Plane & Pilot
Tuesday, June 1, 2004

Devastating Details

Even minor maintenance mistakes can be fatal

One consequence of the nation’s economic downturn and the accompanying slump in general aviation was that some maintenance shops were forced to consolidate or close down, and many mechanics had to consider alternative careers. The result for airplane owners was the increased difficulty in obtaining high-quality maintenance services at a reasonable cost. For example, a friend of mine was unable to use his single-engine retractable for more than four months because of an ongoing landing gear extension problem. After one expensive repair, the problem recurred. In the interim, the maintenance shop had changed hands, personnel and landing gear expertise issues needed to be sorted out, and the airplane sat on the ground untouched. In another anecdote, the owner of a cabin-class twin was surprised to discover that the instrument shop he had used for years no longer was in business, and the technician in whom he had absolute confidence had moved to a distant airport. The result: increased costs in time and money for work he could trust.

Some owners may be tempted to save money by doing more maintenance work themselves within the limits prescribed by the FAA regulations. Others may accept less attention to detail than they’ve demanded in the past and spend less time looking over the shoulders of mechanics and examining paperwork, just to get their airplanes back into service. However, two accident investigations recently completed by the NTSB demonstrate that attention to detail and procedures is vital when it comes to aircraft maintenance, regardless of who does the work.

Beech M35
On May 19, 2001, about 10:32 a.m., PDT, a Beech M35 hit ground obstructions during a forced landing at North Las Vegas Airport (VGT). The aircraft’s engine had lost power. The private pilot/owner was the sole occupant and received fatal injuries. The airplane sustained substantial damage. Visual meteorological conditions prevailed, and no flight plan had been filed.

About 10 minutes after departure, the pilot radioed the local controller at the North Las Vegas Tower and reported that the engine had no oil pressure and that he was returning to the airport. The airplane was cleared to land, but crashed about a ½ mile west of the runway. FAA inspectors conducted an on-site examination of the wreckage, which revealed that the engine oil filter element was hanging free from the filter base by the safety wire and that fresh oil was covering the fuselage underside.

An owner-assisted annual inspection was recently completed. The pilot had changed the oil and filter after the annual inspection. The changed element was taken to the mechanic who had signed off the annual so that it could be cut open for inspection. No mention of the oil filter change was found in the logbook.

In 1998, a remotely located oil filter system had been installed on the airplane. The installation consisted of a gold anodized base, which was attached to the firewall. The filter elements are screwed onto a threaded shaft, which is permanently factory-installed to a depth of about five threads and torqued to the filter base. Investigators noted that the installation of a filter element would require a blind insertion of the element onto the threaded shaft by touch, since the installation location was in the aft engine compartment on the forward side of the firewall. Examination of the filter base revealed evidence of thread damage to the first complete thread. The first five or six threads of the shaft were damaged, with a cross-thread appearance to the first two or three threads. Other threads were damaged and had the appearance of attempted file repair. Both ends of the shaft exhibited internal damage to the bore of the shaft. The mechanic told investigators that, at one time, the pilot/owner commented that he thought the filter base could be modified to accept different filter elements.

The NTSB determined that the probable cause of this accident was the pilot/owner’s failure to correctly install the oil filter, resulting in oil exhaustion and a loss of engine power. Also causal was his attempt to modify the design of the oil filter element adapter by breaking the bond between the base and the threaded shaft, resulting in a loss of torque during filter-element installation.


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