WPR13LA214

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

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.

OAK was at an elevation is 9 feet above mean sea level. OAK was equipped with 4 runways, and an air traffic control tower that was operating at the time of the accident. Runway 27R measured 5,458 by 150 feet. Landing Gear Design All three landing gear were extended and retracted by a single motor, which operated a series of push-pull rods and links. The main landing gear extended in the outboard direction, and each was locked in the extended position by an overcenter positioning of the side braces of its respective bellcrank assembly. Each aluminum bellcrank pivoted parallel to the airplane lateral axis on a steel bolt. Failure of the bellcrank pivot bolt would partially or completely free the bellcrank from its pivot, prevent the side brace from locking in its overcenter position, and permit landing gear collapse under certain loading conditions. Cessna Landing Gear Maintenance Guidance FAA regulations mandate that aircraft maintenance providers "shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable" to the FAA. The Cessna 421 Service Manual (SM) is the document that contains the specific maintenance guidance for that model airplane. A "warning" appeared as a banner across the first page of the Landing Gear section of the C-421 SM. That warning stated "Anytime a landing gear retraction or extension system component has been removed, replaced or tension on the downlocks adjusted, the entire landing gear system must be re-rigged." Cessna originally produced its maintenance guidance in hardcopy but transitioned to a computer-based system termed "Cesview." According to Cessna, Cesview was an application suite that enabled users "to view and manage electronic service and maintenance information manuals." Installation options permitted users to configure the system to their needs, from standalone (on DVD) to internet-based guidance. Users paid fees to obtain subscriptions, which ensured that they had access to the most current and complete information, including all updates and supplements. Additional Cessna Landing Gear Maintenance Guidance As a result of the in-service experience with the airplane, Cessna issued additional or supplemental maintenance-related guidance for the Cessna 421 or Series 300 and 400 airplanes. In November 1993, Cessna issued Supplemental Inspection Document (SID) 32-10-03, applicable to all model 421 airplanes. It concerned inspection, and replacement depending on condition, of each main landing gear bellcrank pivot bolt. That SID specified an initial inspection at 1,000 landings or 3 years, whichever came first, and a repetitive cycle of 500 landings or 3 years. In October 1999, Cessna issued revision 3 of the Continued Airworthiness Inspection (CAI) 32-10-03, applicable to most Series 400 airplanes, including all model 421 airplanes. The original issue date was unable to be determined. The CAI concerned inspection, and replacement depending on condition, of each main landing gear bellcrank pivot bolt. The CAI specified an initial inspection at 1,000 hours, and a repetitive cycle of 500 hours. In June 2002, Cessna issued SID 32-30-05, applicable to all model 421 airplanes. That SID specified an initial "tear down and inspection" of the landing gear retraction system at 7,500 landings or 15 years, whichever came first, and a repetitive cycle of 5,000 landings or 10 years. In addition to those three documents, Cessna issued at least an additional two CAI and two SID inspection documents regarding the Cessna 421 landing gear system. There is no FAA requirement for aircraft in Part 91 operations to track number of landings; tracking an aircraft's number of landings is not the norm in Federal Aviation Regulations Part 91 operations, and the number of landings for the accident airplane was not tracked. N216WA Maintenance Provider Information According to the airplane owner, he almost exclusively utilized Pacific States Aviation (PSA), an FAA-certificated repair station at his home airport in California, for the maintenance and inspection of the airplane. PSA subscribed to Cesview to obtain the maintenance guidance specific to each airplane model. According to information provided by PSA, the airplane inspections were accomplished in accordance with the "PSA annual inspection checklist." Within a few days of the accident, an FAA inspector visited the PSA facility to review their maintenance guidance for the airplane. The PSA maintenance manager and the FAA inspector were unable to locate or access the landing gear SIDs and CAIs in Cesview. They then telephoned Cesview support for assistance, but the Cesview representative was also unable to locate the documents, and was unable to explain why the guidance could not be located. About 2 years after the accident, in response to separate requests of the FAA inspector and the NTSB investigator, the PSA maintenance manager twice again was unable to locate the SID and CAI. Subsequent to that, the Cessna air safety investigator personally guided the maintenance manager through the search process, and the manager was able to access the documents. The reasons for the previous inability of Cesview support to successfully guide the PSA maintenance manager were not determined. Other Maintenance Provider Information A representative of one maintenance provider familiar with the Cessna 421 reported that the Cessna-published rigging process is time-consuming complex, with some "gray areas." He stated that a technician who is familiar with the process would typically require 6 to 8 hours per gear leg to accomplish a complete re-rig of the landing gear system, and that a significant portion of that effort would require a second technician. An operator whose fleet included model 402 airplanes, but not any model 421 airplanes, reported that the bellcrank pivot bolt was not part of the model 402 landing gear configuration. However, that operator reported that it had to reduce Cessna-recommended maintenance intervals on some landing gear components due to its in-service experience. N216WA Landing Gear Maintenance Information In June 2006, the airplane experienced a LMLG collapse on landing. The pivot bolt had failed, but the damage was minor, and the event was not categorized as an accident by the NTSB. At the time of that event, the airplane had accumulated a total time in service (TT) of 3,463.5 hours. Repairs were accomplished by Lane Aviation, an FAA-certified repair station in Ohio. The bolt and bellcrank were replaced with new components, and the airplane was returned to service in June 2007. The current airplane owner purchased the airplane in 2007, after the above-cited repairs. The airplane owner stated that "Prior to my bringing the plane to California it was supposed to be thoroughly inspected and annualed by TAS Aviation in Defiance, Ohio," and that he did "not recall ever having any landing gear problem with the plane" until the subject accident at OAK. The maintenance records indicated that in November 2007, when the airplane had a TT of 3,464.2 hours, an annual inspection and additional landing gear maintenance, including a complete rigging, was accomplished by TAS Aviation. In July 2009, when the airplane had a TT of about 3,535 hours, the spacers for the follow-up rods for the LMLG door were "reinstalled correctly." In November 2010, when the airplane had a TT of about 3,575 hours, a grease fitting for the right main landing gear (RMLG) was found to be missing, and a replacement fitting was installed. The records did not cite the specific fitting that was replaced. The most recent annual inspection was completed in January 2013, when the airplane had a TT of about 3,684 hours. During that annual inspection, the MLG shock struts were "serviced