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PostPosted: Tue Dec 12, 2017 12:40 am 
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Airman Basic

Joined: Mon Dec 11, 2017 8:23 pm
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I absolutely love the Cessna 172 and have a few questions first is the alternate static air system and if so how would I operate this system in the unlikely event I needed to use it. Also are the circuit breakers simulated?

Sincerely-Evan


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PostPosted: Tue Dec 12, 2017 7:44 am 
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Chief Master Sergeant
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Joined: Mon Sep 02, 2013 7:30 pm
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
The alt static air system is modeled and is operated by pulling the red alt static air knob out.

The circuit breakers sometimes do pop sip if you notice an inoperative electrical system, check the breakers as part of your troubleshooting.

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PostPosted: Wed Dec 13, 2017 10:59 pm 
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Joined: Tue Feb 01, 2011 11:21 am
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Would be great if you could overvolt the avionics :)


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PostPosted: Thu Dec 14, 2017 9:59 am 
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A2A Mechanic
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Caldemeyn wrote:
Would be great if you could overvolt the avionics :)
It would be rather unlikely scenario in modernish airplane, for the generator control is pretty robust. In case of 172R, you'd have the regular ACU controlling the voltage to 28.5 volts approximately, and a separate over-voltage protection function that opens the alternator field breaker at 31.75±0.35 volts. Of course, anything can malfunction, though. Fuses/circuit breakers don't protect from over-voltages on their own.

-Esa


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PostPosted: Thu Dec 14, 2017 4:49 pm 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
It happened to me last year, twice in the 172S. Fried the transponder and COM1.

We had to replace a bad alternator and voltage regulator. Only took one time and the field breaker did trip, but it read too late. The transponder and radio failures were difficult to get diagnosed and repaired as they were intermittent.

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Last edited by Oracle427 on Fri Dec 15, 2017 6:46 am, edited 1 time in total.

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PostPosted: Fri Dec 15, 2017 1:41 am 
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Oracle427 wrote:
It happened to me last year, twice in the 172S. Fried the transponder and COM1.
That's interesting to learn, I recall no that kind of issues at all with junction box style regulators newer Cessnas use. Many avionics units have input voltage rating up to 33.0 V, so functioning overvoltage protection should help to protect these somewhat.

Do you recall what was the cause of overvoltage (or if overvoltage indeed was the cause of the occurrence) or if it was ever identified?

-Esa


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PostPosted: Fri Dec 15, 2017 6:46 am 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
I believe the alternator was found to have an internal short. I'm not sure if that should result in damage to a voltage regulator circuit.

The junction box sitting on the firewall had to get swapped out too as we continued to get intermittent alerts until it was replaced.

The transponder and Com1 issues began to manifest shortly after with ATC telling us we were only a primary contact. The radio would work fine and then suddenly cut out. Most annoyingly, they would both work well in the shop. It took a few tries before they finally misbehaved in the shop.

As i recall the transponder was missing some frames in the reply and Com1 had a carrier but no modulation with a very high power draw.

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PostPosted: Fri Dec 15, 2017 9:54 am 
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Oracle427 wrote:
I believe the alternator was found to have an internal short. I'm not sure if that should result in damage to a voltage regulator circuit.
Ah, short in the alternator winding indeed is a candidate, those can and do occur. A big short in the field winding, which in alternator-style generator is rotating and therefore more prone to develop mechanical damage, would generally trip the field breaker. Regulator does have protection for excess field current.

Generally, transients which occur in such circumstances should not damage the equipment (or the other way: the equipment should withstand such transients), but with bad luck, bad things still happen.

Fried equipment due to regulating failure causing an overvoltage would be relatively rare in my guesswork. In my speak, an overvoltage is a steady, or semi-steady state for more than, say, a single generator turn. Or for some fraction of a second, whatever.

Voltage spikes and other transients that occur because of some equipment failure or normal switching events (due to inductance in the electrical system) are a little bit different issue. They can have very high peak voltages, but due to their short duration (milliseconds at most), usually carry very little energy. Modern avionics, in general, have their own internal power electronics that regulate the box's internal voltage levels. This is why separate products for 12 V and 24 V are usually not required nowadays. These power electronics should be able to deal with any transient spikes before they reach any of the truly overvoltage-sensitive components.

Yours is an interesting case, which to me appears to be an alternator failure, perhaps releasing some 'ghosts' in the other equipment.

-Esa


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