Tech Library

PIO: Pilot Induced Oscillation

About PIO(Pilot Induced Oscillation)

A vibration phenomenon that is unintentionally excited by the correlation between the aircraft’s control system and the pilot.
This phenomenon has been observed in the Wright Flyer, the first aircraft, and in the latest fighter aircraft (X-36), and in the worst case, it can lead to a crash.
Although the name gives the impression that the cause is the pilot, the cause is the aircraft (Flight Control System).
PIO is classified into the following four types depending on the cause of occurrence.

CAT.I  Linear Pilot-Vehicle System oscillation
The simplest and easiest to avoid PIO, which rarely occurs in actual operation because it is dealt with at the design stage.(e.g., A-6 Intruder attack aircraft)

CAT.Ⅱ  Quasi-Linear events with nonlinear contributions
PIO caused by nonlinear contributions such as rate limits/authority limits of actuators + linear contributions.(e.g., Space Shuttle / JAS-39 Grippen)

CAT.Ⅲ  Nonlinear PIOs with transients
PIOs triggered by control system mode transitions, etc. They rarely occur and are difficult to recognize, but when they do occur, they always result in a dangerous state.(e.g., YF-22)

CAT.IV PIO by structural modes
PIO generated by structural modes.(e.g., YF-12 Black Bird)

Authority: AIAA2004-6810, D. H. Klyde, “Recommended Practices for Exposing Pilot-Induced Oscillations or Tendencies in the Development Process”

Regulations for PIO

The regulations for PIO are only general requirements for both Part 23/25, as follows. (25.143)

a) The aircraft must be capable of safe maneuvering and movement under the following conditions.

a. Takeoff
b. Ascent
c. Level flight
d. Descent
e. Landing

b) Must be able to smoothly transfer from one flight condition to any other without requiring special piloting skill, attention or control, and without exceeding the load limit multiple, in all anticipated operating conditions, including the following.

a. Engine failure (e.g., critical engine)
b. Change of configuration, including entering and exiting deceleration devices


AC25-7D also states that the current regulations fail to define design requirements to avoid PIO altogether, as follows.

“Operational experience and other factors indicate that modern Part 25 aircraft do not meet 25.143 because of the potential for airplane-pilot coupling (PIO) under certain operating conditions.”