C152 Low Voltage Light

[Okavango]

Hi. I'm low houred PPL and just had a flight this afternoon in a C152 where the Low Voltage Light came on during cruise. I didn't have any services on and as was at cruise power and I'd never seen this happen during training elected to cut my flight short and returned to base. I've never seen the ammeter do much before, but it was pulsing to negative then returning to centre each time. What fault (if any) is there likely to be?

[smo-kin-hole]

The voltage regulator in the alternator might be failing or perhaps the external drive belt. Either way, you were losing the alternator. You will then slowly run down the battery, the radios will fade, and finally your flaps won't work. You can turn the master off and run the plane just fine, saving the battery. Beware of partially functioning flaps, they might not go to 30 degrees and they won't come back up if the battery is too weak.

You made the right decision in quitting the flight.

[BabyBear]

This is a common issue which can often be resolved, in the short term, by recycling the alternator side of the master switch.

[smarthawke]

Was it the 'alternator failure' light (normally placarded 'low volt' on the instrument panel) or a light with 'low volt' on the lens or similar?

The former is fitted by Cessna as standard equipment and is actually an alternator failure light which may be due to an over volt situation (causing the alternator control unit to go off line) or an actual alternator failure. These are normally on the right hand side of the panel out of the direct line of sight of the pilot...

The latter is a UK requirement and actually measures busbar voltage rather than alternator status. Once the alternator output fails, it doesn't take long for any service (beacon, radios etc) to drop the battery voltage below that at which the low volt light will illuminate.

If it was a transient fault then recycling the 'alt' part of the battery master switch may bring the alternator back on line.

You did the right thing by landing. The pulsing of the ammeter (showing a discharge in this case) may be due to the anti-collision beacon - they don't show anything much at the best of times. The minimum in the UK for battery life is 30 minutes worst case scenario - IMC and night. Best to reduce the electrical load as much as possible and land as soon as practicable.

The fault could be the alternator control unit (remote from the alternator and a solid state voltage regulator/over volt relay), the alternator itself or something in the wiring. There is a fair bit about the C152 electrical system in the POH and their should be a supplement to cover the separate low volt light.

[Unusual Attitude]

Had something similar several years ago flying a C172 from Perth to Aberdeen at night, not long after take-off the low volt light came on and no amount of re-cycling / load shedding would put it out.
Fortunately only a short trip and there was enough juice left to power the flaps etc. Seem to remember it was caused by one or two of the alternator phases packing up hence the 'pulsing' of the light.

[SNS3Guppy]

The light indicates that the battery is providing the charge, and not the alternator. The alternator as been taken offline, whether for high or low voltage isn't really a concern; it's simply been taken offline.

The 152 does have separate procedures for a high charge rate vs. a failed alternator, however. In this case, the discharge light is one and the ammeter is showing a discharge, one may consider it a failed alternator and proceed with the low voltage light checklist.

The regulator isn't in the alternator; it's a separate box attached to the firewall. Failure of the regulator isn't uncommon, but then the same may be said of the alternator. An alternator belt may also have failed. I've had the pully come right off in flight before. Checking it should be part of every preflight.

The procedure for the low voltage light in the airplane will be spelled out in the emergency checklist, and should also be found in emergency procedures in the Pilot Operating Handbook. It's not really much of an emergency,but more of an abnormal situation.

The procedure requires turning off the avionics, checking that the alternator circuit breaker is in, then turning off both sides of the master switch. Turn the master switch back on (one side is the battery master, the other is the alternator), and check the ammeter to see that it's either neutral or that it's taking a load. If the low voltage light is out and the ammeter shows that the electrical system is restored, then turn on the avionics and continue. Otherwise, turn the alternator off, reduce electrical load and land as soon as practicable.

[stevef]

Likely cause is a broken field wire terminal at the alternator. An easy fix, if so.