Aircraft Maintenance Incidents on Rise

Fri, Jul 24, 2009 — David Evans

Articles, Featured

Maintenance of escape slides, followed by problems associated with landing gear, are the two biggest issues emerging from a study of aircraft maintenance shortcomings in the UK. Some corrective actions are proposed in the 2009 “Aircraft Maintenance Incident Analysis” prepared by the Safety Regulation Group of the UK’s Civil Aviation Authority (CAA).

Released earlier this month, the report looked at 3,982 maintenance errors reported on jet aircraft weighing above 12,500 pounds take off weight. The reports were from January 1996 to December 2006.

The data was compiled from the CAA’s Mandatory Occurrence Reporting (MOR) scheme in order to identify trends and common causes or factors.

The report looked at three types of deficiency:

Poor maintenance control: an event attributed to an ineffective maintenance control system, such as not implementing an airworthiness directive (AD).

Incomplete maintenance: an event where the prescribed maintenance was prematurely terminated. In these cases, the correct maintenance procedures appear to have been followed, but something was not removed, not fitted or set correctly towards the end of the process.

Incorrect maintenance action: the maintenance procedure was completed but did not achieve its aim. In these circumstances, it appears that an incorrect maintenance procedure or practice was being used. Something was not fitted or set correctly by virtue of not performing the task correctly, rather than as an error of omission. This type of error dominated (see below). Half of the reported occurrences were attributed to incorrect maintenance actions, a quarter to ineffective maintenance control, and a fifth to incomplete maintenance.

Breakdown of 3,982 maintenance error reports (MORs).

Breakdown of 3,982 maintenance error reports (MORs).

The report’s conclusions, and a sampling of its recommendations, indicate the depth of the problem:


“—The most frequent type of maintenance error was incorrect maintenance and, within that category, the most frequent types of error were ‘incorrect fit’ and ‘not set correctly.’

“—The areas of the aircraft most susceptible to maintenance error were, in order, Equipment and Furnishings (ATA chapter 25), Powerplant (ATA 71 to 80), Landing Gear (ATA 32) and Flight Controls (ATA 27). For ATA 25 the most frequent issue was escape slides; for ATA it was roughly equally divided between wheels, gear and brakes; for ATA 27 the most frequent problem area was flaps and slats.

“—The rate of maintenance error MORs has steadily decreased since its peak in 1997 and has leveled since 2004 at 6% of total MORs for aircraft greater than 5,700kg [12,500lbs]. The number of MORs submitted since 2004 has steadily increased. This may be due to the requirements, introduced in 2004, for error management and their formal investigation within the maintenance organizations.

“—There is insufficient information and detail in MORs to identify the underlying causes of maintenance errors. There was only sufficient information available available to complete a Boeing MEDA [Maintenance Error Decision Aid] form in a third of the high risk occurrences investigated by AAIB [the UKs Air Accidents Investigation Branch].


“—It is recommended that a more detailed analysis of the underlying causes … should be documented at the time of data entry or MOR closure … This will benefit not only the CAA but also the individual airlines contributing to the data set.

“—A standardized on-line report form for maintenance error should be introduced to facilitate not only ongoing analysis but also initial reporting, investigation results and data entry …

“—The CAA should give some consideration to the prevalence of cabin issues, particularly with regard to escape slides, passenger seats, oxygen masks and lifejackets and, perhaps, look to airlines which do not appear to have issues in this area as a possibl source of best practice. Although rarely a direct hazard to the aircraft, these issues can pose a direct threat to survivability in the event of an accident.

“—CAA comments regarding MOR closure should state what specific action has been taken so that the use of standard statements is meaningful in future analysis.”

In terms of specific findings, the study found the vast majority of MORS were related to Equipment and Furnishings, and escape slides in particular. This finding may explain the high failure rate of escape slides in actual emergencies. Issues associated with seats mainly concern inadequate attachment to the aircraft structure. Thus, while the industry is fitting 16G seats – up from 9G seats – the improved passenger protection offered by the new seats is undercut by poor attachment to the structure.

Breakdown of Equipment and Furnishing failures, with maintenance of escape slides accounting for nearly half.

Breakdown of Equipment and Furnishing failures, with maintenance of escape slides accounting for nearly half.

Problems associated with Landing Gear were fairly evenly divided between wheels, gear and brakes (see below).


The most frequent problem with wheels was associated with fitting the tire itself, while failure to pull safety pins accounted for most landing gear malfunctions.

The CAA ought to present its findings in a slide show given to airline maintenance directors and maintenance technicians. It is suspected that the technicians would offer insights as to causes and simple, straightforward solutions.

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