Bell 407 Power Loss GOM Ditching: NTSB

The US National Transportation Safety Board (NTSB) has reported on the ditching of a Bell 407 offshore helicopter in the Gulf Of Mexico (GOM) on 11 November 2014.   The aircraft, N373RL operated by Rotorcraft Leasing Company (RLC), ditched following a partial power loss shortly after take off from an installation in the Viosca Knoll 989 (VK 989) offshore block.  The pilot and three passengers were uninjured. The helicopter reportedly sustained no damage during the ditching itself.  It is not clear if it was recovered undamaged but it has re-entered service since.

The Accident

The NTSB say:

…the pilot reported that approximately 30 seconds after takeoff at about 400 feet above sea level, a series of compressor stalls and engine surges began. The pilot adjusted the collective pitch and began a slow decent. After lowering the engine power, the surges and stalls ceased and the pilot’s plan, at this point, was to try to attempt to fly the helicopter back to VK 989. At approximately 250 feet – 300 feet, the pilot began increasing the collective to regain some power, but the engine surges and stalls reoccurred. The pilot reported hearing the low RPM horn and when he observed the rotor RPM gauge (Nr), the Nr was about 90 percent and the power turbine gauge (N2) indicated it was running high; at or near redline. At this point, the pilot made the decision to land the helicopter in the water in the Gulf of Mexico. He fully lowered the collective to salvage the RPM. The engine was still surging at flat pitch so he rolled the throttle to idle and entered an autorotation. The pilot prepared the passengers for the landing and radioed a mayday notification to the operator’s flight following station. He then activated the float inflation handle, pressed the aircraft quick position button, flared the helicopter, and landed on the water. The pilot estimated from the time of the initial compressor stall to water contact was approximately 15 to 30 seconds.

The helicopter did not capsize (although sea conditions are not noted) and the:

…pilot subsequently deployed the life rafts and got a verbal response from all the passengers that they were “ok.” He directed the passengers to collect their belongings and a first aid kit in case it was needed. They got into the life raft on the left hand side of the helicopter.

The pilot boarded the right hand life raft.  It is not clear what ‘belongings’ were taken into the life raft or if that odd instruction is part of the operator’s procedures.  According to the operator’s report:

The Pilot called RLC Operations on the sat phone to let them know his position and that they were all OK. The Pilot observed a fishing boat was making its way to his location from the [Stone EnergyPompano [platform] about 1.5 miles away.  As the boat arrived the Pilot asked the left front passenger to tell the boat to keep their distance for the moment so they don’t make contact with any of the aircraft’s floatation equipment…

The Emergency Locator Transmitter (ELT) did not activate.

The Investigation

Under the supervision of the NTSB investigator-in-charge, the incident [250-C47B] engine was examined at Rolls-Royce… The examination revealed that the engine was shipped without its ECU [engine control unit] and T1 [temperature] sensor.

Disappointing the ECU had been sent to Triumph Engine Control Systems by the operator “for repairs”.

The engine was subsequently fitted with an exemplar ECU and sensor. The engine was test run in a test cell and was found to be operational during testing where at no time was it observed that the ECU requested maximum fuel flow. The bleed valve was found during the test run to be closing early prior to its schedule. Additionally, the engine was found to be seven percent below new engine performance standards, which is two percent below the minimum required by the power assurance chart when the engine is in service. Engine accelerations and decelerations during the test run did not produce any surging.

Subsequently, the ECU examined was examined the supervision of a Federal Aviation Administration (FAA) inspector. An ECU acceptance test  successfully completed and the ECU passed a vibration test. A pressure check was however failed, indicating “an approximate five percent reduction in the maximum fuel flow limit”.  This was passed after the temperature sensor was recalibrated.

The recorded incident data indicated that the engine encountered an engine surge condition during its last flight. N2/Nr rpm both initially droop to about 89 percent. Message coding in the incident recorder data indicated the ECU was requesting maximum fuel flow but an expected increase in N2/Nr did not occur. When collective is reduced, both N2/Nr rpm return to 100 percent and above. The N2/Nr…begin to droop again when the collective was increased near the end of the flight. The incident recorder data showed that the throttle was not rolled to idle until after touchdown on the water. The bleed valve was bench tested by the operator. Testing revealed the bleed valve closed early. Disassembly revealed corrosion was present in the bleed valve and corrosion flakes were found in the bleed valve exit orifice. Witness marks consistent with wear were found on the bleed valve bellows face near the exit orifice.

Rolls-Royce however explained (emphasis added):

…the bleed valve closing early would only reduce surge margin in the Ng (N1) speed range where it would normally be open. The incident recorder data from this event depicts Ng initially at 95 percent and decelerating to 91 percent, at which time the main rotor has drooped enough to trigger the incident recorder (<92 percent Nr). Based on the bleed valve closure chart, the bleed valve would be closed at 95 percent and would not open until just below 88 percent for the given T1 (80 F). The bleed valve would have been closed per specification during the sequence of events leading up to the rotor droop. The engine is decelerating at this point, it is not a surge-inducing condition even with a prematurely closed bleed valve. The potential concern, in reference to engine surging, for a bleed valve, stuck closed, is during engine acceleration.

However, RLC believed the bleed valve was the cause and:

Subsequent to the incident, the operator made the decision to revise the bleed valve overhaul schedule from 1,500 hours to 750 hours… Additionally, the operator’s managers will increase surveillance on engine water wash and chemical wash procedures to insure proper techniques are used when performing these tasks.

NTSB Probable Cause

The NTSB did not agree with RLC, and determined the probable cause was:

Engine power surges during take-off for reasons that could not be determined during post-accident testing of the engine and the engine control unit.


UPDATE 30 July 2016: Helicopter Ops and Safety – Gulf Of Mexico 2015 Update

Aerossurance has extensive helicopter safety, engine performance, airworthiness and accident analysis experience.  For aviation advice you can trust, contact us at:  enquiries@aerossurance.com