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Fuses or Circuit Breakers?

Brian Lloyd (, Cameron Park, CA

[Brianís article is from a post to the RV List regarding the relative merits of fuses and circuit breakers in homebuilt aircraft.  I donít have the space here to reproduce the entire context of the post, but I thought Brianís comments would be interesting to most builders.  Brian is an RV builder and an engineer at Lucent TechnologiesóWebmaster.]

Fuses do not have contact points and a resettable tripping mechanism as do circuit breakers.  A fuse that is not overstressed will have a longer MTBF [mean time between failures; rather like TBO-Ed] than will a circuit breaker.  Oh, and it is a hell of a lot cheaper.

When a fuse blows or a breaker pops, there is a reason.  It means that something is drawing more current than the design called for.  This means that you should investigate the problem but the time for investigation is when the aircraft is on the ground, not while it is still flying.  With a fuse that is out-of-reach there is no impetus to try to troubleshoot when you should be flying the aircraft.

Most people donít think they are smarter than the entire production aircraft industry in all areas.  After all, they arenít designing the RV they are just building per plans.  They arenít that interested in actually designing an electrical system that strays too far from known safe.
Just because Cessna, Piper, and Mooney did it one way doesnít mean that they did it the right way.  They have two problems that we donít: the old way works well enough that they donít think it is worth the money to revisit the design and, if they did change it, some lawyer would use that info to infer that the old design was flawed for the purposes of winning a lawsuit.

If you examine the design of the ďstandardĒ electrical system you will find that it has some interesting failure modes that are likely to take out your whole electrical system.  If you think about it, you can probably figure this out for yourself.

Heck, the traditional air-powered gyro is a perfect example.  Back in the dim recesses of aviation history someone discovered that they could power gyros with air better than with electricity and that a venturi would provide an outstanding source of clean power that would work as long as the airplane was flying.

Nowadays we have high-precision, low-mass, brushless, hall-effect AC/DC motors that are much better for powering iron gyros than is air.  Heck, we have monolythic silicon gyros on a chip and fiber-optic gyros that are much better than our iron gyros; i.e. are about the same cost and have MTBFs measured in hundreds of thousands of hours instead of hundreds of hours; but the industry is steeped in tradition.  This reminds me of what we at the Air Force Academy used to say about the West Pointers:  two hundred years of tradition unhampered by progress.

Airplanes donít usually have the luxury just pulling over to the side of the road when the fuse does blow or the belt breaks.
Right.  We know how these things fail so letís plan for it instead of fixing the problem.  People ďknowĒ that electrical systems are unreliable so they train to deal with an electrical failure rather than spend the same energy figuring out how to eliminate the failure modes from the electrical system.  Go figure.

Another example are people who train to fly partial panel because they know that their vacuum system is going to die and they are going to have to rely on flying needle-ball and airspeed.  Sure I can fly partial-panel but I have also designed my panel so that I have multiple sources of power for my electrical gyros.  So, while I might have a gyro failure, I am not going to have a gyro SYSTEM failure.
Yes, there is more research in the automotive industry than there is in the aviation industry.  I know that electronic ignition SEEMS more complex and less reliable than magnetos, it is, in fact, a lot MORE reliable.

There are better ways to wire an aircraft than the way that was developed back in the 1930ís.

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