Maintenance Error
Maintenance Error
Definition
The unintended failure to carry out a maintenance task in accordance with the requirements of that task and/or not working in accordance with the principles of good maintenance practice.
Description
Aviation industry studies have found that the origin of as many as 20% of all in-flight engine shutdowns can be traced to maintenance error.
Typical maintenance errors include:
- Electrical wiring discrepancies.
- Loose objects left in airplane.
- Incorrect installation of components.
- Fitting of wrong parts.
- Inadequate lubrication.
- Access panels, fairings, or cowlings not secured.
- Fuel or oil caps and fuel panels not secured.
For installation errors on engines, one specific study found the following types of error:
- Borescope plug not refitted.
- Engine-driven pump (EDP) drive shaft seal not fitted.
- Engine attachment bolts incorrectly fitted.
- Anti-ice valves locked out.
- Fire bottle squibs not fitted.
- Fuel pipe not secured.
- Magnetic chip detectors (MCDs) not fitted.
- Prop spinner fitment not completed.
- Fuel control unit (FCU) controls not fitted.
The circumstances in which maintenance error occurs are the focus of human factors methodology.
Analysis of maintenance error data collected by a group of UK Maintenance Organisations found that when the type of error was classified, four categories accounted for 78% of the errors. These were Installation error - 39%, Inattention (damage) - 16%, Poor inspection standards - 12% and Approved data not followed - 11%.
The presentation of this data was accompanied by some solutions for both "people" and "process" for all the main types of error found.
Accidents and Incidents
Valid guidance available
On 6 April 2022, a Boeing 767-300 lost left engine oil pressure whilst eastbound and passing south of Cork and diverted to Shannon after declaring an emergency on account of intended engine shutdown. During the subsequent taxi in, a fire was observed and extinguished and the aircraft towed to the terminal after an initial fire service request for evacuation had been withdrawn. An engine oil leak from a chip detector which had been routinely inspected by a company engineer prior to departure but not reinstalled correctly was found to have caused the leak and thus the loss of oil pressure.
On 5 January 2024, a Boeing 737-9 which had just departed Portland was climbing through 14,800 feet when there was a loud bang followed by a rapid decompression. The cause - the loss of a fuselage plug fitted at an unused door location - was immediately obvious. An emergency was declared and a landing back at Portland was made after a total of twenty minutes airborne. The Investigation is continuing but has found that the fuselage plug involved was installed without being properly secured and noted that the aircraft had only recently been delivered new to the operator.
On 20 July 2021, a Boeing 747-8F experienced a series of problems with excessive engine speed and fire affecting the left outboard engine soon after takeoff from Hong Kong. Although the engine was shut down, the fire continued until just before landing. About twenty minutes after landing, trapped residual leaked fuel then auto-ignited, and that fire was quickly extinguished. The origin of the engine malfunction and continuing airborne fire was identified as improper installation of a component in the engine’s Fuel Metering Unit at build, which caused a fuel leak that was the sole origin of the engine malfunction.
On 25 November 2021, a Fokker F50 departing Helsinki experienced an engine malfunction that resulted in an uncommanded propeller feathering. The associated engine continued to run until shutdown, during which time it began to overspeed. The aircraft landed safely, but the failure experienced was untrained, and this led to both direct and indirect consequences that resulted in a suboptimal crew response to the emergency. The Investigation also highlighted opportunities to improve aspects of the air traffic control emergency response during such emergencies and identified language proficiency certification issues.
On 10 June 2022, on reaching the planned FL330 cruise altitude abeam Meekatharra, a Boeing 737-400 crew observed a 340-foot difference between the two primary altimeter readings. The crew did not advise ATC of the breach of RVSM separation minima, but after identifying which instrument was in error, they returned uneventfully to their departure point, Perth. There, an engineering inspection found residue on all four electrically heated pitot-static probes. It was found that a few days earlier, an engine ground run done without first removing non-standard plastic probe covers had contaminated the probes, with cleaning performed prior to release to service.
Invalid guidance available
On 17 January 2023, an ATR 72-200 in the final stages of a CAT 2 ILS night approach to East Midlands experienced a significant electrical malfunction which disabled one set of primary flight instruments and triggered multiple system status indication failures. These included false system warnings and radio communications problems. The approach was discontinued, a MAYDAY declared and a successful manually-flown diversion to Birmingham was then made. The cause of the electrical malfunction was found to be a wiring defect which was considered to have probably been caused by the incorrect use of mechanical wire stripping tools during third party maintenance.
On 25 March 2018, an ATR 42-500 main landing gear bay door weighing 15 kg detached shortly after a night descent had begun but this was unknown until the flight arrived at Aurillac. The Investigation found that the root cause of the detachment was a loose securing nut which had triggered a sequence of secondary failures within a single flight which culminated in the release of the door. It was concluded that the event highlighted specific and systemic weakness in relevant airworthiness documentation and practice in relation to the lost door and the use of fasteners on this aircraft type generally.
On 28 February 2019, an Embraer E195 abandoned takeoff from Exeter when fight deck fumes/smoke accompanied thrust applied against the brakes. When informed of similar conditions in the cabin, the Captain ordered an emergency evacuation. Some passengers using the overwing exits re-entered the cabin after becoming confused as to how to leave the wing. The Investigation attributed the fumes to an incorrectly-performed engine compressor wash arising in a context of poorly-managed maintenance and concluded that guidance on overwing exit use had been inadequate and that the 1.8 metre certification height limit for exits without evacuation slides should be reduced.
On 19 October 2012, a Jet2-operated Boeing 737-800 departing Glasgow made a high speed rejected take off when a strange smell became apparent in the flight deck and the senior cabin crew reported what appeared to be smoke in the cabin. The subsequent emergency evacuation resulted in one serious passenger injury. The Investigation was unable to conclusively identify a cause of the smoke and the also- detected burning smells but excess moisture in the air conditioning system was considered likely to have been a factor and the Operator subsequently made changes to its maintenance procedures.
On 31 January 2011, a Singapore Airlines Airbus A380-800 was in the cruise when there was sudden loud noise and signs of associated electrical smoke and potential burning in a toilet compartment with a corresponding ECAM smoke alert. After a fire extinguisher had been discharged into the apparent source, there were no further signs of fire or smoke. Subsequent investigation found signs of burning below the toilet floor and it was concluded that excessive current caused by a short circuit which had resulted from a degraded cable had been the likely cause, with over current protection limiting the damage caused by overheating.
Further Reading
- ATSB Transport Safety Report - AR-2008-055: An Overview of Human Factors in Aviation Maintenance, Alan Hobbs Ph.D.
- Airbus Maintenance Briefing Note Maintenance Error Management
- Maintenance Briefing Notes: Maintenance Documentation by Airbus
- Human Factors in Aircraft Maintenance, Colin G. Drury, State University of New York
- Safety Behaviours: Human Factors Resource Guide for Engineers - Chapter 2: Error Management - CASA (Australia), 2013
- Operator's Manual: Human Factors in Aviation Maintenance, FAA, 2014
- Advisory Circular 120-115: Maintainer Fatigue Risk Management, FAA, 2016
- Paper 2009/05: Aircraft Maintenance Incident Analysis, UK CAA, 2009
- CAP 1367: Aircraft Maintenance Incident Analysis, UK CAA, 2016
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