Pitot heat breaker tripping, electrical question
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Pitot heat breaker tripping, electrical question
Was flying along the other day in and out of the clouds, put on the pitot heat and about a minute later the pitot breaker tripped, went to reset and it would not stay in the ON position after a few minutes I reset it and it stayed in the ON position and all back to normal, then a few minutes later it tripped again, I shut off all the lights avionics etc and it still does the same thing. I know I cant be exceeding the alternator load with everything shut of except the pitot heat so does anyone have any ideas as to whats up and why it takes a few cycles to reset properly then lasts about a minute or two then trips again?
Please do not repeatedly reset any circuit breaker.
The best practice is to not reset a popped circuit breaker in flight unless it is servicing an essential for flight safety item, which on a light aircraft is pretty much none of the systems on board
You are just asking for an electrical fire. I suggest you re read the smoke thread in the accidents and incidence forum.....
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Not sure if this is relevant to your particular case, but.....
don't confuse alty output (which is rotational-speed dependent anyway) and breaker current capacity.....most breakers incorporate a bimetal strip, and /or an electromagnetic latch.....the latter will be instantly resettable...the bimetalwill get cooked up by too high a current and open /release the contacts it will take time tocool down to it,s normal shape in order to reset.......
As you've eliminated all other loads on that breaker,and it doesn't trip with any of those, I conclude the fault lies with the pitot -heater and/or associated wiring/switching....a high-resistance earth-fault could pass sufficient current, together with the heater, to trip a breaker.(say the pitot -feed passes through an earthed aluminium bulkhead and the insulation chafes through.)
we are making the assumption that the breaker is correctly rated for the load......If, for example, panel-lightsare wired through a 5-amp trip and then someone later adds another item to take the total load on that breaker,over it's rating, you'll have problems.
Sounds like your repeat tripping is normal where a fault exists....a pitot heater which is partially-shorted (assuming it's wound from resistance-wire) would draw a higher -than-rated current.
disconnect feed at pitot(isolate/insulate) switch on.....NO current should flow/-if it does, look for a shorted-to earth wire but remember an indicator-bulb could be in parallel with the feed to the pitotand show the appropriate current!
reconnect pitot-heat with an ammeter in series(if you're on 12V, they can be found in car scrappers or about a fiver new from a decent car accessory emporium.)...turn on and compare current with rating-plate and with the breaker rating.
As BP has said, it's extremely dodgy to keep resetting a trip.....in theory, the feed to the trip is unprotected , but substantial enough to take the max current the trip could pass ....but, there is a possibility the trip could malfunction and cause a high current to flow, cooking the wiring and anything (other, bundled wires, fuel lines etc.)in close proximity.
Your battery can probably poke in excess of 200 amps down a shorted wire- enough to make it glow white hot before it melts through. (or you turn off the battery master switch.)
don't confuse alty output (which is rotational-speed dependent anyway) and breaker current capacity.....most breakers incorporate a bimetal strip, and /or an electromagnetic latch.....the latter will be instantly resettable...the bimetalwill get cooked up by too high a current and open /release the contacts it will take time tocool down to it,s normal shape in order to reset.......
As you've eliminated all other loads on that breaker,and it doesn't trip with any of those, I conclude the fault lies with the pitot -heater and/or associated wiring/switching....a high-resistance earth-fault could pass sufficient current, together with the heater, to trip a breaker.(say the pitot -feed passes through an earthed aluminium bulkhead and the insulation chafes through.)
we are making the assumption that the breaker is correctly rated for the load......If, for example, panel-lightsare wired through a 5-amp trip and then someone later adds another item to take the total load on that breaker,over it's rating, you'll have problems.
Sounds like your repeat tripping is normal where a fault exists....a pitot heater which is partially-shorted (assuming it's wound from resistance-wire) would draw a higher -than-rated current.
disconnect feed at pitot(isolate/insulate) switch on.....NO current should flow/-if it does, look for a shorted-to earth wire but remember an indicator-bulb could be in parallel with the feed to the pitotand show the appropriate current!
reconnect pitot-heat with an ammeter in series(if you're on 12V, they can be found in car scrappers or about a fiver new from a decent car accessory emporium.)...turn on and compare current with rating-plate and with the breaker rating.
As BP has said, it's extremely dodgy to keep resetting a trip.....in theory, the feed to the trip is unprotected , but substantial enough to take the max current the trip could pass ....but, there is a possibility the trip could malfunction and cause a high current to flow, cooking the wiring and anything (other, bundled wires, fuel lines etc.)in close proximity.
Your battery can probably poke in excess of 200 amps down a shorted wire- enough to make it glow white hot before it melts through.
(or you turn off the battery master switch.)
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gee
what kind of plane was it?
when you were on the ground, did you re set the breaker, turn the pitot heat on and touch the pitot to see if it heated?
things get broken, a short circuit or something else. get a mechanic ( I think you call them engineers, but that is what we call locomotive drivers and science types).
and have the mechanic fix it.
be glad you didn't have a fire, the short could be ssomewhere bad
what kind of plane was it?
when you were on the ground, did you re set the breaker, turn the pitot heat on and touch the pitot to see if it heated?
things get broken, a short circuit or something else. get a mechanic ( I think you call them engineers, but that is what we call locomotive drivers and science types).
and have the mechanic fix it.
be glad you didn't have a fire, the short could be ssomewhere bad
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Update, when sitting on the ground at idle with every bell and whistle turned on the pitot heat works fine (no tripping) and vane heats right up. Ran it for 10 minutes at idle and all works fine, in the air at cruise rpm it trips every time with all the other crap turned both on and off
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with all the other crap turned both on and off
Whether you have other "crap" turned on or off doesn't matter one bit. Your voltage regulator should keep the bus voltage constant regardless of the actual total load (within its design limits obviously). If the voltage remains the same, the current through the pitot heat circuit breaker also remains the same. And it's the current through the CB which eventually trips the CB, not the voltage of the bus, or the total load on the system.
Based on what you've told us, my guess is that it's not the pitot heater itself, but rather some wiring that got damaged and is now causing an intermittent short circuit. But only with the aircraft in motion.
Please do not repeatedly reset any circuit breaker.
The only exception to this might be the alternator or alternator field CB. There may be specific instructions in the POH, to be applied when the alternator might have overheated.
One possible story, though not very likely, is that the regulator does not do its job properly so that at higher rev's the voltage goes up; according to ohm's law this will also increase the heater current, the heater being esxsentially a resistance. If on top of that the heater has lower resistance than intended, or the breaker is too sensitive, tripping could occur.
Ergo: measure and report voltage at various rpm.
Ergo: measure and report voltage at various rpm.
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Have the wiring to the pitot head meggered for any intermittent earthing
Forget all this talk of votage measuring at different rpm's etc etc, not practicle or correct. CB's trip out due to current overload, 10/1 the pitot head has failed as it is nothing more than a resistive device that provides a small amount of heat.
Meggering
Going back to when I was an electrician (air).The use of 250V meggers was common,for example ,had a fault where the positive was tracking across the face of a plastic plug insert that was not found with an Avometer.That was a tail chute problem!
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One possible story, though not very likely, is that the regulator does not do its job properly so that at higher rev's the voltage goes up; according to ohm's law this will also increase the heater current, the heater being esxsentially a resistance. If on top of that the heater has lower resistance than intended, or the breaker is too sensitive, tripping could occur.
One other thing: Isn't a heater element like this some sort of negative coefficient resistor type something thinghy? In other words, as the element heats up, its resistance increases, which means a lower current draw, which means less heat being produced. So in effect this becomes a self-balancing system that keeps a certain temperature automatically?
My money would be on the pitot heater element.
Oh well, let's see what the OP reports back.
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I think there is far too much theory here, just hold the circuit breaker in and after a few seconds the wisps of the smoke will indicate the location of the problem !
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I'm surprised that nobody has suggested establishing the rating of the pitot heater (power in watts at a specified voltage), and from that determining the DC current it should draw when in good condition. Then, having done so, measure the actual current draw. Until this simple test is done you won't know if the heater winding has 1) partially short circuited - this can happen, and will lead to increased current draw but not necessarily failure to heat up), or 2) whether the circuit breaker is tripping at less than its design rating. Breakers can (and do) age, and their current handling will reduce accordingly. The manufacturers will usually list the number of maximum current "trips" before permanent damage will occur. There will also be charts showing the current versus time curves which will trigger the contacts to release. It's just possible that with "all the other crap" switched on, the DC bus voltage is dropping enough to reduce the current draw through the heater circuit, and prevent the breaker from tripping. But until you've established the basics the rest is simply guessing.
Obviously what (if any) of the above the O/P can do depends on the status of the aircraft in question - Permit or CofA...
Obviously what (if any) of the above the O/P can do depends on the status of the aircraft in question - Permit or CofA...
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In practical terms before switching anything on a close visual inspection of the system is needed, points to look for are wires chafing and the ( usually poor quality ) connectors in the system. Also the pitot heat elliment needs to be withdrawn from the pitot head and inspected.
Otherwise you are likely to be troubleshooting by the method I rather sarcastically suggested in my first post on the subject when you switch the system on.
Otherwise you are likely to be troubleshooting by the method I rather sarcastically suggested in my first post on the subject when you switch the system on.
Fifteen minutes or so with Mk1 eyeballs and a multimeter will find a solution rather than pursuing some of the academic suggestions put forward so far.
It's really a very simple circuit.
On the face of it, I'd suggest the element is faulty. Not an uncommon defect.
It's really a very simple circuit.
On the face of it, I'd suggest the element is faulty. Not an uncommon defect.
