What Would You Do (4)
 

I was flying a C172M home that belonged to a friend that had gotten stuck at an airport 100 miles away with a bad mag. Since it was going to take 3 days to get it fixed he left it at the shop and took the bus home. He asked if I could go get the airplane when it was ready. Since I could hop a ride with a friend on a scheduled freight run to the airport where the airplane was this was an easy favour to help out a fellow pilot.

I was in cruise in VFR in controlled airspace and ATC called me and informed me they were no longer receiving a transponder return

What would you do ?

Turn off then turn on the transponder, while waiting to recycle check for a possible electrical issue. Ammeter or volt indicator.
Check circuit breaker. Reset after appropriate cooling down period.
Call ATC and request to proceed without transponder. If denied climb or descent out of said Airspace.

don’t panic, the plane is flying so concentrate on aviation

Is the display on ? If so that rules out power issues.

switch off switch on whilst telling ATC that you are recirculating

give ATC a position report, inc altitude, QNH and intended routing, along with telling them that you are maintaining altitude and track

await their instructions. They would probably prefer a non transponding aircraft to be flying straight and level than be at unknown altitude

keep an eye on ammeter, any other electrical malfunctions and watch/small for smoke from behind the panel

That was exactly what I did. When I checked the ammeter it was indicating a discharge so I told ATC that I had lost my charging system but as I was only 30 minutes from home I would reduce my electrical load and continue. If I lost comms I would phone him on my cel. ATC was OK with that and the radio was fine all the way

The alternator had failed due to a broken wire. The engineer figured that the wire was broken when the cowl was replaced after the mag repair.

So overall not a very big deal but there are a few take aways

1) The older style transponders with a cavity tube like the King KT76, Cessna 300, and Narco AT50 are very sensitive to voltage so they will be the first thing to fail after a charging system failure.

2) Early recognition of the failure gave me plenty of time to deal with it. If I had not checked the a ammeter I probably would have had the radio die just as I was entering the very busy control zone of my destination airport

3) I have terminal and my home base tower phone numbers saved in my cel phone

4) It is a good idea to be extra ​​​​​alert on the first flight after significant maintenance as other problems may have been inadvertently introduced

I thought Cessnas were equipped with a warning light for when the alternator failed? Or it could be something we retrofitted into our Piper 28...

What indications you get after an alternator failure depend on the C172 model. Pre Early 1970's the only indication was a discharge on the ammeter. 1970's M and N models had an "over voltage" warning light. This would come on if the alternator was tripped off line due to high voltage, however if the drive belt broke or there was a broken wire or any other mechanical failure the alternator would be off line and the light would not illuminate. Again the ammeter would be the only way to tell. Airplanes built in 1980's up to today's S model ( non Garmin G1000) have a "low voltage" light. This comes on at around 24.5 volts. Since the charging voltage in these airplanes is 28 volts, if the charging system fails for any reason the voltage drops to battery voltage ( 24 volts) and the light comes on.

I had similar in an Arrow at night, with the owner in the left seat, doing his night rating (an extra in the UK, but not in the US or probably Canada). Anyhow, we were fat, dumb and happy at 3,500' when the RADAR guy said 'just lost your secondary return'. As you say, it was a KT76. A few seconds later, the displays on the other boxes started to flicker. Hit the Master off, then turned everything off in turn, then turned the Master back on. The major effect of turning the Master off was losing the intercom! Pointing and shouting made it work. This Arrow had the benefit of not only an ammeter but a voltmeter, instead of a low voltage light. If we'd had the latter, we'd have been better placed as our complacency hadn't included a scan of the voltmeter, which wasn't illuminated by the cockpit lighting. We turned one com back on and told ATC we had an electrical failure, declared an emergency (a Pan, Pan, do you have those in Canada?), swung round and headed to the nearest airport. The RADAR guy said 'cleared to land any runway'. Nice. Turned off the master again, by this time we could see really well outside with no lights on inside. Then we realised the gear comes down with an electrically driven hydraulic motor. Oh. Master back on, hit the gear switch, got the red light, then 3 greens. Phew! Master back off. Landed just fine, taxy off the runway, met by 3 huge truck painted red with flashy blue lights. Night vision destroyed. Turned master and com back on, got taxy instructions, parked on stand and shut down.
It turned out when the menders got to the aircraft the following day that the alternator regulator had fried. Finished off the night rating later that week, watching ALL the gauges like a hawk!

Postscript.
There's a really amusing story involving the menders and their trip out to our Arrow. They were on a 'round robin' of several jobs at different airfields, in a venerable Cessna 182 (a straight-tail version with mechanical flaps, which is important in this story). They departed from fixing our Arrow with 3 POB, 2 licensed engineers and a mechanic. As they departed, they asked the tower for an early right turn. 'Affirm', she said, so they did a 45 AOB turn just below the level of the Tower windows. 'Not THAT early!' she cried. Anyhow, as they rolled wings level at 500' AGL, the engine went to idle power. Quick as a flash, the pilot pulled ALL the flap and made a decent landing in the only available field, stopping just before the stone wall at the end. If it had been a later model with electric flaps, they would never have come down in time. So, our plucky 3 got out, removed the cowlings and found the throttle linkage had come undone. Now, I thought throttles were supposed to default to full power under these circumstances, but this one didn't. So with all the tools etc they had it fixed and the cowlings back on, just as the local Police and fire service turned up for the major accident, followed by the Air Ambulance. The pilot waved them all away, paced out the field and got his mates to get a lift back to the airfield as he reckoned he'd only get out of the field without them on board. And so it was, back to work, with only the embarrassing story about the 'local pilot hero wrestling the controls' in the local paper a week later.

TOO

Agreed there’s a ‘low volts’ light on the annunciator panel, but that’s not what got my attention one summer morning climbing out of El Paso, Texas in a 172 ‘R’ model. What I did note was the OAT which was 27c even at this level. Even more puzzling, it was slowly decaying.

it was a minute or two before I realised I was reading battery voltage, not OAT! Maybe about then the low volt light came on. A quick 180 and a call to El Paso and time to investigate, I tried cycling the master. Bingo! Power restored. Back on course I exchanged emails with home base (a wonderful innovation back then) I was told it was nothing unusual and ok to continue. Certainly it didn’t happen again in the next 40 Hrs of my round trip to Florida.

When something similar happened in my PA28 in Sweden, the UK CAA mandated low volt light came on immediately. I diverted to Bromma and on final bingo! The alternator came back on, no amount of probing and fiddling on the ground revealed anything and after making suitable preparations continued to Helsinki expecting the power to die at any moment. It didn’t, and a friend came out with an alternator in his hand luggage (!). No more incidents for 30 Hrs or so, and then same again. Regulator, over voltage trip, etc changed out and another 30 Hrs once again the baleful glare of the LV light. This time, instead of just pressing on the alternator field breaker, for some reason I flicked it with my fingernail.

I still have that breaker, on my desk as I write this.