Altitude Response During Landing Approach

An early pilot-induced oscillation experience with the Space Shuttle Orbiter ALT – FF5 landing approach tests (Powers, 1986) led to some concern about the shuttle’s com­bination of large airplane size and distance of the pilot ahead of the center of gravity, and initially reversed altitude response with elevon control deflections. On the shuttle, more than 2 seconds elapses before altitude starts to increase following a step nose-up elevon deflection (Phillips, 1979). F. A. Cleveland (1970) gives a value of 0.8 second for this de­lay parameter on the C-5A Galaxy and speculates that glide path control during landing approach would be unsatisfactory for time delays above 2 seconds.

Cleveland’s analysis of the effects of airplane size gives a linear growth in the altitude delay parameter with size. Time delay would reach 2 seconds on a C-5A scaled up about

2.4 times, requiring direct lift control or canard surfaces. Digital sampling delays in the pitch control circuit and elevon rate limiting are known to contribute to the Space Shuttle’s pilot-induced oscillation tendency. According to Robert J. Woodcock (1988)

extensive, continual training [for the Space Shuttle Orbiter] has resulted in very good landing performance (smooth landings with small dispersion, mostly on long runways, in smooth air).

The conclusion is that large airplane sizes might indeed lead to landing approach prob­lems related to delayed altitude responses following pitch control inputs and special design features to correct the problem.

An alternate criterion for the control response of very large airplanes is the generic re­sponse to abrupt control inputs, as shown in Figure 10.5. Grantham, Smith, Person, Meyer, and Tingas (1987) compared large airplane flying qualities with the generic pitch response requirements of MIL-F-8785C. Maximum effective time delays of 0.10 to 0.12 second are specified in MIL-F-8785C, based on tests of relatively small tactical airplanes. These requirements may be much too stringent for large airplanes in all but very high-gain oper­ations. Satisfactory, or Level 1, flying qualities are obtained in simulation of a Lockheed 1011-class transport for effective time delays of 0.15 second, suggesting even larger accept­able values for superjumbo jets. On the other hand, the effective longitudinal axis dynamics of the Boeing 777 compares with some fighters.

John Gibson (1995) suggests that the observed sluggish response of the Space Shuttle Orbiter is caused by poor control laws as much as airplane size. Increasing pitch rate overshoot in a pullup would improve the observed sluggish flight path angle response. This further suggests that the flexibility of fly-by-wire technology can provide the necessary quickness of response in large airplanes without requiring special features such as direct lift control or canards.