Investigation Summary
The accident sequence began during the climb to 16,000 feet when a non-return valve in the APU system failed to close. This allowed hot bleed air from the Rolls-Royce Spey engines to enter the APU in reverse. The air exited into the airframe plenum chamber at temperatures high enough to ignite the acoustic lining. As the fire intensified, it melted the aluminum partition separating the plenum from the hydraulics compensator bay. Flexible hydraulic hoses failed under the heat, spraying Skydrol fluid into the fire under pressure. The fire then moved upward into the vertical stabilizer, creating a chimney effect. The intense heat destroyed the lower rudder attachment and the elevator control rods. Witnesses observed large sections of the tailplane separating from the aircraft before it plunged into the forest. The cockpit voice recorder captured the captain's final moments as he reported a total loss of control and an unsuccessful attempt to return to Elmira.
Final Conclusions
The NTSB concluded the probable cause was the loss of integrity of the empennage pitch control systems due to a destructive in-flight fire. The fire originated in the airframe plenum chamber, fueled by hydraulic fluid, and progressed into the vertical fin. The root cause was engine bleed air flowing back through a malfunctioning non-return valve and an open air delivery valve, exiting into the plenum at ignition-level temperatures.
Video Analysis
Burning Up From Inside | Mohawk Airlines Flight 40 — A detailed look at the mechanical failure and subsequent in-flight fire that led to the structural failure of Mohawk Airlines Flight 40's tail section.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Crew Experience
Captain
Charles E. Bullock, age 43.
First Officer
Troy E. Rudesill, age 33.
Systems & Failure Modes
Auxiliary Power Unit (APU) Non-Return Valve
A check valve designed to prevent engine bleed air from flowing backward into the APU; its failure allowed hot air to enter the airframe plenum.
Airframe Plenum Chamber
A compartment for the APU installation where the fire originated after acoustic linings were ignited by reverse-flowing bleed air.
Empennage Pitch Control
The mechanical rods and hydraulic systems in the tail that control the aircraft's pitch; these were destroyed by the fire, leading to a total loss of control.
Interesting Facts
- 01The aircraft was nearly new, having accumulated only 2,246 airframe hours since its first flight in 1966.
- 02The aircraft, N1116J, was named 'Discover America'.
- 03The fire destroyed the No. 1 VHF transceiver antenna lead, causing a loss of radio communication with air traffic control.
- 04The tail section was found 400 yards away from the main impact site due to in-flight structural failure.
- 05Mohawk Airlines president Robert E. Peach initially requested an FBI investigation into potential sabotage, though no evidence of such was found.
Safety Actions & Advisories
Airworthiness Directive 68-01-01
Source ↗Issued by the FAA to prevent heat damage or fire in the airframe plenum of the APU installation, requiring modifications to the bleed air system and non-return valves.
Fireproof Barriers and Material Upgrades
Source ↗NTSB recommended providing fireproof barriers to isolate the vertical fin and replacing aluminum partitions in the hydraulics bay with fireproof materials to prevent fluid-fed fires from spreading to critical control structures.
APU Flight Restrictions
Source ↗A temporary restriction was placed on the use of the APU in flight until mandatory modifications to the air delivery and non-return valves were completed.
Check Valve Redundancy
Source ↗Recommendations were made to improve the reliability and redundancy of the non-return valves in the engine bleed air system to prevent reverse flow into the APU.
Known Controversies & Unanswered Questions
Sabotage Allegations
Reference ↗Immediately following the crash, Mohawk Airlines President Robert E. Peach publicly demanded an FBI investigation into potential sabotage, though the NTSB later confirmed the cause was purely mechanical.