What happened
The pilot was unable to start the engine on the ground and requested assistance from a mechanic. The mechanic diagnosed the issue as being related to the ignition switch and decided to disconnect the P-leads, operating under the assumption that this modification would keep the magnetos continually 'hot' to facilitate starting. Despite the P-leads being disconnected, the engine successfully started.
Following takeoff, the pilot did not perform a magneto check. Shortly after leaving the ground, the engine began to vibrate and misfire. The pilot attempted to return to the airport for an emergency landing. However, the engine lost all power completely. During the forced landing attempt, which occurred short of the runway, the aircraft's gear collapsed upon impact.
The investigation
A post-accident examination revealed critical details regarding the mechanic's modification. With the P-leads disconnected, the right leaf spring connector effectively grounded the right magneto. However, the left magneto remained ungrounded for a period of time. Investigators found evidence of arcing between the left leaf spring connector and the magneto case. Additionally, the spark vibrator was found to be weak. No other pre-impact mechanical problems were identified.
Findings
The primary issue stemmed from the disconnection of P-leads which altered the grounding state of the magnetos. The mechanic's procedure failed to account for the leaf spring connectors' role in grounding the system when P-leads are open. This resulted in one magneto remaining ungrounded, leading to erratic engine operation. The weak spark vibrator may have contributed to the initial starting difficulty, but the subsequent power loss was directly linked to the improper ignition system configuration.
Safety message
Mechanics and pilots must adhere strictly to approved maintenance procedures. Modifying ignition systems by disconnecting P-leads without understanding the grounding implications can lead to catastrophic engine failures. Proper magneto checks are essential after any ignition-related maintenance.