Once twice. The first wasn’t pretty.
The second was just after refuelling at EGKA in an “old PA28” just before departing on a Night Solo Nav Ex to finish off my Night Rating. Chemical smoke began venting through the dual master switch, visible with the torch I was using to complete my “after start checks”. I concluded, rapidly that the best place to be was running away, so I goosed the engine, stopped outside the Fire Station, advised ATC of the issue, shut everything down, remembering to leave the park brake disengaged, and fell off the back of the starboard wing was I ran away. Alas, the Fire Station couldn’t start their truck, so I met them half way as they were running towards the aeroplane with large fire extinguishers…
Thankfully, it was just a 24V relay burning out, so the “old PA28” lived to fly another day.
The lesson learnt – if it flys, buy it, don’t rent it, as you can never be too sure who has maintained it previously.
Not spectacular and no PANPAN, but during my PPL, one of my first solo flights to the working area I got an alternator failure. I was still in contact with twr, just before leaving the CTR. I just told them and I got a priority landing opposite runway. Fire trucks were waiting for me on the ground.
I did a proper mayday call just once: on the first instance of fuel servo icing. ATC offered some airport which in retrospect was well out of glide range.
On the 2nd one I just got on with setting up a glide to a nearby airport. I did report loss of power as the first call on the new frequency I had just changed to when it happened – they were quite surprised to get that call!
I believe using prop TKS is the only protection against this, but it remains to be proven (with inlet duct temperature measurements).
Further back, one Pan call when I got a loss of landing gear hydraulics, due to crappy maintenance leaving an internally leaking emergency gear release valve. First post-Annual flight. I called a Pan only to get the radio call in, because I wanted to get back in case there was a big leak (the stuff is a bit corrosive) and everybody on frequency was reading out War and Peace on the radio!
One Pan call after a failed CSU feathered a prop in cruise on a MEP – to say we were diverting to nearest airport. ATC treated it as mayday, asked to squawk 7700 and met with all the blue lights on landing.
Never declared an “emergency”, but have three times asked to go back to departure airport 
First was just after take off on an IFR flight in a PA28. Fuel pressure dropped to zero so I told Socal that I wanted to go back. Felt a bit of a prat as they moved everyone else out of the way and I had priority landing. The mechanics “couldn’t find anything wrong” yet a couple of days later with FI and student onboard and the engine stopped in the tun up area with a fuel issue.
Second time was a rough running engine in a twin over mountains. Right engine started hunting with MP fluctuating dramatically. Bearing in mind this was a knackered old Seminole I thought it wise to go home and have a beer instead.
Third time was in the climb in a DA42. During the climb the right engine power dropped and at 9000 it was down to 75%, while the left remained at 100%. I had an A&P in the back as we were on a rescue mission flying to Sedona to rescue another aeroplane that had blown a tyre. I was happy to continue as I thought it might just be a wastegate issue and we were at a good altitude to clear the mountains but I asked him his opinion and he said we should probably go back. Good enough for me, we did (he then plugged his laptop in and indeed found an issue with the turbocharger of some sort…I didn’t hang around, we went to the pub instead).
None of these was I fearful for my life or anything like that, I’d say they were more precautionary than anything else.
DA42 – LH engine shut itself down after an ECU failure near Exeter (for those who understand the Centurion engines – ECU A fail, switched to B which indicated failed, back to AUTO and engine stopped). ILS to 400 ft followed by a number of firefighters helping us push the aircraft off the runway.
Piper Turbo Arrow – nose gear failed to deploy, partial gear landing. Five fire engines, three police cars and four ambulances (for two crew?)
DA42 – Birdstrike immediately after departure on initial IR skills test. Examiner asked me if it had hit – I reminded him I was under the hood :). LHS prop damaged, engine shut down and single engine landing completed. Did the IR a few days later.
PA31- Chieftain – LHS engine shutdown due to low oil pressure/high temp. Had to execute drift down due to performance issues (very high density altitude).
PA31 – O2 bottle ‘let go’ in rear of passenger compartment, rear door caption illuminated as bottle thrashed around the cabin.
PA31 – Birdstrike on go-around at Jersey – bird entered LH engine intake. Nasty BBQ smell! Engine throttled-back to idle and asymmetric landing completed.
O2 bottle ‘let go’ in rear of passenger compartment, rear door caption illuminated as bottle thrashed around the cabin.
Astonishing!!! Did the valve come out?
I have had a few emergency-type situations but didn’t declare a mayday on them.
On this trip I got my first proper KFC225 autopilot servo burnout. I have had many other failures of the servos but this was the first which filled the cabin with a burning smell. I called ahead and got the message through that the issue was fixed by pulling breakers but if it comes back I will pull the master one and will land non-radio at ETA xxxx. They had an hour’s notice so I got an airport full of fire engines and ambulances (1 POB). Apparently the concern was that I was carrying oxygen.
I got a “more proper” burnout on the last leg of this trip but dealt with it. On the same trip, another servo burnt out on an earlier leg but didn’t generate smell.
I will write up the others when I can remember them
2 pan pans, 1full emergency. One oil pressure drop to zero, precautionary landing. Two cylinders obliterated and valves, rings blown overboard. One door flinging open on takeoff. That was the full emergency. Almost messed it up.
One pan-pan which was never announced due to time and distance. On departure my cabin—attendant came running and shouted ‘fuel, fuel’ just as we got airborne – when I turned my head I could see all 15 pax in the cabin covered in a red substances and therefore knew it wasn’t fuel but hydraulic-fluid. Left flaps at 10, told twr we would make an immediate return and land on the parallel runway, opposite direction (only 4kts tailwind). Leveled the a/c at 500 ft, nice gentle 180 turn and sat her down app 1 minute later. There was fluid enough on the system to go from 10 to full flaps before ldg. Pax was Chinese and couldn’t speak English. I tried to communicate with them afterwards but with zero success. But they each got a free t-shirt and a cap for the inconvenience (+cleaning of their clothes) which did seems to make their day. And 25 minutes after ldg we were all on our way again in a new aircraft.
The times I’ve had critical issues have been right after takeoff, and there was no need to declare an emergency because the tower already understood the requirements. Most recently I had a propeller stuck in low pitch, limiting throttle opening and reducing climb to almost nothing unless I wanted to much over rev the engine. Before that was years ago when a VW engine failed with my father flying. Saying ‘engine failure’ on the radio when on crosswind leg got all the priority needed, and that was the end of the need to communicate.
