History of Flight
On March 18, 2019, at 1531 central daylight time, an Israel Aircraft Industries (IAI) Westwind 1124 airplane, N4MH, was destroyed when it impacted terrain near Yukon, Oklahoma. Both pilots sustained fatal injuries. The flight was conducted as a Title 14 Code of Federal Regulations Part 91 personal flight.
A video surveillance camera at Sundance Airport (HSD) in Yukon captured the airplane’s approach. The video showed the airplane approaching the runway, then beginning to climb, rolling left, becoming inverted, and striking the ground.
Aircraft Information
The airplane was not equipped with a nose landing gear (NLG) ground contact switch, which is designed to prevent in-flight operation of the thrust reverser (T/R). IAI Service Letter WW-2419, dated September 30, 1977, titled “Thrust Reverser Arming Activation,” provided instructions for removing the NLG ground contact switch. The letter stated that compliance was at the “owner’s discretion.”
Wreckage and Impact
The airplane impacted terrain approximately 1,472 ft down and 209 ft west of runway 18. The nose and main landing gear were locked in the extended position. The cockpit landing gear control handle was bent left and nearly flush with the instrument panel, positioned toward the top limit of travel allowed by the handle’s housing, which sustained impact damage. Flight control continuity from the control surfaces to the cockpit was confirmed. Both flaps were extended; the cockpit flap selector handle was in an approximate mid-range position and bent left, with a piece missing from its right side. The housing around the handle was impact-damaged. The left and right flap inboard actuators extended about 9 ¾ inches, and outboard actuators about 6 ½ inches. Extension measurements were taken from a dark area consistent with dirt or grease at the aft end of the pistons to the actuator housing; the dark area was about ½ inch long.
Both engines were attached to their pylons, which were attached to the fuselage. Fan spinners exhibited circumferential scoring, fan blades showed chordwise scratches and leading-edge damage, and bypass and exhaust areas contained earthen debris matching the color of the accident site.
Examination revealed the left T/R door was unlatched and open; the right T/R door was closed and latched. The left and right L-shaped latch hooks that engage the left T/R door showed no bending or tearing. The throttle quadrant and throttle control levers were impact-damaged; the levers were found in the “shutoff” position, and the T/R controls in the “stowed” position. Engine control continuity was established, as was T/R control continuity from the throttle quadrant to the T/R doors. The T/R switches were in the “ON” position.
Electrical testing of the left and right T/R stow microswitches within the throttle quadrant showed the left stow microswitch did not operate within design specifications. Disassembly revealed arc wear from aging. According to the airplane manufacturer, the left T/R wiring was non-conformal with electrical drawings: incorrect wire numbers, missing terminal board connections, missing terminal board cover, improperly secured wires, and excessive wire length. A computed tomography scan of the left actuator “OUT OF STOW” switch found an internal broken rivet, which the manufacturer stated would not have led to a short circuit.
The left T/R retarder Aft Control Box was positioned about 75% of its travel, more than 50% of its stroke. It could be pushed back to 100% travel (max power) but jammed at 75% travel when hand pressure was applied. Removing the Teleflex cable allowed the Aft Control Box to move freely. The left T/R Teleflex cable resisted hand movement compared to the right cable. At a certain travel point, the cable became jammed. Examination revealed the cable sheath fluoropolymer liner was damaged within the length where the cable tip was bound; a 3.37-inch length of liner was missing, consistent with wear loss, mechanical removal, and fracture. The opposite end of the sheath liner had coarse fibrils, consistent with monotonic fracture in overstress. Grease there had undisturbed particulate matter on top of the deposit.
According to the airplane maintenance manual, parts of the Teleflex system were to be lubricated with Dow Corning DC33 white grease, suitable for low temperatures. NTSB Materials Laboratory examination of the left T/R Teleflex cables found black grease within the sheath; most of the cable lacked grease. Operational testing of the T/R system could not be performed due to accident damage.
Additional Information
A pilot who previously owned the accident airplane and had flown Westwind airplanes since the mid-to-late 1970s reported that he attended Westwind flight training in 1978 and continued annual training at the same facility, primarily in Westwinds. The training facility had a Westwind throttle quadrant used to demonstrate that the T/R control could unlock with “aggressive” throttle movements toward idle, which would arm the T/Rs during landing. He had once flown a Westwind to 10,500 ft for a pressurization check, “snatch[ed]” the throttle to idle, and the piggyback came out of its detent. After landing, he could not duplicate it. He said many facilities did not teach that the piggyback could come out of detent, but his training facility did. He had discussed uncommanded T/R deployment with the accident pilot, whom he had trained, before the accident pilot attended the training facility.
The accident airplane’s throttle quadrant was not manipulated by hand during post-accident examination to unlock the T/R control with aggressive throttle movements due to damage.
Flight Recorders
The airplane was equipped with a cockpit voice recorder (CVR) required by its type certificate and Part 91 regulations. Part 91 required turbine multiengine aircraft with six or more passenger seats and requiring two pilots, manufactured before April 7, 2010, and operated under Part 91, to have a CVR recording at least the last 30 minutes. The accident aircraft was manufactured in 1978. When the CVR is deactivated or removed, it retains only the most recent 30 minutes. Download by the NTSB Vehicle Recorders Division showed no heat or structural damage; audio was extracted normally. The last recording was from November 7, 2007. The accident flight was not recorded.