NTSB Identification: CHI92LA137.
The docket is stored on NTSB microfiche number 47305.
Accident occurred Thursday, April 30, 1992 in DOUSMAN, WI
Probable Cause Approval Date: 07/29/1993
Aircraft: BELL 47-G2A, registration: N52HB
Injuries: 1 Serious,1 Minor.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

THE FLIGHT INSTRUCTOR (CFI) REPORTED THE DUAL INSTRUCTIONAL FLIGHT PROCEEDED WITHOUT INCIDENT FOR THE FIRST HOUR. WHILE RETURNING TO THE AIRPORT IN CRUISE FLIGHT, A SUDDEN AND VERY VIOLENT VIBRATION OCCURRED. THE CFI TOOK THE FLIGHT CONTROLS AND BEGAN AN AUTOROTATION TO AN OFF-AIRPORT SITE, STRIKING TREES DURING THE DESCENT. INSPECTION OF THE ROTOR HEAD DISCOVERED A MISSING SWASH PLATE DRIVE LINK BOLT, WHICH ALLOWED THE DRIVE LINK TO DISCONNECT. BOTH THE CFI AND STUDENT RECALLED CHECKING THE BOLT AND ASSOCIATED NUT AND COTTER PIN PRIOR TO THE FLIGHT AND FOUND NO ANOMALIES. THE BOLT WAS NEVER LOCATED.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

THE IN-FLIGHT LOSS OF THE SWASH PLATE DRIVE LINK BOLT WHICH ALLOWED THE SWASH PLATE DRIVE ASSEMBLY TO DISCONNECT.

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