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From: "Ian Dewhirst" <ianddsl@magma.ca>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: To Fuse or not to Fuse
Date: Tue, 3 May 2005 22:18:25 -0400
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Bill wrote "If fuses were safer, that is what they would put in commercial
aircraft."

I just can't resist...

Aren't you the fellow who is putting a rotary engine in an experimental
aircraft that you are building in your garage?

On a more serious note, a stalled fuel pump i.e. the armature is no longer
rotating, will not blow a properly sized fuse, I live in Canada, during the
80's I worked as a auto mechanic in an Audi dealership.  Cars would get
towed in with water in the sump of the tank where the fuel pump was located.
The water would be frozen, we would bridge the fuel pump relay, weight 2 or
3 minutes for the pump to melt the ice, turn the key and drive it in.  I
have never, ever, seen a vehicle come in on a hook because of a blow fuse.
I have seen quite a few that have failed fuel pump relays so I would suggest
that folks avoid those.

If you have two pumps, two fuses, two quality switches, and two separate
wire runs (different grommets etc.) and you blow both fuses on the same
flight you are one unlucky SOB, or a very sloppy electrician.

Cheers -- Ian


-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]On
Behalf Of Bill Dube
Sent: Tuesday, May 03, 2005 6:15 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: To Fuse or not to Fuse


>>Bob  says,
>>"When the failure manifests itself by opening the breaker or
>>fuse likelihood of recovering the system by replacing a
>>fuse or pushing in a breaker is very, very small."
>>
>>         Beneath it all, this is what all the fuse versus circuit breaker
>> discussions hinge on.  It is an incorrect assumption, in my experience.
>>
>>         A very large percentage (but by no means all) of electrical
>> problems in vehicles are intermittent in nature. You can very often
>> reset the breaker and restore the critical system long enough to safely
>> land the aircraft. A fuse does not give you this option, at least not in
>> a timely manner.
>
>To be realistic, don't most of the multi-reset stories start in old/poorly
>maintained/poorly designed/etc aircraft?

         Since our airplanes do not come off an assembly line with millions
of hours on identical prior production aircraft, they might possibly fall
under the category of "poorly designed" since each of our airplanes is
pretty close to a prototype with none of the bugs worked out (at least
during the fly-off hours.) None of our freshly-built airplanes, especially
those with a one-off non-standard engine installation could be called a
"mature" design. I would never call a design "complete" until there were
many failure-free hours on the prototype. You always change something in
the design after the prototype is built and you often find out the design
flaws the "hard way."

         Even well-designed, well-maintained aircraft have electrical
problems. To quote "The Breakfast Club", "The world is an imperfect place.
Sometimes screws just fall out."

         Both Tracy and I (and others) have given examples of how resetting
a CB made all the difference in averting (or at least deescalating) an
in-flight emergency. These examples may be anecdotal, but the fact that
several people in a small group have had similar experiences strongly
suggests that resettable CBs add some degree of safety.  They certainly do
not detract from safety.


>I really think that the cornerstone(s) of his endorsement is more along
>the lines of 1. you don't have to have a 50 year old design &  'aircraft
>(poor) quality' factory electrical system in a homebuilt

         A  50 year old design is a "well-proven" "well-tested" and
"mature" design that has stood the test of time. Unexpected failures are
less likely in such a product. There is a Darwinistic culling of critical
design flaws in mass-produced (or oft-built) aircraft. :^)

>  and 2. you can design critical-to-flight systems  to have automatic or
> flip-a-switch backups. The reality is (as he's stated many times)
> *nothing* is failure proof. His philosophy is that you design the system
> with redundancy to trump any single failure.

         The desire to go with fuses is primarily motivated by cost and
weight, not by safety. Breakers are safer. That is why there are no fuses
in commercial aircraft. If fuses were safer, that is what they would put in
commercial aircraft.

         Many (by no means all) homebuilders are motivated primarily by
cost, and not so much by safety. That is why fuses appeal to many
homebuilders. If circuit breakers were as cheap (and small) as fuses,
almost no one would select fuses. In an aircraft application, there is no
advantage to a fuse aside from cost and weight.


>Everyone's got to make their own choices but it helps to start with the
>right assumptions.

         It is more about deciding on what is most important to you, cost
(weight) or safety.  The safety difference is not non existent, contrary to
what is suggested by Bob. You must determine what that additional degree of
safety actually is, and then decide if it is worth the money and weight to
use a CB instead of a fuse.

         As much as the safety folks hate to admit it, it always comes down
to the cost/benefit ratio. At some level, you must choose accept a given
degree of risk for a given degree of cost savings. I personally think that
CBs add a degree of safety that is worth the extra money they cost for
flight-critical systems. For things like the CD player, a fuse would be OK
with me, but not for the fuel pump.


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