February 9, 2024 at 9:06 pm #189521
I recently had new props and a new panel installed. Prior to the new installation my prop amp meter worked fine. The only problem was that is was on the right side of the panel and I could not see it. When I had the new panel installed I had it moved to the left so it would be in my field of view. Now the meter comes off the stops but never indicates in the green. I have tested the deice pads per the MX manual and they are heating properly. I can here the timer under to copilots seat cycling every 20 sec. I don’t know if moving the meter some how changed the resistance and now the meter reads to low or if the system is not functioning properly. Like I said the prop deice pads are heating up. Is it really that important to have that deice amp meter reading the green? Thanks for the help…February 10, 2024 at 12:14 am #189522adancy1Participant
Are you testing with the engines running, or on battery power?February 10, 2024 at 9:37 am #189529
Yes and no. If the engines are running I get just a little bump on the amp meter. To test the deice boots for heat I do that per the MX manual with the motors off just using the battery/external power.February 10, 2024 at 3:51 pm #189546sthibault1Participant
You asked: Is it really important to have the de-ice amp meter reading in the green?”. I would say most definitely yes!
If your amp meter and prop heat system is working properly, the needle will be in the green arc during all four cycles. If I am watching the amp meter and the needle indicates slightly below the green arc during one of the 20 second stages, this could mean that one of the pads is not heating up, which can lead to ice building up on that single prop heat pad and creating a prop imbalance and vibration. Have seen this (needle reading slightly below the green arc during one stage) dozens of times on clients airplanes.
Have also seen the needle spike fully to the right, followed by dual alternator failure. And know a pilot you had his prop heat needle spike fully to the right, it remained in that position (mine did not), he could not turn off the prop heat with the switch, his cabin filled with smoke, had to make an emergency descent, could not shut off the battery, bundles of wires were welded together, and the fire department had to cut the cables to the battery.
Bottom line – I want my prop heat amp meter to work properly.February 11, 2024 at 1:11 am #189560gsilver1Participant
I moved my ammeter to the pilot side a number of years ago. The ammeter is a shunt on the actual wires that supply the de-ice boots on the props. I would have a competent A&P with electrical background check that the wiring was extended to the pilot side using proper gauge wire (as I recall it is fairly heavy gauge wire, probably at least 16 gg). The current is actually flowing thru those wires en-route to the de-ice boots so it must be sized to carry the land. For the 421B the MM says the current should be 11-18 amps. Timer cycle should be 34 seconds +/- 6 seconds. Also, check the security of the screws on the back of the ammeter and perhaps burnish the shunt metal if there appears to be oxidation. Using a clamp ammeter verify that the current flowing to the prop de-ice boots matches what is called out in the Maintenance Manual. If everything checks except the ammeter being in range you can test the shunt and ammeter with a comparable electrical current and a clamp ammeter to calibrate. You definitely want to have it reading correctly. I believe this is a safety of flight issue.
Good luck, Gary S. Silver, M.D., A&P/IA, Owner for 28 years of N678DB Cessna 421B Farmington, UTFebruary 12, 2024 at 6:37 pm #189624rwelshParticipant
The original post did not say what model or year his plane was. I know how the 340 is wired but not sure of the other models.February 12, 2024 at 9:12 pm #189632
1982 421C. Have spoken to a very knowledgeable shop here in Texas. Sayes that even the smallest amount of corrosion can cause this. Am going to remove and clean gauge and then bench test This week. If that does not work preferred air parts has rebuilt ones for a descent price. Thanks for all the help..
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