Accident Overview
On June 3, 2016, at approximately 1345 eastern daylight time, a Bell 407 helicopter, registration N4999, operated by PHI Air Medical, was substantially damaged during a forced landing to a parking lot following a partial loss of engine power during initial climb near Mountain City, Tennessee. The commercial pilot incurred minor injuries, while the two crewmembers and one patient were not injured. The on-demand air medical flight was conducted under 14 Code of Federal Regulations Part 135. Visual meteorological conditions prevailed, and a company flight plan was filed for the flight that originated from the parking lot, destined to Johnson City Medical Center Heliport (TN91), Johnson City, Tennessee.
Pilot Actions and Sequence of Events
The pilot stated that during initial climb, at about 125 feet above ground level, he heard a loud "bang" accompanied by a left yaw and rapidly increasing measured gas temperature (MGT). The pilot advised the crewmembers that he was rejecting the climb and landing immediately. He maneuvered the helicopter right to avoid ground personnel, leveled the helicopter with cyclic control, and used all available collective to cushion the landing.
Aircraft and Engine Damage
The helicopter was equipped with a Rolls Royce (Allison) 250-C47B, 650 horsepower turboshaft engine. Examination by a Federal Aviation Administration (FAA) inspector revealed that the helicopter landed hard, spreading the skids and causing substantial damage to the fuselage.
Data downloaded from an engine monitor showed that during the accident takeoff, the MGT increased beyond the maximum for takeoff while power turbine rpm decreased. Examination of the engine at the operator's facility revealed a partial separation of the compressor diffuser, which allowed intake air to leak prior to combustion. The compressor diffuser was forwarded to the engine manufacturer's facility for teardown examination under FAA supervision.
Maintenance Findings
Examination of the compressor diffuser showed that a portion of the aft plate had separated near a braze joint. Review of engine manufacturer specifications indicated that the aft plate should have been a single component, not two components joined by brazing. Maintenance records revealed that the compressor diffuser had been overhauled by a repair facility and installed on the accident helicopter about 98 hours of operation prior to the accident.
According to the FAA principal maintenance inspector (PMI) of the repair facility, a machinist had joined a second ring to the compressor diffuser aft plate using a brazing process. There was no approval from the engine manufacturer or the FAA for that process. It is possible that the machinist performed the procedure to correct a mistake made during a dimensional shim repair, which was approved through the FAA designated engineering representative (DER) program. The machinist no longer worked for the repair facility and had left the company prior to the accident.
Repair Facility Practices
Further review by the FAA PMI revealed that the repair facility had authorization through the FAA DER program for approximately 4 years to repair compressor diffusers. During that time, the facility had performed repairs on 52 other compressor diffusers, mostly common repairs to vanes, with 17 being dimensional shims to the aft plates. The machinist in question had not worked on any of the other 17 similar repairs.
According to the Quality and Engineering Manager at the repair facility, ultrasonic testing failed to identify the unapproved repair. Immediate corrective action included suspending dimensional shim repairs indefinitely. The facility also ensured that a three-tier approval process (engineering, quality, and management) was required for any fixture, tooling, drawing, or process before it was performed, and that all necessary personnel were trained in that method.