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Don't Be a Fuel FoolDon't Be a Fuel Fool

Depending on fuel gauges is not a good idea Research shows that just about every other day a general aviation airplane is involved in an accident in the United States because of fuel starvation, exhaustion, or contamination. This statistic doesn't even include incidents or unreported events.

Depending on fuel gauges is not a good idea

Research shows that just about every other day a general aviation airplane is involved in an accident in the United States because of fuel starvation, exhaustion, or contamination. This statistic doesn't even include incidents or unreported events. Unless a component in the fuel system fails and all the fuel is lost overboard, there's no excuse for this all too common predicament. People are hurt, airplanes are damaged, the FAA issues violations, and, perhaps worst of all, the pilot in command never lives it down.

I ran out of gas and am fortunate to be here to pass on the lesson I learned. On July 19, 1983, the engine on my Piper PA-12 went quiet while over the Kenai National Wildlife Refuge, which is located west of Anchorage. The engine stopped 30 minutes after I had taken off on a VFR flight from Alaska's Girdwood Airport to Soldotna Airport. How did it happen?

I ran out of gas because I didn't do a good preflight. I assumed I had enough fuel based on my calculations. I knew the Lycoming O-235 in my airplane burned exactly 5.9 gallons per hour when leaned during cruise. The month before the incident, I had flown that same airplane from Rockport, Texas, to Soldotna. I had 51 hours of recent cross-country time in that airplane and I could count on 5.9 gallons per hour. The airplane carried 36 gallons of gas. Short hops around the area had clicked off 4.6 hours since my last fill-up. I did the numbers and figured I had enough gas to make one more 45-minute flight. What I did not factor into the equation was the increase in fuel consumption during the seven takeoffs in those 4.6 hours. AOPA asked its members for their fuel-exhaustion stories. More than 50 stories were received.

Trust but verify

"I don't leave the ground unless I can reach the level of fuel by inserting a finger, dipstick, or straw tube in and know how much fuel is aboard." — Terry Sherlin AOPA 709396

As AOPA members wrote in with their fuel-exhaustion stories one thing became very clear — trusting airplane fuel gauges is a gamble that can trap even the most careful pilot. Aaron Glassman, AOPA 1290125, wrote a graduate research paper that studied 100 pilots and their ability to estimate the amount of fuel remaining in an aircraft's tanks based on visual inspection. Here are some of his preliminary findings:

  • 88 percent of the surveyed pilots think their fuel gauges are "consistently accurate."
  • 28 percent of the pilots were conservative in their estimates — there was more fuel aboard than they thought. Seventy-two percent estimated too high — there was less fuel than they thought.
  • Only 16 percent of the aircraft in the study had a calibrated dipstick. Only half of the pilots with these sticks "always" use them.
  • 28 percent of the airplanes had fuel-flow meters. Most of these pilots "always" use it.

Glassman recommends filling the tanks all the way up, or using a calibrated dipstick. Filling the tanks only works if the tanks are in good shape, as our next example illustrates.

"Apparently the bladders were partially collapsed and did not hold the original capacity." — Bert Vos AOPA 160750

Vos ran out of gas on a runway. A subsequent fueling showed his tanks held 46 gallons instead of the 52 gallons specified in the handbook.

Never assume the fuel burn or the tank capacity of an airplane that's just been purchased. An AOPA member who had purchased a Piper Seneca three weeks before the following incident found out the hard way that the two bladder-type outboard tanks — which hold 30 gallons total — had been removed sometime in the past. Instead of 123 gallons, there were only 93 gallons usable. Both engines quit short of the planned destination. Fortunately the pilot made a successful dead-stick landing. He assumed there was enough fuel aboard for another 45 minutes of flight.

The unknown zone

"My depending upon the fuel gauges as being accurate put me in a very dangerous situation." — Rodney J. Caron AOPA 1135471

Fuel-system capacity and consumption figures in owner's manuals and pilot's operating handbooks (POHs) should be regarded as fiction until the figures are verified.

The only way to determine the fuel-system capacity is to completely drain it and then refill it. This is an excellent time to make a calibrated dipstick. The dipstick can be made from a smoothly sanded wooden broom handle or dowel. If you want something a little fancier, many pilot shops sell pre-marked clear plastic dipsticks. Park the airplane on a level spot and slowly fill the tanks in 5- or 10-gallon increments. Cut a notch in the wooden stick at each level. Make a couple of dipsticks — this is a time-consuming task and the data for your airplane are not available elsewhere. Because of the long narrow shape of many fuel tanks, the effects of wing dihedral, and the filler cap location, dipsticks are often useless at lower fuel levels.

Some experienced pilot friends poked fun at me when I showed them the fuel flow/totalizer I had purchased. They heckled me with an emphatic, "Duh," as they pointed at their wristwatches. Their low-tech fuel consumption technique (gallons per hour times hours) always works while higher-tech tools such as a totalizer are not foolproof.

Orin Koukol, AOPA 36983, passed on the story of a Cessna P-210 that crashed short of the destination airport on the flight home from the purchase because the ferry pilot didn't understand that the fuel quantity total displayed by the fuel flow/totalizer system in the airplane was not reliable when the master electrical switch was turned off during flight. During the flight the master switch was turned off and then back on to recycle the airplane's alternator system, and then turned off to save battery power, before being turned back on during landing approach. The engine quit short of the airport and was destroyed during the emergency landing. Fortunately no one was injured.

Frozen fuel lines

"That's when the engine quit; I was at 600 feet. I later concluded that I had a partial blockage of the right tank fuel line due to ice. I now have a new checklist item. If the temperatures are below freezing, I perform a full-power runup on each tank and check the fuel pressure gauge for full fuel pressure from both tanks before taking off." — Stephen Early

Early made it safely back to the runway that night. Water enters GA airplane fuel systems in at least three ways. Leaky fuel caps are the primary entry point. Moisture in the atmosphere condensing on fuel tank walls is the second. Less common is water introduced during fueling.

Install "umbrella-type" fuel caps if they're available for your airplane. Replace fuel cap seals and o-rings often. If possible, keep fuel tanks full to cut down on the amount of condensation on the fuel tank walls, although the amount of water condensing in the tank is small. Always topping the tanks also reduces passenger payload (see " Fill 'Er Up?" December 2005 Pilot).

Anytime there's the slightest suspicion about the possibility of water in the fuel system, take action. Sump all the tanks and the gascolator repeatedly. If the sump drains are frozen, don't assume that there's only a little water in the tank. Move the airplane to a heated hangar until the sumps work normally. If your normal practice is to take two or three samples from each fuel tank, take 10 at each quick drain, then physically shake the plane to move every bit of water toward the quick drain — push up and down on each wing by applying pressure at the wing spar, or move the airplane to another place on the ramp that ensures the sump is the lowest point in the fuel tank.

Then take 10 more samples until you're satisfied that there's no water in the fuel. If you still have doubts, add isopropyl alcohol — not the diluted version that drug stores sell as rubbing alcohol — at a ratio of two fluid cups, or one-half quart for every 12 gallons of avgas. The alcohol absorbs the dissolved water. This creates a burnable mixture, as well as lowers the freezing point of the water/alcohol mix. It's best to add the isopropyl alcohol when adding fuel (recommended by Cessna in all its aircraft service manuals, including those for bladder-equipped airplanes).

The headwind factor

"Headwinds had been stronger than forecast, and as a result my trip had taken longer and I'd used more fuel than I'd planned." — David Troup AOPA 3571174

Wind forecasts are at best educated guesses. Higher-than-forecast headwinds always play havoc with flight planning, especially when fuel reserves are slim. Faced with declining fuel reserves and slow groundspeeds, pilots can do one of two things. Landing short of the planned destination and refueling is the most logical solution, but more than one member told of pushing on — seemingly spurred on by some unexplainable need to get to the original destination.

Pilots also are tempted to reduce power, thus lowering fuel consumption. This strategy doesn't save much fuel. Do the math yourself. I got the following results when I reduced the power in my Piper Comanche from 75 percent to 55 percent as I flew into a 20-knot headwind toward a destination 150 nautical miles away. At 55-percent power, airspeed sags to from 137 to 113 knots — fuel consumption drops from 10 to 7.4 gallons per hour. Subtract the 20-knot headwind, and groundspeeds fall to 117 at 75-percent power and 93 knots at 55-percent power. Fuel usage was almost equal, but the lower power setting added nearly half an hour to the flight.

Maintenance factors

"My mechanic and the local FBO operator asked me to run the engine 'wide open.'" — Ron Jones AOPA 1170500

Jones ran one tank of his Grumman AA-1 Yankee out of fuel when his attention was diverted to monitoring the break-in during his first flight after a major engine overhaul. He thought the engine had failed. He blames all the excitement on three things — he was concentrating on the engine, the fuel consumption was higher than he expected, and he hadn't filled the fuel tanks before takeoff because he was staying close to the airport.

Maintenance was mentioned in several fuel-exhaustion stories. There are two common scenarios. One is fuel exhaustion caused by fuel-system leaks because of maintenance miscues. One member recommended that the first flight after maintenance should be flown within gliding distance of the airport and be no longer than 15 minutes.

The second fuel exhaustion scenario resulted from the combination of diverted attention and the higher-than-expected fuel consumption during engine break-in.

"I was burning about 12 gallons per hour or less for cross country flying. On that flight I burned over 16 gallons per hour. I had less than six minutes of powered flight left to go [when I landed]. I learned many things [that day] that I have since integrated into my checklists and flying habits." — Kevin Murray

Murray took off on a four-hour leg during a new-engine break-in period. Looking back after landing, he realized that he had failed to lean the mixture.

Distractions and solutions

Fuel starvation occurs often. It takes place when pilots forget to switch tanks and the engine quits when one tank runs dry. All the symptoms of fuel exhaustion are there — the sudden quiet when the engine stops, accompanied by spiking adrenaline levels — but each member admitted that distractions were always present.

Douglas Davidson wrote with his hint for fuel management. Switch tanks from left to right and back according to the position of the minute hand of the pilot's watch or on-board clock. When the minute hand moves to one minute after the hour, switch to the right tanks — at 31 minutes past the hour switch to the left side. When the minute hand and the fuel selector aren't pointing in the same direction, a tank switch has been missed. Even when a system is in place, often a reminder is needed. Members who wrote in all suggested some sort of written record-keeping when it came to tracking fuel-selector changes.

Many portable and panel-mount GPS units can be programmed to sound a "switch fuel tanks" alert. Approach countdown timers also can be pressed into duty as tank-selector timers.

"I knew that I had not switched to the fullest main tank for landing as directed by the POH. I ran the engine out of fuel with 23 gallons of usable fuel. It was that simple." — John Conley AOPA 1144222

Conley got distracted during his landing checklist by a radio call and because he didn't fully understand the importance of the fussy management procedures for the four-tank fuel system of his 1957 Beechcraft Bonanza.

Nonstandard systems

Many members reported near starvation or momentary power loss problems that they blamed on nonstandard cockpits, something encountered when switching airplanes in the rental fleet.

One member remarked on being surprised when his fuel-injected Teledyne Continental engine quit. Fuel-injected TCM engines always draw more fuel out of the selected tank than the engine uses, and always return this excess fuel (called "vapor fuel") to a designated fuel tank. A few airplanes return the vapor fuel to the tank that's selected but many don't. The pilot has to understand which tank the vapor fuel goes to, and manage his fuel to provide a place for it. If proper procedures aren't followed, the vapor fuel will fill the tank and flow overboard out the tank vent line, resulting in higher-than-book fuel usage.

Flight planning

"My Grumman, with a capacity of 22 gallons usable, took 21.3 gallons that day. Flight planning involves more than just measuring distances on a chart. A few phone calls that day (to planned landing sites to check fuel availability) would have saved me from a genuinely dangerous situation." — Don Hagist AOPA 630912

The ability of an airplane to transport friends and families to small out-of-the-way airports is one of the biggest benefits of owning and flying private airplanes. But fuel availability at these airports isn't always dependable. Many members reported close calls resulting from not being able to get fuel at a remote airport. The lesson? Add phoning ahead for fuel availability to your preflight checklist.


Almost every member provided a recommendation. Many said the experience strengthened their belief in using checklists, while others stressed how practicing engine-out procedures had saved them. As for my story, well, I trimmed my airplane to best glide speed, gave Anchorage Approach control a call with cross bearings off the Anchorage and Kenai VORs, and found a safe landing area. I was lucky that day.

E-mail the author at [email protected].

Links to additional information about fuel exhaustion, starvation, and contamination, and a link to the AOPA Air Safety Foundation Fuel Management Hot Spot, may be found on AOPA Online.

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