Thorp Air Command - T18.net

At last 100/Ann inspection I decided to instal a carb temp guage I've had sitting around for some time. Installation went OK, blank out of carb and probe installed. Power on and all worked well until I went for a flight. Take off climb and cruise all good and I saw temps that I expected. The surprise came when I pulled power in the circuit and discovered the throttle would not fully close! It would come down to about 13-1500 rpm and thats it. I could put power back on ok but same problem when closing throttle. Well a wide circuit and approach and when over the numbers pulled the mixture to land. As we all know a Thorp wont land with that much power on. Start up and taxii in fortunately on grass so didn't have to ride the brakes much.
I thought you dummy you have let the prob wiring get tangled with the throttle lever. Once pushed back in the hangar off with the cowl but there was no evidence if the wiring fowling AND the throttle would close as normal?! Any way tie wrapped the cable to several points and went for another flight, same thing! Dang!! Made up a stainless bracket to really ensure the wiring could not get near the throttle linkage, inspected the full length of the throttle system and there was nothing unusual. Another flight, the same problem!!!@#%. Running out of ideas, have another carb which I could check the location of the prob in the carb throat as I thought if it protrudes in too far it may fowl the butterfly but there is no way this could happen. Only thought left is that if ice forms on the prob and grows upwards (only needs about 3/8") that would stop butterfly fully closing and ofcourse once shut down for 10 mins or so the ice would melt and allow full movement.


My next plan is to remove the probe and blank then see if problem still exists. Any bright ideas would be appreciated . Ah forgot to mention this is a O-320 A2A bog standard.

Tony Schischka
ZK-VMS
T18C

Any chance the probe itself is too long and getting in the way of the butterfly valve?

Did you try carb heat while flying?
Cubes

If it were me and I am , I would remove the probe and try it in the other carb to see if the guilty bastard is hanging things up !

RB

Possible vacuum leak around temp probe that is letting air into carb throat even when butterfly is totally closed??

Thanks all. A bit of a brain scratcher. Yes tried carb heat. I think best to remove probe to prove if it is the cause then take it from there.
Tony S
ZK-VMS

When you installed the probe, did you remove the carb. ? If so you may be getting a air leak when things get warm. You can spray something like WD - 40 around the carb. with the engine running and see if there is a change in RPM or running smooth. Be very carful when doing this ! Secure aircraft to known strong tie down, Think about hands or head into prop and if too much spray is used, smoke +++ can be a result.

Phil
118BC

Well problem solved though I have a very red face as I caused the problem.
I pulled the cowl and disconnected the throttle linkage to ensure the Teleflex cable wasn't the problem which it wasn't. Throttle lever still not able to be brought back to idle stop. Remove filter banjo and peered up the carb throat. Low and behold there was a piece of lock wire hooked on one of the venturi support arms and extending up past the butterfly which stopped the throttle fully closing! poked my finger up and touched it and it fell into my hand. Throttle now moved through full range and normally.
Investigation identified the lock wire had come from the locking on the four filister head screws holding the inner filter housing to the carburettor. I remember when I was fitting it during 100hr that I locked the screws THEN discoved I had locked one set backwards (ie. the locking was assisting the screws to undo!) I cut the lock wire which dropped to the floor but must have bounced into the filter housing sitting ready to go on. I fitted the filter and did not see the lockwire!! See the pic below of the offending wire. I would like to thank all who gave useful advice and I hope we can all learn something here! I'll get all back together tomorrow and go for (hopefully) an uneventful flight.
Cheers
Tony S
ZK-VMS

I'd rather be lucky than good!

I'm glad you found it. Also glad you remained calm and landed ok with a stuck throttle.
Could have been worse.
Cubes

Bet you won't do that again ! The airlines are just as guilty ! Personal knowledge involving a contract Spirit Airlines DC-9 leased to the Navy . A lot of FOD ( safety wire , etc. ) found in the belly of the AC . Like Cubes said , you kept a cool head with a jammed throttle !!

RB

All back together and went for a quick flight, glad to report "ops normal".
I love flying my T18!

Tony S
ZK-VMS

So the lesson here (for me) is to not leave anything directly below the Aircraft that needs to be reinstalled. As something may drop into that other part when working above it.