AWB 00-7 Issue 1 - Circuit breaker resetting procedures
Circuit breaker resetting procedures
AWB 00-7 Issue 1, 19 July 2001
All AOC holders.
All maintenance personnel.
All flight crew.
The purposes of this bulletin are:
- to summarise CASA guidance regarding resetting tripped circuit breakers (CB); and
- to remind personnel of the dangers associated with inappropriate resetting of tripped CB's.
Specific objectives are:
- To ensure that operators with aircraft having an operating weight of more than 5,700Kg:
- incorporate CB resetting procedures generated by manufacturers (see note 1 ) into the manuals used by flight crews and maintenance personnel; and
- have pertinent training programs.
- To ensure that operators with aircraft having an operating weight of less than 5,700Kg:
- operating RPT services, incorporate CB resetting procedures generated by manufacturers (see note 1) into the manuals used by flight crews and maintenance personnel; and
- have pertinent training programs.
- To increase awareness in all flight crews and maintenance personnel of safety information and recommended procedures generated by manufacturers in respect to resetting tripped CB's.
Note 1: Where the manufacturer does not specify or generate procedures the operator is required as part of the aircraft maintenance program to prepare those requirements, the appropriateness of those policies and procedures may be assessed by an appropriate CAR 35 Authorised Person or Design Organisation in consultation with an appropriately rated person from a Flying Operations background. These Policies and Procedures are then independently approved into the Operators Aircraft Maintenance Program/Flight Operations Manual
Historically, flight crews, maintenance personnel, and aircraft ground personnel have viewed the resetting of a tripped CB as a relatively common occurrence in operations. Resetting a tripped CB is often thought to have no adverse results. However, a review of Service Difficulty Reports involving tripped and reset CB's reveals that the opposite is sometimes true. Smoke, burned wires, electrical odours, arcing, and loss of related aircraft systems have been reported as a result of resetting tripped CB's. These findings are supported by other National Airworthiness Authorities.
Aircraft manufacturers normally provide guidance in their flight crew operating manuals (FCOM), maintenance manuals, and aircraft servicing manuals that enables flight crews and maintenance personnel to perform their tasks with a high degree of safety. This guidance addresses the resetting of tripped CB's. Operators meeting the criteria in paragraph 2 A & B should ensure that specific CB resetting procedures based on manufacturer's guidance are reflected in their manuals. CASA has developed the following summary statement to emphasise the importance of exercising caution in resetting tripped CB's and to recommend certain general safety practices.
The following statements summarise CASA's policy in respect to resetting tripped CB's. The overriding message is one of caution. Operators should develop training programs and guidance material/procedures for use by flight crews, maintenance personnel, and aircraft ground personnel in which company policies and procedures with regard to resetting tripped CB's are clearly stated and readily available. Pertinent policies and procedures should promote general awareness of safety concerns associated with resetting tripped CB's and should stress the importance of strict adherence to specific safety guidance generated by the manufacturer. Example statements include:
- General: There is a latent danger in resetting a CB tripped by an unknown cause because the tripped condition is a signal that something may be wrong in the related circuit. Until it is determined what has caused a trip to occur, flight crews, maintenance personnel, or aircraft ground servicing personnel usually have no way of knowing the consequences of resetting a tripped CB. Resetting a CB tripped by an unknown cause should normally be a maintenance function conducted on the ground.
In-Flight: A tripped CB should not be reset in flight unless doing so is consistent with explicit procedures specified in the approved operating manual used by the flight crew or unless, in the judgement of the captain, resetting the CB is necessary for the safe completion of the flight.
NOTE: A detailed maintenance logbook write-up is a proven safety practice for tracking purposes, and may provide maintenance personnel with the key to prompt trouble-shooting and effective corrective action on the ground. That write-up should include the following:
- the conditions existing when the CB trip occurred
- the conditions existing when the CB was reset
- the results of resetting the CB
- On-the-Ground: A CB tripped by an unknown cause may be reset on the ground after maintenance has determined the cause of the trip and has determined that the CB may be safely reset. A CB may be cycled (tripped or reset) as part of an approved trouble-shooting procedure, unless doing so is specifically prohibited for the conditions existing. If an operators minimum equipment list (OMEL) contains procedures that allow a tripped CB to be reset, then the same cautions with reference to resetting tripped CB's identified elsewhere in this bulletin also apply.
Fuel Systems: CBs associated with the Fuel Pump Circuit or Fuel Quantity Indicating System (FQIS). Special caution is appropriate where fuel pumps and/or FQIS are involved, because of the possibility that arcing might lead to ignition of fuel or fuel vapours. CASA has issued airworthiness directives (AD) and manufacturers have issued Service Bulletins affecting certain aircraft makes and models that:
- prohibit the resetting of fuel boost pump CB's in-flight;
- prohibit resetting a fuel boost pump CB while the aircraft is on the ground, without first identifying the source of the electrical fault; and
- because of similar arcing potential, resetting FQIS CB's should be likewise restricted.
Cognisance should be taken of modifications carried out to the aircraft in the assessment of those policies and procedures.