History of Flight
On September 6, 2016, at about 1340 eastern daylight time, a Sikorsky S-61N helicopter, registration N805AR, was destroyed when it impacted a field after experiencing a dual loss of engine power while in a hover near Palm Bay, Florida. The airline transport pilot, commercial-rated copilot, and a maintenance crewmember were fatally injured. The helicopter was registered to EP Aviation LLC and operated by AAR Airlift Group under Title 14 Code of Federal Regulations Part 91 as a post-maintenance flight. Visual meteorological conditions prevailed, and a company visual flight rules flight plan was filed for the local flight that departed Melbourne International Airport at 1324.
According to the operator, the helicopter's fore/aft pitch servo had recently been removed and replaced. Three functional check flights (FCF) were required. Two FCFs were completed uneventfully on the day of the accident, and the crew was conducting the final FCF when the accident occurred. One of the maneuvers during the final FCF was rearward flight at a computed airspeed of 20 knots. The cockpit voice recorder (CVR) indicated that the flight crew performed two rearward FCF maneuvers during the accident flight.
Video from a ground witness recorded the helicopter performing the first rearward maneuver about 200 ft above ground level. Correlation with the CVR showed the helicopter flying rearward at 1337:25, with the copilot stating rear speed of 15 knots. At 1337:40, rear speed was 20 knots. Two thumping sounds were recorded at 1337:42 and 1337:44 when rear speed was about 31 knots, but corresponding sounds were not visible in the witness video. The video then showed the nose pitch down as the helicopter transitioned from rearward flight to a forward left 90° turn and continued forward in straight and level flight. During the recovery maneuver, an event caused a 2-second region of overdriven audio on the CVR. This occurred during a left pedal turn while the helicopter was approaching 90° to the rearward flightpath. The helicopter then flew an orbit with increasing altitude, and the video ended.
According to the CVR, the pilot took over control at 1336:41. The copilot, who was receiving flight training, identified the thumping sounds as a compressor stall, and the pilot agreed. The pilot then informed the maintenance crewmember they were returning to MLB. At 1338:02, the crew discussed the perceived compressor stall and engine exhaust gas temperatures. At 1338:26, the pilot said they would try the maneuver again in a different direction relative to the wind (with wind off the nose), and the maintenance crewmember agreed. At 1339:46, while recovering from rearward flight, there was a change in background noise identified as a compressor stall by the maintenance crewmember. The cockpit area microphone audio was overdriven again from about 1339:45 to 1339:48. At 1339:48, the copilot stated "AFCS is back on" while another change in background noise consistent with decay in drivetrain rpm occurred. The recording ended at 1339:55.
There were no known witnesses to the impact.
Personnel Information
The pilot in the left seat held an airline transport pilot certificate with a rating for rotorcraft helicopter and commercial privileges in airplane single- and multi-engine land and instrument airplane. He also held a flight instructor certificate with ratings for rotorcraft helicopter and instrument rotorcraft. His most recent FAA first-class medical certificate was issued on December 5, 2015. According to the operator, the pilot was hired in 2012 and completed required company training. He had about 6,347 total flight hours, with 5,743 in helicopters and 1,780 in the same make and model. He flew 114 hours in the 90 days and 25 hours in the 30 days preceding the accident, all in the same make and model.
The copilot in the right seat held a commercial pilot certificate with ratings for rotorcraft helicopter and instrument helicopter. His most recent FAA second-class medical certificate was issued on May 16, 2016. He was hired on July 31, 2016, and had completed ground training but was still in flight training. He had 4,090 total flight hours, all in helicopters. Before the accident, he had no flight experience in the same make and model but was qualified as second-in-command.
Aircraft Information
The 41-seat capacity, tricycle-gear helicopter, serial number 61717, was manufactured in 1974. It was powered by two 1,500-horsepower General Electric CT58-140-2 turboshaft engines. The helicopter was maintained under a continuous airworthiness program. Its most recent inspection was a phase five check completed on August 25, 2016. At that time, the airframe had 40,296.2 total hours. The No. 1 engine had 708.9 hours since major overhaul (23,235 hours since new), and the No. 2 engine had 4,520.2 hours since major overhaul (26,259 hours since new). After the phase five check, the helicopter flew about 1.2 hours during two previous FCFs. At the end of the second flight, the crew reported total fuel onboard was 2,200 lbs.
Meteorological Information
At 1353, the recorded weather at Melbourne International Airport, about 8 miles north of the accident site, included wind from 070° at 11 knots, visibility 10 miles, and few clouds at 5,000 ft.
Wreckage and Impact Information
The helicopter came to rest upright in a field with no debris path. The wreckage was oriented on a magnetic heading of about 190°. A postcrash fire consumed the cockpit and cabin. The tail boom transition section exhibited partial thermal damage, and the tail boom remained intact. The five main rotor blades and five tail rotor blades remained attached to their hubs. The blades exhibited signatures consistent with low rotational energy at ground impact. Four of the five main rotor blades had partial thermal damage, and one had thermal damage along its entire span. One tail rotor blade was fractured about 1 ft outboard of the attachment bolt; the outboard section was found on the ground next to the tail rotor. Another tail rotor blade was partially separated about 1 ft outboard, with its tip embedded in the ground. Drivetrain continuity was confirmed between the main transmission and the tail rotor gearbox.
Both engines remained attached to the airframe and exhibited fire damage. Examination of the engines revealed that the first stage compressor blades of both engines had little or no leading-edge damage. Both engine fuel control units were found with their control shafts in the "FLIGHT" position. Both engines were separated from the main gearbox at the aft end of the high-speed shaft. A borescope inspection of the main gearbox showed no thermal damage on internal components; interior coatings were in good condition. Gear teeth exhibited normal wear patterns, and no anomalous damage was observed. The main rotor drive system operated normally when manually rotated.
Examination of Freewheeling Units and Engines
Freewheeling Units (FWU): The left and right FWUs were examined at the manufacturer's facility. The right FWU engaged when rotated in the drive direction and disengaged in freewheeling. No anomalous damage was observed on the right FWU. The left FWU remained disengaged and freewheeled in both directions. The splined nut showed no damage. Bearing cage pins were found in the retracted position. Circumferential scoring was found in the bores housing the pin sleeves. Pin "A" required force to remove, but after reinstallation the pin-and-spring mechanism operated normally. Pin "B" remained compressed for X-ray examination. Circular contact marks were seen on the bearing cage tangs, consistent with pin contact during impact, and were more pronounced than on the right FWU.
No. 1 Engine Teardown: Application of 25 ft-lbs of torque on the starter dog did not move the engine core. The stator vane actuator (SVA) pilot valve exhibited no damage; the feedback cable was connected but could not be manually actuated and was in a position consistent with the SVA closed. After removing the accessory gearbox (AGB), rotation attempts failed. The centrifugal fuel purifier (CFP) external splines were undamaged. After CFP removal, manual rotation of the AGB showed minimal rotation with binding. The lubrication pump could not be manually rotated. After removal of the fuel pump, manual rotation of the AGB became successful. The second stage turbine rotor blades showed soot deposits but no damage. The first stage turbine rotor blades also had soot deposits, heavier at the 12 o'clock position. Compressor stages had soot deposits and no evidence of foreign object debris ingestion. Variable guide vane (VGV) tips were scored between stages 2, 3, and 4. Stator vane impressions were found on soot on compressor spool surfaces. Power turbine blades showed discoloration and rough surfaces. The fuel pump examination revealed thermally degraded gaskets, corrosion deposits, and small metallic globules. The pump drive gear teeth had red deposits, and axial scoring was found on the pump housing.
No. 2 Engine Teardown: After AGB removal, torque on the starter dog did not move the core. The lubrication pump could not be manually rotated. Fuel pump drive splines were undamaged. After fuel pump removal, neither the pump nor AGB could be rotated by hand. The second stage turbine blades had no damage; the first stage shroud had no rubbing marks but had staining in blade shape. The second stage nozzle had cracks near the leading edge root on three vanes, and a trailing edge crack. First stage turbine blades had no damage. Compressor blade bending opposite rotation was observed in several stages. Corrosion was found on IGV and VGV vanes. The spool face for the third stage VGV had rubbing of negligible depth. Small metallic globules were found on pump gears and bearings. The pump gears had iridescent coloration but no damage. No evidence of blockage at the pressurized fuel outlet.
No. 1 Fuel Control Examination: Soot was on the exterior, no cracks or melting. The pressure regulating valve (PRV) housing retaining clip was fractured at one end, but lockwire remained attached. Fuel density adjustment dial tab was bent upward and offset about 40° from the hard stop. The PRV dial was at the JP-5 setting. The Ng governor splines were intact, with typical play. The Nf tachometer drive cable would not rotate freely. The SVA rod protrusion measured about 0.883 inches. Emergency throttle depth was about 1.695 inches. Corrosion was present on rack and pinion gears, but no other damage. The metering valve support could be rotated by hand (normally torqued to 110 in-lbs). The metering valve surfaces had corrosion; its position was about 3.56 inches, consistent with a lower power setting or engine spooldown. The metering valve appeared seized, and the flow window opening was consistent with the minimum flow stop. The spool and sleeve of the metering valve had corrosion, and the spool was seized. Internal components of the fuel control generally exhibited corrosion.
No probable cause was stated in the provided source.