What happened
On March 18, 2019, an Israel Aircraft Industries 1124 airplane, registration N4MH, was involved in a fatal accident near Yukon, Oklahoma. The aircraft was operating as a private Part 9/1 flight.
Video surveillance from Sundance Airport (HSD) captured the aircraft during its approach to the runway. The footage showed that as the airplane reached the approach end of the runway, it began to climb, rolled to the left, became inverted, and subsequently impacted the terrain. The accident resulted in 2 fatal injuries for the two pilots on board.
The investigation
Investigators examined the wreckage and found that the left thrust reverser (T/R) door was unlatched and open, while the right T/R door remained closed and latched. The nose and main landing gear were locked in the extended position, and the wing flaps were extended.
Mechanical examination of the throttle quadrant and thrust reverser components revealed several issues:
- The aircraft was not equipped with a nose landing gear ground contact switch, a component designed to prevent the in-flight operation of thrust reversers.
- Electrical testing showed the left stow microswitch in the cockpit throttle quadrant failed to operate within design specifications, with evidence of arc wear due to age.
- The left T/R Teleflex cable showed signs of resistance and damage, including a missing section of the fluoropolymer liner.
- The left T/R retarder Aft Control Box was found stuck at approximately 75% of its travel.
- The left T/R electrical wiring was not conformal with official electrical drawings and featured missing connections and excessive wire length.
Additionally, the cockpit voice recorder (CVR) was found to be inoperable, as the last recording dated back to November 7, 2007. Toxicology results showed subtherapeutic levels of diazepam in the pilot, but investigators determined the medication was taken several days prior and was unlikely to have caused impairment.
Findings
- The asymmetric deployment of the left thrust reverser caused a left roll and yaw.
- The lack of a nose landing gear ground contact switch, combined with the age-related failure of the stow microswitches, allowed the thrust reverser to deploy in flight.
- The unairworthy condition of various thrust reverser system components contributed to the loss of control.