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In my plane for everything electrical I installed, I asked myself: can I fly without this item? And accordingly decided on CB or fuse.
A CZ builder had followed the plans and had installed the electric nose gear fuse in the "per plans" location under the passenger seat.
He takes a passenger for a flight, but on retraction, his nose wheel is cocked , hits the bottom of the fuselage and pops the fuse. Something goes wrong with the manual override too. Now he asks the passenger to unbuckle, turn around and put his butt against the windshield, so he can replace the fuse while flying the plane at the same time. If he had spent few dollars on a CB, he could have pushed it in and flown with the gear down.
Buly
On Feb 18, 2007, at 7:35 AM, Thomas y Reina Jakits wrote:
Hi Al,
interesting!!
Next chance I have I will try to find out more about this (reasoning for use of fuses over CB's in this helicopter).
I don't really buy the "repair after trip" -reason as the result is the same.
Whatever caused the fuse or CB to trip, it should be repaired. What really happens is that the fuse is replaced - if it doesn't trip immediately, no one will look any deeper into it, than a "look" (unfortunately...), .... until it trips again.
A lot of trips can be caused by changing environment (heat, humidity, etc.) that bring the conditions for over current just to the trip point, but will not under normal conditions.
However I am no specialist, not even building anything at this time - so, I will try to find out for this specific application (AS350B3)
Thomas
PS: First thing I am going to do is to consult the Maintenance/Repair manual....
----- Original Message -----
From: Al Gietzen
To: Rotary motors in aircraft
Sent: Saturday, February 17, 2007 6:10 PM
Subject: [FlyRotary] CBs and fuses
Eurocopter Factory Instructors are VERY COMPETENT in Eurocopter Helicopter Systems!!
There is absolutely NO doubt that he knows the difference between fuses and breakers and WHY either one would be used.
Thomas;
Certainly not impugning the competence of the flight instructors; just suggesting the design engineers may have other reasons than cost. One may be reliability. The reliability of CBs goes down significantly after they have experienced a ‘trip’. I recall reading one experts opinion that a CB should be replaced after a ‘trip’. Yeah; seems extreme. The modern philosophy of circuit design is to design for fault tolerance, so a fault; once causing the protection device to open, is left for repair when you land.
The key for using fuses is fault tolerant circuit design and proper selection and sizing of fuses. Having done this; studies suggest that the fuse offers higher reliability and lower cost.
There certainly reasons for either CBs or fuses, and I have both in my plane. For my flight critical circuits I opted for simple, fault tolerance and fuses. I wouldn’t argue with someone making a different choice.
FWIW,
Al
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