Rough Engine- Was It Carb Ice Or Vapour Lock?

Rough Engine- Was It Carb Ice Or Vapour Lock?
On a recent flight I experienced a really rough running engine, with a loss of power. It could have been Carb Ice Or Vapour Lock, but we'll never really know. What I do know is that if it was vapour lock (something that early Cessna 172s are susceptible too), you really need to know how to fix the problem before you become another forced landing statistic.
Here is the AD:
72-07-02 CESSNA: Amdt. 39-1415. Applies to the following airplanes:
MODELS SERIAL NUMBERS AFFECTED
172 28000 thru 29999
172 36000 thru 36999
172 46001 thru 46754
172A 46755 thru 47746
172B 17247747 thru 17248734
172C 17248735 thru 17249544
172D 17249545 thru 17250572
172E 17250573 thru 17251822
172F 17251823 thru 17253392
172G 17253393 thru 17254892
172H 17254893 thru 17256512
172I 17256513 thru 17257161
172K 17257162 thru 17258855
Compliance: Required as indicated, unless already accomplished.

To reduce the possibility of engine power interruption at altitudes above 5000 feet caused by vapor formation in the fuel lines, accomplish the following:

(A) Effective now, the airplane must be operated on a single fuel tank immediately upon reaching cruise altitudes above 5000 feet.

(B) On or before April 1, 1972, install at the fuel selector valve applicable Cessna placards P/N's 0509021-1, 0509021-2 or 0509021-3 as provided with Cessna Service Letter SE72-7, dated March 17, 1972, or any FAA-approved equivalent placard which reads as follows: SWITCH TO SINGLE TANK OPERATION IMMEDIATELY UPON REACHING CRUISE ALTITUDES ABOVE 5000 FEET.

(C) Compliance with the provisions of Paragraphs A and B is no longer required when the fuel system has been modified by the installation of applicable Cessna Kit No. SK172-31B or SK172-32 referenced by Cessna Service Letter SE72-7, dated March 17, 1972, or by the accomplishment of any equivalent method approved by the Chief, Engineering and Manufacturing Branch, FAA, Central Region.

This amendment becomes effective March 25, 1972.

This summer (June 2023) I'm flying in a 2,086nm fundraiser for Hope Air, an organisation that helps lower income Canadians or those who live in remote areas access healthcare by providing flights (through private pilots and airlines) to larger urban centres.

⚠️ NOTE: These videos are edited for time and entertainment. Editing removes context and can remove safety checklists in the interest of time. Do not use these videos for, or in lieu of flight training.⚠️

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[DannyCreech]

Good job Glen. I shared this video with a former Student of mine who owns an older C-172. Also, very smart move kicking that CFI out of the aircraft and never flying with him again for saying the POH is always right. That is a stupid CFI.

[goldierides]

Thank you. I'm training in a 1960 Skyhawk, ready to take my check ride. I was unaware that this could be an issue, and I have always run on both.

[JohnUllrey]

Very well done good sir! The data from your glass panel is really an amazing thing to have. My dad was an Air Force Navigator and he liked to say: "You never want to run out of Airspeed, Altitude, or Ideas at the same time."

[TJF]

For us non pilots, I had to look up POH. Pilots Operating Handbook. Great video. I admire your transparency, how your research and your attention to detail.

[orvjudd]

Great critical thought processes. The FAA is a bureaucracy. Made up of humans who make mistakes. I would feel comfortable flying with you any time.

[zoe..d]

Peefect display of top level airmanship on display here.
Not afraid to deep dive into a situation regardless of preserving ones own reputation.
The goal is learning, making well informed decisions about changes required. Highlighted the issues surrounding increasing irrelevance of aircraft documentation as modifications become a part of your plane.
Last but not least, you were able to communicate this extremely clearly so others will go home safely.

Bravo Sir.

[togadriver]

Thanks Glen. I own a 1957 C-172. I flew it for the better part of 500 hours in many conditions before experience vapor lock. I have experienced it twice, and both occasions with the tank on both above 5000 feet.

I think you’ve given the best explanation here that I have seen. A few points to add. I could not understand the mechanism of what would cause the vapor lock and how switching to single tank operation somehow made it better. According to the folklore on the Internet, Cessna apparently tried using glass tubes to see if they could replicate the problem and supposedly could not. After some consideration, I believe that’s probably true. The fact is that glass tubing does not conduct heat nearly as well as the aluminum fuel lines do. By switching to single take operation, you are effectively doubling the fuel flow through the tubing between the fuel tank and the fuel selector, thus allowing less opportunity for fuel bubble vapors to form and if they do, flushing them out of the system quicker.

Thanks for your videos and helping us fly safely!

[MrDdaland]

Well, you answered one thought I had (Mogas and possibly getting gas with ethanol)
Main thing is- you had TIME to work through the problem. You kept flying the aircraft, you had a plan for landing in case you needed, and kept your head while trying to analyze the problem
Well Done!

[XMarkxyz]

I think this is one of the best video's you've made, first because it's always somewhat hard to admit having an issue (even though I wouldn't say it was your fauld and you come out of it really good), second because it was very informative and might turn out useful to people who risk having the same issue and also because we as humans learn something new when something unexpected happens. So by all means well done. By the way my guess is as good as everyone's else but I'd say it was vapour lock by the evidences you show us.
I don't know how youtube actually works, I mean who does, but maybe you might add the plane's model and year to the title too so people with the same model can find the video more easily, just a suggestion.

[KriLL]

Outdated information being held as gospel is such a big issue in a lot of fields, because it was printed a long time ago and either people don't know it's not correct anymore, or the official process to update/amend it hasn't caught up etc.

[kazflight]

Thank you for sharing! I would have never thought about vapour lock in anything carbureted, I've been taught that it only applies to fuel injected engines. Excellent reminder about the static nature of POH's. Great job on the cause check, glad to see you were able to land safe and continue your flight.

[GingerThePlane]

Very timely episode, we recently had a student pilot experience what’s likely to be carb ice here at KBLI. It resulted in a forced landing in a nearby field, she did wonderful and both pilot and plane will fly another day. Thanks for the reminder, carb heat and fuel management is simple in a 172 but should never be taken for granted. 👍

[johnhanes]

Thank you for sharing your experience. Experience is the best teacher. On our own the valuable lessons come right after we needed it the most. Best to learn from those who have gon before us. Again Thank You.

[jtstuff]

Wow! This just happened to me in my 1961 172C!! Clear day 7500', 2300 rpm, leaned a bit, on both tanks running great. Then the rpms faded and it nearly died. I did the same as you, carb heat on mixture full rich... I throttled back and luckily was over KRBG. I began descending, engine idling fine with carb heat on...at 4000' I wanted to test it, full power full rich, carb heat off...again not good. At that point I circled and landed. A&P cleaned the plugs, replaced 3 of them and I flew back home no problem. I was not aware of the >5000' single tank AD. I wasn't aware that carb ice can happen at 2300 rpm, my home A&P thinks I was too running ROP and it fouled my plugs...or was it vapor lock? Either way, I've learned a lot in the past few days! Thanks Glen!

[MSIk]

I couldn't even keep my cool when my tire disintegrated on the 401 express... hats off to you for not panicking high up in the air. Stay safe, Glen.

[chrisjohnson]

Is there a way your dynon can give you a reminder if you get to 5000ft it will issue a master alarm switch to one tank...If you forget at cruse... Also did the dynon give you a master alarm or anything that the engine was running out of normal parameters??? GREAT VIDEO...

You and Blancolierio need to get together someday...

[cmflyer]

That POH is a POS! Great story and lesson Glen!

[sip]

That CFI would have a "does not compute" moment if he/she had to fly a older piper product. Best glide? You figure it out? Grass take off distance? What's grass? Tail wind takeoff/landings? Crickets.... LOL.

[richardcox]

Awesome video. I got carb icing in my 172B over Honolulu once at 2000ft about 1 mile away from some moisture in the air. I had difficulty maintaining the cleared altitude of 2000ft through the Class B airspace so I turned around and RTB at Kalaeloa. While descending out of 2000ft I pulled the carb heat and the engine started running smooth again. I found out later from my mechanic that as the air goes through the carburetor the temperature drops about 30 degrees, which is how you can get carburetor icing in 70 degree weather in Hawaii. Scary lesson to learn while flying over a metropolis like Honolulu.

[mikes]

Thank you for sharing your story. I agree that many pilots will far too often use the POH/AFM as a crux, with the completely incorrect belief that it is infallible and that 50-70 years of advancement and better understanding is irrelevant.

One small nitpick.... it's Brechin, not "Brechlin" (no 'L').