Magneto misstep

Knowingly taking off with a mechanical problem--regardless of the reason--is always a bad idea. On August 22, 2004, the pilot of a Cessna 172 was seriously injured, and his passengers were killed when they crashed shortly after takeoff from Decorah Municipal Airport in Decorah, Iowa.

Another Cessna pilot at the airport saw an “older Cessna 172” back-taxi on Runway 29. He then heard the pilot perform the runup and thought “it sounded like the pilot was cycling through a constant speed propeller” and that the “rpm sounds were not those of a 1700 rpm magneto and carburetor heat run-up check that a typical Cessna 172 should be expected to make.” He said the rpm drop seemed very high in comparison to the allowable.

The pilot began his takeoff run, but (as seen by the witness) the Cessna did not accelerate normally. The engine sounded smooth; however, it did not appear to be developing full power. The pilot made no attempt to slow or stop the airplane. The aircraft was observed rotating twice and then lifting about three feet off the runway before coming back down, left wing high, and bouncing into the air at the end of the runway. The Cessna gained little altitude and then disappeared behind some large trees.

The wreckage was found five miles north of Decorah Municipal Airport.

Both magnetos were removed from the wreckage for inspection. The right magneto did not reveal any mechanical defects, producing sparks when rotated. The left magneto, however, did not. Also, the attachment screws for the contact points could be rotated with little effort, proving the magneto was malfunctioning. No record of maintenance on the magneto could be located.

The private pilot had 139.5 total hours, all of which were in Cessna 172s. He had flown 5.6 hours in the last year and 2.3 hours in the last 30 days.

The NTSB determined the cause of this accident to be the pilot’s intentional operation of the airplane with a defective magneto, his failure to abort the takeoff due to degraded aircraft performance, and his failure to obtain or maintain airspeed, which resulted in a stall/spin and subsequent impact with the ground.

A primary purpose of the preflight runup is to ensure that the magnetos are functioning properly. By ignoring the abnormal rpm indications, this pilot endangered himself and his passengers. If you find a discrepancy during your preflight or runup, cancel the flight until maintenance personnel can investigate the problem.

To gain a better understanding of engine and propeller operation and maintenance, take the AOPA Air Safety Foundation’s Engine and Propeller Online Course and read its companion Safety Advisors: Engine Operations and Propeller Safety.