NTSB Identification: LAX91LA090.
The docket is stored on NTSB microfiche number 44647.
Accident occurred Friday, February 01, 1991 in AGUA CALIENTE, CA
Probable Cause Approval Date: 03/31/1993
Aircraft: RIDDELL LANCAIR 235, registration: N17S
Injuries: 2 Serious.

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 AIRCRAFT WAS ON CLIMB OUT WHEN A LOUD BANG WAS HEARD, FOLLOWED BY AN ENGINE OVERSPEED THEN ENGINE STOPPAGE. AFTER THE FORCED LANDING, THE PILOTS FOUND THAT BOTH WOODEN PROPELLER BLADES HAD SEPARATED FROM THE STEEL HUB ASSEMBLY. THE PROPELLER HUB WAS REMOVED FROM THE ENGINE AND SENT TO THE MANUFACTURER FOR FUNCTIONAL ANALYSIS AND DISASSEMBLY UNDER THE SUPERVISION OF FAA INSPECTORS. THE MANUFACTURER STATED THAT ALL INTERNAL COMPONENTS WERE FUNCTIONAL, HOWEVER, THE CAM ADJUSTMENT FOR THE HIGH AND LOW PITCH STOP SETTINGS HAD BEEN ALTERED FROM THE FACTORY SETTINGS. THE BLADE BUTT ENDS RETAINED IN THE HUB WERE REMOVED AND SUBMITTED TO THE FOREST PRODUCTS LABORATORY OF THE US DEPARTMENT OF AGRICULTURE FOR ANALYSIS. ACCORDING TO THE LABORATORY THE WOODEN BLADES FRACTURED AND SEPARATED DUE TO A HYDROLYSIS INTERACTION BETWEEN THE WOOD AND THE ADHESIVE CAUSED BY EXCESS MOISTURE AND HEAT. THE HYDROLYSIS IS A CHEMICAL DECOMPOSITION PROCESS OF THE WOOD AND GLUE RELATIONSHIP.

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

THE FAILURE OF THE PROPELLER BLADES DUE TO A WEAKENING OF THE WOOD STRUCTURE DUE TO ACID HYDROLYSIS. A FACTOR IN THE ACCIDENT WAS THE INADEQUATE MAINTENANCE OF THE PROPELLER ASSEMBLY BY ALLOWING EXPOSURE TO EXCESSIVE MOISTURE AND HEAT.

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