Need for Coupled Approach
At this point in the .ight test, it was obvious that the lateral control was suf.ciently sluggish that a pilot could not be sure of making a successful approach on the .rst try with a signi.cant level of turbulence. An ILS-coupled PCA mode had been developed and evaluated on the B-720 PCA simulation at NASA Dryden and on the medium twin-jet simulation at NASA Ames (ref. 7), and it worked very well. Therefore, the ILS-coupled mode was incorporated into the MD-11 PCA system. Honeywell added this capability using the existing MD-11 ILS-coupled logic and displays as much as possible. This capability was to be a concept demonstration only, and no attempt was to be made to optimize the ILS-coupled system. Requirements were to capture the ILS localizer (LOC) .rst within a 30° intercept and from below the glideslope. The glideslope to be .own was adjustable from 0.5 to 1.5 dots (1 dot is 0.35° ) below the glideslope to keep the thrust required well above idle. A simple two-step auto.are capability was also added. Transition from glideslope control to the .rst .are was smoothed by a .lter with a 2-sec time constant. Because sluggish lateral control was the impetus for the coupled approach, the capability to .y a LOC-only approach was also added. In this mode, the pilot would couple to the LOC for lateral guidance and would use the FPA thumbwheel for glidepath control.
The standard MD-11 ILS glideslope control logic includes integration of terms including true airspeed and attitude, which were not optimized for this concept demonstration. The objective was to demonstrate improved control suitable for an emergency landing down to 20 ft AGL, with a goal to make actual landings.
PCA-ILS Approaches
The ILS-coupled PCA logic was checked and approaches were .own. Two were conducted at Edwards on runway 22, and seven were performed at Palmdale, California on runway 25. Figure 32 shows the Edwards and Palmdale ILS information and typical ground tracks .own. The nominal Edwards ILS glideslope was 2.5° ; at Palmdale, it was 3.0° . Because the nominal PCA ILS-coupled approach was .own one dot (0.35° ) below the glideslope, the nominal glideslope for PCA approaches at Edwards was 2.15° , while at Palmdale it was 2.65° . All the approaches were .own down to predetermined decision heights and terminated with an automatic PCA system go-around using the TOGA go-around button similar to normal MD-11 operation. The conditions .own were Flaps 28 with aircraft trimmed for a speed of approximately 1.4 times the stall airspeed, and in the two-engine PCA con.guration with the center engine at or near idle. All approaches were .own in smooth air with light winds.
The ILS localizer at Palmdale was noisy starting at about 1000 ft above the ground and continuing on down to about 100 ft, but the ILS .ltering in the logic effectively smoothed the noise. Localizer intercepts up to 30° were found to be captured immediately, but larger intercept angles resulted in overshoots. (The PCA localizer capture speci.cation was for intercepts less than 30° .) One capture at a 50° intercept had three overshoots before stabilizing at 500 ft AGL.
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Initially, the auto.are logic was a .rst .are to a .ightpath angle of –1.5° at 100 ft and a second .are to –0.5° at 50 ft. These .ares were raised for initial tests by 100 ft to see their effect without entering ground effect. High sink rates (~11 ft/sec) were experienced at 100 ft AGL; so a change was made to raise the .rst .are to 130 ft. Tests were then .own to 100 ft and 70 ft go-around altitudes. Go-arounds typically resulted in 50 to 60 ft loss in altitude.
The last ILS approach at Palmdale was .own without a PCA go-around at a speci.c altitude but with the safety pilot briefed to not allow a touchdown. Airspeed was about 153 kn. At 130 ft, the .rst .are reduced the FPA to –1.5° . At 100 ft AGL, sink rate was 11 ft/sec. At 30 ft AGL, the –0.5° FPA second .are further reduced the sink rate and, as predicted by the simulator, the airplane leveled off at 10 ft AGL and .oated for approximately 10 sec. At this point, with a sink rate of 2 ft/sec and still on the centerline, the safety pilot made a small elevator input to prevent touchdown, and initiated a go-around.
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