Rotorhead Posted September 22, 2020 Report Posted September 22, 2020 Flying a 81 M20J around and encountered visible moisture below 10C. Put pitot heat on and received an instantaneous low volts warning, volts dropped on all eye-brow lights and the pitot-heat switch went to the 'off' position at the same time. Switched the 'Pitot Heat' and 'Strobe' Klixon switches since they are the same amps and same issue. Possibly a bad ground? Anyone experience this in flight? Any ideas? Currently at Spokane far away from home station.
carusoam Posted September 22, 2020 Report Posted September 22, 2020 Welcome aboard, rotorhead... Sounds like you are attacking things randomly... If your Pitot Heat is using a lot of available power... it won’t be related to normal grounds or circuit breakers... these things have no resistance on a good day... When their resistance increases, the flow through the circuit decreases... It does sound like electrons are shorting to a ground without tripping a CB or fuse... You can start at the heater wires at the pitot tube and chase wires back to the panel... and start at the other end... where the CB is and chase wires from there... If you are handy with a volt meter, you can probably find where something isn’t working correctly... Good time to consult with a mechanic... loose electrons can be hazardous. If the pitot heater is failing, it may be over heating as well... PP thoughts only, not a mechanic... Best regards, -a-
N201MKTurbo Posted September 22, 2020 Report Posted September 22, 2020 It is shorted somewhere. If the lights are dimming when you turn it on, it isn’t the breaker. Did it work OK when you preflighted the plane?
RobertGary1 Posted September 22, 2020 Report Posted September 22, 2020 8 hours ago, carusoam said: Welcome aboard, rotorhead... Sounds like you are attacking things randomly... If your Pitot Heat is using a lot of available power... it won’t be related to normal grounds or circuit breakers... these things have no resistance on a good day... Not sure about that. The pitot heat normally draws a lot of power. In fact looking for the drop in amperage is generally the acceptable way to test them. The OP's may be excessive but no resistance isn't a likely goal. -Robert
carusoam Posted September 22, 2020 Report Posted September 22, 2020 2 hours ago, RobertGary1 said: Not sure about that. The pitot heat normally draws a lot of power. In fact looking for the drop in amperage is generally the acceptable way to test them. The OP's may be excessive but no resistance isn't a likely goal. -Robert Good catch... so busy with the spelling issues... I missed a word... or two... ’A lot more power than usual....’ would have been more descriptive. Thanks for the help... Best regards, -a- 1
Rotorhead Posted September 28, 2020 Author Report Posted September 28, 2020 Back at home and found these issues. Hoping this is the only culprit and nothing else in the wing.
carusoam Posted September 29, 2020 Report Posted September 29, 2020 Great find RH! Thanks for the follow-up... 1) Looks like the results of heating the wires really hot... melting the insulation... could be caused by the pitot heater itself..? 2) Looks really unusual as if the insulation failed on a twisted pair of wires.... possibly by rubbing over something...? What are these connected to? Seeing if orienting the pic changes anything... Best regards, -a-
Rotorhead Posted September 29, 2020 Author Report Posted September 29, 2020 That is also connected to the pitot probe. Anyone know where to find these types of leads on the interwebs??
N201MKTurbo Posted September 29, 2020 Report Posted September 29, 2020 1 hour ago, Rotorhead said: That is also connected to the pitot probe. Anyone know where to find these types of leads on the interwebs?? If you are talking about the wire, it looks like 16 or 18 gauge tefzel twisted pair. You should verify the gauge. I would just splice the wire where it is chafed and heat shrink the repair. The problem appears to be caused by the wire rubbing on something. that wire should be secured so it doesn't rub on stuff. Mooney uses little clips that clip on to the skin stiffeners. get a couple of those and secure the wire properly. 1
MB65E Posted Tuesday at 03:57 PM Report Posted Tuesday at 03:57 PM Why did Mooney ground the other side of the Pitot through two connectors in the wing and panel all the way back to the cockpit? Also, is the pitot tube polarity sensitive? I would think it’s just differential voltage. Troubleshooting a J models system today. thanks, -Matt
EricJ Posted Tuesday at 05:21 PM Report Posted Tuesday at 05:21 PM 1 hour ago, MB65E said: Why did Mooney ground the other side of the Pitot through two connectors in the wing and panel all the way back to the cockpit? Also, is the pitot tube polarity sensitive? I would think it’s just differential voltage. Troubleshooting a J models system today. thanks, -Matt Aviation applications seem to be paranoid about grounds, and tend to put more than needed. I suspect this is just for redundancy, but there may be some static/lightning mitigation going on as well. The pitot heater is just a resistive heater, so it doesn't care about polarity. A specific pitot tube may be made with one side grounded to the tube or mount, though, so it may be worth checking that. 1
MB65E Posted Wednesday at 03:00 AM Report Posted Wednesday at 03:00 AM Thanks! I was able to correct a ground issue today by routing another ground wire. It’s insane to send the ground through two connectors. The unobtanium switch was bad too. Alan Fox to the rescue. Installing a WX31 10a switch breaker to get by until the factory switch arrives. -Matt
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