Cessna 421A N216WA accident near Oakland, California

Probable cause

A failure of the left main landing gear pivot bolt in overstress during landing.

NTSB analysis

The pilot and pilot-rated owner reported that during the approach for a full stop landing, they both independently verified that the landing gear was extended via the cockpit indicator lights. The touchdown was normal, but immediately thereafter, the left main landing gear collapsed, and the airplane exited the left side of the runway. Postaccident examination of the airplane revealed that a bolt that functioned as the pivot axis for the landing gear actuation and downlock mechanism had failed in overstress. The airplane experienced a previous failure of that left main gear pivot bolt about 6 years prior to this accident.

The pivot bolt was the subject of two dedicated inspection procedures that were first issued by the airplane manufacturer about 14 years after the airplane was manufactured, and about 20 years prior to the accident. In combination, the intervals for those two inspection procedures were based on hours in service, calendar years, and number of landings.

Review of the maintenance records revealed that the bolt had failed when it had accumulated about 227 hours in service, over the almost 6 years. The records did not indicate whether the bolt had been inspected in accordance with the manufacturer-recommended dedicated inspection, which specified intervals of 1,000 hours, 3 years, or 500 landings, but those inspections were not mandated by the FAA.

A 2006 investigation by the Spanish investigation agency CIAIAC of a Cessna 421 landing gear collapse determined that a pivot bolt failure was caused by loading in excess of the design criteria, which was precipitated by improper adjustment (referred to as "rigging") of the extension-retraction mechanism. The Spanish report cited multiple previous similar bolt failures that resulted in incidents and accident.

As part of this subject investigation, a limited survey of events subsequent to the Spanish incident revealed several additional similar landing gear collapse accidents in model 421 airplanes, both in the U.S. and other countries. In some of these cases, improper rigging was cited as the cause, while some remained undetermined as to the cause. Although the historical evidence strongly indicated that the pivot bolt failure in this subject airplane was the result of improper rigging, accident damage precluded the determination of the airplane's pre-accident rigging status.

The CIAIAC issued a formal safety recommendation to Cessna to improve its rigging instructions, but Cessna did not implement any corrective actions as a result of that safety recommendation. As a result of this subject accident, the FAA initiated an effort to reduce the inspection intervals on the pivot bolt, and emphasize the need for proper rig checks on an annual basis, for the Cessna 421 airplane.

Text from the NTSB. Open the NTSB docket for WPR13LA214 for the full record.