Pilot Debrief

STINSON 108 near Newport, PA — 2020-09-17

Final reportERA20LA320
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Date
2020-09-17
Location
Newport, PA, USA
Airport
NONE
Aircraft
STINSON 108
Registration
N336C
Category
Airplane
Highest injury
Minor
Fatalities
0
Phase of flight
Cruise

Probable cause

A total loss of engine power during cruise flight due to a disconnected carburetor mixture control cable which allowed the carburetor mixture lever to migrate to the cut-off position during flight.

NTSB narrative

The flight instructor stated that he and the pilot fueled the airplane, departed, and flew for about 1.5 hours when, while cruising at 3,500 ft, the engine “started to sputter.” He checked the magnetos, adjusted the mixture, switched fuel tanks, and pumped the primer and throttle. When he pumped the primer, the engine would “run for about 5 seconds then quit,” and the propeller continued to windmill. Unable to restore power to the engine, he elected to execute a forced landing to a field. During the landing rollout, the airplane impacted a berm resulting in substantial damage to the fuselage. Postaccident examination of the engine revealed that although the cockpit mixture knob was set to full rich, the carburetor mixture control cable was disconnected at the carburetor and the carburetor mixture lever on the carburetor was in idle cutoff position. The hardware that would have attached these components was not located. Given this information, it is likely that hardware holding the mixture control cable to the mixture control arm loosened and departed during flight, allowing the disconnected mixture control arm to migrate to the cut-off position, resulting in a total loss of engine power. Since both the accelerator pump and the primer system bypass the carburetor and delivers some fuel directly to the engine even when the mixture control is in the “cutoff” position, this would explain why the instructor reported momentary bursts of power when he pumped the primer or throttle.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Properly securing the carburetor mixture control cable before flight.

NTSB coding

Evidence available

  • Photos
  • 9 docket documents
View NTSB final reportView NTSB docket

Docket documents9

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