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A three-engine ILS-coupled approach to 50 ft AGL was also .own (.g. 37), and worked well. In this case, the center engine was used for short-term pitch control, with a thrust command opposite to the wing engines, washed out with a 4-sec time constant. The wing- and center-engine EPRs may be observed going in opposite directions in the time between 10 and 20 sec when the bank angle was essentially zero and no differential thrust was being commanded. At 50 ft AGL, as planned, the pilot disengaged PCA and made a normal landing. Note also that speed was held within 1 kn with the center engine active. Pilots commented that pitch control on this approach was the best they had seen, although the air was smooth and the full potential of this mode was not tested (ref. 12).
As mentioned earlier, the ILS-coupled PCA system was designed to demonstrate feasibility and potential improvement and was not a fully developed autoland system. The .ightpath control below 600 ft exhibited some small deviation from the glideslope and resulted in scatter in touchdown sink rate and location. Figure 38 shows an overlay of three ILS-coupled approaches and shows a somewhat repeatable deviation initially below the glideslope beginning at 600 ft AGL. This deviation is caused by integrators in the ILS logic that were not optimized in this application. In addition, the transition from glideslope to .rst .are command had a 2-sec lag .lter. This .lter allowed random variations in sink rate at the 130 ft .rst .are to continue for 2 sec, increasing scatter in touchdown point and touchdown sink rate.
These ILS-coupled approaches and landings were made in November under near ideal weather conditions, light winds and smooth air. Attempts to .nd turbulence at Yuma, Palmdale, and Edwards, were unsuccessful. To evaluate the performance of the ILS-coupled PCA system in more severe weather, simulator and computer analyses were performed. These analyses showed that safe ILS-coupled landings could be made in turbulence levels up to moderate. Figure 39 shows an FDS ILS-coupled approach with severe turbulence and a 15 kn wind 45° off the nose. Note that airspeed excursions of 10 kn result from the turbulence. An updraft caused a deviation above the glideslope, but the system compensated well. An upset during the .rst .are was corrected, with touchdown at 4 ft/sec near the centerline with a 5° bank.
LOC capture
3000
Radar altitude, ft AGL 2000 1000
0 .75
Glideslope .50 deviation, deg .25
0 0 Flightpath angle, – 2 deg
–4
170
165 Airspeed,
160
kn 155
150
1
Localizer 0 deviation, dots
–1
–2
240
220
Magnetic track, deg 200
180 20
Bank 10 angle, deg 0
– 10
1.3
1.2
EPR 1.1
1.0
.9 0
2 1 Dots 0
(a) Entire approach and landing.
600
400 Altitude, ft AGL 200
0
0
–1
Flightpath angle, deg
–2
–3
160
155 Airspeed, kn
150
145
2
Bank angle, 0 deg
–2
8
Angle of attack, 6 deg
4
1.3
1.2
EPR 1.1
1.0
.9
Touchdown, 5.5 ft/sec, 1300 ft beyond threshold, 10 ft right of centerline
(b) Final 60 sec of ILS-coupled hands-off .are and landing. Figure 35. Concluded.
Altitude, ft AGL
Flightpath angle, deg
Airspeed, kn
Bank angle, deg
EPR
2000 1500 1000 500 0 2
0
–2
–4
205
200
195
190
185
2
0
–2
1.15
1.10
1.05
1.00
.95
0
Time, sec
970619
Radar altitude, ft AGL
Flightpath angle, deg
Airspeed, kn
Bank angle, deg
EPR
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Development and Flight Test of an Emergency Flight Control System Using Only Engine Thrust on an MD-11 Transport Airplane(35)
