AS THE AIRPLANE ENTERED A MOUNTAIN PASS, THE PILOT NOTICED DECREASING ENGINE MANIFOLD PRESSURE. UNABLE TO CLEAR HIGHER TERRAIN, HE MADE A FORCED LANDING IN DEEP SNOW. POST-ACCIDENT EXAMINATION REVEALED THE RIGHT IGNITION HARNESS WAS DETERIORATED AND THE IGNITION LEADS HAD ARCED. THE LEFT IGNITION HARNESS (SLICK 1200 SERIES) HAD BEEN ALTERED WITH THE USE OF UNAPPROVED BENDIX 'PANCAKE' GROMMETS. THE AIRPLANE HAD FLOWN 129 HRS SINCE THE LAST ANNUAL INSPECTION. CONDITIONS PREVENTED RECOVERY OF THE WRECKAGE UNTIL 6/27/91. TOTAL TIME ON THE ENGINE WAS 1,714 HRS. RECOMMENDED OVERHAUL INTERVAL WAS 1,800 HRS.
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
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