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Cowl Door Separation Leads to Fuel Cell Penetration on Twin-Engine Turboprop (N770M)

Greenbay, WI, US

On July 22, 1991, a Beech B200 (registration N770M) was involved in an aviation accident near Greenbay, WI. No fatalities were reported. Investigators recorded the probable cause as: INADEQUATE AIRFRAME DESIGN, WHICH RESULTED IN THE IN-FLIGHT SEPARATION OF THE LEFT ENGINE'S OUTBOARD AFT UPPER COWL DOOR. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 8 related events involving the same aircraft type or operator are linked below.

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

During cruise at 8,000 feet MSL, the left engine's outboard aft upper accessory case cowl door separated, struck the left wing, and penetrated the fuel cell, prompting a precautionary engine shutdown and uneventful landing.

Incident Overview

A twin-engine turboprop airplane was cruising at 8,000 feet mean sea level when the left engine's outboard aft upper accessory case cowl door separated from its mount. The detached cowl door struck the top surface of the left wing, outboard of the engine nacelle, and penetrated the wing's fuel cell. This penetration resulted in fuel siphoning from the damaged cell.

Pilot Actions

In response to the fuel siphoning, the pilot performed a precautionary shutdown of the left engine. The airplane was subsequently landed without further incident.

Relevant Airworthiness Directive

Airworthiness Directive (AD) 91-18-11, effective September 3, 1991, required structural modification of the aft upper cowl assembly doors to prevent in-flight separation.

Structural failure

Related events

Same aircraft family

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Frequently asked questions

What happened in the 22 Jul 1991 Beech B200 accident near Greenbay, WI?

During cruise at 8,000 feet MSL, the left engine's outboard aft upper accessory case cowl door separated, struck the left wing, and penetrated the fuel cell, prompting a precautionary engine shutdown and uneventful landing.

Were there any fatalities in the 22 Jul 1991 Beech B200 accident?

No fatalities were recorded in this accident.

What aircraft was involved and where did it happen?

The accident on 22 Jul 1991 involved a Beech B200, registration N770M, at Greenbay, WI.

What was the probable cause of the accident?

INADEQUATE AIRFRAME DESIGN, WHICH RESULTED IN THE IN-FLIGHT SEPARATION OF THE LEFT ENGINE'S OUTBOARD AFT UPPER COWL DOOR.

More accident analyses

Investigation report by the U.S. National Transportation Safety Board (NTSB) historical archive. Original record: https://carol.ntsb.gov/event/20001212X17488. This page is a structured re-presentation; facts and quotes are in the National Transportation Safety Board (NTSB), United States.