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Left-Rear Engine Cowl Door Separation and Rotor Blade Damage on Aerospatiale AS355 During Medevac Abort (N101UC)

Billings, MT, US

On October 29, 1991, an Aerospatiale AS355F1 (registration N101UC) operated by Omni Flight Helicopters, Inc was involved in an aviation accident near Billings, MT. No fatalities were reported. Investigators recorded the probable cause as: THE UNLATCHING AND SUBSEQUENT OPENING INFLIGHT OF THE LEFT-REAR ENGINE COWL DOOR. A FACTOR CONTRIBUTING TO THE ACCIDENT WAS THE FAILURE OF COMPANY MAINTENANCE PERSONNEL TO EXECUTE THE MANUFACTURER'S SERVICE BULLETIN. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.

Sourcesthe U.S. National Transportation Safety Board (NTSB) historical archivePrimary reportUpdated 2026-07-04Data APIEditorial standards

During a medevac flight abort, an Aerospatiale AS355 experienced a shudder attributed to gusty winds. Post-landing inspection revealed a separated left-rear engine cowl door and main rotor blade damage requiring replacement. The crew had reported a cowl door latch issue before the flight.

Incident Overview

Shortly after the company initiated an abort of a medevac flight, the flight crew of an Aerospatiale AS355 noticed a shudder, which they attributed to gusty wind conditions. The aircraft landed at Billings International Airport, Montana, without further incident.

Post-Flight Findings

Following the landing, a portion of the left-rear engine cowl door was found to have separated from the aircraft. Additionally, the main rotor blades sustained damage that necessitated their replacement.

Pre-Flight History

The crew had previously notified company personnel of a problem with the cowl door latch. They also performed a preflight inspection of the cowl prior to the accident flight.

Manufacturer Awareness

The manufacturer was aware of the issue and had issued a relevant service bulletin. However, the service bulletin had not been implemented on the accident aircraft.

Maintenance error

Frequently asked questions

What happened in the 29 Oct 1991 Aerospatiale AS355F1 accident near Billings, MT?

During a medevac flight abort, an Aerospatiale AS355 experienced a shudder attributed to gusty winds. Post-landing inspection revealed a separated left-rear engine cowl door and main rotor blade damage requiring replacement. The crew had reported a cowl door latch issue before the flight.

Were there any fatalities in the 29 Oct 1991 Aerospatiale AS355F1 accident?

No fatalities were recorded in this accident.

What aircraft was involved and where did it happen?

The accident on 29 Oct 1991 involved an Aerospatiale AS355F1, registration N101UC, operated by Omni Flight Helicopters, Inc, at Billings, MT.

What was the probable cause of the accident?

THE UNLATCHING AND SUBSEQUENT OPENING INFLIGHT OF THE LEFT-REAR ENGINE COWL DOOR. A FACTOR CONTRIBUTING TO THE ACCIDENT WAS THE FAILURE OF COMPANY MAINTENANCE PERSONNEL TO EXECUTE THE MANUFACTURER'S SERVICE BULLETIN.

More accident analyses

Investigation report by the U.S. National Transportation Safety Board (NTSB) historical archive. Original record: https://carol.ntsb.gov/event/20001212X18400. This page is a structured re-presentation; facts and quotes are in the National Transportation Safety Board (NTSB), United States.