Welcome Guest. Sign in or Signup

1 Answers

Cessna 172S Induction Overboard Drain Line

Asked by: 2888 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.

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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!

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.