Oakland, CA, USA
N216WA
CESSNA 421A
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.
HISTORY OF FLIGHTOn May 1, 2013, about 1445 Pacific daylight time, a Cessna 421A, N216WA, sustained substantial damage as a result of a left main landing gear collapse during landing rollout at the Metropolitan Oakland International Airport (OAK), Oakland, California. The airline transport pilot and the pilot-rated owner were not injured. The personal flight was operated in accordance with 14 Code of Federal Regulations (CFR) Part 91. Visual meteorological conditions prevailed, and no Federal Aviation Administration (FAA) flight plan was filed for the local flight that had departed Buchanan Field (CCR), Concord, California, about 1330. According to the pilot, earlier in the day, he had conducted several touch-and-go landings at CCR. In a second flight later that day, he and the airplane owner departed CCR, conducted a touch-and-go at another airport, a low approach at a third airport, and then planned to land at OAK. During the approach to OAK runway 27R, the pilot and the owner independently verified that the landing gear was extended by observing the landing gear position indicator lights. The touchdown was normal, but immediately thereafter, the left wing dropped, and then the airplane veered to the left. The airplane exited the left side of the runway, and came to rest upright. The pilot and owner exited the airplane, and observed that the left main landing gear (LMLG) had collapsed. An FAA inspector examined the airplane shortly after the event. He reported that a bolt that functioned as the pivot axis for a landing gear extension and retraction component had failed, and that a LMLG bellcrank assembly that was part of the extension and retraction mechanism was also damaged. The bellcrank and bolt were retained for further examination. PERSONNEL INFORMATIONAccording to the pilot, he held an airline transport pilot certificate with multiple ratings. He reported that he had a total flight experience of approximately 6,720 hours, including about 200 hours in the accident airplane make and model. His most recent flight review was completed in October 2012, and his most recent FAA third-class medical certificate was also issued in October 2012. AIRCRAFT INFORMATIONFAA information indicated that the airplane was manufactured in 1969, and was equipped with two Continental Motors GTSIO-520 series engines. It was acquired by its current owner in October 2007. METEOROLOGICAL INFORMATIONThe OAK 1453 automated weather observation included wind from 310 degrees at 12 knots, visibility 10 miles, clear skies, temperature 26 degrees C, dew point 9 degrees C, and an altimeter setting of 30.02 inches of mercury. AIRPORT INFORMATIONFAA information indicated that the airplane was manufactured in 1969, and was equipped with two Continental Motors GTSIO-520 series engines. It was acquired by its current owner in October 2007. WRECKAGE AND IMPACT INFORMATIONThe airplane struck a taxiway sign, and came to rest off the left side of runway 27R. The gear collapse resulted in left engine propeller blade contact with the pavement, which damaged those blades. The left fuel tip tank and left wing, including the spar and aileron, incurred substantial damage. Portions of the left main landing gear linkage were also damaged. One lug on the upper landing gear leg was fracture-separated, and the pivot bolt that installed through a bushed hole at the apex of the bellcrank and the landing gear trunnion was fractured. The forward clevis lug of the lower end of the main landing gear bellcrank was also fracture-separated. Damage to the landing gear and airplane structure precluded the determination of the airplane's pre-accident landing gear rigging status. ADDITIONAL INFORMATIONLanding 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." The records indicated that when the airplane had a TT of about 3,690 hours, all 16 "brake pads" were replaced. The dates associated with that activity were somewhat discrepant with the date of the accident. The records indicated that the activity was "opened" on April 29, 2013, and "closed" on May 6, 2013, which was 5 days after the accident. The reasons for this discrepancy were not able to be determined. No other information regarding any other landing gear maintenance or inspection activities was obtained. The available records did not indicate how many landings the airplane had accumulated. The subject bolt failure at OAK occurred about 6 years after the bolt was installed, when it had accumulated a TT of 226.5 hours. None of the maintenance records subsequent to the installation of that bolt explicitly or implicitly cited compliance with Cessna SID or CAI 32-10-03. Detailed Component Examination Results The landing gear bellcrank assembly was sent to the National Transportation Safety Board Materials Laboratory for examination. The assembly (part number 5041001-1) consisted of an aluminum forging with a dual clevis lug provision at each end, and a single bushed pivot hole at the bellcrank apex. One of the clevis lugs from the lower bellcrank end was fracture-separated, and was not recovered. The fracture face was consistent with an overstress fracture. The bolt fracture occurred in the shank portion of the bolt, approximately 0.3 inches from the bolt head. The majority of the bolt remained captive in its hole in the bellcrank apex due to some bending deformation, and retained its washer, nut, and cotter key. The fracture face was consistent with an overstress fracture by shearing. Stamping on the bolt head indicated that it was part number (PN) NAS6204 26D, which was the close-tolerance version of the part number that superseded the original design bolt PN NAS464 P4-26. The as-found bolt was an acceptable replacement for the design-specified bolt. The bolt hardness, as measured on the flats of the hexagonal head, averaged a Rockwell C Scale value of 38. The measured hardness was within the allowable range of 36 to 40 for NAS6204 series bolts. A detailed report is contained in the public docket. Previous Similar Events On April 6, 2006, a Spanish-registered Cessna 421 (registry EC-IPO) experienced a left main landing gear collapse on landing. The Spanish investigative agency (CIAIAC) report cited the shear failure of the bellcrank pivot bolt as a principal factor in the accident. That report cited 11 previous similar events on Cessna 300- and 400-series airplanes, including 3 others in Spain, 1 in the UK, and 7 in the US. A search of the NTSB accident database between April 2006 and January 2015 revealed at least four other C-421 MLG collapse events that were possibly attributable to the failure of the bellcrank pivot bolt. In one event, the airplane sustained landing gear damage, including a failed pivot bolt, which was very similar to that of the subject accident; that event was attributed to improper rigging of the landing gear. Two events were insufficiently documented to determine whether a pivot bolt failure occurred. In the fourth event, the airplane sustained landing gear damage, including a failed pivot bolt, which was very similar to that of the subject accident; that investigation had not been completed as of the writing of this report. A survey of the NTSB database revealed that in addition to those 4 accidents, since 2006, there were an additional 32 accidents and 5 incidents involving Cessna 300 or 400 series airplanes that involved landing gear problems. The investigation did not further evaluate those events for relevance to the subject accident, but the list of those events is available in the public docket for this accident. Spanish CIAIAC Report and Safety Recommendation As part of its final report regarding the investigation of EC-IPO, the CIAIAC issued Safety Recommendation (SR) 11/08 to Cessna Aircraft. In SR 11/08, the CIAIAC recommended that Cessna improve the 300- and 400- Series airplane landing gear rigging guidance in the relevant maintenance manuals. According to Cessna, no changes were made to the rigging guidance in response to the CIAIAC recommendation. According to CIAIAC representatives, and in accordance with ICAO Annex 13 SARPs, the NTSB was notified of the incident shortly after it occurred, and a copy of SR 11/08 was provided to the NTSB when it was issued. In 2015, internal NTSB searches and queries were unable to locate any NTSB records regarding this event, the CIAIAC report or safety recommendation, or any NTSB follow-up activity. Communications in 2015 with personnel from the responsible FAA Aircraft Certification Office (ACO) in Wichita, Kansas, and the FAA Safety Recommendations Divisio
A failure of the left main landing gear pivot bolt in overstress during landing.
Source: NTSB Aviation Accident Database
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