Narrative:

I was conducting an instrument training flight as a flight instructor with one other person onboard. Conditions along the route of flight were IMC; with ceilings variable between 500 ft and 4000 ft. Tops were being reported as variable between 17000 ft and 21000 ft. Upon reaching [our destination]; we flew an ILS to a touch and go; before turning back east bound and resuming our flight plan back to [the departure airport]. Our filed altitude on the way was 9000 ft; and in the climb-out through 8000 ft; we received an annunciator light for 'altitude inoperative' and 'lo bus' and also noted the ammeter had dropped to 0. We immediately ran the checklist and attempted to reset the alternator by switching it back on (per the checklist). This did not remedy the situation. I then advised approach of the situation and requested vectors back to [the airport]. Approach cleared us direct; but also offered that the nearest airport was ZZZ. I requested ZZZ and at began looking for approaches in electronic flight charts; but none were published. I asked ATC whether approaches were available; and they informed me that I have my choice of RNAV 18 or RNAV 36. The GPS in our aircraft had the approaches; and winds favored 36 so I requested the RNAV 36. [Approach control] acknowledged my [condition]; and cleared me direct to the IAF at 4000 ft. I proceeded direct to the fix and shortly thereafter advised [approach] that we were going to intercept the final approach course between the if and IAF instead. Without the approach being published; I requested altitudes from [approach control] who gave me the altitude between the if and the IAF as 3300; so I descended to 3300. During this time; I also switched off all lights; my second GPS; and dimmed the pfd/mfd to reduce the electrical load. Upon intercepting the course; [approach] told me to switch to CTAF and cancel IFR with flight service on the ground. I switched to CTAF and announced my position as an emergency aircraft. We descended out of 3300 MSL after the IAF and upon reaching roughly 800-900 AGL; we saw the ground and the runway threshold. At 600-700 AGL we were clear of clouds and continued visually to the runway. Shortly before landing; we saw a fire truck pulling onto the ramp. The landing was uneventful and safe; and we taxied over to the ramp where we were marshaled in. I cancelled with flight service after exiting the aircraft; then began coordinating with my company. The maintenance facility opened the engine cowling and found that the part of the pulley system connecting the belt to the alternator itself had sheared off in flight; so the belt was no longer connected to the alternator. This came as a surprise to me; as we didn't hear any unusual noises or feel any vibrations during any part of the flight; so my guess is that the structural failure occurred suddenly; though I have no way of confirming this for sure. Overall; I believe the situation was handled professionally and expeditiously by all parties involved; approach was prompt and helpful; and the services at ZZZ were timely and they took every precaution. I can't speak on behalf of the true cause of the structural failure that caused the emergency situation; nor did the nature of the cowling of the aircraft allow me to physically see and inspect the alternator components prior to flight; but I can say that I believe the situation was ultimately handled well and that there was no harm to persons; aircraft; or airport property as a result.

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Original NASA ASRS Text

Title: A Flight Instructor reported an alternator failure during an IFR training flight. The flight diverted and completed an uneventful landing.

Narrative: I was conducting an instrument training flight as a flight instructor with one other person onboard. Conditions along the route of flight were IMC; with ceilings variable between 500 ft and 4000 ft. Tops were being reported as variable between 17000 ft and 21000 ft. Upon reaching [our destination]; we flew an ILS to a touch and go; before turning back east bound and resuming our flight plan back to [the departure airport]. Our filed altitude on the way was 9000 ft; and in the climb-out through 8000 ft; we received an annunciator light for 'ALT INOP' and 'LO BUS' and also noted the ammeter had dropped to 0. We immediately ran the checklist and attempted to reset the alternator by switching it back on (per the checklist). This did not remedy the situation. I then advised Approach of the situation and requested vectors back to [the airport]. Approach cleared us direct; but also offered that the nearest airport was ZZZ. I requested ZZZ and at began looking for approaches in Electronic flight charts; but none were published. I asked ATC whether approaches were available; and they informed me that I have my choice of RNAV 18 or RNAV 36. The GPS in our aircraft had the approaches; and winds favored 36 so I requested the RNAV 36. [Approach Control] acknowledged my [condition]; and cleared me direct to the IAF at 4000 ft. I proceeded direct to the fix and shortly thereafter advised [Approach] that we were going to intercept the final approach course between the IF and IAF instead. Without the approach being published; I requested altitudes from [Approach Control] who gave me the altitude between the IF and the IAF as 3300; so I descended to 3300. During this time; I also switched off all lights; my second GPS; and dimmed the PFD/MFD to reduce the electrical load. Upon intercepting the course; [Approach] told me to switch to CTAF and cancel IFR with flight service on the ground. I switched to CTAF and announced my position as an emergency aircraft. We descended out of 3300 MSL after the IAF and upon reaching roughly 800-900 AGL; we saw the ground and the runway threshold. At 600-700 AGL we were clear of clouds and continued visually to the runway. Shortly before landing; we saw a fire truck pulling onto the ramp. The landing was uneventful and safe; and we taxied over to the ramp where we were marshaled in. I cancelled with flight service after exiting the aircraft; then began coordinating with my company. The maintenance facility opened the engine cowling and found that the part of the pulley system connecting the belt to the alternator itself had sheared off in flight; so the belt was no longer connected to the alternator. This came as a surprise to me; as we didn't hear any unusual noises or feel any vibrations during any part of the flight; so my guess is that the structural failure occurred suddenly; though I have no way of confirming this for sure. Overall; I believe the situation was handled professionally and expeditiously by all parties involved; approach was prompt and helpful; and the services at ZZZ were timely and they took every precaution. I can't speak on behalf of the true cause of the structural failure that caused the emergency situation; nor did the nature of the cowling of the aircraft allow me to physically see and inspect the alternator components prior to flight; but I can say that I believe the situation was ultimately handled well and that there was no harm to persons; aircraft; or airport property as a result.

Data retrieved from NASA's ASRS site 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.