Cessna 172SP Fuel System
Fuel System Failure
The Cessna 172SP (C172) is a high wing aircraft that utilizes a gravity fed fuel delivery system to the fuel injected engine. Fuel flows from the two tanks located inside of the two wings. The fuel from either both tanks, or just one then travels into fuel selector tank. Depending on whether the pilot wants to burn fuel from both tanks equally or change tanks will determine which tank they will draw fuel from. The fuel then travels through a couple more mechanical devices before the engine driven fuel pump uses pressure to suck fuel into each piston head of the engine.
The C172 has 2 kinds if fuel pumps, one is an engine driven pump and the other is an electrical (AUX) fuel pump. The reason why we have two fuel pumps powered by different systems is due to redundancy. If we lose the engine driven fuel pump it does us no good to have an extra system that could have the exact same failure. This is the main reason why we have two fuel pumps based off two different power sources. The most critical point of failure for the fuel system will be on initial takeoff along with on short final for landing. The reason these can be thought of as the most critical points of failure would be the fact that we don’t have enough altitude to maneuver for a safe landing. This is heighted due to the C172 only having one engine and having poor performance depending on the weather conditions available.
How Do Pilots Mitigate This Possible Failure?
As pilots it’s our responsibility to be prepared for this failure and memorize the engine failure on takeoff/inflight checklists. This kind of checklist is considered memory items because in the event of this failure actually happening it’s critical we follow the checklist immediately. We simply don’t have enough time to grab the checklist and read it one item at a time. One of the first steps on that checklist is to turn on the AUX fuel pump. This could the point of failure within the fuel system. The engine driven fuel pump only has a certain length of life on the part and if you don’t change the pump it could very well fail. Another issue could be contamination within the fuel. As pilots we wait at least 5 minutes after refueling. We do this because any water/contaminates which are heavier than fuel will float to the bottom of the tanks. We then test or “sump” the fuel to make sure the fuel is not contaminated. Another mitigation strategy would be testing your AUX fuel pump before engine start. Since the pump is loud it is easy to hear if it’s working correctly. Lastly, it’s important to have a game plan before we even have the engine started. I like to take a couple of minutes and go onto google maps and look at where I can land after I initially takeoff. This helps because if I’ve never been to this airport I’m not certain of any places to land near me.
References
Notebook, C. (n.d.). Aviation Fuel Systems. Retrieved November 08, 2020, from https://www.cfinotebook.net/notebook/operation-of-aircraft-systems/aviation-fuel
SpecialVFR, E. (Director). (2016, September 9). Aircraft Systems — 05 — Fuel System [Video file]. Retrieved November 8, 2020, from https://www.youtube.com/watch?v=rya4YFDpsPs&ab_channel=ERAUSpecialVFR
