Pilot Counsel

IFR fuel requirements

June 1, 2006

John S. Yodice and his associates represent AOPA and its membership in aviation legal matters.

FAR 91.167, "Fuel requirements for flight in IFR conditions," is an operational rule that instrument pilots need to review from time to time, especially because of a few nuances and interpretations that sometime escape notice. As with many of the operational rules that we review in this column, they are excellent guides to safe practices, but we are also aware that the question of compliance doesn't usually come up until after an accident or incident. In the case of this rule, if an aircraft runs out of fuel, the FAA will be particularly interested in investigating compliance with FAR 91.167. So, for a number of good reasons, here goes our review.

Paragraph (a) of the rule provides that "No person may operate a civil aircraft in IFR conditions unless it carries enough fuel (considering weather reports and forecasts and weather conditions) to: Complete the flight to the first airport of intended landing; Except as provided in paragraph (b) of this section, fly from that airport to the alternate airport; and fly after that for 45 minutes at normal cruising speed or, for helicopters, fly after that for 30 minutes at normal cruising speed."

Paragraph (b) allows a pilot to eliminate from his or her computation the fuel necessary to fly from the first airport to the alternate if the first airport is expecting good weather and it has an instrument approach. In that case the basic rule is simply to carry enough fuel to land at the destination airport plus a 45-minute reserve (30 minutes for helicopters). Here are the precise conditions that must be met to eliminate the leg to the alternate: The first airport must have a standard instrument approach procedure (or a special instrument procedure issued to the aircraft operator). And, for at least one hour before and one hour after the estimated time of arrival, the relevant weather reports/forecasts must indicate that the ceiling will be at least 2,000 feet above the airport elevation and visibility will be at least three statute miles. For helicopters, at the estimated time of arrival and for one hour after that, the ceiling must be expected to be at least 1,000 feet above the airport elevation, or at least 400 feet above the lowest applicable approach minima, whichever is higher, and the visibility must be expected to be at least two statute miles.

Here is a nuance that sometimes escapes notice. These IFR fuel requirements apply only to flight in IFR conditions. An IFR flight that will be conducted completely in VFR conditions need only meet the VFR fuel requirements of FAR 91.151, which are, during the day at least a 30-minute fuel reserve, and at night at least a 45-minute fuel reserve.

And here is nuance that rarely escapes notice but certainly merits special mention. In calculating the fuel requirement a pilot must assume "normal cruising speed," taking into account weather reports and forecasts and weather conditions. Clearly, while the rule may be read to mean normal cruising airspeed, the expected groundspeed must be computed as well. The winds that are forecast and reported, and the computed headwind components, must be taken into account in computing the fuel needed at "normal cruising speed."

But a more difficult question is the interpretation of the word normal in the phrase "normal cruising speed." We know that there is a whole range of fuel consumption figures for various cruise configurations of an aircraft. 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. There is not much legal authority on this point. The most conservative answer is no, unless you are using that same fuel-efficient cruise configuration in planning for the flight itself. This answer is based on a decades-old FAA interpretation dealing with a similar airline fuel-reserve requirement. The interpretation is arguably not right on point. What was at issue was the need to standardize a procedure for the airline fleet. However, the interpretation does give us a pretty good insight into FAA thinking. In that interpretation, the FAA told the airline that it should calculate the required fuel reserve using "the cruising power most often used." One could 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 regulation, 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.

Here is a case containing another interpretation that could be helpful in understanding the fuel-reserve requirement. How does one determine the available fuel load onboard the aircraft? Again there is not much law or interpretation for us, except that in an enforcement case several years ago, the FAA and the NTSB were critical of a pilot's reliance on "eyeballing" the fuel quantity in the tanks of a rented aircraft, and they were also critical of the pilot's relying on the fuel gauges of that aircraft. They said that when a fuel tank is not full, or up to a calibrated tab, the best way to establish the amount is with a calibrated dipstick. In this case the pilot, who happened to be a captain for a major airline, had his ATP certificate suspended for 30 days for violating FAR 91.167 (as well as FARs 91.103 "preflight action" and 91.13, "careless or reckless"). The pilot explained how carefully he computed his fuel requirements, including the fuel burns according to the manual. His problem was that he relied on the fuel gauges and eyeballing the fuel in the tanks. As we cautioned earlier, this was a hindsight analysis because the pilot ran out of fuel, crashing the Piper Archer that he was piloting. The case does illustrate the cautious approach that the government agencies expect us to take in determining available fuel. Query: What about the fuel gauges that have proved reliable to a pilot with experience in that aircraft? Or a fuel totalizer, or the like?

On the other hand, merely landing with insufficient fuel, by itself, does not constitute a violation of FAR 91.167. This is a frequently asked question that has a happy, reasonable answer. In a past case, the NTSB made clear: "Inasmuch as the [FAA] did not establish that the [pilot's] planned flight time was invalid or unreasonable, it is of no consequence, for purposes of determining whether the [pilot] began the flight with the fuel reserves required by FAR section 91.167, that the flight took longer than was anticipated and, as a result, the aircraft landed with less fuel than had been estimated."

So, there you have it: the rule and some nuances and interpretations and all. Happy landings!