Dry Tanking

Running your tanks bone dry and topping them off is the only way to know how much gas is really aboard. The results may surprise you.


by Paul Bertorelli

As a means of making sure it will never happen, I have a joking arrangement with several of my pilot friends: if I ever run an airplane out of gas, they are to track me down and unceremoniously beat me senseless with a baseball bat. No questions asked. A couple of years ago, I came close enough to requiring the Louisville Slugger therapy to find the idea not that funny, in retrospect.

Fortunately, I had a Plan B and the low-fuel issue was resolved without excessive angst. Even though it was a cheap lesson, I found it both irritating and humbling to be brought up short on what was a fairly carefully planned flight. On the upside, the incident reinforced the notion that the only way to find out how much fuel an airplane holds is to run the tanks alternately dry and measure the top-off values.

New Airplane
We had just purchased a new-to-us Mooney 231, which I was ferrying from Florida to our homebase, then in Connecticut. I have a few hundred hours in the K-model, quite a bit of it instructing. Nonetheless, I accepted the brokers offer of a checkout with another CFI to refresh myself on power settings and procedures. Im also aware of one peculiarity of this model: it takes effort to get the tanks filled to their stated capacity.

Because of the Mooneys relatively high dihedral, the tanks seem to trap air bubbles when theyre being filled. Further, spring-loaded anti-siphon flappers inside the filler necks make squeezing gas into the tanks more difficult. You can top the tanks, but it takes effort; shaking the wings and allowing settling during fueling helps. Ive found that some K-models are worse in this regard than others. Ours is as bad as any Ive seen.

On the trip north from Florida that day, non-stop was out of the question so I planned a stop in Salisbury, Maryland, a distance of 761 miles. At 150 knots TAS, that would be about a five-hour leg, with another 20 minutes for the climb. The K-model carries 78.6 gallons, with 75.6 gallons supposedly usable. In some Mooney models, all of the fuel is usable and some owners argue that the same is true of the K-model. In any case, our K-model capacity doesnt match the book specs.

In our airplane, there are no fewer than five ways to track fuel quantity. This includes two digital totalizers-a Shadin and a Hoskins-mechanical gauges in the wings, the in-panel gauges, a low-fuel warning light for each tank and last, the all-important tiebreaker: the clock. It would take an idiot to run out of gas in this airplane and I almost proved it.

After takeoff, I leveled at cruise altitude-9500 feet northeast-bound, the best altitude for winds-and the groundspeed steadied up at 160 knots on a fuel flow of 11.5 GPH. Knocking off the fuel used in climb, I had 70 gallons aboard, or six hours for a 4.7-hour leg. In sparkling clear weather, a bit over an hour of reserve is comfortable for me, especially going into an area thick with airports, as is the east coast.

The flight was going swimmingly until the left tanks low fuel warning came on over Norfolk, well ahead of where my watch and the two totalizers said it should. The low-fuel lights illuminate with about four gallons remaining in that tank, give or take. According to my watch, there should have been eight gallons in that tank and both totalizers agreed within a few tenths of my time calculations.

I dismissed this as that tank not having been topped properly. I knew I had 13 gallons in the right tank and I had only 30 minutes to go, so I switched to the right side. I got across the mouth of the Chesapeake Bay before the right low-fuel light illuminated. I studied both lights for a minute or two and had the waffling argument with myself that I suspect every pilot whos run out of gas would understand. Why dont those lights agree with the totalizers and my watch? They must be wrong. After a minute or so contemplating that ridiculous reasoning, a bolt of reality struck like lightning.

Under the right wing was Accomack County Airport, in Virginia. I spiraled down from 9500 feet, landed and taxied to the pumps. The verdict: the totalizers and my watch said there had been about 20 gallons aboard; according to the amount pumped into the airplane at Accomack, there were only seven gallons.

What explained the 13-gallon, hour-plus difference? One reason was immediately obvious; the other became apparent a year later: The fueler hadnt topped the airplane properly. Although the tanks looked full during pre-flight, they were easily five or six gallons shy, maybe more. I had not been present when the airplane was fueled and should have been. Because I didnt fly the airplane on extended-range trips where having the maximum amount of fuel aboard was absolutely necessary until many months later, I didnt run the tanks dry to check their capacity. When I did, I got an unpleasant surprise.

Dry Tanks
Dry tanking is the only way to truly know a fuel tanks capacity. You can run them dry in the air or drain them on the ground and then re-fuel on a level ramp to determine capacity accurately. Although many pilots are nervous about running a tank to the point that the engine sputters and dies, theres no reason it cant be done safely. Obviously, you need to pick your venue. Dont try it on takeoff or in the pattern but at a high cruise altitude in good weather. Nothing wrong with doing it over an airport either, just in case.

Theres no need to let the engine silently windmill, either. Once you sense the surge of impending tank exhaustion, thats good enough. Switch to the opposite tank or one with adequate fuel and continue to a landing. Naturally, there needs to be plenty of fuel in the tank youre switching to so youll have the margin necessary to land safely.

Some pilots routinely run a tank dry as a means of standard fuel management. The reasoning is that if one tank is dry, all the remaining fuel is in the available tank and thats the only way to know exactly how much you have.

If thats your view, be my guest. But the logic of this escapes me. Theres always a slight zone of doubt about how much gas is in a tank and, assuming youve done one dry-tank calibration, having all your remaining fuel in one or the other tank doesnt change that. Personally, Im not about to run a tank dry with passengers in the airplane because its just not worth making them nervous about a sputtering engine. In truth, a sputtering engine isnt my idea of a good time, either.

But its worth it at least once to determine what the tanks really hold. A year later, I finally got around to it in our Mooney. I ran the left tank dry first and filled it. This exercise also allowed me to calibrate the point at which the low-fuel warnings illuminate, which turned out to be just as advertised, between three and four gallons.

I got a rude surprise at the pumps, however. With 1.3 gallons of invisible and theoretically unusable fuel, the empty tank should have taken 37.8 gallons to fill. It took only 34.8 gallons. Similarly, after running the right tank dry, I got a bit less than 35 gallons into it. And it took 10 minutes of shaking the wing and worrying the nozzle around to the point that it wouldnt accept any more gas. Bottom line: the fuel totalizer was calibrated to assume 75 gallons usable, the actual total was closer to 69 gallons, or six gallons fewer. That doesnt sound like much-its only eight percent of stated tank capacity-but its 30 minutes of flight time.

Of more consequence is the fact that in the K-model, the tanks can look topped visually but will easily accept another three or four gallons per side. Adding it all up, I could see why a baseball bat was looming on my flight over Virginia.

As I thought about this tank capacity issue, I talked myself into believing I must have done something wrong when I checked the tank capacity. So I repeated the tank-dry run on the left side and duplicated the results within a few tenths of a gallon, this time after letting the airplane sit overnight to settle the fuel levels and re-topping in the morning.

Does It Matter?
Does any of this matter? Do you really need to know fuel capacity down to the last tenth? Unless you fly max-range trips (See Septembers issue of Aviation Safety), the answer is no. If you fly no more than a three-hour leg with five hours in the tanks, fill the tanks to capacity and dont worry about the fuel or trying to stuff in the extra tenths. But if you fly the occasional four-hour-or-more leg on the same tankage, you ought to know what the tanks hold and the zone of doubt should be as narrow as you can make it.

If you down-fuel in order to carry more payload, knowing the actual tank capacity is even more critical because you wont be able to start with the known data point of full tanks. In this scenario, fuel totalizers can be a help or a hindrance. In the incident cited above, the fuel totalizer was dutifully calculating flow accurately, but its totalizer function was blinded by an inaccurate starting on-board fuel volume.

Totalizers can keep track of added fuel for down-fueling but be careful of accumulating errors that will multiply each time you refuel and use the add-fuel function on the totalizer. The running error can work in your favor or against you but there may be no way to determine which is which until you top carefully again and start with a known quantity.

Some owners use sight gauges-essentially calibrated hollow plastic tubes-to dip tanks manually to determine the fuel level. (The Fuelhawk, a commercial product, is one such device. Its sold by various online pilot supply outlets.) These are reliable and accurate if calibrated correctly and, once again, that may require dry tanking to set a known baseline for the tank capacity. Once thats established, visual dipping is accurate if the airplane is level.

An instructor I know made his own fuel dipstick for a Cherokee using a wooden dowel with notches carved in it, in five-gallon increments. This requires dipping the tanks every five gallons during topping and cutting a notch or marking the stick accordingly.

How Much Reserve?
Once you know how much fuel you have, you can then decide how little youre willing to land with. Here, you can and probably should ignore the FARs. For daylight VFR operations, FAR 91.151 specifies planning a flight to land with no less than 30 minutes of fuel; at night, the requirement is 45 minutes. Except for special purpose operations, say ag application or the like, a 30-minute reserve is probably cutting it too close, no matter how accurate your calibration. Personally, I use an hour of reserve for day or night VFR in areas rich with airports. IFR reserves are obviously more generous, depending on weather and alternates.

Its unrealistic to set a reserve figure and then declare youll never eat into it. But just how much youre willing to erode your reserve is a fine-point judgment call. For instance, on a recent return flight to Florida, I hit bingo fuel in the descent over Sarasota airport, with the destination airport 10 minutes away. Land or continue? Conditions included clear weather with little traffic so I completed the flight. With 30 minutes to go, Id have landed, re-fueled and just eaten the delay.

When setting your reserve, you also need to determine the power setting to use. Some of the big-bore engines can easily be run at settings yielding a fuel flow calculated to make a Middle East oil minister wince. Yet, a moments inattention resulting from a nuisance passenger or ATC asking if youre ready to copy a new clearance can literally double the amount of dinosaur juice being burned. The moral is to pay close attention to power and mixture settings and the resulting fuel flow-especially when trying to squeeze that last nautical mile out of your tanks-to make sure that 30 minutes of gas you planned to have on the ground at your destination isnt being wasted.

With fuel prices at all-time highs, I suspect fuel exhaustion incidents will increase as pilots stretch gas to an airport where its cheaper to buy. Theres nothing wrong with flights that use all but the last tank dregs, as long as you know how much you really have aboard, how much youll need to reach the destination and have a nearby landing alternate.

After my dry-tank calibration, it occurred to me that many pilots who run out of fuel probably dont think theyre stretching but are profoundly surprised when the engine quits. And fuel totalizers, accurate stopwatches and whiz-bang computer planners-not to mention a Louisville Slugger–wont help a bit unless you know exactly how much gas you have at engine start-up. Make it a point to find out.

Also With This Article
“Checklist: Running Tanks Dry”
“Tools Of The Trade”

-Paul Bertorelli is editor of Aviation Consumer and editorial director for Aviation Safety.


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