LAX93LA263 · Sill/Thomas LONG EZ
Sill/Thomas LONG EZ · Accident: total loss of engine power (mechanical failure) en route
From Mammoth Yosemite Airport (MMH) to Van Nuys Airport (VNY)
Event
- NTSB case
- LAX93LA263
- Event typeAccident: someone killed or seriously injured, or the aircraft substantially damaged (NTSB definition). Incident: an occurrence that affected or could have affected safety, short of that. Glossary
- Accident
- InvestigationHow far the investigation had got when the record was published: preliminary, ongoing, final or completed. Unknown where the source does not say. Glossary
- Completed
- API-reported fatalities
- 0 · all aircraft and ground
- Ground fatalities
- 0
- OperationWhat kind of flying it was, grouped by the rules it flew under: airline (US Part 121), air taxi and commuter (Part 135), general aviation (Part 91 and similar), military or government. Glossary
- General aviation
- WeatherVisual meteorological conditions (VMC): good enough to fly by looking outside. Instrument conditions (IMC): cloud or low visibility, flying by instruments. Glossary
- Visual Meteorological Cond
Sill/Thomas LONG EZ
- Aircraft type
- Sill/thomas LONG EZ
- Category
- Airplane
- RegistrationThe aircraft's tail number, such as N12345 or G-ABCD. Registrations are reissued, so the same one years apart can be a different aircraft. Glossary
- N977ST
- Operator
- Unknown
- Onboard fatalities
- Unknown
- Route
- From Mammoth Yosemite Airport (MMH), MAMMOTH, CATo Van Nuys Airport (VNY), Van Nuys, CA
- Aircraft age
- Not recorded
- Flight rulesThe regulations the flight operated under: in the US, Part 91 (general aviation), Part 121 (airlines), Part 135 (air taxi and commuter) and others; for flights abroad, the NTSB's coarser commercial or non-commercial code. Glossary
- Part 91: General Aviation
- Phase of flightThe stage of the flight when things started to go wrong: standing, taxi, takeoff, initial climb, en route, maneuvering, approach or landing. Glossary
- En route · cruise
- First occurrenceThe single coded event the NTSB judges best describes what happened (records from 2008 on). Older records name the first occurrence in the sequence instead. Glossary
- Total loss of engine power (mechanical failure)
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- Substantial
Cause areas
- Engine assembly, valve, exhaust › Failure, total
- Engine assembly, valve, exhaust › Overtemperature
- Pilot in command › Maintenance, inspection, not performed
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
THE PILOT STATED THAT HE WAS IN LEVEL CRUISE AT 10,500 FEET MSL WHEN THE ENGINE SUDDENLY QUIT. HE MADE A FORCED LANDING ON OWENS DRY LAKE, WHICH WAS SOFT FROM RECENT RAINS. THE NOSE GEAR COLLAPSED AND THE AIRPLANE NOSED OVER INVERTED. POST CRASH EXAMINATION OF THE ENGINE REVEALED THAT THE EXHAUST VALVE HAD FAILED IN THE NUMBER FOUR CYLINDER. THE VALVE HEAD HAD SEPARATED FROM THE VALVE STEM. EXAMINATION OF THE VALVE SPRING AREA REVEALED OVERHEAT SIGNATURES IN THE FORM OF OIL COKING AND OXIDIZATION, AND CARBON DEPOSITS. THE NTSB METALLURGY LABORATORY FOUND THAT THE VALVE STEM WAS DEVOID OF CHROME PLATING IN THE AREA OF THE FRACTURE, WAS REDUCED IN DIAMETER, AND HAD A SIGNIFICANTLY DECREASED MATERIAL HARDNESS, ALL PRINCIPALLY DUE TO HEAT EXPOSURE. POST ACCIDENT EXAMINATION OF THE LOG BOOKS AND THE FAA OPERATING LIMITATIONS REVEALED NO RECORD OF A REQUIRED CONDITION (ANNUAL EQUIVALENT) INSPECTION SINCE THE AIRCRAFT WAS PLACED INTO SERVICE. EXAMINATION OF THE ENGINE REVEALED MANY AREAS OF SUBSTANDARD MAINTENANCE PRACTICES.
Probable cause
the failure of the number four cylinder exhaust valve due to excessive heat exposure and lack of maintenance.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record LAX93LA263
- Event ID
- 20001211X12659
- Case number
- LAX93LA263
- Dataset
- historical-pre2008
- Source SHA-256
- 89e2df6d848e7ab3b42e7d7c7a03ff403303057a26ca04c95b3ea67201c3ce74
Source notes (4)
- Unreviewed is an editorial label, not an investigation status. API-sourced is not report-checked or human-reviewed. Explicit event totals are used without summing aircraft injury tables; onboard allocation is withheld. Unknown values remain unknown. The operation category is mapped from the NTSB-reported FAR part and has not been reviewed.
- Filled from the NTSB case API where the bulk record had no value: investigation status. Values present in the bulk record are kept.
- The date is the local date, which is the same as the UTC date the NTSB stores.
- API snapshot SHA-256: 18098ec6218fdc9845aa48396702a9650c3c219b06afb9422736b86ff7d19954; retrieved 2026-09-21T22:54:44.680Z.