NTSB Factual Report

On April 2, 1996, at 1058 hours Pacific standard time, a Powell Glasair homebuilt experimental airplane, N59LP, collided with trees and the ground following a loss of control during the base-to-final turn at the Pine Mountain Lake Airport, Groveland, California. The aircraft was owned and operated by the pilot. Visual meteorological conditions prevailed and no flight plan was filed for the local area personal flight. The aircraft was destroyed in the collision sequence. The private pilot, the sole occupant, sustained fatal injuries. The flight originated from the airport about 4 minutes prior to the accident.

Ground witnesses said the aircraft departed straight out following takeoff. About 4 minutes later, it made a downwind entry to the pattern "low and very close to the runway." The witnesses consistently estimated the aircraft's altitude as between 200 and 300 feet agl. One witness said the aircraft was so close that at first he thought the pilot was going to make a downwind landing on the runway. The witnesses said the aircraft made a tight turn from downwind to base. During the base-to-final turn, the right wing dropped and the aircraft entered a nose-down vertical descent to ground impact in a tree. The witnesses said they heard engine sounds throughout, however, they were described as "not full power."

The pilot holds an FAA Airframe and Powerplant mechanics certificate. Immediately prior to the accident flight, the pilot completed a condition/annual inspection on the aircraft and the airframe and engine logbooks were annotated to reflect the inspection. During a telephone interview, the aircraft co-owner stated that they had been having problems with the throttle cable and the pilot was going to order a new one.

An FAA airworthiness inspector responded to the accident site and interviewed witnesses and examined the wreckage. The engine portion of the wreckage examination was conducted with the assistance of a technical representative from Textron Lycoming Engines. Their respective reports are attached to this report.

The FAA inspector reported that he found no discrepancies with the aircraft's control system. Fuel was found in the aircraft tanks and all lines to the carburetor.

The engine crankshaft rotated easily by hand, with compression developed in each cylinder. Accessory gear and valve train continuity was established throughout the engine, with normal valve lift developed. Both magnetos were timed to 25 degrees before top dead center and produced strong sparks at each ignition lead. The examination report notes that the spark plugs exhibited normal operating signatures and the induction system was free of obstructions. No discrepancies were found with the fuel pump. Disassembly of the carburetor revealed that the clip which secures the accelerator pump plunger to its actuation shaft was missing and the pump was inoperative.

The throttle cable was examined in detail. The cable housing/actuating shaft at the carburetor end was found separated from the cable sheath, exposing the unsupported inside cable. A piece of welding rod was found bent around the housing end and taped to the sheath. The cable was removed from the aircraft and subjected to a series of operational tests. For the tests, the cable was supported by hand at each housing end and then the cockpit end was actuated, with no resistance on the carburetor end. In both push and pull directions, the carburetor end was observed to move intermittently. On some occasions, the carburetor end would move an amount corresponding to the cockpit end input; however, at other times, the carburetor end would only move slightly if at all.

The pilot sustained fatal injuries in the accident and an autopsy was conducted by the Tuolumne County Sheriff/Coroner with specimens retained for toxicological analysis. The results of the toxicological tests were negative for alcohol and all screened drug substances.

NTSB Probable Cause

the pilot's decision to fly the aircraft with a known mechanical discrepancy in the throttle linkage, which resulted in a partial loss of engine power; and his failure to maintain an adequate airspeed while maneuvering for an emergency landing, which resulted in an inadvertent stall/spin. Inadequate maintenance/annual inspection was a related factor.