Funny thing -
Cheers. I fly a North American Yale (Harvard's 'uncle' with a Wright
Whirlwind 425hp) from The Can. Warplane Museum locally and have done
since we finished it in 1992.
First time out of the circuit last June, we were enroute to an airshow
about one hour away, and were 25 minutes into the flight when I got a
thump in one or both rudder pedals. It was an instantaneous 'rap', as if
someone had whacked the back of the pedal with a small hammer.
I said to my buddy, "you feel that?". He hadn't - he's in the back seat
sorting out maps, so I had him put feet to pedals.
After ten minutes he was beginning to question my contact with reality,
when it happened again. Five minutes later we took TWO whacks with the
engine smooth as a kitten. I turned back and he began to point out large
roads and suitable fields.
During our return this began to occur more often and in greater spurts,
so that by the time we got back to base, the tower were ready to call
out the rescue crew.
Ground runups brought nothing, so the complaint raised eyebrows but
little reaction, except that it seemed to affect only the port magneto
and applicable plugs. We rewired the plugs (rear for port mag) and runup
was fine. Second long trip was fine till about 25 minutes and guess
what? Definitely port mag so we took the mag part and found dirt and
loose material. Cleaned it up and runup was OK.
This time I stayed in the circuit at high power, and sure enough after
20 min. or so we got thumped again. On return, the boss stripped the
inbuilt condenser and we tested it under heat. There it wasw. When the
engine had transferred enough heat to the mag, the condenser began to
break down, and one miss was enough to thump the pedals under foot.
Replaced the condenser, reinstalled and she purrs again just like she
always had. So mebbe it ain't all gasline contamination. Just like ol'
Barney used to say, "3 things, it's gas, spark or timing".
Here endeth the lesson.
Happy landings
Ferg #A064
|