Aviation Accident Summaries

Aviation Accident Summary ANC21LA061

Skwentna, AK, USA

Aircraft #1




The pilot reported that during takeoff from an off-airport gravel bar on a river, the airplane experienced a partial loss of engine power. The pilot rejected the takeoff, and the airplane subsequently encountered a “bump,” which pitched the nose down. The pilot then made a hard right turn to avoid going into the water, and the airplane nosed down and impacted the ground with the left wing, which resulted in substantial damage to the left wing. Postaccident examination revealed that the No. 2 cylinder would not produce compression. Removal of the No. 2 cylinder revealed damage, including a hole in the piston that was consistent with detonation and/or preignition. The reason for the detonation and/or preignition could not be determined based on the available evidence.

Factual Information

On July 16, 2021, about 1230 Alaska daylight time, a Cessna 172, N7137T, sustained substantial damage when it was involved in an accident near Skwentna, Alaska. The pilot and one passenger received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that while enroute, the engine experienced a partial loss of power. He ran emergency checks with everything checking “good” according to the check list. He opted to make a precautionary off-airport landing on a gravel bar in the Yetna River. After an uneventful landing, he proceeded to perform more checks according to the check list. He verified all control movements and checked the fuel levels and fuel sumps. The pilot made several run-ups to confirm the engine was making appropriate power, found the power to be within the limits, and setup for takeoff. During takeoff from the gravel bar, as the nose of the airplane lifted from the surface, the pilot applied additional flap to allow the airplane to become airborne. At this point, the engine experienced a partial loss of power. He then rejected the takeoff, reduced power to idle, retracted the flaps, and applied maximum brakes. The airplane subsequently encountered a “bump,” which pitched the nose down. Subsequently, the pilot made a hard right turn to avoid going into the water, and the airplane nosed down. The airplane’s left wing impacted the ground, and the airplane came to rest upright. The pilot and passenger were able to exit the airplane without further incident. The airplane sustained substantial damage to the left wing. A postaccident engine run was completed by a mechanic under the supervision of a Federal Aviation Administration (FAA) inspector, and the mechanic noted that the engine was running rough. The mechanic reported that the engine was leaking significant oil and that the appearance of the exhaust system was consistent with the engine “burning” significant oil. During start up the oil pressure immediately reached 35 pounds per square inch. The engine was run-up to 1,700 rpm, and it was “not smooth” but otherwise normal. The magneto check and the carburetor heat check were normal. Another mechanic further examined the engine at a later date under the supervision of a National Transportation Safety Board investigator and performed a compression check. Compression was obtained on all cylinders with the exception of the No. 2 cylinder, which indicated 0 compression. The No. 2 cylinder was removed, and the cylinder, piston, and rings were found damaged. A hole in the piston was observed behind the rings, and a portion of the piston head was fractured. The combustion dome of the No. 2 cylinder exhibited a slight sandblasted like appearance. The FAA’s Pilot’s Handbook of Aeronautical Knowledge states that “an overly lean mixture causes detonation, which may result in rough engine operation, overheating, and/or a loss of power.” The Handbook further explains detonation and preignition as follows: Detonation is an uncontrolled, explosive ignition of the fuel-air mixture within the cylinder’s combustion chamber. It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves. In less severe cases, detonation causes engine overheating, roughness, or loss of power. Detonation is characterized by high cylinder head temperatures and is most likely to occur when operating at high power settings. Common operational causes of detonation are: - Use of a lower fuel grade than that specified by the aircraft manufacturer - Operation of the engine with extremely high manifold pressures in conjunction with low rpm - Operation of the engine at high power settings with an excessively lean mixture - Maintaining extended ground operations or steep climbs in which cylinder cooling is reduced Detonation may be avoided by following these basic guidelines during the various phases of ground and flight operations: - Ensure that the proper grade of fuel is used. - Keep the cowl flaps (if available) in the full-open position while on the ground to provide the maximum airflow through the cowling. - Use an enriched fuel mixture, as well as a shallow climb angle, to increase cylinder cooling during takeoff and initial climb. - Avoid extended, high power, steep climbs. - Develop the habit of monitoring the engine instruments to verify proper operation according to procedures established by the manufacturer. Preignition occurs when the fuel-air mixture ignites prior to the engine’s normal ignition event. Premature burning is usually caused by a residual hot spot in the combustion chamber, often created by a small carbon deposit on a spark plug, a cracked spark plug insulator, or other damage in the cylinder that causes a part to heat sufficiently to ignite the fuel-air charge. Preignition causes the engine to lose power and produces high operating temperature. As with detonation, preignition may also cause severe engine damage because the expanding gases exert excessive pressure on the piston while still on its compression stroke. Detonation and preignition often occur simultaneously and one may cause the other. Since either condition causes high engine temperature accompanied by a decrease in engine performance, it is often difficult to distinguish between the two. Using the recommended grade of fuel and operating the engine within its proper temperature, pressure, and rpm ranges reduce the chance of detonation or preignition.

Probable Cause and Findings

A partial loss of engine power due to detonation/preignition damage to the No. 2 cylinder. The reason for the detonation/preignition damage could not be determined based on the available evidence.


Source: NTSB Aviation Accident Database

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