Maintenance Corner

Since my area of expertise is electrical stuff, the first few articles will be on some of what I’ve found keeping our 1962 Cessna 175B flying, and what I’ve found helping others fix electrical problems.

The 30+ years that I’ve worked as an electrical engineer helps me analyze and deductively analyze and fix electrical problems without ‘shot-gunning’, saving a lot of money and time. Also, this is an open invitation to other members who want to write an article that will be of interest to our member pilots.
So first, some Quick Notes :

  • With the cold weather, check your tire pressures. Per Goodyear, our tires lose 1-2 psi for a 10 deg F Drop. So, when we went from summer temps of 90deg F, to 40 deg F, a change to 50 degrees, your tires have lost up to 10 psi, enough to give you a ‘pinch flat’ with the sidewalls bulging.
  • With the ‘lots of darkness’ we have this time of year, if you use a PAR36 landing light, consider using the Q4509, the 100 hr quartz version of the 4509 tungsten lamp, which actually only lasts about 10 hrs, running on 14.2 volts. Also, the 4595 as a taxi light is a great choice. At 300 hrs life and a wide beam, it works great. I use both and they’re still alive after 3 yrs and 200+ hrs!
    (NorCal Flight Center has these in stock!)

Have You Pre-flighted Your Battery?
With pilots flying with more electronics and relying on them more, we are depending on the electrical system to keep all of this stuff alive. The ‘heart’ of the electrical system is the battery. Since most of us fly with a 12 volt, lead-acid battery, that’s what this will cover.
We’ll look at the other parts in future articles.

Quick notes – an aircraft battery usually has a life of 3-5 years. If yours is older than that, it’s probably lost much of its amp-hr capacity, and may not be airworthy.!
BTW -The lead acid battery was invented in 1859 by French physicist Gaston Planté. So it’s 156 year old technology and still works well.!

Is your battery safe to fly?
There are 2 main tests for a battery – state of charge, and reserve capacity.

State of charge is the percent of its stated amp-hour capacity, and is easily measured as its voltage.
A fully charged ’12 volt’ battery will measure 13.0 volts at 100% state of charge . A battery that has ‘0’ charge, will measure 12.0 volts or less. A ‘rule of thumb is to use the ‘tenths’ part of the voltage to indicate the charge. So, at 12.7 volts, a battery is at about 70% state of charge. 12.4 volts is 40%, and so on.

Measuring the battery voltage must be done with an ‘open circuit’, that is, with no load on the battery and after 2-3 hours of being charged. You can’t just switch on the master and read the voltage on your panel, assuming that you have a voltmeter. The master contactor and panel instruments will put a few amps of load on the battery, and you’ll get an inaccurate, lower voltage reading.

You’ll need to use a digital multimeter. Inexpensive meters are available on Amazon. Use the DC volts, 20 volt setting. Get access to the battery and touch the meter probes to each battery terminal. If you get a reading of 12.7 volts or more, you’re good-to-go. If the voltage is less than 12.7, the battery need a recharge. Connect a charger with 2 amp capacity and charge it until the voltage reaches 14.5 volts, about 3-4 hours.

After engine start, monitoring the battery/bus voltage is important to see that the battery is being recharged properly and that your alternator is supplying the loads ok. But, most certified aircraft don’t have a volt meter. The FAA specifies an ammeter, or load meter, as on a Piper. If you don’t have a voltmeter, you can buy a small digital meter that plugs into the ‘cigar’ lighter socket. If you don’t have a 12 volt socket, you can easily install one, and plug in one of these small digital meters. I use one like this.

So, after engine start, you should see a positive charge rate on the ammeter, and the voltage climbing to 14.2-14.5 volts within a few seconds. We’ll look at ‘what if it’s not showing a positive charge and/or the voltage is not rising to 14+ volts quickly ’, in a future article.

The key to a fully charged battery is the voltage regulator, which controls the alternator output voltage and limits the maximum current. It should be set to 14.2 to 14.5 volts, with the engine running at about 1,000 rpm and normal loads.

Most voltage regulators are adjustable and it only takes a few minutes to make the adjustment. But it takes 2 people to safely do this safely. If yours is not adjustable and you’re not getting at least 14.0 volts, there may be other problems; the wiring, alternator, or the battery itself. A shorted cell will cause a loss of 2.2 volts, and the battery will not charge above 12 volts. More on the rest of the electrical system in future articles.

Reserve capacity is how many amp-hours your battery is capable of supplying to the loads. This requires an easy test, but you’ll have to recharge the battery immediately after this test before you fly. Your battery has a stated amp-hour capacity, defined as the load current it will supply for 20 hours, to a point where the voltage is down to 10.0 volts. So, a 20 amp-hour battery will supply a 1 amp load for 20 hours. But because of internal resistance, higher currents will cause some of this energy to be lost as heating of the battery. So, a 10 amp load won’t be sustained for 2 hours.

The test, without damaging the life of your battery. This is for a new Concorde AGM battery (BTW, this is all they sell now):

  1. After a one-hour rest from initial charge per the instructions, 13.0 volts.
  2. Check with landing light 100 watt, (10 amps) for 2 minutes.
  3. With load in place: 12.4 volts.
  4. After removing load: 12.8 volts.
  5. After a 2-hour recovery: 12.9 volts.

If the voltage doesn’t get back to at least 12.6 volts, the battery has lost enough capacity to not be airworthy.

What kills a battery?

  • A low state of charge. If a battery is left for days, with a voltage of less than 12.5 volts, it will sulfate and permanently lose capacity.
  • Leaving the master on, or other load that discharges the battery below 12.5 volts.

Note – never jump-start a dead battery, or hand prop the engine to start with a dead battery [unless it’s an emergency].

If you do a jumpstart, 3 bad things can happen:

  1. Once the engine is running, the alternator will recharge the battery at its max capacity , 50-80 amps, which will overheat the battery and can cause the plates to warp and short a cell.
  2. The alternator is working at max load, trying to recharge the dead battery and supply all the loads too, so it can overheat and damage the diodes, or bearings
  3. If you do fly with a dead battery and the alternator fails, (see above), the battery is at a very low state of charge and may only supply power for a few minutes. If you’re flying a retractable gear aircraft, when you select the ‘gear up’, the battery has no capacity to supply the high current and the alternator won’t either, so it may trip off, leaving you with a total loss of power, of your doing.

What to do? Connect a battery charger designed for aircraft batteries, or one that will limit the charge to 10% of the rated capacity [ 2 amps for a 20 amp-hour battery], and let it recharge for 10 hours. Be sure that the charger will limit the voltage to 14.5 volts. Most cheap chargers can kill a battery with excessive voltage, over 15 volts. But if you monitor the voltage and keep it below 14.5 it’ll be ok.

So, take care of your battery and you’ll get a full, useful life from it, and it won’t let you down when you need to rely on it, beyond cranking your engine to life!

I’d be happy to answer any questions that you have on electrical issues and will help troubleshoot problems that you may have. Email me.

For more info, see the following references.