ERA16FA311

A dual loss of engine power for undetermined reasons after the pilot's improper decision to attempt another maneuver after recovering from a perceived compressor stall, rather than returning to the airport.

The helicopter flight crew, consisting of a pilot, copilot, and maintenance crewmember, was performing 20-knot rearward flight about 200 ft above ground level as part of a post-maintenance functional check flight (FCF). While the pilot was flying and recovering from the first rearward flight maneuver, unusual sounds were heard, which the flight crew identified as a compressor stall. The pilot then told the maintenance crewmember that they were returning to their home airport; however, after discussing compressor stalls and engine exhaust gas temperatures with the copilot, the pilot changed his mind and told the maintenance crewmember that they were going to try the maneuver again in a different direction relative to the wind (with the wind off the nose). While the pilot was recovering from the second rearward flight maneuver, there was a change in background noise, which the maintenance crewmember identified as a compressor stall. About 2 seconds later, there was another change in background noise, consistent with a decay in drivetrain rpm, which was followed by the helicopter descending and impacting the ground. Although the crew identified the sounds and loss of power as compressor stalls, a sound spectrum study could not characterize the gas generator speed (Ng) behavior, due to overdriven audio on the CVR, to determine the engine anomaly the crew identified as a compressor stall. It is likely that the cause of the overdriven audio is related to an engine anomaly. At the same time the engine anomaly occurred, the sound spectrum revealed Nr quickly decayed due to a dual loss of engine power coupled with a high collective setting. Because main rotor speed (Nr) decayed at the same time the overdriven audio occurred, it is likely both engines lost power nearly simultaneously. No anomalous damage to the engines was found that would have explained the dual loss of engine power and the main rotor drive system did not exhibit evidence of preimpact mechanical malfunction. Fuel exhaustion was unlikely given the estimated fuel load at the time of the accident. After the overdriven audio, the rate of Ng decay for each engine appeared similar to engine Ng decay when at a lower power setting. Due to a lack of a flight data recorder or a cockpit image recorder, the behavior of the engines as well as the position of the cockpit engine control levers at the time of the anomaly could not be determined. During postaccident examination, the left (No. 1) freewheeling unit (FWU) was found to rotate freely in both directions of rotation instead of rotating freely in only one direction as it was designed to do. Had the left FWU failed in flight resulting in a loss of drive between the No. 1 engine and the main gearbox, the No. 1 engine power turbine would have experienced an overspeed due to the sudden loss of load. A No. 1 engine power turbine overspeed would also have resulted in an overspeed of the AC generators due to a through shaft connecting the left FWU and the AC generators. However, during the accident loss of power event, the frequencies of the main gearbox planetary mesh and the AC generators did not diverge but remained synced, which is not consistent with a left FWU overspeed. Therefore, the anomalous finding of rotation in both directions for the left FWU is likely a postaccident artifact. Review of the rotorcraft flight manual (RFM) emergency procedures revealed, "2. If an abnormal engine conditions occurs such as engine stall, flame-out, or overtemperature, transition to single engine flight or landing…."

The helicopter was equipped with a Universal CVR-120 solid-state CVR that recorded 120 minutes of digital audio. Specifically, it contains a two-channel recording of the last 120 minutes of operation and a four-channel recording of the last 30 minutes of operation. The four channels that should have been recorded during the last 30 minutes of operation were; one for the pilot, one for the copilot, one for the maintenance crewmember, and one for the cockpit area microphone (CAM). However, only two channels were recorded on the accident CVR during the last 30 minutes, with one channel being the CAM and the other channel a mix of pilot, copilot, and maintenance crewmember. The helicopter was not equipped with a flight data recorder nor was it required to be. The CVR exterior sustained heat and fire damage, but the interior crash-protected case did not sustain damage. The memory board did not exhibit heat damage; however, the ribbon cable that connected the memory module to the main circuit board sustained heat damage and was replaced. Also, several electronic components were replaced to get the memory to playback successfully. The digital audio was downloaded, and a transcript was prepared for the accident flight. Additionally, about 9 minutes preceding the accident flight was transcribed; during this time, the helicopter was on the ground, and the pilot was calculating the weight and balance and center of gravity for the accident flight. A sound spectrum study was performed on recorded audio from the CVR. The study revealed that engine frequency could be correlated to Ng, and both the main gearbox planetary mesh and the AC generator electrical bleed-through noise frequencies could be correlated to main rotor speed (Nr). Additionally, CVR frequencies were compared to frequencies obtained from the witness video and provided a time correlation between the video and CVR. Plots were prepared to depict Nr and Ng through the two rearward flight maneuvers of the accident flight. The plots revealed that during the recovery from the first rearward flight maneuver, Nr decayed from about 102% to 100% and then oscillated between 104% and 101% before steadying near 102%. The Ng for both engines (average between both engines) increased from about 95% to 96% and then oscillated between 94% and 89% before returning to 95%. The plots for the recovery from second maneuver revealed an increase in Nr from about 100% to 101%, followed by a linear decay to about 32% over a span of 9 seconds (the end of recorded data). Just before this Nr decay began, the Ng for each engine was about 95 to 97%. The Ng could not be characterized for a 3-second portion of overdriven audio, after which the Ng for each engine decayed linearly from about 83 to 88% to 71 to 76% over a span of 6 seconds. The engine manufacturer provided the National Transportation Safety Board with historical data of Ng reduction rates following a loss of engine power on CT58-series and CT7-series engines. The Ng reduction rate for the accident helicopter was slower compared to all but one of the previous events, which involved a CT7-9 engine where a fire handle was pulled while the engine was running at high power on the ground. Although the engine manufacturer did not have historical compressor stall data for the CT58-series engines, their compressor stall data on the CT7-series engines were not consistent with the data recorded during the two events. Lastly, the Ng reduction rate for the accident helicopter appeared comparable to that seen in an accident involving G-BBHM that the United Kingdom (UK) Air Accidents Investigation Branch (AAIB) investigated. In the accident involving G-BBHM, which was equipped with a flight data recorder (FDR), the No. 2 engine Ng decay was based on the crew retarding the cockpit engine control lever to "flight idle" after an engine fire warning light illumination, and subsequently to the "cut-off" position. Based on historical data of compressor stalls from the engine manufacturer, the plotted Ng data was not consistent with a compressor stall. (For more information, see the Spectrum Study in the public docket for this accident.) Regarding engine failures, the rotorcraft flight manual (RFM) emergency procedures stated, in part, "if an abnormal engine condition occurs such as engine stall, flame-out, or overtemperature, transition to single engine flight or landing, confirm the malfunctioning engine has been correctly identified, and retard that engine to ground idle while proceeding as follows:…" There was no published height velocity diagram for a dual-engine loss of power in the RFM. The RFM contained a height velocity diagram for a single-engine loss of power. Review of the diagram revealed instructions to avoid hovering below 300 ft. The RFM did not contain procedures for a loss of dual-engine power in either a hover or during rearward flight, and according to the operator, the company's training did not include loss of dual-engine power in a hover. Review of the FCF instructions in the maintenance manual revealed that one of the maneuvers was to fly rearward at 20 knots. The instructions allowed the maneuver to be completed at a "0 – 1000 feet altitude…" At 1353, the recorded weather at MLB, which was located 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. The State of Florida District 18 Medical Examiner's Office, Rockledge, Florida, performed autopsies on all three crewmembers. The cause of death for all three crewmembers was noted as multiple blunt force injuries. The FAA Bioaeronautical Science Research Laboratory, Oklahoma City, Oklahoma, performed toxicological testing on all three crewmembers. The results were negative for alcohol and drugs for the copilot and maintenance crewmember. Review of the toxicology report for the pilot revealed detected diphenhydramine in urine and heart blood and ibuprofen in urine. Ibuprofen is an anti-inflammatory analgesic available over the counter. Diphenhydramine is a sedating antihistamine available over the counter. The level of diphenhydramine detected in the pilot's blood was below the therapeutic range and not reported. 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. In addition, the pilot held a flight instructor certificate with ratings for rotorcraft helicopter and instrument rotorcraft. The pilot's most recent Federal Aviation Administration (FAA) first-class medical certificate was issued on December 5, 2015. According to the operator, the pilot was hired in 2012 and completed all required company training. At the time of the accident, he had accrued a total flight experience of about 6,347 hours of which 5,743 hours were in helicopters and 1,780 of those hours were in the same make and model as the accident helicopter. The pilot had flown 114 hours and 25 hours during the 90-day and 30-day periods preceding the accident, respectively; all of these hours were flown in the same make and model as the accident helicopter. 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. According to the operator, the copilot was hired on July 31, 2016. He had completed company-required ground training and was in the process of completing flight training at the time of the accident. The copilot had accrued a total flight experience of 4,090 hours, all of which were in helicopters. Before the day of the accident, he did not have any flight experience in the same make and model as the accident helicopter but was qualified to act as second-in-command. The 41-seat capacity, tricycle-gear helicopter, serial number 61717, w