A combined US Navy and Marine Corps test team has identified the warning signs which will help pilots of Boeing F/A-18s avoid the danger of losing the aircraft during a "falling-leaf" departure from controlled flight.

The team estimates that at least eight F/A-18s have crashed because of this and other out-of-control events. It believes that many could have been avoided if pilots had identified the situation earlier.

In some cases, the team says, pilots misinterpreted the out-of-control mode and activated the automatic spin recovery mode (ASRM) which actually "-delays recovery from the falling leaf".

The falling-leaf mode is similar to a spin with a high yaw rate, cyclic angle of attack, large side force, roll and yaw in phase and reversals in roll, yaw and pitch.

During a series of test flights in which an F/A-18 was deliberately made to depart from controlled flight, the team identified out-of-control modes ranging from the falling leaf to a post-departure gyration and an oscillating spin.

In nearly every case, the biggest cue was a "-high side force felt simultaneously with low acceleration. It left the pilot feeling light in the seat and pressed up against the canopy". The situation, which was "extremely noticeable in the cockpit", occurred with high sideslip and low angle-of-attack but produced a unique pattern for each departure mode.

For the falling leaf, the pilot's biggest cue was side forces from both sides, and recovery was accomplished by putting the stick fully forward. With an oscillating spin, several side force cues were felt on both sides before the aircraft departed from controlled flight.

In this case, the aircraft was recovered by putting the stick in the direction of the command arrow produced on the head-up display by the ASRM. The only cue for a post-stall gyration was "maybe one" side-force event, but recovery was made by simply releasing the controls.

The team originally planned to "-do a video and put it out to the fleet", but discovered that "-it contains a lot of things that are forbidden in the manuals". As a result, it will now visit units as a team to explain its findings.

Further work, under the code name Project Screamer, is also planned which may eliminate low angle-of-attack departures from controlled flight and falling leaf events altogether. This is expected to be accomplished by incorporating sideslip feedback into the F/A-18C/D flight control laws.

Source: Flight International