Narrative:

I was conducting a flight of a patient and her spouse. We were on an IFR plan with initial northerly vectors out of the class B. We were given direct to a fix then on course; having just leveled at 10;000 and having just exited the class B. I lost all audio in the headsets. I switched the intercom to the emergency setting and regained audio. I attempted to contact ATC to advise. I saw the transmit symbol come active on comm 1 when I keyed the mike but heard no side tone and received no response. I then smelled something burning - smelled like phenolic; such as when a pc board is melting. I blind broadcasted my intention to divert to the nearest airport - and my intention to commence an emergency descent. Within 5 seconds of the transmission attempts smoke began to enter the cockpit. I shut down all avionics immediately. At this point I thought I was going to be dealing with a cockpit fire. I unlatched the extinguisher between the seats and increased the descent rate to maximum. I took off the headsets and briefed the two passengers; advising that the aircraft engine was fine and that we would be making a rapid landing and that while the rate of descent would be uncomfortable; we would be ok. If I had possessed a functioning radio at this point; I would have declared an emergency. I reached ZZZ still at about 7;000 ft and commenced a steep spiral over the approach point. The smoke seemed to have stopped; but I decided to consider the likelihood of a fire to be imminent and continued the rapid descent. I had a bit of time so I retrieved a handheld comm and attempted to broadcast on the ATC frequency; and then on the CTAF. I heard no response on either. I had a good opportunity to observe traffic as I descended. There was one C172 in the pattern. About 2;500 ft I elected to swing out to perform a normal downwind pattern entry. This was completed without event and the landing was normal. I turned off at the first taxiway; got clear of the active and shut the aircraft down. We immediately exited the aircraft. There was no sign of smoke or further trouble. I retrieved a handheld comm and attempted to raise approach. No luck with that. I was assisted by the good people at the FBO on the field. They led me to a phone and I phoned the number for ATC. I spoke with a controller and I informed her that we had to divert and that we were safely on the ground. I believe this call was made within 10 minutes of the landing. Things I learned - the intercom panel had a minute of abnormal operation en route on the first leg. I advised ATC of this issue. The panel then resumed normal operation. I checked it out on the ground and it was ok. In retrospect I should have been more suspicious of that component. It functioned during the climb and for the first couple of minutes of cruise. I then heard a rhythmic thumping that I thought was the avionics fan failing. I now believe it was the intercom panel dying and the thumping was probably something I heard through the headsets. It was a low frequency sound and it seemed to be coming from everywhere at once. When the intercom panel ceased operating normally; I should have elected to divert. This would have gotten us on the ground 3 or 4 minutes earlier and perhaps would have allowed me to communicate with ATC. Also; although I had a handheld comm in the front of the plane; I did not have the jacks allowing it to be used with the headset. I attempted to use the comm without the headset. I do not know if any of my transmissions were received because I could not hear any response. I think it would be advisable to simply leave the converter jacks plugged into the handheld comm at all times. I think it would also be advisable to have an antenna jack installed in the aircraft that would allow the handheld to use the aircraft antenna. An avionics tech on the field pulled the stack and I have learned that the problem did indeed lie in the intercom panel. The power supply failed and melted part of the board. This was the origin of the acrid smoke. The cooling fan was fine. Once the avionics power was cut there was likely little risk of a fire. Had it remained on; who knows? I was aided in my decision to quickly cut avionics power by the fact that I did not appear to be able to use the radios anyway. Like most folks I use an ipad in cockpit now. I did not have a paper AFD with me. It is very important to have quick access to a phone number for ATC. I started by calling the 1-800-WX-brief line but got frustrated with the robovoice labyrinth. I was very glad to be able to find a paper directory in the FBO facility.

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Original NASA ASRS Text

Title: C206 pilot at 10;000 FT experiences communications failure followed shortly by a burning electrical smell and smoke. All avionics are shut down and the reporter commences an emergency descent to the closest airport. During the descent the smoke begins to dissipate and a safe landing ensues.

Narrative: I was conducting a flight of a patient and her spouse. We were on an IFR plan with initial northerly vectors out of the Class B. We were given direct to a fix then on course; having just leveled at 10;000 and having just exited the Class B. I lost all audio in the headsets. I switched the intercom to the emergency setting and regained audio. I attempted to contact ATC to advise. I saw the transmit symbol come active on Comm 1 when I keyed the mike but heard no side tone and received no response. I then smelled something burning - smelled like phenolic; such as when a PC board is melting. I blind broadcasted my intention to divert to the nearest airport - and my intention to commence an emergency descent. Within 5 seconds of the transmission attempts smoke began to enter the cockpit. I shut down all avionics immediately. At this point I thought I was going to be dealing with a cockpit fire. I unlatched the extinguisher between the seats and increased the descent rate to maximum. I took off the headsets and briefed the two passengers; advising that the aircraft engine was fine and that we would be making a rapid landing and that while the rate of descent would be uncomfortable; we would be OK. If I had possessed a functioning radio at this point; I would have declared an emergency. I reached ZZZ still at about 7;000 FT and commenced a steep spiral over the approach point. The smoke seemed to have stopped; but I decided to consider the likelihood of a fire to be imminent and continued the rapid descent. I had a bit of time so I retrieved a handheld comm and attempted to broadcast on the ATC frequency; and then on the CTAF. I heard no response on either. I had a good opportunity to observe traffic as I descended. There was one C172 in the pattern. About 2;500 FT I elected to swing out to perform a normal downwind pattern entry. This was completed without event and the landing was normal. I turned off at the first taxiway; got clear of the active and shut the aircraft down. We immediately exited the aircraft. There was no sign of smoke or further trouble. I retrieved a handheld comm and attempted to raise Approach. No luck with that. I was assisted by the good people at the FBO on the field. They led me to a phone and I phoned the number for ATC. I spoke with a Controller and I informed her that we had to divert and that we were safely on the ground. I believe this call was made within 10 minutes of the landing. Things I learned - The intercom panel had a minute of abnormal operation en route on the first leg. I advised ATC of this issue. The panel then resumed normal operation. I checked it out on the ground and it was OK. In retrospect I should have been more suspicious of that component. It functioned during the climb and for the first couple of minutes of cruise. I then heard a rhythmic thumping that I thought was the avionics fan failing. I now believe it was the intercom panel dying and the thumping was probably something I heard through the headsets. It was a low frequency sound and it seemed to be coming from everywhere at once. When the intercom panel ceased operating normally; I should have elected to divert. This would have gotten us on the ground 3 or 4 minutes earlier and perhaps would have allowed me to communicate with ATC. Also; although I had a handheld comm in the front of the plane; I did not have the jacks allowing it to be used with the headset. I attempted to use the comm without the headset. I do not know if any of my transmissions were received because I could not hear any response. I think it would be advisable to simply leave the converter jacks plugged into the handheld comm at all times. I think it would also be advisable to have an antenna jack installed in the aircraft that would allow the handheld to use the aircraft antenna. An avionics tech on the field pulled the stack and I have learned that the problem did indeed lie in the intercom panel. The power supply failed and melted part of the board. This was the origin of the acrid smoke. The cooling fan was fine. Once the avionics power was cut there was likely little risk of a fire. Had it remained on; who knows? I was aided in my decision to quickly cut avionics power by the fact that I did not appear to be able to use the radios anyway. Like most folks I use an iPad in cockpit now. I did not have a paper AFD with me. It is very important to have quick access to a phone number for ATC. I started by calling the 1-800-WX-BRIEF line but got frustrated with the robovoice labyrinth. I was very glad to be able to find a paper directory in the FBO facility.

Data retrieved from NASA's ASRS site as of July 2013 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.