1. 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

2. 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

3. 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

4. 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.

5. 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

6. 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”

7. 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

8. 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