An Israel Aircraft Industries’ IAI 1124 Westwind business jet rolled over and crashed while on approach to Sundance Airport (HSD) in Yukon, Oklahoma, on March 18, 2019. A video surveillance camera at the airport recorded the aircraft on short final approach pitching up, rolling left and then becoming inverted before descending to the ground.

When investigators examined the wreckage, they found the left thrust reverser deployed. It wasn’t hard to see what happened—the thrust asymmetry caused the pilot to lose control. Finding out why it happened was more difficult.

There had been another Westwind crash in 2014 in which a thrust reverser inadvertently deployed. Despite an extensive investigative effort, the NTSB could not figure out why the device deployed. The accident at HSD was different. The agency did some excellent forensic lab work and came up with the answer.

The twinjet, registered N4MH, was owned by fixed-base operator Sundance Airport FBO. It departed the airport on the afternoon of March 18 carrying two pilots and two passengers. Their destination was Northwest Florida Beaches International Airport (ECP), in Panama City, Florida. After dropping the passengers there, the jet returned to Sundance Airport.

Skies at HSD were clear when the flight began its approach to Runway 18. Winds were out of the south at 7 kts, and the temperature was a mild 68 deg. F. The pilots did not report any indications of trouble and both were familiar with the aircraft.

At 1537:15 CDT, the jet was stabilized at 124 kts 117 ft. above the runway’s approach end. Twelve seconds later, GPS data showed the airplane still stable and only 22 ft. above the runway. Three seconds after that, the jet had climbed 19 ft. and slowed to 109 kts. It had begun its fatal roll. It was inverted when it struck the ground; the two pilots were killed.

Safety investigators found the landing gear and flaps extended and the left thrust reverser door unlatched and open. They confirmed that there was flight control continuity from the control surfaces to the cockpit controls. They found the throttle quadrant to be damaged, but no indication that the thrust reverser had been deployed. The thrust levers were in shutoff, the toggle switches were on, and the thrust reverser controls were stowed. There was control continuity from the throttle quadrant to both the fuel control units and the thrust reverser doors.

No Obvious Explanation

With no obvious explanation for why the system deployed, it must have looked to the investigators like a repeat of the 2014 Westwind accident. In that case, the NTSB had concluded the right thrust reverser had deployed “for reasons that could not be determined because post-accident examination of the airframe and engine thrust reverser system did not reveal any anomalies.”

The 2014 accident in Huntsville, Alabama, occurred during takeoff, with the right thrust reverser opening just after liftoff. The jet was equipped with a cockpit voice recorder (CVR). At 33 ft. radio altitude, the CVR recorded a rattling sound that continued to the end of the flight. It was the sound of the thrust reverser deploying.

A pilot proficiency examiner (PPE) was conducting check rides for two contract pilots who worked part time for the owner of the aircraft. All of the pilots had ATP certificates and were highly experienced; the PPE had more than 28,000 hr. of flight time and the pilot in the left seat had nearly 20,000 hr. However, the PPE had been involved in two prior accidents. Both took place during check rides and the probable cause of both pointed to the PPE’s actions.

IAI officials said at the time that they knew of no accidents caused by inadvertent thrust reverser deployment and, after combing their own data base, NTSB investigators couldn’t find any either. In addition, during certification flight tests, IAI test pilots had shown the airplane was fully controllable at cruise or takeoff with a thrust reverser deployed.

Investigators removed the left and right thrust reverser actuators and control valves from the Huntsville jet and scanned them with x-ray computer tomography (CT). They then disassembled the actuators and examined the components on a test bench. The locking pawls were in place and there was no evidence of failure or malfunction of the actuators.

There was one irregular mechanical issue that couldn’t be explained. A throttle retarder system was supposed to reduce thrust to idle within 4 to 8 sec. if a thrust reverser deployed inadvertently. A sound spectrum analysis showed that it had malfunctioned, only reducing thrust to 84% N2. “Contributing to the accident was the excessive thrust from the right engine with the thrust reverser deployed for reasons that could not be determined during post-accident examinations and testing,” the NTSB said in its probable cause finding.

That excessive reverse thrust from the right engine might have been enough to invalidate the controllability assumptions made by the IAI test pilots. Even so, it appeared that the two Huntsville pilots just lost control of a jet they should have been able to control.

Investigators still couldn’t explain why the thrust reverser deployed, but there were clues in two of the interviews they conducted. The chief pilot of the operator in the 2014 accident told investigators that could be possible to inadvertently deploy a thrust reverser if you “snatched the throttle back really quickly.” He said he had observed the PPE retard a throttle immediately after takeoff on check rides.

Another Westwind pilot also commented in an interview that the PPE was “aggressive.” He knew from experience that if the training pilot pulled back the thrust lever aggressively and there was “slop” in the system, the thrust reverser could have deployed. He knew of maintenance practices that allowed “piggyback (reverse) levers” to be loose. If that was the case, moving the power lever to idle would cause the piggyback lever to come out of the stow detent.

The word “slop” is not a precise technical term, but it hinted at the possibility of a mechanical issue in the throttle quadrant. However, the cockpit of the Huntsville jet was badly fragmented and fire damaged and investigators did not probe the throttle quadrant beyond documenting control positions.

Shoddy maintenance is implicated in the malfunction of the Westwind’s thrust reverser system, in Part 2 of this article.