Bernoulli And The Blue Fluid

THUNK! As you rotate and lift off, you hear a healthy noise from the top of the plane, followed by a rattle. As you scan your instruments, you notice the fuel level in the left tank is now less than half, and falling right before your eyes! As you strain to look through the windows behind you, you can see a blue plume trailing off your left flap.

That blue plume is your fuel, which is being sucked right out of the fuel tank by the same lift that is causing your plane to fly. Worse yet, if you don’t get the plane down on the ground FAST, it won’t be flying much longer, because the low pressure above the wing will suck the left tank dry, then start on the right tank through the equalizing line!

Fuel caps help to keep contaminants out of your fuel system, but they also have a second — and far more important — purpose…

Think about how your airplane flies. As the wing moves through the air, the motion creates a low-pressure zone above the wing that pulls the airplane up with the force of a couple thousand pounds. Note: Your fuel only weighs six pounds per gallon. When the force up is equal to the force down, we achieve level flight. However, it is important to remember that there is always low pressure on the top of the wing (as long as air is moving over the wing). That low-pressure zone can cause some real headaches if you don’t follow your checklist and confirm that your fuel caps are securely in place.

You are renting your favorite airplane from the local FBO. As you arrive, you see the fuel truck pulling away from the plane, and with it, the stepladder you need to check how much fuel is in the tank. The fuel truck guy is headed for home, but he stops by and lets you know that he topped both tanks and the plane is ready to go.

You strain to climb up the strut, but as you place your foot on the strut, it slips, leaving an impressive scratch from a rock in your shoe. Not wanting to do any further damage, you climb back down and hit the masters to check the fuel gauges. Both spring to the full position, reassuring you that the fuel truck driver told you the truth, so you finish up your preflight, and start the engine. Your taxi to the active is uneventful. Then, after your takeoff roll … well, see above.

If you lose a fuel cap in flight, and are losing fuel, prompt action is required to avoid an off-field landing. Head for the nearest airport, and inform them of the situation. Follow your emergency procedures in your POH, and get the plane down on the ground. Critical: Remember that the fuel won’t stop siphoning until the lift on the wing stops — and that won’t happen until the plane is on the ground.

The fastest way to avoid this problem is to confirm that your fuel caps are secure prior to each flight — by physically touching them. Whether high wing or low, this task, which is called out in virtually every checklist on the planet, will help to keep the fuel where you want it!


It is a familiar story. A CFII was climbing into an airplane at the field, after his student had checked it out on preflight. The private pilot was working towards his instrument rating in his own plane, a mid-1970 Cessna Skylane, which was reasonably well-equipped for the mission.

The CFII asked his pilot if he had checked that the fuel caps were secure, to which the pilot replied that of course he had, right after the fuel truck had finished filling the tanks. What the pilot didn’t say was that he visually checked the caps from the ground. The fuel truck driver had courteously placed both caps in their holes, but didn’t turn them to lock them into position, because he knows most pilots want to visually check the amount of fuel in their tanks.

The aircraft taxied to the active runway, and departed from the airport. As the plane rotated, the pilot and CFII heard two loud thumps. The CFII craned his head around, looking for the source of the clatter, when he noticed two blue fuel plumes, one from each tank, trailing the airplane. He called the plumes to the pilot’s attention, who immediately turned back to the airport and landed.

On landing, the fuel truck came by and dutifully refilled the tanks. The plane had left the ground with full fuel on board, roughly 79 usable gallons. It took over 65 gallons to refill the plane from this short five minute trip — at a cost of around $163!

The fuel truck driver and the pilot made errors, but the pilot in command has the responsibility to maintain the safety of the flight. Had this event resulted in an accident, the CFII would have been held responsible, as he did not follow the checklist, and verify the fuel caps were secure prior to takeoff.

Follow your checklist, and make sure your fuel caps are secure before each and every takeoff. The fuel you keep in your tanks (and the money that stays in your pocket) will be your own!