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For a PPL, if the alternator goes out and I'm running on battery power, the typical advice is turn off what you can to conserve power. But what exactly is safe/unsafe, or order in which to turn off equipment?

I fly a Piper Cherokee. So in that case, I'm thinking the order in which I can do things is as follows:

  1. Call out that I've had an alternator failure, either to traffic or ATC.
  2. Turn off lights
  3. Turn off avionics
  4. Turn off pitot heat (Technically where I fly, it's blazing hot and we never need pitot heat, but I'm listing it as number 4 as it's more of a flight functionality/safety issue)
  5. Fuel pump?

My concern is #5. Cherokee is a low wing and when we come into land, we always keep the fuel pump on. So I'm guessing it's never okay to turn off the fuel pump and just fly the plane. However much time I get with the fuel pump, I'll take.

Once all power is gone, then I can proceed with power-off landing procedures.

Is this the right way to think about it?

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    $\begingroup$ A comm receiver needs lots less power than transmitting. Depending on circumstances, keeping a comm receiver & your transponder powered might be useful. Don't play 20 Questions with ATC, though -- that can drain your battery quickly. $\endgroup$
    – Ralph J ♦
    May 17, 2022 at 20:18
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    $\begingroup$ See also this thread for discussion of the mechanical fuel pump in the PA-28: aviation.stackexchange.com/q/79542/7532 Just losing the electric pump won't starve the engine of fuel. $\endgroup$
    – Ralph J ♦
    May 17, 2022 at 20:21
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    $\begingroup$ The fuel pump should be turned off on the climb check. Turn it off, it is only used as a backup in case the engine-drive fuel pump fails. $\endgroup$
    – wbeard52
    May 18, 2022 at 4:42

3 Answers 3

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I experienced this once in a night IFR training flight in VMC. The aircraft was a Cessna 172RG Cutlass. I was on an ILS when I noticed the low voltage light on. I decided to wait until missed approach to troubleshoot.

As I went missed, I checked the low voltage light again at full throttle. It was still on. After leveling off, I notified my CFI of the problem. We pulled the checklist and attempted to reset the alternator. No improvement. So we notified ATC of the issue and asked to return to our home airport, not far away. Then we tried to reduce electrical load.

There was not an awful lot we could turn off. We were in airspace underlying Class B, so we didn't want to turn off any nav lights, beacon, or strobes. Nor did we want to turn off the transponder. Although the plane had an auxiliary fuel pump, it was not used in normal conditions and we did not have it on. We did turn off unneeded avionics, such as the second nav/comm, DME, and ADF. We also dimmed down the panel lights and used red-lens flashlights to read the instruments.

Another factor is that on the missed approach, I actually forgot to retract the landing gear. This was a good thing, since the landing gear uses an electrically-driven hydraulic pump. Retracting the gear would have consumed a lot of power, and we might not have had enough left to get it down again. Manual extension would have worked, but I am glad it was not necessary.

We had no difficulties landing, though the radio was getting a bit crackly by then. I can't remember if we extended the (electric) flaps. Odd thing though, when I pulled the throttle to idle in the flare, the alternator came back online. Strange.

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Any advice you get is situational and what is discussed could not be the best advice in a different scenario.

There is an emergency procedure in the PA28 checklist that covers electrical failures. I would run that checklist and I agree it says to reduce electrical loads

If I am in day VFR in class G or E airspace (United States). I would turn off all lights, radios, transponder, pitot heat, fuel pump, etc. As I got towards my landing airport, I would turn on the radios and self-announce my intentions.

If I am in class C or D airspace or already talking with approach control, I would let ATC know and keep one comm radio on for communications.

If I was at night, I would keep on my position lights but most likely turn off my strobe lights. I would also land at the nearest suitable airfield.

Reading about the systems in the POH, the PA28 uses roughly 30 amperes to power nighttime equipment with radios on. Perhaps, someone with knowledge of a 12-volt battery in a Cherokee could provide a suitable operational time in that condition.

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One thing that helps to inform a decision about how much load to shed when losing an alternator is to understand how much battery capacity your plane has. If you know your battery can power your equipment for a predictable amount of time, you can keep equipment running improving safety, communications and situational awareness.

Each year during my annual, I run a battery test capturing data from my G3X Touch PFD. I run this test with the full panel on, fuel pump off (high wing plane) and external lights off.

From my data, I estimate that my primary battery (yellow trace) will easily last an hour powering my full panel including radios, GPS IFR navigator, and transponder. The backup battery (blue trace) is not powering the preceding three items, but does power the non-IFR GPS in the G3X, will last for 6 hours - far longer than my fuel.

Primary and Backup Battery Under Load

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  • $\begingroup$ My school’s C172S POH promised 45 mins on the main battery, plus another 30 minutes for avionics on the standby battery. I never tested them to confirm, but it did make me think about the problem differently. $\endgroup$
    – StephenS
    Nov 18, 2022 at 21:47

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