NTSB Factual Report

HISTORY OF FLIGHT

On September 19, 2013, about 2127 central daylight time, a Eurocopter AS350B2 emergency medical service (EMS) configured helicopter, N810LE, experienced a hard landing in a field following a total loss of engine power near Canton, Mississippi. The airline transport pilot incurred serious injury and the two medical flight crewmembers and a passenger incurred minor injuries. The helicopter was owned and operated by Metro Aviation Inc., under the provisions of Title 14 Code of Federal Regulations Part 135 as an on-demand Emergency Medical Services flight. The intent of the flight was a patient transfer. Night visual meteorological conditions prevailed for the medical patient transfer flight and a company visual flight rules flight plan was filed. The flight originated from a private hospital helipad in Ackerman, Mississippi, at 2054, with the intended destination of Jackson, Mississippi.

According to the pilot, about 10 minutes after departure, a low engine oil pressure indicator warning light illuminated and the oil pressure gauge indicated between 0 and 2 psi. He noted that there was no rise in the oil temperature indication or engine temperature indicator and continued toward the intended destination while attempting to verify the loss of engine oil pressure. Several minutes later the pilot noticed a low torque indication and again noted that there was no rise in the oil temperature or engine temperature. About 25 minutes after departure, the pilot saw a rise in the engine temperature gauge and subsequently lowered the collective. He then observed the engine temperature rising at a higher rate and located an open field in order to perform an off airport landing. The engine lost total power prior to reaching the open field and the pilot performed an autorotation. After impacting the ground, the pilot performed the emergency shutdown procedures. In addition, the pilot stated that he was using night vision goggles during the accident flight.

According to a witness, as the helicopter was departing the helipad there was "dark black" smoke coming out of the engine exhaust at the back of the helicopter. The smoke was "heaviest when [the helicopter] tilted to go west." In addition, the helicopter started to make a "clanking" noise as it accelerated away from the helipad. As it departed the area, the helicopter continued to fly forward without making any type of turn.

PERSONNEL INFORMATION

According to the pilot and Federal Aviation Administration (FAA) records, he held an airline transport pilot certificate for airplane multiengine land, a commercial pilot certificate for airplane single-engine land, helicopter, and instrument helicopter. In addition, he held a flight instructor certificate for airplane multiengine and single-engine land, helicopter, and instrument. He also held an advanced ground instructor, instrument ground instructor, and flight engineer certificate. He held a first-class medical certificate, which was issued on August 8, 2013. He had accumulated 3,380 total hours of flight experience, 1,200 hours of flight experience in helicopters, and 77 of those hours were in the same make and model as the accident helicopter.

AIRCRAFT INFORMATION

According to FAA records the helicopter was issued an airworthiness certificate in 2010 and was registered to Metro Aviation Inc. on May 27, 2010. It was powered by a Turbomeca Arriel 1D1 series, 732-shaft horsepower engine.

The helicopter's most recent maintenance inspection, under the approved inspection program, was completed on September 13, 2013, which was about 12 flight hours prior to the accident. The maintenance inspection included a replacement of the second stage turbine disk (T2) in the engine. The engine manufacturer required that a maintenance center level III technician had to perform the replacement of the T2 on the engine. The maintenance technician was required to have and did have manufacturer specific engine training and utilize a maintenance technical instruction manual provided by the engine manufacturer for the T2 replacement. During the replacement of the T2, the maintenance manual stated that the preliminary procedures required the technician to "clean the rear bearing – free turbine nozzle guide vane assy." During the maintenance, the rear bearing had been judged "clean" by the maintenance technician and no cleaning had been carried out. However, review of manufacturer's maintenance procedures required a systematic cleaning of the rear bearing when the T2 was replaced every 4,000 cycles no matter the condition of the bearing. The recorded cycles on the T2 was 3990.58 at the time of the T2 replacement.

Engine Oil System

The engine oil system consisted of a tank, which held 5.7 liters of oil, pumps, a filter, strainers, a cooler, breather, and indicating devices. The gas generator rear bearing was lubricated by oil passing through an external tube, then through a restrictor, and a tube screwed into the bearing housing. The oil coated the bearing, lubricated it, and the bearing housing was sealed by pressurized labyrinth seals. Then the oil traveled by gravity to the bottom of the housing, through a tube in the bottom of the casing, an external tube, through a strainer, then finally through the scavenge pump. During the lubrication process, the air/oil mist that resulted from lubrication passed through a tube connected to the rear bearing housing and an external tube, which vented any excess air/oil mist or over pressurization of the gas generator rear bearing housing into the exhaust of the engine.

COMMUNICATIONS

During the accident flight, flight following was being conducted on the helicopter by a communications center, as part of the operator's protocol. Two times during the accident flight the pilot was contacted by communications center personnel in order to verify if the flight was proceeding without any anomalies. Approximately 14 minutes prior to the destination, which was about halfway through the accident flight, the pilot reported that "operations were normal."

WRECKAGE AND IMPACT INFORMATION

The helicopter came to rest upright in an open field that was surrounded by trees. Postaccident examination of the landing gear crosstubes revealed that they were bowed downward. The tailboom separated from the helicopter and came to rest just aft of the fuselage. The main rotor blades remained intact and attached to the rotorhead.

TESTS AND RESEARCH

Engine Examinations

Under the supervision of an FAA inspector, the engine was removed from the airframe, and shipped to the manufacturer in Grand Prairie, Texas. On October 11, 2013, technical representatives from Turbomeca USA, American Eurocopter, and Metro Aviation Inc. examined the engine under the supervision of an NTSB investigator.

The engine was disassembled during the examination. The five modules of the engine were separated and continuity was confirmed through the reduction gearbox, accessory gearbox, and free turbine modules. The gas generator rear bearing exhibited heat damage. Due to the heat damage, the rear bearing housing was disassembled with a plasma torch in order to examine the rotating assembly. Carbon was found in the scavenge pipe and union from the rear bearing housing. No anomalies other than the heat damage, which is consistent with a lack of lubrication, were detected.

An additional examination was performed on October 31, 2013, under the supervision of an FAA inspector in order to examine the TU208 strainer. An obstruction of solidified oil carbon was found in the rear bearing chip detector housing union of the oil return pipe of the rear bearing located just prior to the strainer.

Detailed notes documenting the engine examination and teardown are available in the official docket of this investigation.

Vehicle Engine Multifunction Display (VEMD)

The VEMD was installed on a training bench on October 11, 2013, and power was applied. The flight reported complete for the accident flight, which was about 32 minutes in duration.

The VEMD revealed an analytical inconsistency between the NG (gas generator rotation speed), torque (TRQ) and the gas temperature at the gas generator turbine outlet (T4) parameters, at 05 minutes, 31 seconds, which concerned the TRQ information that appeared in yellow on the screen. The last consistent TRQ parameter values recorded two seconds prior to the failure was 71.9 percent and 70.0 percent at the time of the fault detection. The VEMD recorded the same analytical inconsistency between the NG, TRQ, and T4 parameters, on the right module, at 05 minutes, 32 seconds.

The overlimit detected by the VEMD was a T4 Temperature excedance, which the maximum value reached 1215 degrees C during the starting phase. According to the manufacturer, the starting phase can correspond with two phases of the flight. It corresponded to the beginning of a flight when the NG increased from 10 percent to 60 percent and at the end of the flight when the NG decreased from 60 percent to 10 percent. In this instance, the temperature was recorded during the later of the phases due to the time it was recorded, when the NG decreased from 60 percent to 10 percent.

Detailed notes documenting the VEMD examination and teardown are available in the official docket of this investigation.

Recorded Flight Data

The North Flight Data Systems Outerlink Voice and Video Recorder (OVVR) was capable of recording video, flight data, and up to 6 audio channels. Data was stored on a Compact Flash (CF) card mounted inside the device as well as an SD card located in the Quick Access Recorder. The downloaded data was related to engine operation during the entire accident flight. A graphical report was made by the NTSB recorders lab of the North Flight Data Systems OVVR that was found onboard the helicopter. The NTSB recorders laboratory report illustrated several parameters for the including, the Engine Oil Pressure, Oil Temperature, Torque, T4 Temperature, rotor rotation speed (NR/NF) and NG against time.

The data indicated that the helicopter departed about 2056:30 CDT. At 2058:30, the oil pressure decreased from approximately 4 bar to around 0.2 bar for the remainder of the flight. Then, the gas generator speed began to drop at 2124:25 and the oil temperature and turbine outlet temperature began climbing at 2125:10. The last data was recorded at 2125:26.The engine torque reading dropped to approximately 0 at 2058:30, along with the drop in oil pressure. The engine oil temperature gauge measured the oil temperature from a switch that was located downstream of the oil filter. In addition, the engine temperature was measured by a sensor located at the gas generator turbine outlet. On this engine, the torque reading is driven by oil pressure. A detailed report regarding the recorded flight data can be found in the official docket of this investigation.

Flight Manual Emergency Procedures

According to the airframe manufacturer's flight manual, it stated that if an abnormal engine parameter was observed, to check the "ENG P" light, and if the light was illuminated, to check the oil pressure gauge, then land immediately if the gauge indicated "low or nil" oil pressure.

Cockpit Emergency Checklist

According to a checklist found in the cockpit of the accident helicopter, the checklist stated that if the "ENG P" light was illuminated, the pilot was to "Land ASAP."

ADDITIONAL INFORMATION

According to the pilot, he had an intermittent oil pressure gauge issue in a different helicopter a "couple of weeks" prior to the accident flight so "[he] thought this was a similar issue at first."

According to FAA Publication "Pilot's Handbook of Aeronautical Knowledge," (FAA-H-8083-25A) that "for the past several decades, research into how people actually make decisions has revealed that when pressed for time, experts faced with a task loaded with uncertainty, first assess whether the situation strikes them as familiar. Rather than comparing the pros and cons of different approaches, they quickly imagine how one or a few possible courses of action in such situations will play out. Experts take the first workable option they can find. While it may not be the best of all possible choices, it often yields remarkably good results.

The terms naturalistic and automatic decision-making have been coined to describe this type of decision-making. The ability to make automatic decisions holds true for a range of experts from fire fighters to chess players. It appears the expert's ability hinges on the recognition of patterns and consistencies that clarify options in complex situations. Experts appear to make provisional sense of a situation, without actually reaching a decision, by launching experience-based actions that in turn trigger creative revisions.

This is a reflexive type of decision-making anchored in training and experience and is most often used in times of emergencies when there is no time to practice analytical decision-making. Naturalistic or automatic decision-making improves with training and experience, and a pilot will find himself or herself using a combination of decision-making tools that correlate with individual experience and training."

NTSB Probable Cause

The pilot’s improper decision to continue flight with a low engine oil pressure warning light instead of landing immediately in accordance with the flight manual emergency checklist. Contributing to the accident was the maintenance technician’s failure to clean the rear bearing in accordance with the engine manufacturer’s guidance when replacing the 2nd-stage turbine wheel, which resulted in an obstruction of solidified oil carbon in the rear bearing chip detector housing union, subsequent oil exhaustion, and engine failure.