Fuses for night VFR question

Discussion in 'Maintenance Bay' started by 4RNB, Jan 18, 2021.

  1. Dan Thomas

    Dan Thomas Final Approach

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    And yet, airliners, those big machines that are responsible for carrying hundreds of people at a time safely to a destination, use breakers exclusively. Lots of them.

    I wonder why that is so?

    And as I have mentioned before, I have found fuses failing just because they were old. The fuse strip is trapped between the glass tube and the caps, and those connections oxidize and present resistance that heats the fuse and fails it even when it's not anywhere near max load. I have also found breakers that fail due to oxidizing contacts; they also are a thermal device and resistance causes heating. However, the old breakers were 40 years old, and switches suffer the same fate whether they're fused or breakered. The fact remains that age hits everything hard. And the fact remains that a popped breaker is visible, a fuse is not unless it's removed and inspected. Ever try inspecting a 2-amp fuse in the dark?

    POHs give advice on resetting breakers. So does this: https://www.faasafety.gov/files/notices/2009/Dec/SAIB_CE-10-11.pdf

    An excerpt:

    Current guidance for part 25, Transport Airplanes in AC 25-16, Electrical Fault and Fire Prevention and Protection that has been accepted for small airplanes, is to recommend that no pilot should reset any circuit breaker more than once.

    Pilots can keep replacing fuses just like they can keep resetting breakers. It's a training issue, not a fuse-versus-breaker issue. Worse, a blown fuse is too easily replaced by a larger fuse, or wrapped with aluminum foil, to get the system going again and risking a fire. Impossible to do that with a breaker.
     
  2. LesGawlik

    LesGawlik Pre-takeoff checklist

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    You could have just pinched your empty fingers togethers and said "Here's one." When the DPE asked you how you used the space between your fingers, say "Just find me the fuse holders on this plane and I'll show you."
     
  3. Snowmass

    Snowmass Line Up and Wait

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    As an engineer I think I can safely say that engineer are often terrible at dealing with human beings in their designs. Think the Boeing MAX. Or maybe the trying to get an appointment for a covid vaccination via the computer (dumb design raised to the power of dumb). And having panels full of CBs is part of the same human interface ignorance. When a CB pops or a fuse blows in the air fix the problem safely back on the ground. But with a fuse blows at least you are putting in a brand new part. With a CB you are dealing with a possibly defective old part (like I was when it would fail and restore and fail again).
     
  4. 3393RP

    3393RP En-Route

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    You haven't shared the particulars of the self resetting breaker, like load, age, flight conditions, etc. That would be helpful.

    Regarding the weakening of a breaker after multiple trips, do share your source for that, and the specifics. It seems to me that a breaker that fails because of repeated trips is a symptom of a larger problem. Why is it tripping, and why hasn't corrective action been taken, i. e. reduction of the load? How many times does a breaker trip before this claimed phenomenon occur?

    If resetting a circuit breaker involves a fire hazard, so does replacing a fuse. For me, doing either would depend on the circumstances and the advisability of doing so.

    Lastly, I would much rather be glancing down and resetting a breaker in hard IMC over separating a fuse holder, fumbling around for the proper fuse, installing it, and resecuring the twist top in that same IMC. The opportunity for the onset of spatial disorientation is much greater in the latter situation.
     
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  5. Snowmass

    Snowmass Line Up and Wait

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    I really don't know how far I will take this since the fundamental point is to NOT try to restore power in flight per the FAA. I really think that it is a waste of time to discuss with anyone who is not interested in the truth but in trying to win an argument.

    The CB that failed and restored and failed was from generator to buss. It first happen on an enviro mission in Costa Rica where the battery would not charge. I took generator out and the Cessna shop said the generator was fine. I then replaced the regulator and still no charge. I was about to haywire a couple of storage batteries to get back to the U.S. at least. I started the engine again to taxi to a tie down and the system started charging again. I completed my mission and flew home. I then flew locally and charging would fail for a short while and then restart before I could find the cause. Finally, on the Vegas-Phoenix trip it stayed failed so after a full shop charge I flew home and traced the bad component. I kept flying flights where battery reserve would be enough since It was impossible to find an electrical problem when there is no problem. After it permanently failed it was simple to trace the cause to the CB.

    The hard IFR statement makes no sense unless it is an argument against glass with a single point of failure. In my case it would make little difference since my wet pump driven AH and DG would be plenty to fly by. I would only might lose the electrical rate gyro (of the T&B).

    My experience with a CB losing capacity come from my car where a re-occurring short required replacing the headlight CB since the original would no longer handle the load. The cause of the short is interesting and took a long time to find. The tail light bulb is also the brake light bulb with two filaments. The upper filament which is the tail light would sag after while due to being white hot and touch the brake light filament and overload the circuit popping the CB.

    All my new electronics was installed with fuses.

    End of story.
     
  6. Dan Thomas

    Dan Thomas Final Approach

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    Many breakers feed more than one component. If I was in busy airspace or in IFR and a radio breaker popped, I'd turn off the radios I didn't need, inform ATC on the radio I did need, and work from there. Panel lights often run off the nav light breaker, if it pops you could shut the nav lights off so you could still have the panel lighting instead of a flashlight in your mouth. In both cases, if the breaker popped a second time you have the choice of doing without that circuit or making the decision to try it again because the risk of going without it is worse than the risk of a smoking circuit. Popping in the middle of an ILS approach to minima, and losing the ILS, comes to mind. Sure wouldn't want to be fooling with fuses there.

    Trip-free breakers cannot be reset right after the pop. They have to cool internally before the bimetal conductor in them reverts to its proper shape. Breakers that open and reset themselves would be either those breakers hidden behind the panel (like those found on some cigarette lighters), which were designed to auto-reset, or some much older tech than the trip-free. Once a trip-free opens it stays open until it cools and the pilot resets it.

    People who only fly day VFR in D or E or G airspace have few worries about this stuff. Other people have to make hard decisions.
     
  7. Snowmass

    Snowmass Line Up and Wait

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    Each radio should have its own circuit and if you don't then rewire. A fire inside the cockpit is the worst thing you can have which is why the FAA says don't reset. The overwhelming cause of of a cockpit fire is electrical (people that smoke won't live long in any case). The chances of a landing without a radio are much better. You could just descend at minimum airspeed and flare at a few hundred feet and live in fact you can stop in 4 feet at around stall speed and not even be seriously injured in a C-172 which is what happened here. 172 crash.jpg