Pilot-Induced Oscillations (PIOs) represent a critical challenge in flight control systems, characterized by negative feedback between the pilot and the aircraft. When a pilot manipulates the controls to achieve tighter control, it effectively increases the gain of the controller, the pilot. If the reduced-order model used for control design doesn’t match the phase behavior of the actual aircraft, PIOs can be induced. PIOs occur when control inputs become 180 degrees out of phase with the aircraft response, leading to a delayed reaction by the pilot to stimuli. Control system lags, particularly during landing, are a significant factor, as these lags can create PIOs. Fly-By-Wire (FBW) systems can offer a faster feedback loop, which helps in preventing PIO events, but aircraft with sidestick controllers are notably more prone to PIOs due to their high sensitivity.
The occurrence of PIOs has been documented across many aircraft development programs in the US and Europe, often leading to program time and cost overruns. In light aircraft, excessive elevator authority and light stick forces can lead to sudden and unexpected PIOs. These oscillations are typically caused by excessive lag or deadband in the control system, where there is significant delay or insensitivity around the neutral position of the control stick. Recovery or mitigation methods, such as frequency-dependent gain reduction filters, are often employed but usually at the expense of reducing the aircraft’s agility. Common PIO problems include bobble and tracking oscillations, with typical pilot response times ranging from 1 to 2 seconds, making oscillations around this frequency particularly problematic. The degree of nonlinearity in the control system’s response and characteristics like high stick sensitivity, excessive time delay, or phase lag also contribute to PIO susceptibility, independent of the specific flight control mechanization.
Understanding and mitigating PIOs is essential for flight control system designers to ensure aircraft stability and control. Technological solutions, such as advanced FBW systems and frequency-dependent gain reduction filters, play a crucial role in addressing these issues. By accurately modeling the aircraft’s phase behavior and implementing effective control strategies, the risks associated with PIOs can be significantly reduced, leading to safer and more reliable aircraft performance.
History of Digital Fly-By-Wire – Examined PIOs
SAAB Gripen – first test aircraft crashed due to PIO.
SAAB Gripen PIO– modifications were made in light of the PIO crash.
F-22 PIO – Initial FCS caused PIO crash
F-8 DFBW PIO – F-8 FBW testbed PIO
Reduced Order Models – accurate models are important for preventing PIO
F-4 Phantom PIO Accident – aircraft breakup due to excessive structural loads at supersonic speeds.
Sidestick Controller – aircraft with sidesticks are more prone to PIO
C-17 PIO – C-17 PIO film
Space Shuttle PIO
Phase Lag – causes errors in DFBW flight control systems
M2F2 accident – lateral/directional PIO caused an interruption in a critical phase of flight, causing the aircraft to crash during landing.
J-35 Draken PIO Tendencies
[[Type 1 Higher-order Locked-In PIO]]
F-15 Active Flight Control System – PIO was not considered during development
High AOA Wing Rock – may be caused by PIO
HL-10 Limit Cycle Tests – PIO tendencies during approach and landing
Speed Stability – can be caused by a tight pilot gain for glide slope tracking
Type 2 PIO
F-16 PIO taxi-test – first flight
Eurofighter Roll Command Path – PIO tendencies are avoided with notch filters.
F-16 VISTA (X-62)– can show pilots dangerous PIO conditions and different control strategies in a single sortie.
F-4C Longitudinal Dynamics – Could cause a PIO loss of control if the bellows failed.
Hang Glider Aerodynamics – hang gliders can exhibit PIO tendencies
half-p Hop– PIO due to gyro dynamics of a rigid rotor
X-29 Flight Control System – exhibited unstable responses with rate-limiting of the ailerons
Artificial Feel Systems– can go unstable and cause PIOs at lower gains
F-117 PIO – discovered during flight testing
LAHOS Study – determined that an excessive delay between the pilot actions and the aircraft response
HQDT Test – good way to expose latent PIO tendencies.
F-14 ARI Control System C – could have a higher PIO risk due to high gains
DIGITAC – flight controls with integrators are susceptible to a loss of stability due to large servo nonlinearities
BP6.11 – PIO should be avoided at all circumstances
F-18 Legacy Hornet – F-18A encountered dramatic PIOs.[^8]
Roll Ratchet – high-frequency roll-axis PIO
MQ-1 Landing PIO – caused by datalink delay
JAS-39 Airshow Crash – also caused by PIO
Sources
- P. Dr. Hamel, “Advances in Aerodynamic Modeling for Flight Simulation and Control Design,” Jan. 2001
- E. Field, “The application of a C* flight control law to large civil transport aircraft,” 1993, Accessed: May 15, 2023. [Online]. Available: https://dspace.lib.cranfield.ac.uk/handle/1826/186
- AIAA2023
- B. J. Goszkowicz, “Sidestick Controllers During High Gain Tasks”.
- RTO-TR-029
- “Wing Rock Prediction Method for a High Performance Fighter Aircraft”.
- INCAS − National Institute for Aerospace Research “Elie Carafoli” B-dul Iuliu Maniu 220, Bucharest 061126, Romania iursu@incas.ro, U. Ioan, and T. Adrian, “Dealing with actuator rate limits. Towards IQC-based analysis of aircraft-pilot system stability,” in INCAS BULLETIN, Jun. 2013, pp. 53–72. doi: 10.13111/2066-8201.2013.5.2.7.
Backlinks:
Control System Sensitivity
Deadband Control
Disadvantages of DFBW
Downside of Bang-Bang Control Systems
Limit Cycles
Phase Lag
Unit Delay in Z Domain