Tuesday, May 12, 2009
Should You Reset A Circuit Breaker?
Revisiting and revising old ways of doing things
|The NTSB says it’s time to rethink something most GA pilots learned early in their training: If a circuit breaker trips while you’re flying, it’s okay to reset it after allowing a minute or two for it to cool, even if you have no idea what caused it to trip and cut off electrical power to a particular circuit. |
WHEN TO RESET? The information in operating handbooks may conflict with FAA advice, leaving pilots in a confusing situation.
The NTSB says it’s time to rethink something most GA pilots learned early in their training: If a circuit breaker trips while you’re flying, it’s okay to reset it after allowing a minute or two for it to cool, even if you have no idea what caused it to trip and cut off electrical power to a particular circuit. Only if the breaker trips a second time should you not try to restore power by resetting it—so the thinking has been. This still appears in operating handbooks put out by aircraft manufacturers, as well as in literature on aircraft operations.
Largely as an outgrowth of the in-flight fire and crash of a Swissair MD-11 in 1998, and an FAA Advisory Circular (AC) issued in 2004, air carriers reviewed the procedures followed by their flight crews in the event of a tripped circuit breaker. Guidance was updated to inform flight crews that if the cause of the breaker trip is unknown, or if the breaker controls a nonessential circuit, it should not be reset. Aircraft operated under Part 121 commonly have indicators, such as circuit breaker markings or coloring or segregated placement of specific circuit breakers in the cockpit, showing which circuit breakers are critical.
The NTSB suggests that many Part 91 pilots and operators may not know about AC 120-80, which dealt with in-flight fires and largely was aimed at air carrier crews, or may be unsure of whether to follow the advice in the AC or conflicting guidance provided by an aircraft manufacturer. It’s time for the FAA to issue new guidance aimed at Part 91 pilots.
In AC 120-80, the FAA expresses concern about resetting a circuit breaker (unless a reset is called for in specific procedures or the captain decides it’s necessary for the flight’s safe completion). It also says that crewmembers should remember not to use a circuit breaker as a switch to perform procedural functions unless doing so is specified in approved company procedures or the manufacturer’s operating procedures.
The FAA notes that circuit breakers are slow-acting devices and may not offer sufficient disconnect protection during events such as arc tracking or insulation flashover. Arc tracking is a phenomenon in which a conductive carbon path is formed across an insulating surface. The carbon path provides a short circuit path through which current can flow (electrical arcing). Insulation flashover, a result of arc tracking, is an instantaneous burn-through of an insulated wire with the possibility of continuing the burn into surrounding wires. The high temperature of an electrical spark causes a wire’s insulation to fail, thus allowing more arcing to occur, which can destroy even more insulation and, ultimately, an entire bundle of wires. The effects of electrical faults can include, in addition to wiring damage, component overheating, toxic fumes, smoke, fire and holes melted into the metal.
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