Emergency Sikorsky S-76D Landing Due to Fumes (Air Ambulance N761AF of Arkansas Children’s Hospital)

The US National Transportation Safety Board (NTSB) has recently (5 Oct 2021) opened the public docket on an accident that occurred to Sikorsky S-76D air ambulance N761AF of Arkansas Children’s Hospital on 15 May 2019.  The helicopter received substantial damage after an event that occurred in the cruise flight near Morrilton, Arkansas.  None of the 6 occupants were injured.  NTSB have since issued their probable cause so we have updated the article below.

The Accident Flight

The NTSB safety investigation report explains that 43 minutes into the flight the pilot experienced fumes in the cockpit.  He turned off the environmental control system and commenced a descent.  He recounted that:

Within approximately 10 seconds, system visual and aural warnings indicated smoke within the aft baggage compartment so I initiated an emergency descent and landing to the [Morrilton Municipal] KBDQ airport declaring an emergency…[and]…requesting crash/fire/rescue from the town managing KBDQ be dispatch to the uncontrolled airport. I alerted the Arkansas Children’s Hospital communications center of the situation and made a landing and shutdown at KBDQ without further complication.

The Safety Investigation

The NTSB explain that…

…examination of the helicopter by a FAA inspector revealed that the exhaust duct from the No. 2 engine was disconnected and not in its seated position.

Arkansas Children's Hospital Sikorsky S-76D Detached No 2 Engine Exhaust (Credit: FAA via NTSB)

Arkansas Children’s Hospital Sikorsky S-76D Detached No 2 Engine Exhaust (Credit: FAA via NTSB)

Exhaust from the No. 2 engine entered the compartment containing the tail rotor drive shaft and resulted in heat damage to drive shaft and surrounding areas.

Arkansas Children's Hospital Sikorsky S-76D External Heat Damage (Credit: FAA via NTSB)

Arkansas Children’s Hospital Sikorsky S-76D External Heat Damage (Credit: FAA via NTSB)

The exhaust ducts are attached using two bolts secured at 110 ft-lbs of pressure. Upon inspection of the No. 1 engine, as well as the operator’s second helicopter, all bolts were found partially disengaged and not tightened to the specified torque value.

Lock wire is not required to be applied to these fasteners.  The NTSB concluded that:

The maintenance history of these components was not determined, but given the available information, it is likely that they were improperly secured, which resulted in their loosening and subsequently allowed the exhaust duct to become unseated.

NTSB Probable Cause

The improper securing of the exhaust duct bolts, which resulted in the duct becoming unseated and substantial heat damage to the tail rotor drive shaft.

While this doesn’t explain why, the NTSB have classified this accident:

  • Aircraft Fasteners – Incorrect service/maintenance
  • Aircraft (general) – Incorrect service/maintenance
  • Personnel issues (general) – Maintenance personnel

Another Exhaust Duct Accident – A More Dramatic Outcome (Agusta A109A G-DNHI, 9 October 2006)

The UK Air Accidents Investigation Branch (AAIB) report that Agusta A109A G-DNHI was also in the cruise when…

…an engine exhaust duct separated from the helicopter and struck the tail rotor assembly, causing the tail rotor gearbox to also separate. After an initial yaw to the right, the pilot regained limited control.

Agusta A109A G-DNHI Flight Path (Credit: AAIB)

Agusta A109A G-DNHI Flight Path (Credit: AAIB)

However, a further sudden yaw, possibly associated with a partial structural failure of the upper vertical stabiliser, prompted an immediate autorotative descent, which culminated in a successful forced landing.

Agusta A109A G-DNHI Wreckage (Credit: AAIB)

Agusta A109A G-DNHI Wreckage (Credit: AAIB)


The investigation established that a [Mormon / U-band] clamp attaching an exhaust duct to the left engine had failed, due to stress corrosion cracking, allowing the duct to disconnect from the engine.

Agusta A109A G-DNHI Mormon Clamp (Credit: QinetiQ via AAIB)

Agusta A109A G-DNHI Mormon Clamp (Credit: QinetiQ via AAIB)

Two AAIB safety recommendations were raised.

Safety Resources

The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn.   You may also find these Aerossurance articles of interest:



Aerossurance has extensive air safety, operations, SAR, airworthiness, human factors, aviation regulation and safety analysis experience.  For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com