The UK Civil Aviation Authority (CAA) is urging industry to look seriously at the upkeep of airliner escape slides to increase passengers' chances of survival in the event of a crash, following a study into the chief causes of maintenance-related incidents on aircraft.

"Although these are rarely a direct hazard to the aircraft, these items can pose a direct threat to survivability in the event of an accident," says the CAA in the report, recommending that future efforts to solve persistent problems here should focus on airlines with few problems in this area as a possible source of best practice.

Types of Maintenance Error

Poor maintenance control - when dysfunctional or inadequate maintenance control produces maintenance errors:

Airworthiness data that is ambiguous, incorrect or conflicting

Airworthiness Directives and other mandatory tasks that have been incorrectly or improperly controlled

Certification ie omission of, or improper

Component robbery - inadequate control of parts removed from one component or aircraft to be fitted to another

Configuration control - inadequate control of the design or build standard of the aircraft or component so that it remains within the approved type design standard

Inadequate control of allowable deferred defects

Inadequate tool control - ie hammers, torches, pliers left in the aircraft

Incorrect interpretation of the aircraft minimum equipment list.

Unapproved modification of aircraft or component, or failure to control modification

Failure to adequately control scheduled tasks required to be performed on an aircraft or component

Omissions or incorrect entries in technical log.

Incomplete Maintenance - when activity is prematurely terminated, resulting in components being:

Not fitted - ie panels not properly closed wire locking omitted loose caps.

Not set correctly - ie engine anti-icing valves left in incorrect position O2 generator door latches in test position.

Not removed - components that should have been removed ie blanks landing gear pins slide bottle pins.

Incorrect action - when the completed procedure does not achieve aim:

Incorrect fit - ie seals damaged on fit cross connections

Incorrect part

Incorrect procedure

Incorrect repair

Not fitted - ie no oil in the IDG.

Not set correctly - ie wheels not deflated for shipment, incorrect positioning of valves and circuit breakers

Poor maintenance practice

Procedure not adhered to

Not removed - ie unapproved blank left in place speed tape over fuel drain hole.

Published late last year, the study analysed a selection of maintenance-related incidents on jet aircraft above 5,700kg (12,555lb) maximum take-off weight, logged under the requirements of the CAA's Mandatory Occurrence Reporting (MOR) scheme in an effort to identify trends, themes and common causes or factors.

A 2005 pilot study had already developed a taxonomy through which to classify maintenance events and this was applied to the latest effort, which established three first level maintenance error types with data further broken down in terms of second level descriptors (see Types of maintenance error, left).

The latest study found that just under half of occurrences analysed were attributable to incorrect maintenance actions, with the remainder credited equally to ineffective maintenance control and incomplete maintenance.

Areas of most concern

Air Transport Association (ATA) Chapter headings which classify aircraft systems were also allocated to the data. The frequency of the top three headings reveals that by far the most frequent issue concerned equipment and furnishings, with escape slides featuring prominently as a particular area of concern.

The findings of the study did provide some good news: the number of maintenance-related MORs as a percentage of the total submitted to the CAA appears to have decreased steadily from 2000 to 2005 - attributed partly to the UK authority's ongoing efforts to promote human factors awareness training, guidance and policy within the industry. The largest decrease was seen in the area of incorrect maintenance actions.

The real challenge thrown down by the study, however, was the need to improve the consistency and comprehensiveness of data to support future trend analysis and identification of the underlying causes of maintenance error.

The study says that the most frequent type of error was incorrect maintenance and, within that, the most frequent types of mistakes were "incorrect fit" and "not set correctly". The areas of the aircraft most susceptible to maintenance error were (see pie charts P32), in order, equipment and furnishings (19.22%), combined engine and powerplant (15.04%), landing gear (11.01% and flight controls (8.99%). Within equipment and furnishings, the most frequent issue was escape slides for landing gear it was almost equally divided between wheels, gear and brakes while for flight controls, the main problem area was flaps and slats.

A review of all high risk MORs for the period covered by the study revealed that maintenance error was the primary causal factor or a contributory factor in 6% of incidents.

A breakdown of the high risk maintenance-related MORs was broadly similar to lower risk occurrences, the only significant difference being that errors in the area of equipment and furnishings did not feature significantly in the high risk category - unlike the three individual chapters most frequently involved which were, in order: landing gear, flight controls and engine. Importantly. when taken together, the combined engine-related ATA Chapters equalled the frequency of high risk maintenance MORs involving landing gear.

"Given the high number of maintenance errors occurring while performing and managing tasks associated with [these areas], and that these feature prominently in high risk occurrences, it is recommended that the CAA investigate how to reduce such errors," it says.

The study, importantly, recommends that a more detailed analysis of the underlying causes is developed, capturing and documenting data at the time of entry or MOR closure.

"There is insufficient information and detail in MORs to identify the underlying causes of maintenance errors. There was only sufficient information available to complete a Boeing Maintenance Error Decision Aid (MEDA) form in a third of the high-risk occurrences investigated by the UK Air Accidents Investigation Branch," it says.

Standardised approach

This more detailed analysis, the CAA says, should be combined with a more standardised approach to data entry regarding applied logic, language and categorisation that would lead to a more robust and reliable data set, as well as significantly supporting future data interrogation and trend analysis. "This will benefit not only the CAA but also individual airlines contributing to the data set," says the CAA.

The study also recommends that a standardised database should be introduced to support not only ongoing analysis but also initial reporting, investigation results and data entry. Standardised entry fields, constrained free text and online reporting from the airlines and maintainers would, it says, greatly increase data reliability and reduce the requirement for speculation and deduction.

The recommended system would allow for initial reporting of an event and would remain open on the system until the reporter had concluded their investigations and gone back to record why the event happened. "Consideration should be given to ease of use, robustness and comprehensiveness of any database design," says the CAA. Ongoing year-on-year data analysis would identify and monitor themes and trends and also allow aircraft type to be included as a variable for analysis.

The report concludes that should this foundation be laid, it would allow the CAA to state what action has been taken to prevent recurrence so that the use of standard statements becomes meaningful in future analysis.

"Further studies should consider extending the analysis to include aircraft below 5,700kg MTOW given that they account for the majority of air accidents within the UK and, indeed, further still to helicopters," says the CAA.

mro graph


Source: Flight International