"Low Voltage Warning"

From Bill Bell, Redtail 28

(10/29/05)

First printed from AOPA ePilot

Training Tip from Redtail 28 -- LOW VOLTAGE WARNING

You are in cruise flight on a daytime cross-country when a warning on the panel catches your eye. Electrical power is being discharged and is not being replenished by the alternator. In some aircraft, you are alerted by a low-voltage light and the ammeter showing a discharge. In others, the total load on the alternator is displayed on the ammeter; alternator failure is indicated by a zero load. Meanwhile, the airplane is flying along strongly, thanks to the dual magnetos that are independent of the electrical system.

As the pilot, you have some decisions to make. "If the alternator has failed, must you land immediately? That depends on your situation, but for visual daytime flying the answer is 'not necessarily.' Battery life is the primary concern," Ralph Butcher explained in his "Insights" column in the May 2001 AOPA Flight Training.

Study your aircraft's pilot's operating handbook (POH) checklist for handling low-voltage occurrences. For example, alternator failure in a Piper PA-28-140E is detected by a zero reading on the ammeter. The pilot verifies the problem by activating an electrically powered device such as the landing light. If no increase in ammeter reading appears, the POH gives these steps: Reduce electrical load, check alternator circuit breakers, switch off the "alt" switch (in many aircraft this switch is half of the split-type master switch), then turn it back on after a brief interval. If the alternator remains offline, "maintain minimum electrical load and land as soon as practical. All electrical load is being supplied by the battery." Note the Butcher article's suggestion for protecting avionics from voltage spikes during this procedure.

How the pilot manages this load-shedding chore will determine whether electrical power is available for communication and navigation until landing, writes Steven W. Ells in his "Airframe and Powerplant" feature in the April 2003 AOPA Pilot.

On a related subject, generators once were the mainstay of light-aircraft electrical systems. They have been supplanted by alternators, which are generally lighter and more reliable over a broader engine-power range. The systems are compared in Chapter 5 of the Pilot's Handbook of Aeronautical Knowledge. Know what's installed in your aircraft, and have a plan in case the bank account that is your electrical-power supply ever threatens to become overdrawn.