What happened
On May 15, 2019, at approximately 18:33 CDT, a Sikorsky S-7lam helicopter, registration N761AF, was operating as a Part 135 on-demand air medical flight near Morrilton, Arkansas. Approximately 43 minutes into the cruise phase of the flight, the pilot reported smelling fumes in the cockpit. In response, the pilot turned off the environmental control system and began a descent. Shortly after, visual and aural warnings indicated smoke in the aft baggage compartment. The pilot declared an emergency with air traffic control and performed an emergency descent, landing the aircraft without further incident. There were no injuries among the pilot, two crewmembers, or three passengers.
The investigation
An inspection conducted by a Federal Aviation Administration inspector revealed that the exhaust duct for the No. 2 engine had become disconnected and was no longer in its seated position. This allowed exhaust from the No. 2 engine to enter the compartment housing the tail rotor drive shaft, causing heat damage to the shaft and the surrounding area.
While the exhaust ducts are intended to be secured with two bolts torqued to 110 ft-lbs, investigators found that the bolts on the No. 1 engine were also partially disengaged and not tightened to the required torque. Furthermore, an examination of the operator's second helicopter showed that the bolts securing both of its exhaust ducts were also not tightened to the specified torque value.