From the 172R POH, Section 3: Emergency Procedures, Electrical Power Supply System Malfunctions:
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INSUFFICIENT RATE OF CHARGE
NOTE
The low voltage annunciator (VOLTS) may come on and
ammeter discharge indications may occur during low RPM
conditions with an electrical load on the system, such as
during a low RPM taxi. Under these conditions, the
annuciator will go off at higher RPM.
If the overvoltage sensor should shut down the alternator and trip
the alternator circuit breaker (ALT FLD), or if the alternator output is
low, a discharge rate will be shown on the ammeter followed by
illumination of the low voltage annunciator (VOLTS). Since this
may be a "nuisance" trip out, an attempt should be made to
reactivate the alternator system. To reactivate, set the avionics
master switch to the OFF position, check that the alternator circuit
breaker (ALT FLD) is in, then set both sides of the master switch to
the OFF position and then to the ON position. If the problem no
longer exists, normal alternator charging will resume and the low
voltage annunciator (VOLTS) will go off. The avionics master switch
may then be returned to the ON position.
If the annunciator illuminates again, a malfunction is confirmed.
In this event, the flight should be terminated and/or the current
drain on the battery minimized because the battery can supply the
electrical system for only a limited period of time. Battery power
must be conserved for later operation of the wing flaps and, if the
emergency occurs at night, for possible use of the landing lights
during landing.
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During our Thursday morning flight, the low voltage annunciator did not illuminate, although the entire annunciator inset flashed once before the ammeter started reading low. The discharge was not present just before takeoff, and takeoff and climb were performed at full throttle. I don't think the "NOTE" applies.
The ammeter was showing a discharge. The ALT FLD breaker was tripped. No failures or abnormalities were observed on any instruments or avionics using electrical power. I did not think to check the voltage on the voltage/OAT/timer instrument.
The mentions of discharging ammeter and tripped ALT FLD breaker in the POH all indicate that the Low Voltage annunciator should be on, flashing at first, then solid on until the problem is resolved, but it wasn't. The annunciator came on during preflight test, so the annunciator itself is capable of lighting.
How I wish I had had the POH on my iPad! A quick search would have brought me to the above section and proper procedure for dealing with what in all likelihood is a "nuisance" trip out. The proper procedure is basically to shut it all down and start from scratch:
1. Kill the avionics (the big power users).
2. Reset the breaker.
3. Kill both battery and alternator with the master switches.
---At this point, the circuits are open and electricity is not being delivered.
4. Turn the master (both sides) back on to connect battery and alternator.
--- Incrementally bring things back to life. Check for neither charge nor discharge and verify no annunciators and the breaker is in. At this point, what is receiving power? The turn coordinator, the voltage/OAT/timer, panel lighting according to knob setting, beacon/strobe/etc as per the switches, flaps when actuated, fuel pump when activated, and that's all I can think of.
5. Turn on the avionics master.
--- It could be a good idea to start with some of the items off. For Class C flying at KAUS, transponder and radio are required. If being conservative, I might turn off the GPS and COM/NAV2 and leave the autopilot off. (Note to self: investigate how to get back to KAUS without radio or transponder. An IFR clearance gets you your cleared route even in the event of lost comms. At this point in my training, I'd land at Smithville or Lockhart and call home for advice!)
This forum has some interesting information about the circuits (as well as a lot of hot air and attitude, sorry). And discussion of whether resetting a breaker in flight is a good idea. Resetting it repeatedly is obviously ill-advised, but the POH itself indicates this particular issue can be a nuisance rather than an emergency.
Takeaways:
- Be more thoughtful before rationalizing an abnormal indication.
- Have the POH in searchable form!!!!!
- Continue the flow/checks that identified the abnormal indication so promptly.
As for the fog in the turn coordinator, there are a number of reports of this in forums across the web and the recommendations are (1) do nothing, it'll be fine when the humidity goes down, or (2) moisture in an electrical instrument is not good, so repair or replace. It's not my plane, so I have to make a determination of whether I feel comfortable as PIC to continue using it. For now I think I will continue to fly 2MA, but will keep an eye on it.
