Circumstances
On 6 August 1986 at approximately 1750 hours local time (UTC+2), a Piper PA-18 '150' (registration HB-PEK) departed from Runway 01 at Lausanne-La Blécherette Airport for a local flight with one passenger. After a ground roll of about 170 meters, the aircraft lifted off but failed to climb. The pilot observed that the engine was not delivering full power. The aircraft flew at an altitude of roughly 5 meters for approximately 600 meters, then banked to the left and made hard contact with the ground. The landing gear collapsed and the aircraft came to rest against a pasture fence outside the runway.
Injuries and Damage
Both occupants were uninjured. The aircraft sustained damage to the landing gear and propeller. No ground damage occurred.
Established Facts
The pilot held a private pilot license that had expired on 17 July 1986; it was renewed the day after the accident. No evidence indicated that the pilot was not in good health at the time of the flight.
The aircraft was equipped with all valid required documents. The last inspection was performed on 20 June 1986, after which the aircraft logged 46 flight hours with no reported engine anomalies. The aircraft's weight at takeoff was near the maximum allowable, and the center of gravity was within limits.
Weather at the airport: wind 190°/2 knots, visibility >10 km, clear sky, temperature 26°C, relative humidity 41%. The runway was mowed, dry, and hard. The aircraft was fitted with skis, which were in the raised position during takeoff.
Post-accident checks on site revealed no technical defects. The carburetor heat and mixture controls were in the 'cold' and 'full rich' positions, respectively. Electrical circuits were off and the fuel shutoff valve was closed. Fuel tanks were full.
Technical investigation focused on the fuel system, carburetion, ignition, and engine. All individual components functioned normally except for two spark plugs (on different cylinders) that were heavily fouled. The engine had accumulated 1,792 hours of service. A bench test with all accessories in the post-accident condition showed no failures or abnormal power loss.
The exhaust system, which was original to the aircraft, was in poor condition. The internal tube (baffle) was torn and deformed to the extent that it almost completely obstructed the exhaust gas outlet.
Analysis
No mechanical or electrical defect could explain the power loss except for the exhaust system condition. The baffle, a cylindrical grid where exhaust collectors converge, had its holes enlarged by hot corrosive gases over time, leading to wall rupture. Under heat, the wall expanded and deformed, obstructing the outlet tube. This increased back pressure, reducing engine power. This phenomenon is well-known and was addressed in a technical communication (CT-C FM 11.820-30) issued on 30 April 1982 by the Federal Office of Civil Aviation (OFAC) with instructions for prevention and detection.
The pilot stated he was aware of the low engine power after lift-off but did not check the tachometer and continued flying at low altitude to accelerate until the runway slope caused ground contact. An immediate decision to reduce power and land would likely have avoided the consequences of the power loss.
Probable Cause
The accident was due to the following causes: late decision to abort the takeoff after an engine power loss, and poor condition of the engine exhaust system.