The PAARS is an automatic aircraft recovery system that operates at all attitudes and speeds.[1] It sets the autothrottles and recovers the aircraft to slight climb.[1] It was the first auto-recovery system installed for a fighter in general use.[1] The PAARS is an all-attitude recovery system to prevent pilot disorientation.[2] It generates commands to the FMS outer loop.[2] The recovery maneuvers emulate a lucid pilot recovering from the same attitude.[2] It uses a sector scheme to schedule the maneuvers.[3]Sector 3 uses a 1g roll as measured in the normal acceleration direction.[3] The system protects against pilot disorientation due to sensor/display failure, vertigo, or distraction due to a high workload.[3] This is the nominal arrangement for 0 roll rate with an airspeed above 200 kts.[3] The location of sector 3 is defined by the deadband in the pitch control path.[3]
The PAARS is housed in the autopilot/autothrottle portion of the NIAC computer.[3] It integrates the autopilot and the autothrottle with a completely digital implementation that generates normal acceleration, roll surface, and throttle position commands.[3] PAARS is disabled during ground and in-flight refueling operations.[3] It is activated by a button or a paddle switch on the control stick.[3] INS data is used if the INS and AHRS agree, but the AHRS data is used if there is disagreement between the two attitude data providers.[3] If invalid data is provided from both attitude sources then PAARS is unable to be activated.[3] The terminal recovery condition results in a shallow, nominal climb when the landing gear is retracted, and a steeper climb when the landing gear is retracted:
| Condition | Value |
|---|---|
| \(\phi\) | \(0.0\ deg\) |
| \(\theta=5.25\ deg\) | GEAR UP |
| \(\theta=9.25\ deg\) | GEAR DOWN |
| CAS=300 kts | GEAR UP |
| CAS=200 kts | GEAR DOWN |
The display shows the minimum altitude expected to be reached during the recovery.[1] This is recalculated at 32 Hz and provides the pilot information on if a safe recovery can be made.[1] The PAARS needed to have dual redundancy to perform after a pilot disorientation caused by sensor failure.[4] Sector 1 involves a 180 deg/s roll to wings level.[5] Any pilot stick commands applied during the recovery were blended with the recovery commands.[5] This system saved the life of a pilot when a FLIR camera, normally slaved to the pilots head movement stayed locked in a sideways position.[5] This caused disorientation and the pilot activated the PAARS system to keep the aircraft level.[5]
Here are some example animations of a PAARS recovery from simulation test data.
This first example is a recovery from a nose-low inverted attitude. This would be able to save the aircraft from a collision with terrain if there is enough altitude to recover.
In this next recovery we start from a nose-high condition and we can see that the aircraft rolls inverted to pull the nose down toward the horizon and then rolls-out of the maneuver
This final recovery is from a near vertical attitude. This is dangerous because the aircraft is able to depart controlled flight in a steep vertical climb if the airspeed is allowed to decay while not triggering the AOA limiter.
[[F-117 PAARS Boundary Shift for Landing Gear]]
[[F-117 PAARS Boundary Shift for Airspeed]]
[[F-117 PAARS Boundary Shift for Altitude]]
[[F-117 PAARS Boundary Shift for Roll Rate]]
[[F-117 PAARS Architecture]]
[[F-117 PAARS Minimum Altitude Calculation]]
[[F-117 Autothrottles]] – used in the PAARS
[[Davinci Resolve Overlay Animated Graph]]
[[JAS-39 Maneuver Load Limiter]] – also has a wings-leveling function
Sources
- [1] “An overview of the F-117A avionics flight test program.” Accessed: Jun. 25, 2024. [Online]. Available: https://arc.aiaa.org/doi/epdf/10.2514/6.1992-1228
- [2] R. Colgren, D. Enns, R. Colgren, and D. Enns, “Dynamic inversion applied to the F-117A,” in Modeling and Simulation Technologies Conference, New Orleans,LA,U.S.A.: American Institute of Aeronautics and Astronautics, Aug. 1997. doi: 10.2514/6.1997-3786.
- [3] “Pilot activated automatic recovery system on the F-117A.” Accessed: Jul. 02, 2024. [Online]. Available: https://arc.aiaa.org/doi/epdf/10.2514/6.1992-1126
- [4] S. R. Combs, R. C. Loschke, and G. J. Tauke, “Flight management system on the F-117A,” in Proceedings of the IEEE 1992 National Aerospace and Electronics Conference@m_NAECON 1992, Dayton, OH, USA: IEEE, 1992, pp. 510–516. doi: 10.1109/NAECON.1992.220523.
- [5] RTO-TR-029
Backlinks
[[Auto-GCAS]]
Deadband Control
[[Dual Redundant Flight Control System]]
[[F-35 Auto Recovery Mode]]
F-117 Aerodynamics
[[F-117 Autopilot]]
[[F-117 Avionics]]
[[F-117 Flight Control Laws]]
[[F-117 Flight Management System]]
[[Thermal Cameras]]