19
18
Altitude,

ft x10–3  17
16

15
4

2
Flightpath angle, 0 deg
–2
–4
100

Magnetic 0 track, deg 260
160
20

Bank
angle, 10
deg

0
240
220 Airspeed, kn 200
180
1.5
1.3
EPR
1.1
.9 0 200 400 600 800
192 Stab. Right  212 195 – 2.5° G/S 120° turn Outboard aileron Outboard elevator Measured Command Left Center Command Measured  Inboard aileron Inboard elevator Speed change with center engine 
　
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