The Cessna 120-140 Association Forum

This is not about not trusting your mechanic, its about the stats of how many accidents happen right after planes are worked on. Annual just got done and I did a super thorough pre-flight with cowl off, all looked good. Went to fly and no rpm rise during run-up when I turned carb heat off, yes I test our carb heat by turning it off during run-up momentarily instead of on for a second. So I called a no-go. It was bitter cold but I pulled filter and the carb heat unit was visually operating. One of the guys around the terminal figured it was brutal cold and I just didn't have enough heat in the muffs yet... Made plausible sense but not totally as she was thoroughly preheated and I tried it more than once and not my first time flying her in the cold.

Tried again today, very moderate temps, same thing- no drop... I tried multiple times, and again called a no-go. This time I popped the whole cowl off. I did not catch this visually on the preflight, as it looked fine until I ran my hand over it. The springs got screwed up in the Skat tubing, assuming during maintenance got bumped or something. The tubing instantly crushed to nothing when I ran my hand over it. So I'm assuming it may have not looked all kinked but as soon as it went to suck air through it, the tubing collapsed denying the carb of heated air...

So a good reminder that after Mx to super inspect, fly solo and close to the airport for a bit to make sure all is good. This was a really simple "oops" that could have easily turned into a dead-stick landing with as much as our lil Continentals make ice... Also a reminder, that run-up checks aren't to be cursory or shrugged off if they fail or are even "off" a bit... I could here a minor tone change in the engine when I applied it... One could assume, "Oh with these old tachs its prob just that as I can hear a difference, I need to get to xyz, its fine" If it ain't right, it ain't right nor should it be assumed airworthy...

Thanks, Ray!!

Thanks for posting your experience! I always find these to be great reminders that if something doesn't seem right, stop and get it checked out.

Follow up: This absolutly confirms and reminds one to stick to your guns if you don't feel something is right!

We replaced that SCAT tube and theres no doubt it needed it, and went to fly and still no drop/rise with carb heat. I thought I could hear a tone change in engine but before annual had always got a visible drop. Everyone around the airport kept telling me it was just so cold the mufflers weren't hot enough. I flew last winter and that never happened, I went to mechanic and said something is still not right, he was very agreeable to check it over again but thought the terminal's collective knowledge that its just so cold out was really probably the issue.

NOPE!!! The flapper door for the carb heat was not closing all the way when applied full on. It was closing but left enough open to suck enough unheated air that I wasn't getting any noticeable heat... They readjusted it and back to a nice drop. Took her up today and with an -2500 density altitude and a headwind I thought they had swapped engines on me-she was like a supercub! She lifted off by the time the tail came up which felt like about 50 feet and climbed out at 1200fpm.

BUT THE POINT IS, I KNOW MY MACHINE, I KNEW IT WASN'T RIGHT, I HAD MULTIPLE VETERAN PILOTS TELLING ME IT PROBABLY WAS JUST FINE, I MADE THE RIGHT DECISION AND REFUSED TO FLY HER UNTIL SHE WAS RIGHT! ITS YOUR BUTT UP THERE, IF IT AIN'T RIGHT ITS BACK TO THE HANGAR-PERIOD.

Great post !

Thanks for sharing.

One more info: one of the 32 C120/C140/140a imported to Argentina had an accident due to ice build that could not be removed by carb heat.
Fortunately, the pilot did a fortune landing, was uninjured and the machine had only minor damage.
SN is 12.417 and registration is LV NSO.

The investigation showed that all the heat box was not the Cessna original, and no logbook entry was found for any repair.

I never had problems in 22 years with my 140. And it drops near 75 RPM when I check heat drop with CHT at about 250 Farenheit.

I am still curious to see a pic showing an original heat box, to check if all in my plane is right. In 2018 annual we changed the command cable.
I will double check my tubings on next oil change. It will be before next winter. I have an oil cartridge STC installed, but I still change oil every 25 hours.

Kind regards,
Horacio