DESPITE A PCA DESIGN envelope of about 150-250kt (280-460km/h), limited to 10,000ft (3,000m) and below, the thrust-only system has been explored well beyond the original flight envelope. The guest-pilot demonstration flights were focused appropriately around the landing pattern, but test results gathered well away from the original design envelope suggest additional PCA-system potential.
By moving fuel forward or aft, the MD-11's centre of gravity can be significantly altered. Flight with the PCA system engaged was conducted as high as 30,000ft, with the centre of gravity as far aft as 32% and also at a mid-range 24%. Guest pilot PCA demonstration flights, were conducted with the centre of gravity, in the mid-range.
Airspeeds as high as 360kt and Mach 0.83 were flown. PCA system recoveries from upsets were initiated and successfully performed, from a 45° bank angle and 10° dive at 12,000ft. More extreme conditions were evaluated in the simulator with success dependent upon the level of upset introduced and altitude available. The PCA system always directed the aircraft in the proper recovery direction upon engagement. Given the thrust authority available to the PCA system, it appears that the pilot can rely on the system for an automatic, hands-free recovery.
Since none of the MD-11's normal flight controls (except for stability augmentation and yaw damping, which were turned off) was inhibited during the PCA system evaluation, the control surfaces could not float, and remained fixed. Simulating a total hydraulic failure at 20,000ft with the throttles matched, all hydraulic pumps were secured. Test pilots reported a benign transient in the form of a mild pitch-up, resulting in a 20-25kt loss in speed. Retarding the number-two (centre-engine) throttle allowed the wing engines to modulate for better pitch control. Although the ailerons floated up without hydraulic pressure, the stabiliser, elevators and rudders did not move. The aircraft was flown with the PCA system engaged, but without hydraulics for 25min during which 360¡ turns, a 5,000ft descent, and an alternate landing-gear extension were performed.
The gear extension caused another mild pitch-up transient and an accompanying 10kt speed reduction. The PCA system compensated well for the mild pitch-down caused by re-engagement of the hydraulic systems. Approaches with the PCA system engaged, was flown with only the leading edge slats extended, and also with neither slats nor flaps extended.
Pilot reports were favourable, although the approach speeds were excessive. Three-engine PCA system operations were also performed. With all three engines available, the PCA system used the number-two engine for speed control, while the wing-mounted engines continued to provide pitch and yaw/roll control.
Evaluations are now being performed with four-engine propulsion control in the MDC C-17 simulator at Douglas Aircraft, Long Beach. MD-11 PCA software. The results appear promising.
Source: Flight International
