Narrative:

We were on a dual cross-country training flight where I was the instructor. This was the first time I was flying with this particular student. We had planned and were executing a cross-country flight; a distance of approximately 55 NM out and then return. As we were descending to enter the traffic pattern at the turn around airport; I noticed the radio lights flicker momentarily. I didn't think much about it immediately; but a few seconds later I happened to look over toward the student; and then I noticed the low voltage light was on; and the ammeter was showing a discharge from the battery. I informed the student of what I saw; and we then noticed that the alternator field circuit breaker had popped open. We decided the student would continue flying the aircraft while I did some troubleshooting. I explained that we were going to shut off all the electronics; reset the master switch; reset the breaker; and then turn on the electronics again. We did so; but the breaker popped again; and the discharge was still occurring. At this point we were on final approach; and so we elected to do a touch-and-go and continue back to our home airport where repairs could be made. I felt fairly certain that we noticed the alternator failure as soon as it occurred; and thus we should have plenty of battery power to make it back without needing to operate with the transponder; radios; and lights inoperative for more than fifteen minutes. The student continued to fly the aircraft; and I asked him to locate and execute the appropriate electrical system failure checklist. The checklist is essentially a repeat of all the steps we had already taken; but we followed it; still to no avail. I then explained my plan of action and solicited any comments/feedback. We decided that we would fly approximately halfway back with the electrical system completely shut down. At a particular VFR checkpoint that was already on our navigation log; we would turn on the battery power and continue with radios and transponder on (no lights though); and then turn to enter the class east airspace that is inside the 30 NM mode-C veil around class B airspace. At that point; we planned to contact the TRACON; declare an emergency; and ask for flight following back to our home airport. We felt confident we could navigate without entering the class B airspace. Upon declaring an emergency; we of course offered the typical information: nature of emergency; number of souls; fuel remaining; etc. The controller offered us other nearby airports for landing; but we felt the best course of action was to continue to our planned destination. We were cleared to enter the class B airspace as needed; and though we did alter our course slightly in that direction; I don't believe we did enter the class B. We reported the destination airport in sight when we were approximately 5-6 NM away. We were handed off to the tower and cleared to land (we decided not to use any flaps). The tower asked if we needed any assistance from the emergency vehicles and personnel that were standing by; and we declined. Overall I am pleased with how all the events unfolded and played out. We noticed the failure very shortly after it occurred (we're not sure at the moment what may have caused it). We followed recommended corrective procedures/checklists. We did not perceive; and I believe rightly so; any extreme threats. We communicated well with each other and followed reasonable CRM practices. I did think twice before declaring an emergency; as I wasn't immediately sure if I should. However over the last few years; I've read and heard so many stories from people who; in retrospect; decided they should have done so; that I decided it was probably the best course of action; especially considering there was a real possibility; however small; and that we could have lost all electrical power. I would have liked to have had more clear communications with TRACON; but we did limit our transmissions in order to conserve battery power. If these circumstances were to repeat; I can't think of anything that I would have done much differently.

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Original NASA ASRS Text

Title: A C172 Instructor noticed an electrical system anomaly caused by a generator failure so power was shutdown to conserve the battery until approaching their destination where they successfully communicated with ATC near Class B airspace.

Narrative: We were on a dual cross-country training flight where I was the Instructor. This was the first time I was flying with this particular student. We had planned and were executing a cross-country flight; a distance of approximately 55 NM out and then return. As we were descending to enter the traffic pattern at the turn around airport; I noticed the radio lights flicker momentarily. I didn't think much about it immediately; but a few seconds later I happened to look over toward the student; and then I noticed the low voltage light was on; and the ammeter was showing a discharge from the battery. I informed the student of what I saw; and we then noticed that the alternator field circuit breaker had popped open. We decided the student would continue flying the aircraft while I did some troubleshooting. I explained that we were going to shut off all the electronics; reset the master switch; reset the breaker; and then turn on the electronics again. We did so; but the breaker popped again; and the discharge was still occurring. At this point we were on final approach; and so we elected to do a touch-and-go and continue back to our home airport where repairs could be made. I felt fairly certain that we noticed the alternator failure as soon as it occurred; and thus we should have plenty of battery power to make it back without needing to operate with the transponder; radios; and lights inoperative for more than fifteen minutes. The student continued to fly the aircraft; and I asked him to locate and execute the appropriate electrical system failure checklist. The checklist is essentially a repeat of all the steps we had already taken; but we followed it; still to no avail. I then explained my plan of action and solicited any comments/feedback. We decided that we would fly approximately halfway back with the electrical system completely shut down. At a particular VFR checkpoint that was already on our navigation log; we would turn on the battery power and continue with radios and transponder on (no lights though); and then turn to enter the Class E airspace that is inside the 30 NM Mode-C veil around Class B airspace. At that point; we planned to contact the TRACON; declare an emergency; and ask for flight following back to our home airport. We felt confident we could navigate without entering the Class B airspace. Upon declaring an emergency; we of course offered the typical information: nature of emergency; number of souls; fuel remaining; etc. The Controller offered us other nearby airports for landing; but we felt the best course of action was to continue to our planned destination. We were cleared to enter the Class B airspace as needed; and though we did alter our course slightly in that direction; I don't believe we did enter the Class B. We reported the destination airport in sight when we were approximately 5-6 NM away. We were handed off to the Tower and cleared to land (we decided not to use any flaps). The Tower asked if we needed any assistance from the emergency vehicles and personnel that were standing by; and we declined. Overall I am pleased with how all the events unfolded and played out. We noticed the failure very shortly after it occurred (we're not sure at the moment what may have caused it). We followed recommended corrective procedures/checklists. We did not perceive; and I believe rightly so; any extreme threats. We communicated well with each other and followed reasonable CRM practices. I did think twice before declaring an emergency; as I wasn't immediately sure if I should. However over the last few years; I've read and heard so many stories from people who; in retrospect; decided they should have done so; that I decided it was probably the best course of action; especially considering there was a real possibility; however small; and that we could have lost all electrical power. I would have liked to have had more clear communications with TRACON; but we did limit our transmissions in order to conserve battery power. If these circumstances were to repeat; I can't think of anything that I would have done much differently.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.