The Shuttle had a low L/D glide ratio with high induced drag.[^1] The center of pitch rotation was ahead of the pilot’s position in the cockpit.[^1] This meant that the pilot would not be able to perceive a change in flight path until a second after the control input.[^1] On the first PIO incident (STS-3) A cross-wind caused an overshoot of the final approach with a late stabilization.[^1] This put the aircraft into a high-gain situation on short final which caused a PIO to start just as the orbiter touched down.[^1] This caused a 1.5 cycle oscillation that caused the nose to slam down on the runway.[^1]
The second PIO incident had a touchdown point 500 feet beyond the runway threshold.[^1] This caused a skip and ballooning behavior as the pilot thought that he would overshoot the touchdown point.[^1] A PIO started 7 seconds before the first skip with continuous elevon limiting, and a lateral PIO started 5 seconds before the first touchdown.[^1] After the first skip a Lateral PIO occurred that was followed by other skipping and hopping motions caused by overcontrol.[^1]
These PIO tendencies were predicted using the Smith-Geddes Criterion.[^1] It was corrected by using a frequency-dependent filter that reduced the gain as the frequency of the pilot input increased.[^1]
The PIO was caused by a 270ms delay in the flight control system loop.
[[Disadvantage of Elevons]]
[[Shuttle Pitch SAS]] – PIO was a disadvantage of the rate command/attitude hold system.
- RTO-TR-029[^1]
- ivlyersSpaceShuttleFlying[^2]