When attempting to get a twin engine aircraft charging system to parallel correctly the previously described test will come in very handy. One of the most common causes for aircraft charging systems to not share the load is an imbalance in the performance of the alternators or generators. The full field test allows the service technician to determine if these units will output relatively close to the same under the same conditions. It is necessary to full field each unit and compare their out- put at the same rpm. Running the engines at 2000 rpm will provide enough speed for full output and give the technician a very good idea if they are close to equal.

If the aircraft in question is equipped with amp meters these can be used to monitor the output during the full field phase of this test. If cockpit amp meters are not available the technician should use an inductance (clamp around) type amp meter. This should be connected around the main output wire at each alternator during the engine run-up. At 2000 rpm the alternator outputs should be within 10 %.

After determining that the alternators are capable of providing very close to the same amount of amperage, it is time to adjust the paralleling feature using the voltage regulators. First start one engine. Under a no load condition adjust the voltage to whatever voltage is called for in the aircrafts maintenance manual. Now shut down this engine and start the other. As before, without load, adjust this side to exactly the same voltage. Next start both engines and bring them to 2000 rpm. Turn on the avionics, landing light and pitot heat. Monitor the amp meters. They should now be very close to sharing the load. If not, make final tweak changes at the lower of the two regulators.

Most paralleling problems are caused by either a severe unbalance in the alternators or defective or out of adjustment regulators. Following the test procedure in the manner outlined above should bring the charging system back into parallel. If not feel free to call us at our toll free number (1-800-634-0190) to seek additional help.

John and Mike Evans