The Ejection Site

The YF-22 Articulating ACES II
During the Demonstration/Validation (DemVal) phase of prototype production for the YF-22 aircraft (produced by Lockheed Aeronautical Systems (LASC) with partners Boeing Military Airplanes and General Dynamics Fort Worth), a very unique ejection seat was designed. The YF-22 ACES II seat was modified from a standard A-10/T-46A version to have a particularly unique feature. In modern combat with the extremely dynamic manuevering ability of new aircraft the pilot must be able to survive and be in control of the aircraft while being exposed to rapid and high changes in G-force. There are two traditional methods of dealing with this problem. One is the use of the combined anti-g-suit and g-straining manuever (which involves a physical tensing of certain muscles while breathing in a certain manner), the other is placing the seat in a reclining posture. Both methods were examined by the project teams, with a new style anti-G-suit being issued for the former, and the modification to the ACES II seat for the latter.

The concept was simple (as so many engineering concepts are), but required much work to develop. The seat was to be installed in a traditional semi-upright posture with a seat back angle of 15 degrees, but to have the ability of changing the seat back angle to 55 degrees either by the pilots choice, or possibly automatically selected by the flight control computer at the onset of rising Gz. Many factors had to be considered during the design phase, including:

This is only a sample, many other factors were considered as well.

Weber Aircraft was chosen at the vendor to provide the seats. The seat as built was a fairly standard seat with B-2 style flip up pitot sensors, new machined side panels, a hinged/rotating back panel, moving seat pan, modified survival kit, lap belt tension adjustment, and the articulation/retraction system. The articulation system used a electrical motor to drive the seat pan up and forward, pulling the seat back with it. The motor drive was coupled by a drive shaft to the rudder pedal adjusting mechanisms. The seat back was hinged at its upper end, directly behind the pilot's shoulders. This arraingment was developed by intensive evaluation with mockup and human subject evaluation at General Dynamics.

The net result was that at the choice of the pilot (The automated system being defered), the seat pan could be adjusted so that the seat back angle was any position between 15 and 55 degrees. This allowed the pilot to almost lay prone in the aircraft during high-G manuevering, but resume an upright position for other flight duties. In the event of an emergency requiring ejection while the seat was in the reclined mode a pyrotechnic retract mechanism was included in the design. It was this mechanism which delayed the use of the articulation until late in the program. This system was designed to retract the seat to the 15 degree upright posture in 30-700 milliseconds and was integrated with the center pull ejection handle. The seat was eventually fully cleared for articulated use except during takeoff and landing.

Pilots reports were generaly positive, especially in regard to seat comfort with the seat in the 34 to 45 degree position. There were concerns, however with such details as increased difficulty in 'checking 6', and reaching certain controls. There also were some safety concerns raised by reports that the ejection handle blocked some portion of the lower multi-function display with the seat reclined, and one pilot reported that his knees contacted the underside of the instrument panel (this occured while the seat was fully reclined and the pilot raised the seat to get better visibility).

Overall, the minimal time to test the complete system, and the added costs, weight, and complexity caused the seat to be omitted from the final design. Hopefully, the concept of escape system articulation will be examined again in the future as it could provide for improved pilot capability and comfort.

Information and diagrams for this page taken from "The YF-22 Articulating Ejection Seat" by Gordon P. Cress, Lockheed Aeronautical Systems Company. A paper presented to the SAFE organization.

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