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9 Answers

91.205 Night VFR Fuses Req: Complete Set or 3 of Each Kind

Asked by: 4696 views
Aircraft Systems

What does it mean to have a complete set of fuses? (I assume it's like having a set of circuit breakers: one for each, except fuses are irreplaceable).

What does it mean to have 3 fuses of each kind? What is the "kind" being referred to? And why 3, instead of say, 2 or 5?

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9 Answers

  1. Mark Kolber on Jun 09, 2014

    except fuses are irreplaceable

    Not quite. Fuses are very replaceable. Even more replaceable than breakers because they are pilot-replaceable, even in flight.

    I assume you are talking about the 91.205(c)(6) requirement to have spare fuses for night VFR. The rule is there because fuses can be replaced in flight, while breakers cannot.

    And note that “accessible” part of the reg. As an example, if I recall correctly, there are only 3 fuses in a 172P model, none of which are accessible in flight (ther are located in the battery compartment.

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  2. Sam Dawson on Jun 10, 2014

    You may have fuses of different resistance and it is important not to mix them up. There have been some Cessna 150 crashes where using the wrong type of fuse- a fast blow verses a slow blow- have been a contributing cause.

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  3. pautpy on Jun 11, 2014

    So, when the regs refer to “3 fuses of each kind,” it means that each equipment would have 3 fuses of difference resistances?

    What would be the purpose of having 3 fuses of each kind?

    I appreciate your responses!

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  4. Sam Dawson on Jun 11, 2014

    No, it does not mean each equipment would have three fuses of each type. It means you would need three fuses of each type required.

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  5. pautpy on Jun 12, 2014

    Could you give me an example? How would it looks like?

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  6. Best Answer

    Mark Kolber on Jun 12, 2014

    pauutpy, since the airplane you are flying may not have a pilot accessible fusebox in the cockpit (most have primarily circuit breakers), lets use your car as an example. I’ll assume you know where the fuses are in your car – typically below and to the left of the dashboard in a left-hand drive car. If you don’t know what I’m talking about use google image and search for something like “toyota fuse box.” An aircraft that uses fuses rather than circuit breakers would look the same. A Cessna 172 “fuse panel” would have all the stuff a Cessna 172 circuit breaker panel does, except it would have fuses rather than breaker buttons.

    Here’s an example from a homebuilt: http://www.docthrock.com/panel_files/NelsonFusePanel.jpg if my attempt at embedding doesn’t work) Like a car, these fuses are color-coded for their amperage,

    Let’s suppose your car (or fuse-operated 172) uses 7.5, 10, and 20 amp fuses. The reg requires you to have three of each. Why? Well, just like the simple home-built in the photo and your care, more than one fuse of each type is being used, so you need more than one backup.

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  7. pautpy on Jun 15, 2014

    So, the reg simply requires some spare fuses that you can just access in the cockpit, in case a fuse blows out and you need to replace it? For example, you can just keep the exact number and type of fuses in a bag in the baggage compartment OR in the case of a 172 using 5, 7.5, 10, 20 amp fuses, three 5 amp, three 7.5 amp, three 10 amp, and three 20 amp fuses (a total of 12 fuses) in your flight bag?

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  8. Mark Kolber on Jun 16, 2014

    Depending on the specific aircraft, of course, there is usually not going to be a need to carry spare fuses in your flight bag. As previously mentioned, the only fuses in a 172P are in the engine compartment (not accessible, so not an issue). OTOH, if you check earlier models which used fuses rather than breakers, spare fuses were in a location within the glove box. Search location of spare fuses C172 and you;ll come across some early manuals that discuss this.

    Bag in the baggage compartment?I’m trying to picture a pilot climbing over the seats into the baggage compartment to replace a fuse in flight.

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  9. pautpy on Jun 16, 2014

    I an example of carrying spare fuses in the baggage compartment because I figured you’d just land ASAPractical and replace a fuse (hopefully it’s not one critical to flight), but I can see how that would be problematic. But overall, I finally understand what the reg is all about. Thanks for your replies, Mark.

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