This aircraft was involved in a PIO during landing after a flutter excitation test that started 13m above the runway and led to a crash on April 25, 1992.[1] The PIO started 40 feet above the runway and caused the aircraft to impact the runway after 4-5 oscillations.[1] This PIO was caused by rate and position saturation of the control surfaces as well as time delays in the flight control system.[1] Upon analysis against the Smith-Geddes handling criterion, the F-22 was found to be PIO prone in the flaps-up up-and-away control law at low speeds.[1] The landing gear was retracted at the same time as the afterburner was ignited to initiate a go-around.[2] These caused significant mode switching in the flight control laws.[2] A discrete gain change caused an excessive nose down pitch causing the pilot to reverse the stick.[3]. The pilot was holding nose up trim that required operating near the forward deflection limits for the stick.[4] With gear-up the thrust vectoring allowed for greater commanded pitch rates.[4] The gear-up nose down gradient was developed for nose down recoveries from the post-stall AOA region.[4] Rate limiting of the horizontal tail and thrust-vectoring nozzles occurred.[4] An FFT analysis was used to determine the PIO handling metrics including Gibson Phase rate, Bandwidth, and Smith-Geddes Criterion.[4]
In the video below, you can see the PIO start at 1:13
The graph below shows the gradient change.[4]
To remedy the system, the design goals for the CAP and damping ratio are nearly identical along with the stick force gradient.[4] Also the thrust vectoring is kept on for the gear down flight control law.[4]
[[F-22 Smith-Geddes Criterion]]
[[F-22 Handling Qualities Simulator]] – used to try to re-create the PIO event
[[BP4.13]] – handling qualities should be identified when modes are changed
[[YF-22 Handling Qualities]]
Sources
- [1] RTO-TR-029
- [2] D. M. R. Anderson and A. B. Page, “UNIFIED PILOT-INDUCED OSCILLATION THEORY VOLUME III: PIO ANALYSIS USING MULTIVARIABLE METHODS”.
- [3] NATO, Ed., Flight Vehicle Integration Panel Workshop on Pilot Induced Oscillations: = (Atelier sur le Pompage Piloté). in AGARD advisory report, no. 335. Neuilly-sur-Seine: AGARD, 1995.
- [4] J. Harris and G. Black, “F-22 control law development and flying qualities,” in 21st Atmospheric Flight Mechanics Conference, San Diego,CA,U.S.A.: American Institute of Aeronautics and Astronautics, Jul. 1996. doi: 10.2514/6.1996-3379.
Backlinks
[[F-22 Flight Control System]]
[[Mode-Blending]]
Pilot-Induced Oscillations