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FredFenster

FredFenster

Pre-takeoff checklist
Joined
Apr 18, 2013
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429
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Manitowoc, WI
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Display name:
Greg L
I'm having an issue with a toggle switch, its for the pitot heat in a '68 Cherokee 140. I can turn it on, pitot blade will heat up but after maybe 5-10 minutes the switch will flip itself to the off position. I can turn it back on and after a few more minutes it is off again. There's a "10" pressed into the end of the switch, so I don't know if that means there's a 10 amp breaker built into the switch or its just rated for 10 amps. There is a separate breaker for the pitot heat that is not tripping which makes me think bad switch. Anyone have any ideas if I'm dealing with a bad switch or an actual short? :confused:
 
It's tripping. Gotta diagnose the fault now. (Don't keep reseting it.) The poh might tell you about its use?
it might be like this one
 
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Let'sgoflying! said:
It's tripping. Gotta diagnose the fault now. (Don't keep reseting it.) The poh might tell you about its use?
it might be like this one
Click to expand...

The one in your link sure looks like the side profile of what I can see in the panel. I'll have to look at the POH tonight. I wonder if it ever worked right, I had it installed 10 months ago but never flew with it turned on until this past weekend. I'd check it during preflight to make sure it was getting warm but that wasn't long enough to trip it apparently.
 
FredFenster said:
The one in your link sure looks like the side profile of what I can see in the panel. I'll have to look at the POH tonight. I wonder if it ever worked right, I had it installed 10 months ago but never flew with it turned on until this past weekend. I'd check it during preflight to make sure it was getting warm but that wasn't long enough to trip it apparently.
Click to expand...

OK, so you had a heated Pitot tube installed that probably takes 10 to 15 amps controlled by a switch with a built in 10 amp circuit breaker?

If so, there is no surprise here that it trips, right?

What, exactly, is the spec for the Pitot tube?
 
Capt. Geoffrey Thorpe said:
OK, so you had a heated Pitot tube installed that probably takes 10 to 15 amps controlled by a switch with a built in 10 amp circuit breaker?

If so, there is no surprise here that it trips, right?

What, exactly, is the spec for the Pitot tube?
Click to expand...

I'll have to dig through the paperwork that came with it tonight and find the spec.
 
Or maybe it's a timer switch like they are using in cars now days... does it always stay on about the same length of time???? Say 10 mins...:dunno:
 
Twin_Flyer said:
Or maybe it's a timer switch like they are using in cars now days... does it always stay on about the same length of time???? Say 10 mins...:dunno:
Click to expand...

When I finally figured out to pay attention to it, I'd say it was 2-3 minutes and it was off. I know right where the sheet is that should tell me how many amps it should be drawing, its in the box with the old pitot blade at the hangar. Forgot to grab it when I was there the other night, but I'm going there again tomorrow.
 
Finally got my answer. Pitot heater spec is 15 amps. Looks like I need a new switch for sure, hopefully nothing else like wiring..
 
FredFenster said:
Finally got my answer. Pitot heater spec is 15 amps. Looks like I need a new switch for sure, hopefully nothing else like wiring..
Click to expand...

I would check the wire gauge, the breaker is spec'd to protect the wire.
Wrong breaker size and you risk heating the wire.

Maybe I missed it but how did we get to this point of considering changing the switch? Did someone put the wrong one in? You can check the wiring diagrams to see what it should be. Unless this is some sort of modification. Same with the device; the pitot heater, was it changed out?
 
GlennAB1 said:
You may, what is the circuit breaker for that circuit rated at?
Click to expand...

Apparently the circuit breaker is 10 amps built into the switch. I thought there was a separate breaker, but apparently I "mis-remembered" :D. There's a spot on the panel labeled "pitot heat" where the breaker apparently would be with a factory new installation but there is no breaker in the hole above it.
 
Let'sgoflying! said:
I would check the wire gauge, the breaker is spec'd to protect the wire.
Wrong breaker size and you risk heating the wire.

Maybe I missed it but how did we get to this point of considering changing the switch? Did someone put the wrong one in? You can check the wiring diagrams to see what it should be. Unless this is some sort of modification. Same with the device; the pitot heater, was it changed out?
Click to expand...

Looks like the wrong one was put in. The pitot blade was originally unheated, I had a heated one installed last year.
 
FredFenster said:
Looks like the wrong one was put in. The pitot blade was originally unheated, I had a heated one installed last year.
Click to expand...

Seeing the installer put in the wrong breaker/switch it would be a good idea to look at the printing on the wire and see what gauge was used. If they did the wrong size wire, you could have problems down the road. Ie, fireball in the sky.
 
Certified plane and certified mechanic. The FAA has certified that nothing can go wrong EVAR!
 
AC43.13 Chapter 11 explains it all. I agree with Dave, size the wire(s) and choose a breaker that will open before any of the wires connected to it can exceed their design temperature.

I don't understand why there would be two over current protective devices in series in a circuit, if the wires are all the same size.

The breaker for pitot heat was probably originally installed at the power bus bar (so it protected the maximum amount of wire). It's possible someone with old or incomplete maintenance info had trouble identifying (or paying) for the same switch the OEM installed. If it's not an original pitot tube installation, is there any mod paperwork for the changes?
 
someone installed the wrong switch/breaker look in your log book and ask who signed it off to replace it with a 15 or 17.5 amp one when he gives you a bill throw it on the floor and **** on it
 
OK, If I read correctly, you have had installed a heated pitot tube where none existed before. The pitot draws ~15 amps, and the breaker/switch that was installed to operate it is a 10 amp breaker. #1 whoever installed the system went the easy route, by thinking you could use a circuit breaker as a switch, and #2 used too small a breaker. AC43.13 mentions that one must not use a circuit breaker as a switch, as there will be similar problems develop over time. add to that the fact that the value of the breaker is too low anyway, and there ya go.
The proper installation would have been a 15 amp circuit breaker, (in the breaker panel) and a seperate switch, capable of carrying at least 15 amps continuous load. a 20 amp rated switch would be good. And also the proper gauge wiring for the circuit.
 
bluerooster said:
OK, If I read correctly, you have had installed a heated pitot tube where none existed before. The pitot draws ~15 amps, and the breaker/switch that was installed to operate it is a 10 amp breaker. #1 whoever installed the system went the easy route, by thinking you could use a circuit breaker as a switch, and #2 used too small a breaker. AC43.13 mentions that one must not use a circuit breaker as a switch, as there will be similar problems develop over time. add to that the fact that the value of the breaker is too low anyway, and there ya go.
The proper installation would have been a 15 amp circuit breaker, (in the breaker panel) and a seperate switch, capable of carrying at least 15 amps continuous load. a 20 amp rated switch would be good. And also the proper gauge wiring for the circuit.
Click to expand...

That would be it. I just paid a local mechanic to fix it this week, $50 later and I've got a 15 amp switch/breaker just like what was in there. At least the wiring was heavy enough. I would've spent more than that on gas just to fly the plane to the original guy to fix it.
 
bluerooster said:
AC43.13 mentions that one must not use a circuit breaker as a switch, as there will be similar problems develop over time. add to that the fact that the value of the breaker is too low anyway, and there ya go.
Click to expand...

Shorty --

I believe that reference was meant for breakers of the tit-nipple variety, but the switch variety were built to be used as a switch AND a breaker. They don't have a particular problem with being used as a switch, although they are MUCH more expensive than a regular klixon variety.

Jim
 
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