Cessna 172S N951JA accident near Chattanooga, Tennessee

Probable cause

A partial loss of engine power due to a fouled spark plug in engine’s the No. 4 cylinder. Contributing was the flight instructor’s failure to maintain aircraft control during an attempted go-around following an unsuccessful landing approach in IMC, which resulted in an aerodynamic stall and impact with terrain.

NTSB analysis

The pilot was receiving instruction from the flight instructor in preparation for the pilot’s instrument practical test. Shortly after departing on the flight, and while in instrument meteorological conditions (IMC), the pilot noted that the engine’s rpm decreased to 2,300, so he declared an emergency and requested to return to the departure airport. The air traffic controller provided the pilot with radar vectors to land, but once he and the flight instructor visually acquired the runway, the airplane was near its mid-point and the airplane was flying too fast to land. Because the engine was still producing power, the flight instructor took control of the airplane and performed a go-around. After the flight instructor initiated a climb the airplane’s airspeed began to decrease as she tried to turn back toward the runway. During this maneuvering, the pilot and flight instructor stated that the engine lost power completely and the airplane’s airspeed decreased to 40 knots. The airplane then descended nose down into trees and impacted the ground.

Review of data recorded by the airplane’s primary and multifunction flight displays revealed that although the pilots perceived a progressive loss of engine power after takeoff, the airplane’s engine continued to produce power throughout most of the flight and during the attempted go-around. Specifically, the recorded data showed that the engine rpm remained generally between 2,200 and 2,300 rpm (the maximum rpm for this engine was 2,700) following the initial reported power loss, and that the engine power had increased to about 2,135 rpm during the go-around attempt. Oil pressure, fuel flow, and other engine parameters were generally consistent with the recorded engine power output.

Postaccident examination of the engine revealed several discrepancies, including anomalies associated with the recently overhauled right magneto, a lead-fouled No. 4 spark plug, (a small piece of metal was found wedged between the center electrode and prongs), and evidence of heat distress and wear in the No. 4 cylinder. Follow-on testing of the magneto and fuel servo did not reveal any evidence of any conditions that would have prevented normal engine operation, and there was no evidence of a catastrophic engine failure. These findings, along with the recorded data, suggest that while engine performance was degraded, there was not a complete loss of engine power as reported by both pilots.

The flight data also showed that following the go-around, the airplane entered a climbing left turn while the airspeed steadily decreased. Over the final seconds of recorded data, the engine rpm remained near 2,150 rpm while the airplane's pitch attitude increased and the airspeed decayed to approximately 40 knots.

Based on the available evidence, the partial loss of engine power initially observed by the pilots was the result of the lead fouled spark plug. The pilot subsequently conducted a precautionary landing approach in IMC that did not place the airplane in a position where it could land safely. The reduction in engine performance due to the fouled spark plug most likely increased the challenge of executing the subsequent go-around, but the recorded data indicated that a loss of control and subsequent aerodynamic stall resulted primarily from the decaying airspeed and increasing pitch attitude rather than a total loss of engine power.

Text from the NTSB. Open the NTSB docket for ERA25LA052 for the full record.