Runway excursion Accident Cozy Mark IV R N4518S,
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ASN Wikibase Occurrence # 227193
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Date:Wednesday 4 January 2017
Type:Silhouette image of generic COZY model; specific model in this crash may look slightly different    
Cozy Mark IV R
Registration: N4518S
MSN: 0329
Year of manufacture:2012
Total airframe hrs:223 hours
Fatalities:Fatalities: 0 / Occupants: 1
Aircraft damage: Substantial
Location:Napa, CA -   United States of America
Departure airport:Napa, CA (APC)
Destination airport:Napa, CA (APC)
Investigating agency: NTSB
Confidence Rating: Accident investigation report completed and information captured
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. 

Probable Cause: A partial loss of power due to missing apex seals. 



Accident investigation:
Investigating agency: NTSB
Report number: WPR17LA048
Status: Investigation completed
Duration: 2 years and 6 months
Download report: Final report
Other occurrences involving this aircraft

4 January 2016 N4518S Private 0 Napa County Airport (KAPC), Napa, CA sub
Runway excursion


Revision history:

15-Jul-2019 18:04 ASN Update Bot Added

Corrections or additions? ... Edit this accident description

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