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Cessna 172S Induction Overboard Drain Line

Asked by: 3318 views Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Private Pilot, Student Pilot

Hi everyone,

I was going through the C172S fuel systems, and as most of you might have noticed, the line running down the firewall called 'induction overboard drain line' which from the text book explains as follow :-

"~prevent any fuel from the fuel/air control unit from settling in the induction system when the pilot over primes the engine, an overboard drain line redirects fuel from the induction elbow back towards the firewall,"

"~this arrangement is made so as to avoid a potential fire with the close proximity of the exhaust stack."

 

I would like to know how fuel comes out of the fuel/air control unit, drops down through the drain line, and lastly, how fire could result if this was not installed on Cessna 172S.

The book explanation itself is insufficient to understand in detail.

 

Steve.

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1 Answers



  1. WidgetPilot95 on May 28, 2020

    I am by know means a mechanic, and I am more less assuming from past experiences, but if you have ever seen an airplane over primed, fuel pours out of the cowling. My old flight school had multiple engine fires om startup due to over priming.

    I believe, the fuel/air control unit contains a check valve, after a certain amount of fuel is within the unit, the check valve will allow the fuel to return through the over board lines.

    If i had to guess, it would prevent fuel from dripping out of the cowling due to a flooded cylinder, potentially close to the hot exhaust pipe underneath. It’s also important to understand that it doesn’t necessarily take away the complete risk of engine fires. Over priming to a certain extent will cause excess fuel in the cylinders and result in fuel escaping from the cowling creating a dangerous situation, regardless of the overboard drain lines.

    Hope this somewhat helps!

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