FlarePath

What happened

NTSB summary, verbatim

As the helicopter departed from the oil platform's helipad, witnesses heard a "pop" followed by a high-pitch screeching noise coming from the back of the engine. The helicopter nosed-over and dove into the water with the emergency floats extended. Examination of the engine revealed that one of the second-stage turbine disk blades had liberated due to a high-cycle fatigue (HCF) crack. The crack propagated until the blade separated due to material overload, which resulted in extensive damage to all of the third-stage turbine disk blades and most of the fourth-stage turbine disk blades. Metallurgical analysis of the liberated blade revealed no material anomalies. According to the engine manufacturer, there was no history of second-stage turbine blade failure due to HCF at the same spanwise position as the accident blade. A dynamics analysis of the second-stage turbine wheel revealed no mode crossing within or above the normal engine operating range that would be consistent with HCF initiation at the point where the blade failed. Several potential failure scenarios were also evaluated; however, none yielded any evidence to support the blade failing as it did. The engine was not equipped with an engine-monitoring system that could have identified any anomalies that may have initiated the HCF crack or contributed to the failure of the second-stage turbine blade. Although the surviving passengers stated that the platform was not venting methane at the time of the accident and an eyewitness said he did not see methane being vented, platform operating records revealed that methane was being vented continuously on the day of the accident. However, the actual volume of methane released at the time of the accident and whether it was ingested into the engine on takeoff could not be determined. The HCF crack was a pre-existing condition and was not the result of a sudden ingestion of methane on the day of the accident. A low-level of diphenhydramine, a sedating antihistamine, was detected in the pilot's blood and urine, but it could not be determined if it impaired the pilot's ability to operate the helicopter.

Photographs

5the aircraft, and what the investigators found

Wreckage and site documentation · 5 photographs from the docket

Photographs are NTSB docket attachments — works of the US government, in the public domain. Captions are the Board’s own.

Probable cause

the Board's determination

A total loss of engine power due to the liberation of a second stage turbine blade near the blade root due to a high-cycle fatigue crack and subsequent overload. Although extensive testing and materials analysis was performed, the reason for crack initiation could not be determined.

Occurrence sequence

2 stepsNTSB coding · CICTT taxonomy

1 · Takeoff
Loss of engine power (total)
defining event
2 · Emergency descent
Collision with terr/obj (non-CFIT)

Findings

1 causal · 0 contributing

CAUSE
Aircraft › Aircraft power plant › Engine (turbine/turboprop) › Turbine section — Failure

NTSB analysis

from the final report

As the helicopter departed from the oil platform's helipad, witnesses heard a "pop" followed by a high-pitch screeching noise coming from the back of the engine. The helicopter nosed-over and dove into the water with the emergency floats extended. Examination of the engine revealed that one of the second-stage turbine disk blades had liberated due to a high-cycle fatigue (HCF) crack. The crack propagated until the blade separated due to material overload, which resulted in extensive damage to all of the third-stage turbine disk blades and most of the fourth-stage turbine disk blades. Metallurgical analysis of the liberated blade revealed no material anomalies. According to the engine manufacturer, there was no history of second-stage turbine blade failure due to HCF at the same spanwise position as the accident blade. A dynamics analysis of the second-stage turbine wheel revealed no mode crossing within or above the normal engine operating range that would be consistent with HCF initiation at the point where the blade failed. Several potential failure scenarios were also evaluated; however, none yielded any evidence to support the blade failing as it did. The engine was not equipped with an engine-monitoring system that could have identified any anomalies that may have initiated the HCF crack or contributed to the failure of the second-stage turbine blade. Although the surviving passengers stated that the platform was not venting methane at the time of the accident and an eyewitness said he did not see methane being vented, platform operating records revealed that methane was being vented continuously on the day of the accident. However, the actual volume of methane released at the time of the accident and whether it was ingested into the engine on takeoff could not be determined. The HCF crack was a pre-existing condition and was not the result of a sudden ingestion of methane on the day of the accident. A low-level of diphenhydramine, a sedating antihistamine, was detected in the pilot's blood and urine, but it could not be determined if it impaired the pilot's ability to operate the helicopter.

BELL 206L 3PANTHER HELICOPTERS INC