An extended time in service in the 'A' windscreen filter location appeared to increase the risk of an electrical arcing event in that filter.
Aircraft manufacturer comment
In its consideration of alternate strategies to address the failure of the aircraft's 'A' windscreen electrostatic filter, the aircraft manufacturer determined that there was no apparent trend in relation to the age of the failed filters. Similarly, the manufacturer considered that the failure condition was not predictable, and therefore placing a life on the filters was not practicable.
Additional aircraft manufacturer comment
In its response to the draft safety action that was proposed by the ATSB, the aircraft manufacturer advised that, in accordance with its procedures, the classification of the event was 'MAJOR' but that, given the total flight hours of the BAE 146/RJ of over 10 million hours, the electrostatic filter failure rates were 'within acceptable levels for this failure classification.' In regard to the possibly age-related nature of the 'A' windscreen electrostatic filter failures, the manufacturer noted that, although the first aircraft was delivered in 1986, the failures were confined to units that were manufactured in or after 1987.
Component manufacturer
The manufacturer of the electrostatic filter believed that the 'A' windscreen electrostatic filters became susceptible to failure after extended periods in service in the moisture-laden environment associated with that filter's location. Advice was provided by the manufacturer that 'the unit's location in the aircraft may generate a combination of environmental factors detrimental to the unit's service life.'
Additional component manufacturer comment
In its response to the draft safety action that was proposed by the ATSB, the component manufacturer advised that, in its opinion, placing a time in service limit on 'A' windscreen filters 'would be the most prudent action to avoid repeat incidences similar to that contained in the report'.
In addition, the component manufacturer carried out an examination of its internal design and drawing modifications records for the electrostatic windscreen filter, including since the inception of the BAe 146 aircraft. That examination found no correlation between any design or production changes to the electrostatic filter and the late 1980s period.
ATSB comment
Despite the disparity in the aircraft and component manufacturers' conclusions with regard to the influence of 'A' windscreen electrostatic filter time in service on the risk of an electrical arcing event in that filter, an extended time in service in the 'A' filter location appeared to increase that risk.
Whereas, to date, the existing engineering and other defences had minimised the consequences of electrical arcing events in the 'A' windscreen electrostatic filter, it appears that there may be an opportunity to reduce the likelihood of future electrical arcing events in those filters as a result of the consideration of an appropriate time in service for filters in that location.