The fuel requirement for flight in VFR conditions is an important rule that we don’t have much occasion to look at in detail. So, here goes. FAR 91.151(a) says that: “No person may begin a flight in an airplane under VFR conditions unless (considering wind and forecast weather conditions) there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed (1) During the day, to fly after that for at least 30 minutes; or (2) At night, to fly after that for at least 45 minutes.”
There is not much question of interpretation about how to account for wind and weather in estimating flight time and fuel requirements. Pilots are trained to do it, and they do it all the time in practice. But one phrase that does raise an interesting question is, what is normal cruising speed? We know that there are a whole range of fuel consumption figures for various cruise configurations of an airplane. Would it be legal to select the most fuel-efficient cruise configuration in figuring the required reserve? After all, you could always go to that configuration in flight if fuel becomes a problem.
The most likely answer from the FAA is no, unless you are using that same cruise configuration in planning for the flight itself. It is based on an old FAA interpretation dealing with a similar airline requirement. In that interpretation, the FAA told the airline that it should calculate the required fuel reserve using “the cruising power most often used.” We take from this interpretation that the FAA would not consider it “normal” to use a different cruising speed for computing the fuel reserve than that used for the flight itself. Or stated in another way, the FAA would consider it “normal,” and in compliance with the rule, to use the same cruise speed (and hence the same cruise configuration) for computing the required fuel reserve as you use for the flight itself.
What about landing with less than 30 (or 45) minutes of fuel in your tanks? Is that an automatic violation of the regulation? No. The regulation, by its terms, imposes its requirement at the beginning of a flight. While landing with a short supply of fuel could be some evidence that you began the flight with insufficient fuel, it is not conclusive. It could be explained by unexpected conditions that extended the time of the flight—which is the reason you are required to have a reserve.
How does a potential violation come to the attention of the FAA? The FAA does not typically ramp check general aviation aircraft after they land. The most usual way the FAA finds out about a potential violation is if the agency is called to investigate an off-airport landing and discovers empty fuel tanks. Or, as in the following case, an FAA inspector just happened to notice something unusual.
According to the FAA inspector, he was on an airport investigating an unrelated matter when he happened to notice a Cessna 180 being towed by automobile from the runway area of the airport to the apron in front of a skydiving operation. He decided to check. He approached the pilot of the aircraft, who was part owner of the skydiving operation, and asked her if she had a problem. She said yes, she had an oil pressure light come on during flight, so she had shut down the engine and landed. The inspector looked the airplane over and questioned whether the problem could have been fuel exhaustion.
The pilot insisted that could not have been the problem. They got a ladder and dipped the fuel tanks in the wings of the high-wing aircraft. The operator said she found fuel. The inspector dipped the tanks himself, and said he found the right tank completely dry and only a few drops of fuel in the left tank.
The ultimate result was an FAA order suspending the operator’s ATP certificate for 90 days, charging that she had begun a flight with insufficient fuel. The FAA also charged her with operating an unairworthy aircraft in violation of FAR 91.7(a)—to blunt her defense that she had a subsequently discovered leak because of a faulty fuel strainer. And, lastly, a charge that the FAA always throws in: being “careless or reckless” in violation of FAR 91.13(a). The operator appealed the suspension to the National Transportation Safety Board.
On appeal, she offered evidence that she had begun the flights with sufficient fuel and reserve. She testified to the flight times and altitudes of the parachute hops, the fueling of the aircraft, the fuel consumption of the Cessna 180, and her contemporaneous logging of these details. She was corroborated by her partner who personally fueled the aircraft, and checked the fuel. She offered evidence of the discovery the next day of a leak from a defective fuel strainer plunger, which was then repaired—all to no avail. The board determined that there had been a violation of FAR 91.151(a). The board did dismiss the FAR 91.7(a) charge and reduced the sanction from a 90-day suspension to 30 days.