By David Learmount in London

Agency awaits full response

The AAIB’s progress report says the European Aviation Safety Agency (EASA) has replied to the circuit breaker proposal, saying: “EASA continues to be an active participant in the ageing systems rulemaking process. Industry continues to develop arc fault circuit breaker technology. A regulatory impact assessment will need to be performed before action may be taken.”

The AAIB classified EASA’s response as “partially accepted, open” while the US Federal Aviation Administration’s is “reply awaited, open”. The same classification is accorded to another recommendation to the FAA: “It is recommended that the [FAA] accelerate the publication and adoption of the guidance material produced by the Ageing Transport Systems Rulemaking Advisory Committee on developing an electrical systems standard wiring practices manual, developing an effective wiring systems training programme and on changes to existing maintenance practices and analysis methods, which could be applied to both in-service aircraft and new designs, to ensure adequate consideration is given to potential in-service deterioration of electrical wiring systems.”

Aviation safety agencies on both sides of the Atlantic are moving too slowly on improving electrical wiring safety in older aircraft, says the UK Air Accidents Investigation Branch (AAIB). In its first progress report to the European Aviation Safety Agency on the status of follow-up actions recommended in accident/incident reports, it calls on both the European Aviation Safety Agency (EASA) and the US Federal Aviation Administration to “expedite a requirement for the replacement of existing thermal/mechanical type circuit breakers [CBs] by arc fault circuit breakers”.

The AAIB says it is aware that plans are in progress through both agencies to reduce the likelihood of damage to old wiring looms, but since this will never be completely effective the introduction of more sensitive “intelligent” circuit breakers that recognise momentary arcing is the most critical part of the solution, it insists.

Checking progress on its recommendations from reports on four serious incidents in 2002 and 2003 – three of them to Boeing 737s and one to a Concorde – the AAIB says: “However strenuous the efforts to avoid design and maintenance quality lapses, their essentially random natures make them very difficult to eliminate. There are many reports of wiring loom damage where sustained arcing within/between looms occurred, or probably occurred, where [thermal/mechanical] CBs have failed to operate, or to operate in sufficient time to prevent serious wiring damage and, in some cases, loss of the aircraft. The four incidents reported here present such examples of sustained arcing.”

The lost aircraft were a Trans World Airlines Boeing 747-100 in July 1996 and the September 1998 Swissair Boeing MD-11.

Source: Flight International