Stupid C-172 Light Dimmer Rheostat.

TripleSticks

TripleSticks

Well-Known Member
I toasted one of those stupid rheostats that are for the panel light dimming and the overhead light dimming in a late 70's C-172. It's one of the 2 in 1 rheostats that are mounted in the panel right near the carb heat, mixture and throttle. Anyway. It's a standard electrical part so there's no reason to spend $85 bucks to get it from Cessna. The thing is.... I don't know where to get it.

Does anyone here know where I can get it?

Thanks,
John
 
John....let me try to talk you out of what you are about to do. Pay the 85 bucks and get the appropriate rheostat that is called out for by the parts manual. The part you buy from Cessna is going to come with a little slip of paper that says the FAA blesses the use of that part. Going down to the local radio shack and getting a similar part will not provide you with this. Since you didn't go into any detail as to whether the aircraft is your personal one or not...I'd only assume it is since you are going to buy the part. You may very well go get a new part from Radio Shack and put it in and no probs...never get caught with it and never an issue. On the flip side of that...say you get somewhere and get ramp checked and an airworthiness inspector is checking out your airplane and he notices a different looking knob on your instrument panel and begins questioning it...or you are involved in an incident and you have the opportunity for inspectors to comb through your airplane and find a different looking knob/switch on your instrument panel. This could be some A** pain that you don't want...over what...85 bucks. You may go ahead and get a generic dimmer and then come annual time, the IA that does the inspection will point it out and you'll need to get another (proper) one, so you'll be spending the money anyway. Just my $.02 worth.

Pac Man
 
Hey guys. Just so you know... my first inkling was that it would have to be a Cessna part but after looking in to it I realized that this is just a standard electronic part and should be covered under the FAA Ruling on Replacement Parts.

Here's an excerpt from 14 CFR Part 21, Docket No. AIR-100-9601

The FAA has determined that certain kinds of electrical and electronic parts fit within the limits of the § 21.303(b)(4) exception; these include resistors, capacitors, diodes, transistors, and non-programmable integrated circuits (e.g. amplifiers, bridges, switches, gates, etc.). ... Generally, a standard part may be replaced with an identical standard part, in accordance with the manufacturers maintenance instructions, without a further demonstration of compliance with the airworthiness regulations.

Now... I'm no lawyer so I could be wrong... but that's why I brought it up. After a short search I found that those rheostat's are a huge problem. I figured if there's anyone who has looked in to it and found the standard part then they would be on here. If not. I'll spend $85 bucks for a $2 Rheostat. I just hate to do that when it's not necessary.

BTW. Not my plane. It's a friends and it had a major avionics fry a a couple months ago while I was flying it (leaseback at a flight school). The Voltage Regulator failed at take off power and toasted everything except the ADF. Figures. Huh.

Anyway. It's got all new stuff and we accidently refried the rheostat when we changed the bulb in the mag compass. Can you believe that? How frustrating.

It's all being worked on by a legit avionics shop. All yellow tags and the whole 9 yards. I was just thinking. Why give them the satisfaction (and the money) when I don't have to? All they do is buy a standard part from "????" and mark it up 700%.

Done venting.

j
 
John....good point on the 21.303 reference you cited. I'm at work and will look up my FAR's to see the full reference later on this evening when I get home. Not being a smart A**...just off the top of my head...can you get a resistor that does the dual function of a Cessna rheostat--with the inner/outer knobs. Also...I'm by no means an electronic guru, but when I see a resistor on the list you mentioned, I personally wouldn't associate that with the rheostat on the Cessna panel. I see it more along the lines of a little part that is placed on a circuit board or something like that. I understand the rheostat operates as a resistor, but not connecting it as an allowable item. Just me though.
Sucks that ya'll fried everything, but imagine that....the ADF stood up to the challenge :-)
 
I definitely don't want to claim that I know everything about 21.303 either. It just makes sense though. I used to design medical devices and we were subject to similar regulations. BUT... a standard part is a standard part. You just design around it. Cessna didn't invent that rheostat. They found a standard part, bought it and threw it in the plane.... seemingly without wiring it through a circuit breaker.

I don't know much about electronics either but it certainly seems like a micky mouse set up to me. I don't want to put a different part in there though. I just wanted to know where to find the right part without paying an arm and a leg.

aloft. True. It's not my plane. But... It's my best friends plane and we were both discussing it. I was using singular in the posts when I should have been using plural. We'll probably go up the street and get the Cessna part this time because we can get it today. But I would still like to know where they buy the part so the next time I can just get it there.

j
 
John....hope you know I'm just playing devils advocate. From being around aicraft owners and working on airplanes I found that they are the tightest folks when it comes to putting money into their airplanes--as a general observation on my part. As the rheostat is a simple device....there are many simple additions/modifications that you can do to airplanes--i.e., sun visors, but if you change the type that are in the aircraft, it is a mod and some paperwork would be in order. Just for a simple sun visor. This can be debated around for days, but I was taking the side of caution and don't mean to come across as knowing everything....I have a lot to learn, but if it were my plane, I would buy the 85 dollar rheostat and install it myself....as I can do that.
 
Oh yeah.... I play the devils advocate all the time with my friends. I completely understand and appreciate all input. It's kind of a mute point now because, after digging in to it today, they found that the Rheostat is fine. We just blew a fuse somewhere.

Anyway... I completely agree that we shouldn't be "changing" the plane. That's not really what I wanted to do. I was just trying to find the OEM of that exact part so I didn't have to pay the middleman price. I understand that you can't change the plane without paperwork. That's not what 21.203 says. It says that "a standard part may be replaced with an identical standard part." That's what I was looking for.

Thanks for the help. It's all fixed now and I didn't even have to get that damn 80 dollar Rheostat :)

Off to get a flight in before the crappy weather.

have a good day,
j
 
Replacing standard parts with ones from non aviation sources is legal in some cases. Toggle switches from Cessna are no different from the ones at Autozone in most cases.

However the responsibility is on the mechaninc who signs of on any repair useing that part that the replacement part is actually is identical to the part it is replaceing. Almost all mechanics don't want to take that responsibility and will refuse to use anything other than TSO parts from the manufacturer or supplier.

Completely legal, just not often done.
 
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