N6195Z

On August 1, 2000, at 1430 Eastern Daylight Time, a Piper PA-25, N6195Z, was substantially damaged during an emergency landing on a grass strip near Wildwood, New Jersey. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the local banner-towing flight conducted under 14 CFR Part 91.

According to the pilot, he departed the Paramount Air Airport (JY04), Green Creek, New Jersey, around 1330. After about 45 minutes, he had "completed his run," and decided to return to JY04. While en route, the pilot advanced the throttle to initiate a climb; however, he got "no response from the engine." As he continued the flight toward JY04, the airplane was "loosing power," and smoke appeared from under the cowling. The pilot dropped the banner he was towing to reduce drag on the airplane, and noticed the engine was "overtemping fast." The oil temperature gauge was "approaching red line speed," and the RPM gauge was decreasing; however, the oil pressure remained constant at 80 psi. The pilot prepared for an emergency landing on a grass strip, performed one "circle pass" over the field, and added one notch of flaps. After touchdown, the airplane struck a tree with its right wing, which spun the airplane around 180 degrees.

The pilot reported that just before landing, the oil temperature gauge had exceeded its red line value of 240 degrees, and was indicating 270 degrees.

On August 1, 2000 an on-scene examination of the airplane was conducted by a Federal Aviation Administration (FAA) inspector. The examination revealed that the engine was "very stiff to rotate." The spark plugs were light gray in color, and all produced a spark, when current was applied to them. In addition, metal residue was observed on both oil screens when extracted from the engine.

The engine was removed from the accident site and disassembled in a hangar on August 22, 2000, under the supervision of an FAA inspector. According to the inspector's written statement, the cylinders were removed and observation of the #3 cylinder revealed a hole burned through the top of the piston "consistent with detonation." The oil sump was removed and a fragment of a piston ring and metal residue was observed. The oil pump drive gear and shaft were removed; however, the oil pump idler gear had seized and had to be forcibly removed. The idler gear was unable to be rotated by hand. The engine case halves were then separated and engine rotation could be observed. Visual examination of the bearing shells revealed metal residue.

According to FAA publication AC65-12A, detonation is "the instantaneous and explosive burning of the fuel/air mixture. The explosive burning during detonation results in an extremely rapid pressure rise. This rapid pressure rise and the high instantaneous temperature, combined with the high turbulence generated, cause a 'scrubbing' action on the cylinder and the piston. This can burn a hole completely through a piston."

The advisory circular further stated, "...the presence of severe detonation during its operation is indicated by dished piston heads, collapsed valve heads, broken ring lands, or eroded portions of valves, pistons, or cylinder heads."

Factors affecting detonation were described as design, cylinder cooling, magneto timing, mixture distribution, and carburetor settings.