Aerovodochody L-29 DELFIN N37KF accident near Reno, Nevada

Probable cause

The failure of a modified engine mount to keep the engine properly aligned, which resulted in partial failure of the rudder support structure due to the deflection of the jet engine exhaust.

NTSB analysis

While being flown on an air race qualification/training flight, the experimental jet’s engine support mount, which had been recently installed to accommodate an increased-thrust engine modification, failed to keep the engine properly aligned. The resultant misalignment resulted in jet blast being directed onto and melting a portion of the rudder support structure. Although there was no indication of the problem in flight, during the landing roll, the pilot discovered that the rudder did not function properly, and directional control had to be maintained by alternate means. During the investigation, it was determined that five other racing airplanes of the same make and model had been modified in a similar manner and that there were slight structural variations and areas of poor welding associated with some of the mounts.

As a result of this accident, the manufacturer of the engine modification instituted a program to obtain an independent professional engineering analysis of the engine mount design. Later, in response to another accident, the National Transportation Safety Board (NTSB) issued Safety Recommendation A-12-13, which asked the Reno Air Race Association (RARA) to require aircraft owners to provide an engineering evaluation for aircraft with any major modification. On August 27, 2012, pending receipt of further information showing that the process proposed by RARA specifically requires an owner of an aircraft with a major modification to provide an engineering evaluation that includes flight demonstrations and analysis within the anticipated flight envelope, the NTSB classified the recommendation Open-Acceptable Response.

Text from the NTSB. Open the NTSB docket for WPR11LA265 for the full record.