N304MAMooney Aircraft M20K · 1997

On February 12, 2023, about 0958 central standard time, a Mooney Aircraft Corporation M20K Encore airplane, N304MA, sustained substantial damage when it was involved in an accident near Lakeway, Texas. The pilot sustained no injury. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal cross-country flight.

The pilot reported that he had recently purchased the airplane and he was relocating the airplane to his home base at the Outlaw Field Airport (CKV), Clarksville, Tennessee. During the preflight inspection, the pilot did not observe any anomalies. He reported that the airplane had 57 gallons of fuel onboard at takeoff and the engine oil was at a sufficient level for flight operations.

A review of ADS-B data showed that the airplane departed from the Kestrel Airpark (1T7), Spring Branch, Texas, and traveled to the northeast. The pilot reported that while in flight, the engine had a low manifold pressure reading and then the engine sustained a total loss of power. The ADS-B data showed that the airplane performed a 180° turn as it approached the Colorado River just to the north of Bee Cave, Texas. The pilot then maneuvered the airplane north toward the Lakeway Airpark (3R9), Lakeway, Texas.

Unable to make the airport, the pilot performed a forced landing to a golf course just to the north of 3R9. During the forced landing, the airplane impacted trees and a wood fence, and came to rest upright near a diesel generator. The pilot was able to egress from the airplane without further incident. The airplane sustained substantial damage to the fuselage and to both wings.

A Federal Aviation Administration (FAA) inspector responded to the accident site. A large amount of engine oil was observed trailing rearward on the underside of the fuselage. The engine oil level was checked at the accident site and oil was not observed on the dipstick.

Examination of the airframe revealed flight control continuity. Both wing fuel tanks were found breached and no anomalies were noted with the airframe.

Examination of the engine revealed a hole in the corner of the No. 6 piston; however, the No. 6 cylinder was intact. The No. 6 piston had eroded crown material and a darkened periphery. The engine case was found intact. The top spark plugs from the Nos. 3, 5, and 6 cylinders and the bottom spark plugs from the Nos. 3 and 5 cylinders were found in a “worn out - normal condition.” The right magneto timing was found at 22°; according to the engine manufacturer, it is supposed to be at 20°. The left magneto timing was found at 15°, rather than 20°. The oil breather tube was found intact and the slot was not blocked. Engine oil was found splattered in and around the oil breather tube area, including traveling rearward on the area underneath the oil breather tube. No anomalies were noted with the turbocharger system or the propeller.

Before purchasing the airplane, the pilot had an inspection performed by a mechanic. One of the findings on the undated inspection list stated that, “spark plugs are worn past limits.” Additionally, all cylinders were borescoped and the cylinders were listed as, “all appear normal.” The pilot reported that there were no indications that the magneto timing was checked during the prebuy inspection. The pilot further reported that the airplane had been run with a lean mixture in the past. A review of the engine maintenance records showed that the most recent time the spark plugs were cleaned and gapped, and the magneto timing was checked was during a 100-hour inspection on October 8, 2022, at 2,732.4 hours.

A review of data obtained from a J.P. Instruments EDM-700 unit onboard the airplane at the time of the accident showed an excessively high No. 6 cylinder head temperature reading about 17 minutes before the unit stopped recording data.

The FAA has published the Pilot’s Handbook of Aeronautical Knowledge FAA-H-8083-25C. This document discusses detonation and states in part:

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.

A common operational cause of detonation is listed as:

Operation of the engine at high power settings with an excessively lean mixture.

This document discusses preignition and states in part:

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.

The document further discusses both detonation and preignition and states in part:

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.

Continental Motors has published Mandatory Service Bullet MSB94-8D Magneto to Engine Timing on February 17, 2010. This document discusses the importance of maintaining correct magneto timing and states in part:

Incorrect timing, in addition to producing a rough running engine, can lead to detonation, preignition and internal engine damage or failure.

Failure to properly maintain the magneto, harness and spark plugs will lead to internal engine damage and failure.