Proficient Pilot: Good to the last drop?

To run dry or not to run dry

February 1, 2013

Barry Schiff

There is a controversial technique used by some pilots of piston-powered airplanes that makes it possible to approach and land at a destination with more fuel in the selected tank than would otherwise be possible. This is accomplished by consuming all of the usable fuel in one tank so that all available reserve fuel is in the other (instead of being divided between two tanks). This technique presumably reduces the possibility of inadvertently starving the engine of fuel while using one tank and then having to switch to another during a critical phase of a flight (such as during a landing approach).

In other words, the pilot intentionally runs one tank “dry” (except for unusable fuel) and knows that all remaining usable fuel is in the other.

Such a technique is largely practiced in low-wing airplanes because the fuel-selector valves of these aircraft typically do not have a Both position (as do high-wing aircraft). Selecting the Both position at the end of a flight assures that all usable fuel will be available without interruption.

In my mind’s eye, running a tank dry is an emergency procedure and should be done only when it becomes apparent that a pilot will be landing with a critically low fuel supply and has no other options (such as landing at an intermediate airport). I do not believe that it should be done routinely. Having said that, there are no significant problems associated with intentionally running a tank dry at altitude other than having to acknowledge that flight planning has gone awry and you are attempting to stretch range.

There are two ways to run a tank dry. The first is to simply wait for the engine to fail (hopefully at a safe altitude and with the passengers briefed about what to expect). A better way is to keep a watchful eye on fuel pressure (or fuel flow) and switch tanks at the first fluctuation. You should be able to switch tanks without the engine missing a beat.

A pilot should not be concerned about the engine in a certified airplane failing to restart. During certification trials, the airframe manufacturer is required to demonstrate that when power loss becomes apparent due to fuel depletion from one tank, it must be possible after switching to any full tank in level flight to restore at least 75 percent of power within 10 seconds for single-engine airplanes and 20 seconds for multiengine airplanes. Note, however, that this assurance applies only when switching to a full tank, not necessarily when switching to one with significantly less fuel. Also, 10 or 20 seconds can be an eternity when waiting for an engine to restart.

I have done this numerous times during flight testing in a variety of airplanes, and it never took more than a few seconds for full power to be restored irrespective of the fuel level in the tank used to resupply the engine. This does not mean, however, that there cannot be exceptions.

Does this mean that the engines and fuel systems in homebuilt and Light Sport aircraft offer similar reliability during an attempted restart following fuel starvation? No, it does not.

Is it harmful to an engine when operating at cruise power to suddenly be allowed to fail due to fuel starvation if power is restored within seconds? Candidly, I cannot say. Nor can any of the Continental and Lycoming representatives with whom I spoke about this. All they would say is that the procedure is not recommended, which is not surprising.

Some argue that this procedure is not harmful and cite as proof the longevity of the engines in training aircraft. These engines receive substantial abuse (shock cooling) from frequent and rapid opening and closing of the throttle. This may not be a valid analogy, however. Training aircraft are typically flown much more frequently than privately owned airplanes, a practice known to assist in extending times between overhaul. Besides, such engines typically run cooler and are less complex than the engines in larger, more powerful aircraft. Do any aircraft manufacturers recommend or condone this controversial procedure? I cannot find one that does, although some expressly prohibit it. The pilot’s operating handbook for the Piper Warrior II, for example, states, “Do not run tanks completely dry in flight.”

One way to avoid having to consider using this procedure is to never allow yourself to be airborne with too little fuel. For starters, ignore the FAA’s requirement that flights be planned with a minimum VFR fuel reserve of 30 minutes. My personal minimum is one hour. If at any time you determine that your landing will occur with much less than that, make a fuel stop. As the bromide says, “It is much better to be down here wishing you were up there than up there wishing you were down here.”

Barry Schiff has written more than 1,600 magazine articles and currently is writing his fourteenth book. Visit the author’s website.