Resetting a DC Circuit Breaker
 

Had a good discussion with a high time Learjet pilot about the potential threat of resetting DC circuit breakers. The gentlemen stated he first heard about DC CBs being a threat in Learjet ground school. He also cited Air Canada Flight 797 which had a rear lavatory catch on fire after resetting a toilet motor CB.

Another military pilot was listening in on our conversation, and we both replied that it is common practice in the military to reset a DC breaker if it was not tied to a fuel system.

Prior to researching Flight 797, I've never heard of giving a CB "time to cool", and thinking twice about resetting one unless it was critical equipment for safety of flight.

What is your take on the topic?

At my last 135 job we were allowed to reset them after 5 minutes, my current one three minutes. Though we're only allowed to reset them once.

CBs work off heat and their size is directly related to the current the wire in the system can handle. That's why there is a cool down period. Never, ever just hold down a breaker once it has been reset.

It used to be OK in general aviation to reset a breaker once. This was because on older planes the electrical systems were not particularly sophisticated and might experience breaker-popping surges in routine operations.

121/135 generally have specific rules, typically PIC discretion if the equipment is safety critical.

But given the relatively recent hull losses of two very modern heavy airliners due to apparent cockpit electrical fires I would be very reluctant to tempt fate by resetting a breaker.

Advisory Circular 120-80
 

AC 120-80 gives awesome guidance on this. Sorry I didn't complete a more thorough internet search before posting.

Recently had a wing root light CB popping on one of our FA20's (reset it only once per flight). Was flying with a very experienced (I'd say how experienced, but it'd give him away) aircraft commander and neither one of us thought twice about resetting it.

Never underestimate the knowledge gained from hangar flying with some of the old hands over cold beer on a warm summer evening when the day's flying is done.


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a. Resetting Circuit Breakers In Flight. The FAA reiterates its concern about resetting CBs during flight. Crewmembers may create a potentially hazardous situation if they reset a CB without knowing what caused it to trip. 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 flightcrew or unless, in the judgment of the captain, resetting the CB is necessary for the safe completion of the flight. A detailed entry in the aircraft’s maintenance log is a proven safety practice for tracking purposes, and may provide maintenance personnel with key information to enable prompt trouble-shooting and effective corrective action on the ground.

Air Carrier manuals and training programs should contain company policies and explicit procedures regarding resetting tripped CBs, both during flight and on the ground. The procedures shown in the manuals used by the air carrier’s crewmembers, maintenance personnel and airplane ground servicing personnel should be consistent with the airplane manufacturer’s guidance. Crewmembers should be reminded that a CB should not be used as a switch to perform procedural functions unless doing so is specified in approved company procedures or manufacturer’s operating procedures.