Maintenance Corner

My intention with this series of articles, is to give you pilots/ aircraft owners some useful, practical information that you can use to test critical components and systems and keep your aircraft reliable and airworthy.

So, from last month, did you preflight your battery? Get a multimeter ? Install a voltmeter ? (if not, we’ll have 3-4 of these at the Poker Run raffle)

So now you should know how to use these, run the tests, and are sure your battery is good-to-go!

This month we’ll move on to Regulating Your Voltage, and keeping your avionics alive and working well.

Before all the nitty-gritty, let’s look at how the 2 important parts work together; the alternator and voltage regulator.

These 2 parts form a ‘closed loop control system’ that maintains the buss voltage at the set point, which should be 14.2-14.4 volts, over the range of minimum to maximum loads . If it’s not at these voltages, adjust the voltage regulator.

The alternator is an electro-mechanical amplifier. The output voltage and current depend on the engine rpm and the voltage applied to the field winding by the voltage regulator.

The Voltage regulator monitors the voltage on the main buss and adjusts it by controlling the average voltage applied to the alternator field winding. The rpm determines the ‘amplifier multiplier’ -the ratio of the field current to the output current. (3 amps into the field will yield 60 amps from the armature, at the critical rpm of about 1,000 rpm, which we’ll test next.)

Engine running, alternator/ voltage regulator check
This is a very important test to run to check the ability of your alternator system to supply your maximum loads !
This test is valid for all aircraft with an alternator or generator, whether it’s certified, experimental, using pma or non-pma parts…!! Running this test on DC generator equipped aircraft will probably show the ‘cut-in’ rpm will be in the 1,500 rpm range, or higher.

  • Start your engine and note the buss voltage, [ 14.2 -14.5 V]. It should rise to a stable voltage once the ammeter is close to ‘0’ charge, 1-2 minutes.
  • Run the engine until it will idle, 500-600 rpm.
  • Verify voltage is still close to the voltage above.
  • Turn on max loads – landing light, pitot heat, position, beacon lights, to get about 20 amps or more discharge.
  • The alternator will not be able to supply the loads at this rpm. So, the voltage should drop to about 13 volts, or less.
  • Note the voltage. If it’s in the 13 volts range , the ammeter should be showing a discharge.
  • Now, slowly increase the engine rpm until the voltage rises to 14.0 volts , and the ammeter is showing a charge, or is at least at ‘0’ (positive = needle right of the center ‘0’ marking.) You can repeat this step a few times to get an accurate rpm.
  • This rpm should be less than 1,000 rpm. Write this rpm down for future tests and the loads applied.
  • If it is much above 1,000 rpm, 1,500 or more, turn off a load – pitot heat or landing light.
  • If the voltage is still below 14 volts, or the set point, there is an issue with the charging system and trouble shooting is needed to determine the cause.

(my Cessna with a 50 amp alternator; 900 rpm =14V with beacon, LL, pos lts , radios, transponder, pitot heat. Without the pitot heat, 750 rpm = 14V.)
If it’s not right – troubleshooting. we’ll cover this next month
Inspections and tests

  • Belt tension and condition (Lycoming)
  • all connections – alt, Vreg, alt contactor, current shunt, alt sw, ckt brkr, buss
  • tests; chk voltages, at alt fld term. Alt sw, contactor (if installed),

What to do if you have an electrical failure?

  • cycle the alt switch
  • check that the field and alt. breakers are ‘IN’.
  • If no joy, turn off big loads – lights (LL, pos lts, beacon, transponder)

Leave the radios on. They only pull ½ amp each on ‘receive’. Your battery can run these for 8 hours or more. Transmit as briefly as possible ; 5+ amps in ‘transmit’.
Next month we’ll diagnose what’s failed and determine whether to repair vs rebuild vs replace , which is A&P work, and which of those 3 ‘words’ you use matters.!

Why you should use an alternator vs that old generator
Note: most aftermarket alternators are less reliable than the 3 original equipment alternators; Ford type, Chrysler and Prestolite. The oem alternators will run 3,000 hours, while many of the aftermarket units are failing at 200 to 900 hours..!!
The advantages of using an alternator;

  • Capacity of 50 to 100+ amps at 14 volts
    Note: select an alternator so that you only run it at less than 60% of it’s rated capacity. [ 30 amps for a 50 amp unit]. excessive heating will kill an alt and it’s diodes.
  • reliability (after market alt are less reliable that OE alts.)
  • Life – alternator, about 3,000 hrs., vs a generator, about 1,000 hrs.
    (repairs usually means only replacing the brushes)
  • low rpm output – full output at 1,000 rpm or less. Most generators require 1,500 rpm or more.
  • Noise – switching a 5 amp field circuit vs 30-50 amps of generator armature current.
  • Weight – a few pounds less for an alternator.