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.
In a rotorcraft, there must be enough fuel on board to fly to the first point of intended landing and for 20 minutes thereafter, day or night.
By regulation, then, the pilot must consider weather and wind in estimating the duration of the intended flight. In making the fuel calculations, the pilot must assume that the flight will be conducted at normal cruising speed. A pilot may not consider some lesser, more fuel-efficient speed in calculating the minimum fuel reserve required.
FAR 91.167 specifies the fuel requirements for IFR conditions.
No person may operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to-(1) Complete the flight to the first airport of intended landing; (2) Fly from that airport to the alternate airport; and (3) Fly after that for 45 minutes at normal cruising speed or, for helicopters, fly after that for 30 minutes at normal cruising speed.
Again, the regulation requires that the pilot consider weather and assume normal cruising speed in making the calculations. The regulation also provides some relief from this minimum fuel requirement under limited circumstances. If certain good weather conditions are expected at the destination airport, and if the destination airport has a standard instrument approach procedure, then the pilot does not have to calculate into the minimum fuel requirement the flight to the alternate airport. The weather reports or forecasts for the destination airport must indicate that the ceiling will be at least 2,000 feet above airport elevation and visibility will be at least three statute miles for at least one hour before and one hour after the estimated time of arrival. This is sometimes remembered as, simply, 1, 2, 3.
So, what happens if-after the pilot consults the appropriate aeronautical chart, the applicable aircraft manufacturer's performance charts, and the pertinent weather reports-he determines how much fuel should be on board the aircraft before departure and then puts exactly that amount in the aircraft's tanks? Perhaps this pilot accomplishes the flight but lands with less than the reserve required. In other words, what happens if the aircraft burned more fuel than the pilot expected would be needed based on properly performed calculations? This might occur as a result of some unexpected event that prolonged the duration of the flight, such as an equipment problem or an out-of-the-way vector.
A few years ago, the FAA attempted to argue that the amount of fuel remaining on board an aircraft after it landed could be used to establish that the minimum fuel requirements had not been met. In other words, if the FAA measured the amount of fuel remaining in your airplane's tanks after you landed and found less than the minimum reserve, then you could be considered to have violated the regulations. However, the National Transportation Safety Board declined to accept the FAA's argument, finding that it was incumbent on the FAA to show that the planned flight time was invalid or unreasonable, not that the flight actually took longer and used more fuel than reasonably anticipated. So, it seems that a pilot's reasonable estimates will be what determines whether he committed a violation of FAR 91.151 or 91.167.
When it's time to do your flight planning, be sure to calculate your minimum fuel requirements according to the rules-identify the flight course and distance, apply normal cruise power settings for the particular aircraft, and consider timely weather reports. Then make sure that at least that amount of fuel is in the tanks before your depart. Always remember that the fuel that does you the least good is the fuel left back at the airport.
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