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Aircraft Collides with Terrain East of Seattle After Autopilot Malfunction (N98C)

Snoqualmie Pass, WA, US

On July 17, 1999, a Cessna T-337 G-P (registration N98C) was involved in an aviation accident near Snoqualmie Pass, WA. Investigators recorded the probable cause as: The pilot's failure to maintain clearance from mountainous terrain while maneuvering below low ceilings. Factors include low ceilings and inadequate in-flight planning. 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

An aircraft experienced an autopilot problem en route from Lewistown, Montana, to Olympia, Washington. The pilot canceled the instrument flight plan and proceeded under visual flight rules, later colliding with mountainous terrain east of Seattle.

Incident Overview

While en route from Lewistown, Montana, to Olympia, Washington, on an instrument flight plan, the pilot encountered a problem with the aircraft's autopilot. The pilot elected to cancel the instrument flight plan and divert to Seattle, Washington, for repairs. According to the pilot's statement, the intention was to continue under instrument flight rules until reaching Ellensburg, Washington, and then switch to visual flight rules to fly through a mountain pass.

Witness Account

Approximately 30 minutes after the pilot canceled the instrument flight plan, a witness observed an aircraft with two tail fins flying out of the clouds and proceeding westbound along an interstate highway. The witness, who did not see the crash, reported that visibility was 100 to 300 feet and that the clouds were low to the ground.

Collision

The aircraft collided with mountainous terrain approximately 25 miles east of Seattle, Washington.

Controlled flight into terrain

Frequently asked questions

What happened in the 17 Jul 1999 Cessna T-337 G-P accident near Snoqualmie Pass, WA?

An aircraft experienced an autopilot problem en route from Lewistown, Montana, to Olympia, Washington. The pilot canceled the instrument flight plan and proceeded under visual flight rules, later colliding with mountainous terrain east of Seattle.

What aircraft was involved and where did it happen?

The accident on 17 Jul 1999 involved a Cessna T-337 G-P, registration N98C, at Snoqualmie Pass, WA.

What was the probable cause of the accident?

The pilot's failure to maintain clearance from mountainous terrain while maneuvering below low ceilings. Factors include low ceilings and inadequate in-flight planning.

More accident analyses

Investigation report by the U.S. National Transportation Safety Board (NTSB) historical archive. Original record: https://carol.ntsb.gov/event/20001212X19360. This page is a structured re-presentation; facts and quotes are in the National Transportation Safety Board (NTSB), United States.