MENLO PARK, CA, USA
N8135U
Cessna 150M
The pilot had just departed the airport and was over the San Francisco Bay when the engine suddenly lost power. He attempted to make an emergency landing on a nearby bridge but had insufficient altitude and landed in the water on the shore short of the area. The airplane nosed over when the nose wheel plowed into the mud. Postaccident examination of the engine revealed that the spark plugs were not producing any spark. The 'P-leads' were grounded at a short in a tie-wrapped wire bundle under a grommet at the firewall. The insulation was melted and the inside conductor was contacting the radio shielding of the wire where it was attached to the airframe. Additional examination of the engine found that the rear wall section of the left muffler was missing, with the remaining edges eroded. With the muffler wall missing, hot exhaust gases were directed towards the firewall where the shorted section of the 'P-leads' were located. Maintenance actions had been performed in the engine compartment three times in the past 2 months, with the last maintenance activity 3 days prior to the accident when a cylinder was replaced. The loss of power event occurred on the first flight following return of the airplane to service.
On September 5, 1999, at 1422 hours Pacific daylight time, a Cessna 150M, N8135U, nosed over during a forced landing near Menlo Park, California. The forced landing was precipitated by a loss of engine power as the aircraft was climbing to cruise altitude following departure from the Palo Alto, California, airport. The private pilot, the sole occupant, was not injured. The airplane sustained substantial damage. The airplane was owned and operated by Oakland Flyers, Oakland, California, under the provisions of 14 CFR Part 91, and rented by the pilot for a local area personal flight. No flight plan was filed and visual meteorological conditions prevailed at the time of the accident. The pilot said that he was about 2 miles north of the Palo Alto Airport at 900 feet agl when the engine suddenly lost power. He said that the engine did not choke or sputter. The pilot said that he had 15 gallons of fuel onboard the aircraft. The airplane was over the San Francisco Bay and the pilot turned toward the closest landing area. The airplane had insufficient altitude to make the landing site and touched down short of the shore in the water and nosed over as the nose gear encountered soft mud at the shoreline. The aircraft was examined on September 15, 1999, at the facilities of Plain Parts in Pleasant Grove, California. The engine remained attached to the airframe with no external evidence of catastrophic failure. The propeller remained attached to the engine. The top spark plugs were removed; the electrodes were undamaged and displayed no unusual wear patterns or coloration signatures. Rotation of the crankshaft produced thumb compression in each cylinder, with accessory gear and valve train continuity established. The gas path coloration at the exhaust pipe appeared a light gray with no visible oil residue found. Several unsuccessful attempts were made to start the engine. During subsequent diagnostic checks the spark plugs were determined to not be producing any spark. The "P-leads" were removed from each of the magnetos and a test lead was installed in their place for further testing. The engine then started and ran smoothly. A test of the existing "P-leads" revealed that both of the leads were grounded. Investigators actuated the cockpit mounted ignition switch by hand from the off position to the various detents with the same results. The ignition switch was removed from the panel and disassembled. The ignition switch contacts were clean and free from any signs of defects and with the switch was isolated from the system, the "P-leads" remained grounded. The short was traced and located at a wire bundle at the firewall where the bundle was tie-wrapped at a grommet. Examination of that area revealed that the insulation had melted and the inside conductor was contacting the radio shielding of the wire, which grounded it to the airframe. During the external examination of the engine it was noted that the muffler on the left side of the engine was missing the aft wall section at the rear of the muffler. The muffler wall section was not located in the cowling and the remaining edges of the muffler had an eroded appearance. The exhaust gas path as it exited the rear of the muffler was directed towards the firewall where the shorted "P-lead" harness was located. A review of the engine log revealed that on July 7, 1999, three new pushrods (0.030-inch oversize) were installed at a recording hour meter reading of 0011.6 hours. On August 20, 1999, a compression check of the cylinders was performed at a recording hour meter reading of 0042.0. The compression readings were: No. 1 cylinder 77/80; No. 2 cylinder 79/80; No. 3 cylinder 79/80; and No. 4 cylinder 79/80. The entry noted that the No. 1 cylinder had slight exhaust valve blow by and recommended that the engine be checked in 10 hours. On September 2, 1999, the No. 1 cylinder was removed for repair due to low compression. After the repair, the engine was run-up and the airplane returned to service. The wreckage was released to the insurance company, representing the registered owner on September 22, 1999.
The fatigue failure of the left muffler rear wall, which resulted in hot exhaust gasses being ported onto, and shorting, the magneto p-lead wires. The failure of maintenance personnel to detect the impending failure of the muffler during the recent engine maintenance activity is also causal.
Source: NTSB Aviation Accident Database
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