Investigation Summary
Mercer Airlines Flight 901 was a ferry flight from Burbank to Ontario, California. During the takeoff roll, a fatigue crack in the No. 1 blade of the No. 3 propeller assembly led to the blade's separation. The resulting imbalance caused the No. 3 engine to tear away from its mounts almost instantly. The separated blade sliced through the fuselage, severing hydraulic, pneumatic, and emergency airbrake lines, and struck the No. 2 engine. The crew initially touched down at Burbank but found they had no braking capability or propeller reversing. The Captain elected to perform a go-around to avoid a high-speed overrun into a residential area. During the transit to Van Nuys Airport, the No. 2 engine failed because the blade strike had disabled its oil pump, causing the engine to fill with oil. This specific failure mode prevented the crew from feathering the propeller, resulting in significant parasitic drag. With two engines out on the same side and significant system failures, the aircraft could not maintain altitude and performed a forced landing on a golf course, striking a concrete foundation and a trailer.
Final Conclusions
The National Transportation Safety Board determined that the probable cause of the accident was the degraded performance of the aircraft after two engines failed. The dual failure was precipitated by the in-flight failure of the No. 1 blade of the No. 3 propeller assembly due to a fatigue crack. The crack, which originated in the leading edge under the deicer boot, had not been detected during an improperly performed overhaul. The separation of the No. 3 engine and subsequent damage to the No. 2 engine and flight systems rendered the aircraft unable to reach the runway at Van Nuys.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
First Officer
Systems & Failure Modes
Propeller Assembly
A fatigue crack in the No. 1 blade of the No. 3 propeller led to catastrophic failure and engine separation.
Hydraulic and Pneumatic Systems
Lines were severed by the departing propeller blade, leaving the crew without wheel brakes or emergency air brakes.
Propeller Reversing System
Electrical wiring for propeller controls was severed, preventing the crew from using reverse thrust to stop at Burbank.
Interesting Facts
- 01The No. 3 engine separated from the wing during takeoff due to propeller blade failure.
- 02The propeller blade failure was caused by a fatigue crack hidden under a deicer boot.
- 03The aircraft lost all braking and propeller reversing capability due to severed lines.
- 04The No. 2 engine failed after being struck by debris from the No. 3 engine/propeller.
- 05The aircraft was the prototype Douglas DC-6 (YC-112A).
- 06One person on the ground was slightly injured during the crash.
- 07Ten firefighters were injured during the post-crash fire response, three of whom sustained serious burns.
Safety Actions & Advisories
Propeller Overhaul Inspection Procedures
Source ↗The NTSB highlighted the need for improved inspection techniques during propeller overhauls, specifically ensuring that deicer boots are removed to allow for thorough non-destructive testing (NDT) of the leading edges where fatigue cracks may be hidden.
Location
ACCIDENT SITE · Coordinates taken from the geocoded place name.