NTSB Factual Report

On March 28, 2004, at 1134 Pacific standard time, a Maule M-7-260C, N99KJ, lost engine power during takeoff from a private airstrip near Paso Robles, California. The airplane collided with the ground during the ensuing forced landing attempt. The pilot, the registered owner, was operating the airplane under the provisions of 14 CFR Part 91. The private pilot and one passenger were not injured. The airplane sustained substantial damage. Visual meteorological conditions prevailed, and no flight plan had been filed. The personal flight was en route to Santa Barbara, California.

According to the pilot, the taxi and run-up were normal. During the initial takeoff climb, approximately 1,000 feet above ground level (agl), the engine sputtered. The pilot changed the fuel tank selector from the "BOTH" position to the "RIGHT" position and adjusted the mixture control. Partial power returned momentarily; shortly thereafter, the airplane impacted the ground. During the forced landing on uneven terrain, the right wing impacted the ground. Each of the main tanks contained about 11 gallons; the total fuel capacity for each tank is 21.5 gallons.

The National Transportation Safety Board investigator-in-charge examined the airplane on April 19, 2004 at Plain Parts, Pleasant Grove, California. The top spark plugs were removed and displayed normal operating signatures when compared to the Champion Check-A-Plug Chart AV-27. The left exhaust stack was detached from the left exhaust manifold. The clamp that secured the exhaust manifold to the exhaust stack remained attached to the exhaust stack. Removal of the clamp revealed that the pin which secures the exhaust stack to the manifold was worn and no longer held the two pieces together. The right exaust stack clamp was removed. The securing pin had a tapered appearance. The pin, by installation, is positioned in the flow of exhaust gases. An insulating sleeve covering the fuel line to the electric fuel pump was gray in color and partially melted. Spiral plastic tubing that covered cables was also melted.

The following day, the engine was test run using an external fuel supply. No operating anomalies or malfunctions were noted.

According to the airplane manufacturer, the pin is 1/4-inch diameter and not tapered. The exhaust stack to the muffler, by design, is attached to the exhuast manifold outlet tube using a "U" clamp and pin. The pin engages matching holes in the exhaust manifold outlet tube and the exhaust stack. In the event of a pin failure, the exhaust stack will no longer be secured to the manifold.

The maintenance manual for the airplane states, "Note that the tail pipe [exhaust stack] clamps and the muffler-to-header attachments should not be tightened to the point of rigidity, but should be loose enough to be moved easily by hand."

Vapor lock is defined in the Airframe and Powerplant Mechanics Handbook (AC 65-12A) as a partial or complete interruption of fuel flow. This results in vapor pockets from the premature vaporization of fuel restricting fuel flow through units that are designed to handle liquids rather than gases. The three general causes of vapor lock are the lowering of the pressure on the fuel, high fuel temperatures, and excessive fuel turbulence.

NTSB Probable Cause

fuel system vapor lock due to the failure of the left exhaust stack clamp pin, which caused a separation of the exhaust stack from the muffler and the porting of hot exhaust gases onto the fuel lines.