How To Check Fluids in an Airplane

The quality and level of fluids is important in any mechanical apparatus. On an airplane, most of the controls are manipulated through hydraulic systems. Therefore, aside from the aviation fuel itself, there are a handful of other fluids that you need to check before starting your flight.

On big commercial aircraft, it’s usually the ground crew that does the inspection of fluids on an aircraft pre-flight. A pilot also runs onboard tests for the integrity of the plane’s fluid as a part of his pre-flight checklist. If you are piloting your own aircraft, or studying for your pilot’s license, you will need to take not of your plane’s fluids to ensure a safe and smooth flight.

Basically, the three important pre-flight checks for fluids involve your fuel level, the engines’ oil and your hydraulic fluids.

Engine oil

Open the access door to each of your plane’s engines. You will need to check for the level and quality of the engine oil. Pull out the dipstick and wipe the end dry with a clean piece of lint-free cloth. Put back the dipstick and pull it out again. The resulting liquid should be between the “min” and “max” line on the dipstick. The oil should also be translucent. If it’s black or if it looks muddy, then you will need to change the engine oil.

The engine oil should be changed at a certain interval, so be sure to check with your plane’s user’s manual. The oil is usually changed on the regular maintenance schedule, which is defined by the manufacturer. If you are leasing or renting an airplane, check the service record to see when the last oil change was done.


Most planes have fuel tanks on the fuselage or built inside each wing. You can check the fuel level visually, by opening the cap and peeking inside. You also need to test the fuel for debris and water, which can badly affect the performance of your engine. You will need a fuel tester for this purpose. Plug in the fuel tester to the valve and siphon out the fuel by pulling the plunger. Fuel from the lowest point of the fuel tank will be siphoned into the tester.

Visually inspect the tester for any debris, which you can see floating in the fuel. You can also check for water in the fuel. Since water is heavier than fuel, it will sink to the bottom of the tester, and will create air bubbles along the way.

Hydraulic fluids

Check with the plane’s manual and service record to see when hydraulic fluids were last replaced. If these are beyond their regular maintenance schedule, then have them replaced. You can also check the fluid levels by manually inspecting the reservoir. The fluids should be within the proper level, and should appear clear and without any debris.

Any flight is safe and smooth if your aircraft is in perfect working condition. Be sure to check the quality and level of fluids on your plane, including aviation fuel, engine oil and hydraulic system.


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