AOPA Pilot Magazine - February 2013 - 18

PROFICIENT PILOT

Good to the last drop?

BY BARRY SCHIFF

To run dry or not to run dry

BARRY SCHIFF

has written more
than 1,600
magazine articles
and currently
is writing his
fourteenth book.

18 | AOPA PILOT February 2013

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 highwing 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.”
AOPA
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AOPA Pilot Magazine - February 2013

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