On the bare edge.
It was a warm, hazy morning in the Gulf of Oman. I launched from the deck of USS Abraham Lincoln (CVN 72) on a functional check flight (FCF). The Pro C for the Hornet is an uncomplicated check required after certain kinds of flight-control maintenance. Airborne requirements include flight-control rig checks, a G-warm maneuver, and a loop to verify flap scheduling. The flight is normally expedient and methodical. This particular FCF was scheduled following maintenance on the port leading-edge-flap universal, the starboard rudder servo and port aileron servo.
I completed the on-deck portion of the FCF checklist and the airborne rig checks, then began the G-warm maneuver. The initial 4 G pull to the left was uneventful. As I reversed for the 6 G right hand pull, the aircraft began what I can best describe as a loose-tailed, uncoordinated roll at a high rate.
I couldn't stop the jet at the desired roll angle, overbanking to about 115 degrees. After two or three oscillations, I recovered a wings-level attitude. I had to maintain half left-stick deflection to counteract the aircraft's tendency to roll right. The standby attitude-balance ball was pegged full right. Rudder trim only brought the ball halfway back to center. When I relaxed on the stick, the Hornet immediately and rapidly rolled right. The odd handling characteristics were not accompanied by any indication of a flight-control malfunction from the aircraft warning system. I began climbing and decelerating to reach a safer altitude and more normal airspeed.
As the aircraft slowed, the tendency to roll right decreased until, as I passed through 13,000 feet at 250 knots, control inputs were no longer required to maintain wings-level flight.
With what now appeared to be a properly functioning flight-control system, I decided to further investigate the strange handling. Accelerating to 300 knots, I began a 3 G left hand pull. The flight controls felt solid. I increased speed to 350 knots and pulled to 4 Gs. The heavy, uncoordinated roll returned with violent pitch, roll and yaw oscillations. This time the aircraft was more difficult to upright. Each control input produced an adverse reaction in a separate axis, as the aircraft oscillated from 60-degrees left bank to 10-degrees right every two seconds.
On the bare edge of controlled flight, I knew I wouldn't be able to complete the immediate-action step of releasing the controls should the jet depart. Therefore, for the first time in more than 2,000 Hornet hours, I realized that any indication of departure would signify my entry into an unrecoverable condition and cue my immediate ejection.
Fortunately, with the onset of the heavy roll, I had pulled my throttles to idle. As my airspeed decayed, the oscillations abated. After 20 to 25 seconds, I was able to maintain wings-level flight with left stick input. I continued to slow and did a controllability check. With the flaps extended, the aircraft was controllable and did not require left stick input to maintain wings level. However, the lateral stability was less crisp and the roll rate was slower than normal. I flew a straight-in approach and recovered aboard the carrier.
Postflight analysis of the flight-control failure indicated my left aileron position sensor, a component of the recently replaced aileron servo, had malfunctioned twice in flight. In both instances, the flight-control computer, believing it to be in the full-up position, had continued to drive the port aileron down. After the first occurrence, the sensor began working again, but then failed a second time. It remained inaccurate throughout landing. Decelerating and lowering the flaps allowed the functioning right aileron to mirror the drooping left, explaining the lack of oscillation and roll in the landing configuration. Also, the reduced lateral stability and roll rate I noted was as expected with one of the two ailerons out of commission.
In February, 2011, an updated FA-18C NATOPS was released. Among the new emergency procedures was "Uncommanded Roll/Yaw Excursions with Aileron Hard-Over." I dutifully memorized the sole immediate-action item for this procedure, but I didn't look through the remaining steps or read the discussion in the NATOPS Flight Manual (NFM).
To be honest, the simplistic first step, "Rudder and stick--AGAINST YAW/ROLL," led me to assume the procedure was a common-sense application that didn't warrant further investigation.
DURING THE COURSE of this emergency, it never even occurred to me, or the others involved, that we were experiencing an "Uncommanded Roll/Yaw Excursion." Had we read the NFM discussion, or taken time to fully investigate the flight-control positions, the nature of the emergency would have been obvious. The NFM clearly explains the lack of warning-system indications, the need to alter airspeed to reduce oscillations, and the fact that lowering the flaps greatly reduces the tendency to roll. More importantly, it warns that lateral asymmetries consistent with our normal carrier-launch configuration may produce insufficient lateral-stick authority to arrest the uncommanded roll. Based on the oscillations I had and the close proximity to the water, had this failure occurred during launch it almost surely would have resulted in a catastrophic loss. This occurrence validates the FCF requirement that checks be performed at or above 10,000 feet AGL.
LCDR. HIGGINS FLIES WITH VFA-34.
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|Title Annotation:||flight control rig checks|
|Date:||Nov 1, 2012|
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