SCHOSANSKI JOHN H COZY MK IV R near Napa, CA — 2017-01-04

Probable cause

A partial loss of power due to missing apex seals.

NTSB narrative

The commercial pilot reported that he was practicing touch-and-go landings in the experimental, amateur-built airplane. During the final approach, he noted that he would be landing farther down the runway then he desired, so he initiated a go-around. However, when he advanced the throttle, the engine power decreased. The pilot subsequently landed on the remaining runway, but the airplane overran the runway end and came to rest in a marsh, which resulted in substantial damage to the left wing. A Mazda automotive engine had been modified by the pilot for use in the accident airplane. The postaccident engine examination revealed that two park apex seals had been installed in each of the three corners of each rotor to seal their edges. However, the two top seals from one of the rotor sides were missing; they were not recovered after the accident. According to a Mazda rotary engine conversion guide, this engine is susceptible to premature apex seal failures, which can be greatly reduced by lubrication delivered directly by a two-stroke sump designed to burn with a minimum of deposits; the sump was tested and functioned normally. The investigation determined that the absence of these seals would have resulted in low compression and degraded power. Because there were no other mechanical malfunctions or failures noted that could have affected engine performance, it is likely that the engine lost partial power due to the missing apex seals, which likely failed and were expelled from the engine for reasons that could not be determined.

Analysis

Primary failure mode

Engine power loss

First missed decision gate

Pilot could have aborted the go-around when engine power decreased.

NTSB coding

• Powerplant sys/comp malf/fail · Approach-VFR go-around
• Go-Around (VFR)

Evidence available

• 5 docket documents

Docket documents5

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Excerpt from Rotary Conversion Manualother
• Record of Conversationsother
• NTSB Engine Examination Reportreport
• Weather Reports and Recordsreport

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