Proficient Pilot: Using the unusable

As has been repeated often on these pages, there rarely is a legitimate excuse for running out of fuel. Unfortunately, however, many of us commit that blunder every year. It happens more often than we realize because many of these events luckily do not result in accidents and do not cause reports to be generated. Only those events of fuel exhaustion resulting in bent metal or injury contribute to NTSB statistics.

Pilots know that the total fuel capacity of their tanks is the sum of the usable and the unusable fuel. What if it were possible—following an engine failure caused by fuel exhaustion—to use some or all of the unusable fuel, avgas that otherwise would be left sloshing on the bottoms of our fuel tanks as we glide powerlessly to a forced landing on inhospitable terrain? The truth is that in most cases some or all of the unusable fuel actually is available for us to use.

According to aircraft certification regulations, the amount of unusable fuel in our tanks is determined by flying an airplane in “the most adverse fuel-feed condition” until fuel exhaustion occurs. The fuel remaining in the tanks at that time is then measured and called unusable. This means that if the airplane is flown in a less adverse condition—a different pitch attitude, for example—the engine likely will operate for a longer period of time.

Let’s put this a little differently. The most adverse condition for a particular and popular four-place general aviation airplane is straight-and-level flight. This means that fuel exhaustion will occur during straight-and-level flight sooner than if the airplane were flown in a different attitude or configuration.

Each gallon of accessible and “unusable” fuel could keep the engine running for an additional seven minutes.So how could we put this knowledge to use following an actual power loss caused by fuel exhaustion during level flight? In most cases, you could try raising a wing and maintaining a gentle slip so that some of the unusable fuel would flow to the engine from the elevated tank. When that tank runs dry, you could then reverse the attitude of the airplane (slip in the other direction) to provide fuel flow from the other tank.

In addition, raising or lowering the nose slightly might also help to get some unusable fuel to a fuel-tank port so as to restore engine power.

The published amount of unusable fuel in the example airplane is 3 gallons (1.5 gallons per side). By slipping, you might not gain access to all three of those unusable gallons, but you could likely access some and perhaps most of it. Considering that this airplane typically burns 9 gph in cruise, each gallon of accessible and “unusable” fuel could keep the engine running for almost an additional seven minutes. Those valuable extra minutes could be used to maintain altitude while heading toward a more desirable emergency landing site, perhaps even an airport. It could make the difference between a successful forced landing and one that might not be so pretty.

You can get a sense of what attitude likely would be best for accessing unusable fuel from your tanks by studying the fuel-tank diagram for your airplane and noting where the fuel ports are situated. Your airframe technician has access to these diagrams and can help with this. Consider, however, that some airplanes have no unusable fuel whatsoever while others have a lot. Refer to your pilot’s operating handbook.

Never consider using this procedure following fuel exhaustion when near the ground because the distraction can add extreme peril to your dilemma. Instead, concentrate on flying the airplane and setting up for a forced landing.

Attempting to use unusable fuel is obviously an emergency procedure, something a pilot might attempt in an effort to save his bacon after he’s already screwed up by allowing himself to run out of fuel in the first place. And it should go without saying that planning to use this procedure to extend range is obviously illegal and suicidal.

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