Flight Test Mishap Red Flag Analysis Aircraft/Participants Mishap Synopsis Investigation Synopsis Cause Summary (incl. flawed assumptions, human error, organizational lapses, pre-existing failure, unintended effects) Red Flags for Future Testers Best Practice Lessons

YF-22 PIO Aircraft/Participants TM Aircraft Pilot YF-22, single seat USAF fighter, equipped for telemetry (TM) support Recently completed Demonstration-Validation program Fly-by-wire flight control system (FCS) Pilot–selectable thrust vectoring (TV) capability Standard flight card precaution called for TV DISABLED <10 K’ To be removed upon FCS maturity. FCS gain change during gear retraction causes normal nose-up pitch ~4 deg/sec. Countered by forward stick application Subsequent small 4 deg/sec pitch down normally arrested by relaxing forward pressure Pilot Excellent skill & reputation Many test hours in aircraft, experience conducting YF-22 low approaches TM Test Conductor new & inexperienced Incident mission was first he TC’d Experienced Test Director did not have many YF-22 test missions

YF-22 PIO Mishap Synopsis Edwards AFB Follow-on flying after Dem-Val program Go-around for 2nd low approach Pilot selected afterburner, retracted gear TV was not disabled <10K’ i.a.w. cards With TV ENABLED, TV function automatically activates upon gear retraction With TV gain change, normal forward stick input magnified nose-down pitch rate to ~12 deg/sec Stabilator and thrust vector actuators became rate-limited Result was very low altitude PIO with increasing severity After 4-5 large cycles, aircraft impacted runway essentially in flat attitude with pilot making conscious decision to “fly” aircraft to runway

YF-22 PIO Investigation Synopsis No factor: pilot fitness, weather, malfunction, urgency Previous sim studies showed PIO susceptibility due to actuator rate limiting Residual oscillations were small in power approach config More pronounced oscillations with thrust vectoring enabled Combination of holding forward stick during gain change (due to raising the gear) and thrust vectoring ON triggered PIO During RTB descent, pilot failed to disable TV TC, TD, control room team apparently did not notice or notify pilot to disable TV Doing so would have likely prevented accident.

YF-22 PIO Investigation Synopsis Little or no awareness of PIO susceptibility outside Ft Worth FCS team Edwards team unaware. Holding forward stick during gear retraction done many times No concerns TV worked well throughout FQ testing with very high confidence Testers enabled TV >10K’ during all missions – even if not HQ related. Lead FTE asked FCS engineers about revising procedures to enable TV prior to takeoff through entire flight. All queried saw no issue in doing so, but no action due to ending Dem Val program

YF-22 PIO Cause Summary Proximate cause: Unexpected a/c response with TV engaged during gear retraction Critical team members unfamiliar with vital a/c systems findings Ft Worth team didn’t effectively communicate potential flight control hazards to test team Pilot, TM team let their guard down Complacency brought on by familiarity with control system that functioned flawlessly Failure to be diligent for entire mission, from start to shutdown Pure speculation: if noticed, TM engineers may have deemed TV engagement non-critical because it had been “proven”

YF-22 PIO Red Flags for Future Testers Beware of Abnormal Ops Edwards testers neither aware of nor on guard against PIO susceptibility Inadvertent ops with TV engaged. Communication Error(s) Ft Worth team did not effectively communicate PIO susceptibility findings Bottom line: above led to participants being less prepared & vigilant than needed

YF-22 PIO Best Practice Lessons Testers should not conclude complex systems are safe until fully vetted Effectively communicate any Engineering concerns to test team When TM being used anyway, consider support through entire flight Other incidents showed TM monitoring during non-test portions did or could improve safety Perhaps most helpful with complex single seat aircraft