The test aircraft crashed in 1989 due to PIO which was partially caused by the control surface servo rate limits. The PIO initially developed in roll, then in pitch.[5] Software limits are placed on the control servo commands to prevent the hydraulic servos from rate limiting.[4] When the rate limiter is saturated, the resulting phase shift reduces the closed-loop stability margins and increases the risk of a PIO.[4] The solution to this problem is to either reduce the phase margin or extra delay in the feedback loop. The feedback phase compensation for the controller was based on anti-windup methods. This increased or advanced the phase around the points of nonlinearity. Low-pass filters were used to eliminate the biases and issues caused by high-frequency roll-off. This phase compensation technique was used in place of rate limiters in the control system of the production fighter. Lateral turbulence caused the PIO. The control system augmented the dihedral effect, which made the aircraft sensitive to the roll axis. This caused the rate limits to engage, which increased the control system delay, causing a PIO incident and subsequent crash. The commanded roll rates saturated the elevons in the pitch axis. A footprint of parameterized stick inputs was developed to reduce maneuverability at low speeds. The effect was to create a margin between the footprint and the onset of the rate limits. The trigger was a response to lateral turbulence.[5] The control system augmented the dihedral effect.[5] The “mini-stick” was a contributing factor as it had a skewed axis and could demand full control with very small movements.[5]
For the second accident, the actuator would reverse when the stick is reversed. This reduced the phase lag loss, but it required careful signal blending, and noise reduction.
JAS-39 Airshow Crash– also caused by rate limiting
[[JAS-39 Phase Compensating Rate Limiters]]
[[Rundquist Limiter]] – used on the Gripen
Sources
- [1] “Flight Control Law Design: An Industry Perspective – ppt video online download.” Accessed: Feb. 11, 2023. [Online]. Available: https://slideplayer.com/slide/5738737/
- [2] G. Stein, “The practical, physical (and sometimes dangerous) consequences of control must be respected, and the underlying principles must be clearly and well taught.,” IEEE Control Systems Magazine, 2003.
- [3] RTO-TR-029
- [4] L. Rundqwist and R. Hillgren, “Phase compensation of rate limiters in JAS 39 Gripen,” in 21st Atmospheric Flight Mechanics Conference, San Diego,CA,U.S.A.: American Institute of Aeronautics and Astronautics, Jul. 1996. doi: 10.2514/6.1996-3368.
- [5] 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.
Backlinks
[[Anti-Windup Scheme]]
[[Control Surface Rate Limits]]
[[Dihedral Effect]]
[[Mode-Blending]]
Pilot-Induced Oscillations
SAAB Gripen