CBs – No reset!

April 29, 2009 by Bruce Landsberg

Let’s discuss circuit breakers (CB). We routinely verify during the prestart check that they are pushed in or appropriately disabled. Most of the time this is perfunctory and it’s a boring topic – sorry. Read on.

In the recent landmark accident CBs and wiring were implicated. Note that the rules regarding CBs have changed – In years past, if a CB popped the practice was to let it cool and reset ONCE on the theory that it might be a transient fault. No more. If a CB pops, unless it’s flight critical, do NOT reset. There have been only a few instances of big in-flight electrical fires where the aircraft and lives were lost. In this case, the tab was about $20 million and the lawsuits are flying. Seems like a lot for a simple thermal-mechanical device.

If you smell burning insulation before disabling the appropriate CB, the aircraft should be grounded until the fault has been isolated and the affected wiring replaced. That’s probably going to be expensive. But, if that seems harsh, according to NTSB engineers, once the wiring is hot enough to melt insulation so you can smell it – the insulation is toast, if you’ll pardon the pun. It will no longer function as intended and there is a fair chance that other circuits or equipment may malfunction and cause a fire when you least need it. Don’t forget that on-board fire extinguisher – this is potentially an extremely serious fault. Remember where it is? Can you reach it? Is it properly charged? Again – boring but critical.

Wiring is like props – we take them for granted and they routinely perform with little complaint until something unfortunate happens suddenly. Also suggested reading is ASF’s new In-Flight Electrical Fires Safety Brief on the topic. It’s a short read.

If you have a wiring story, please share it. Many others may benefit.

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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6 Responses to “CBs – No reset!”

  1. Larry Anderson Says:

    Bruce,

    Your advice to not reset CBs seems to conflict with the advice given in In-Flight Electrical Fires. I agree with you. It’s dangerous to second-guess the CB. If the tripped CB prevented a fire, you have just excaped a bullet, No sense in playing Russian Roulet. By-the-way, I am a CFII and an electrical engineer.

    Larry Anderson

  2. Bruce Landsberg Says:

    Larry ….

    One of us is confused – the NTSB’s and ASF’s recommendation is to NOT reset unless the item is flight critical. We’ll leave it to the pilot to determine flight critical but I would err on the side of extreme caution – unless it’s something that will definetly cause a major problem we don’t recommend rest.

    I think we are in raging agreement

  3. John Taylor Says:

    Bruce: You might mention that aircraft with checklists will have procedures wherein CB may be reset, but only when the checklist allows. The Falcon 2000 which I fly is one such airplane. We don’t pull’em, we don’t set’em.

  4. Bruce Liddel Says:

    Obviously, if there is a CB for each individual device on your AC, then yes, of course, I agree, leave the popped breaker alone, especially if you smelled smoke.

    Suppose you are in near IMC (like at night over nowhere under a moonless overcast) in some un-restored classic where some idiot retrofitted a current-hungry device onto the same CB as your turn coordinator and /or pitot heat, and maybe you don’t have an engine-driven vacuum pump. You don’t smell anything, but your breaker pops just as as the power-hungry device (or turn coordinator) indicates a loss of electrical. Yes, I know, you shouldn’t have been there in the first place. Still, it would be mighty tempting to switch off the other device or devices, wait a minute, and try again at least once to restore only the pitot heat or turn coordinator, especially if manually pulling the breaker again (at the first sensation of any odor) is an option. I guess in this scenario the turn coordinator or pitot heat would be considered “flight critical”?

    Not all aircraft are alike – some have stupid stuff done to them. All situations are not alike. One simple rule like “no reset” can’t address every single possibility. As pilot, you’ve got to know your aircraft inside and out, and manage the risks accordingly.

  5. John LeNard Says:

    The rule as I learned it was to turn off all devices on the popped circuit breaker and only then should you reset it. Then you can turn on each device, one at a time, with a good wait between each. Then the one which popped the circuit breaker again was the curlpit.
    Knowing the electrical circuit in today’s airplanes is critical to safe flying.

  6. Terry Scott Says:

    This subject came up and we all agreed CBs are not switches and they don’t work like switches so simulating things by turning then off then on is breaking two big rules. Its not a switch so they are being improperly used and resetting them just because you pulled it with out coming to a full stop landing is the other.

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