As a professional pilot you are also a part-time aeronautical engineer, meteorologist, and risk management analyst. You can add petroleum scientist to that list. Don't believe me? Try this riddle on for size:
How can a Gulfstream V make it from San Francisco to Tokyo in a headwind routinely, but sometimes struggle to make Tokyo to San Francisco with a tailwind? The answer does not have anything to do with routing, hemispheric altitudes, or Byzantine Japanese departure procedures. But it does have everything to do with fuel. And even if you're not flying an ultra-long range aircraft, you need to understand:
- Regional Differences in Fuel Density
- Regulatory and Company Fuel Minimums
- Flight Planning Software
- Fuel Measurement
- When Enough Isn't Enough
- "Min Fuel" versus "I am Declaring an Emergency"
- Personal Fuel Minimums
Fueling operations, from Eddie's aircraft.
Regional Differences in Fuel Density
Figure: Regional differences in fuel density, from Eddie's notes.
The answer to our Gulfstream V riddle perplexed early Gulfstream pilots for years, so much so it was a common complaint against the airplane's fuel system. Every aircraft was tested for a full capacity of 6,118 U.S. gallons. Aircraft attitude during refueling was found to have only a negligible impact on total fuel capacity. The fuel measurement system specification of plus or minus 300 lbs. was bested by every aircraft tested. The engineers had to dig further and came up with the answer: fuel density.
More about this: Fuel Density.
Studies reveal fuel is denser in the Western United States than in the Far East. Given the GV's maximum fuel volume is fixed at 6,118 U.S. gallons, the highest allowed Jet-A fuel density of 6.99 lb./USG would permit a fuel load of 42,764 lbs. The lowest allowable density of 6.46 lb./USG would leave a fully loaded GV with only 39,522 lbs. of fuel. This variation of 3,242 lbs. could reduce the aircraft's range by almost two hours. There's nothing you can do to fix low fuel density except be forewarned: never count on a full load of fuel leaving airports in Asia.
The study cited is from what used to be called the Gulfstream Breakfast Minutes and uses 6,100 U.S. gallons. Subsequent Gulfstream reports correct this to 6,118 U.S. gallons.
If you aren't topping off your tanks, then you're going to need a quick answer for the fuel truck driver's next question: How much?
Regulatory and Company Fuel Minimums
Figure: 14 CFR Fuel minimums, from Eddie's aircraft.
The first consideration to the "how much?" question is physics: getting the airplane from Point A to Point B. Obviously this isn't enough, so we have rules and regulations to ensure we exercise at least a minimum level of caution.
[14 CFR 91, §91.151]
(a) 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.
[14 CFR 91, §91.167]
(a) 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) Except as provided in paragraph (b) of this section, 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.
(b) Paragraph (a)(2) of this section does not apply if:
(1) Part 97 of this chapter prescribes a standard instrument approach procedure to, or a special instrument approach procedure has been issued by the Administrator to the operator for, the first airport of intended landing; and
(2) Appropriate weather reports or weather forecasts, or a combination of them, indicate the following:
(i) For aircraft other than helicopters. For at least 1 hour before and for 1 hour after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles.
FAA Order 8900.1, Volume 3, Chapter 25, encourages non-flag commercial operators to include similar fuel allowances for all overwater, passenger-carrying flights. While not specifically included in this guidance, general aviation operators would be wise to do the same.
[ICAO Annex 6, Part I, ¶4.3.6]
This only applies to commerical operators.
184.108.40.206 All aeroplanes. A flight shall not be commenced unless, taking into account both the meteorological conditions and any delays that are expected in flight, the aeroplane carries sufficient fuel and oil to ensure that it can safely complete the flight. In addition, a reserve shall be carried to provide for contingencies.
220.127.116.11 Turbine-engined aeroplanes. The fuel and oil carried in order to comply with 18.104.22.168 shall, in the case of turbine-engined aeroplanes, be at least the amount sufficient to allow the aeroplane:
22.214.171.124.1 When a destination alternate aerodrome is required, either:
a) to fly to and execute an approach, and a missed approach, at the aerodrome to which the flight is planned, and thereafter:
1) to fly to the alternate aerodrome specified in the operational and ATS flight plans; and then
2) to fly for 30 minutes at holding speed at 450 m (1 500 ft) above the alternate aerodrome under standard temperature conditions, and approach and land; and
3) to have an additional amount of fuel sufficient to provide for the increased consumption on the occurrence of any of the potential contingencies specified by the operator to the satisfaction of the State of the Operator; or
b) to fly to the alternate aerodrome via any predetermined point and thereafter for 30 minutes at 450 m (1 500 ft) above the alternate aerodrome, due provision having been made for an additional amount of fuel sufficient to provide for the increased consumption on the occurrence of any of the potential contingencies specified by the operator to the satisfaction of the State of the Operator; provided that fuel shall not be less than the amount of fuel required to fly to the aerodrome to which the flight is planned and thereafter for two hours at normal cruise consumption.
126.96.36.199.2 When a destination alternate aerodrome is not required:
a) in terms of 188.8.131.52 a), to fly to the aerodrome to which the flight is planned and additionally:
1) to fly 30 minutes at holding speed at 450 m (1 500 ft) above the aerodrome to which the flight is planned under standard temperature conditions; and
2) to have an additional amount of fuel, sufficient to provide for the increased consumption on the occurrence of any of the potential contingencies specified by the operator to the satisfaction of the State of the Operator; and
b) in terms of 184.108.40.206 b), to fly to the aerodrome to which the flight is planned and thereafter for a period of two hours at normal cruise consumption.
[ICAO Annex 6, Part II, ¶220.127.116.11]
This applies to general aviation.
A flight shall not be commenced unless, taking into account both the meteorological conditions and any delays that are expected in flight, the aeroplane carries sufficient fuel and oil to ensure that it can safely complete the flight. The amount of fuel to be carried must permit:
a) when the flight is conducted in accordance with the instrument flight rules and a destination alternate aerodrome is not required in accordance with 18.104.22.168, flight to the aerodrome of intended landing, and after that, for at least 45 minutes at normal cruising altitude; or
b) when the flight is conducted in accordance with the instrument flight rules and a destination alternate aerodrome is required, flight from the aerodrome of intended landing to an alternate aerodrome, and after that, for at least 45 minutes at normal cruising altitude; or
c) when the flight is conducted in accordance with the visual flight rules by day, flight to the aerodrome of intended landing, and after that, for at least 30 minutes at normal cruising altitude; or
d) when the flight is conducted in accordance with the visual flight rules by night, flight to the aerodrome of intended landing and thereafter for at least 45 minutes at normal cruising altitude.
Note.— Nothing in 22.214.171.124 precludes amendment of a flight plan in flight in order to replan the flight to another aerodrome, provided that the requirements of 126.96.36.199 can be complied with from the point where the flight is replanned.
Armed with these minimums we can turn to our flight-planning service to churn out fuel computations printed to the nearest pound, including alternate, holding, and reserve fuel. But before you stake your pilot's license on the number listed under "Total," consider that one or more of these lines could be lying to you.
Flight Planning Software
Figure: Comparing flight plan alternate (KSFO to KSJC) versus full flight plan, from Eddie's notes.
Your flight plan's en route fuel is probably based on an immediate climb and a perfectly scripted descent, as if you owned the airspace. But if you are descending into Teterboro and get the SLAM DUNK ONE arrival to sneak below La Guardia, JFK, and Newark traffic, your ideal fuel burn can go up thousands of pounds. Even a graceful profile descent can include expect crossing clearances that will rob you of fuel. If your flight plan doesn't adjust your arrival altitudes accordingly, you need to adjust your arrival fuel burn.
That isn't the only problem: many flight plan providers compute alternate fuel based on flying a straight line from the destination airport to the alternate, no muss, no fuss.
But your odds of getting a straight line shot to your alternate are not just less than slim; it isn't going to happen. If the weather is such that you need to file an alternate airport, you should have a good idea of how much fuel it will really take, including the missed approach, the expected climb out instructions, and the expected arrival to the alternate airport.
A G450 crew, for example, could plan a trip to San Francisco International Airport with San Jose as an alternate, planning on arriving at the alternate with 5,000 lbs. of fuel to meet all Part 91 IFR requirements. But typical flight planning software counts on getting from the missed approach point at KSFO to landing at KSFC as the crow flies using less than 400 lbs. But the crow doesn't have to negotiate with NORCAL approach; the actual maneuver could take over 2,000 lbs. of fuel. It would be nice to know that before you find yourself at minimums on the KSFO ILS Runway 28R.
Knowing how much fuel you need to get from Point A to Point B, go missed approach, shoot an approach and land at Point C, and then have the legally required fuel reserve gets you an answer to the "How Much?" question. You should compute that answer with the total required minus the fuel on board. Why not just start pumping and stop when the number on the gauge matches the number in your computation? Because the gauge can lie.
Figure: Capacitance probes, from Eddie's notes.
Most aircraft fuel quantity measuring systems use fuel capacitance probes that eliminate the problems with old float designs. A capacitance probe is a metal rod surrounded by a hollow metal tube with an insulator that prevents the inner rod from contacting the outer tube. The probe is mounted vertically so fuel can flow in and out from the bottom and air into and out of the top. An electrical charge is applied and the capacitance is measured. The ratio of fuel to air affects the capacitance and a fuel quantity is derived.
Modern systems tend to use several probes in each tank and multiple computers to ensure the result is accurate. The design specification in a Gulfstream V, for example, is plus or minus 300 lbs. These modern systems appear to be very accurate, but they can be fooled when the temperature of the fuel in the truck is appreciably warmer than the fuel in your wings. Even the best fuel quantity computers can be fooled by variations in temperature within the same tank. Let's say you spent several hours in your Gulfstream V at high altitudes and landed with fuel tank temperatures somewhere south of -10°C. If you take on a full load of fuel from a truck that has been sitting for hours at a higher ambient temperature, the fuel will become stratified: the layer of fuel on top will be less dense than the layer on the bottom. Fuel computers will tend to think the fuel quantity is higher than it really is. A 2007 Gulfstream study revealed the fuel quantity in a Gulfstream V could be 800 to 1200 lbs. low under these conditions. If the airplane sits for about 10 hours on the ramp, the discrepancy goes away. But what if you don't have 10 hours? You still need to add a known quantity of fuel, but don't stop the truck just because the gauge says you have enough. (You might not.)
Even after 10 hours for the fuel temperature to acclimatize you could still find yourself unable to take on a full fuel load because of high ambient temperatures. The warmer fuel gets, the more space it takes and you could run out of volume before you have the necessary fuel by weight. A 2005 study showed that every 10°C increase in temperature can reduce a Gulfstream V's total capacity by 200 lbs. While we assume our jet engines burn fuel by volume this isn't true, our range is determined by the weight of fuel. Density is even more critical than temperature.
The fuel truck driver wants his "how much" question answered by volume, be it gallons or liters. Your aircraft burns the fuel by weight, and that could be in pounds or kilograms. Just because 3,000 U.S. gallons gives you 20,000 lbs. of fuel most of the time in the United States doesn't mean it will everywhere else. Going from the lowest to highest density can change the weight of 3,000 U.S. gallons by over 1,500 lbs. Don't let the fuel truck pull away until the fuel total by weight equals or exceeds that on your flight plan.
When Enough Isn't Enough
Once the fuel truck pulls away and your tanks have what you need to fly the trip, make it to an alternate, and still have enough to satisfy all published fuel minimums, you still cannot relax. If you see a line of airplanes waiting for takeoff, your first thought is likely to be about your estimated time of arrival. But don't forget your excess engine idle time will eat into the expected fuel remaining when you reach your destination. While en route, fuel remaining becomes a concern again when confronted with stronger than expected headwinds or when deviating around weather systems. Finally, remaining fuel on landing needs to be considered after every lap around the holding pattern, and especially after a missed approach. The time to compute this fuel is right after the hold is given, not after four spins of 10-mile legs. Your options run the gamut from holding to finding another place to land. There may come a time when you need to let someone outside the airplane know you are not happy.
"Min Fuel" versus "I am Declaring an Emergency"
According to the Aeronautical Information Manual, declaring "minimum fuel" does not imply a need for traffic priority only that you cannot accept any undue delay upon reaching your destination. In actual practice, it is a way of conveying your discomfort to ATC, nothing more. AIM goes on to say that if you need traffic priority to ensure a safe landing, you should declare an emergency due to low fuel.
Declaring an emergency doesn't cost you anything other than possibly being asked to send a written report in accordance with 91.3. If you need to declare an emergency, do it and be explicit. In 1990 an Avianca Boeing 707 ran out of gas while holding for over an hour waiting to land at New York JFK, killing 73 of 158 on board. The crew let ATC know they were running out of fuel but never used the magic word, emergency, and never got traffic priority.
More about this: Avianca 52.
Personal Fuel Minimums
The best way to avoid having to one day declare minimum fuel is to have a sound set of personal fuel minimums and a logical approach to answering the "how much" question:
- Fuel required to fly the trip,
- Fuel required to execute a missed approach, fly to the alternate, shoot a second approach, and land,
- Fuel to satisfy regulatory and company minimums, and√Fuel to satisfy personal minimums.
There are two schools of thought when it comes to minimums and the wrong school of thought is this: "If the minimum wasn't good enough, it wouldn't be the minimum." You might be happy with an extra 30 minutes of fuel in the day and 45 minutes at night. It is a personal preference but you should think it through. Modern airspace is more crowded today than the day that regulation was written and a 30-minute delay can be considered a short one these days. More fuel is better, to be sure, but to what limit? The answer will be different for an aircraft with a high allowable landing weight and good brakes versus another with narrow landing weight margins and fragile braking systems. Me? I start the bidding at one hour of fuel and work my way higher.
14 CFR 91, Title 14: Aeronautics and Space, General Operating and Flight Rules, Federal Aviation Administration, Department of Transportation
14 CFR 121, Title 14: Aeronautics and Space, Operating Requirements: Domestic, Flag, and Supplemental Operations, Federal Aviation Administration, Department of Transportation
14 CFR 135, Title 14: Aeronautics and Space, Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such Aircraft, Federal Aviation Administration, Department of Transportation
ICAO Annex 6 - Operation of Aircraft - Part 1 Commercial Aircraft, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part I, July 2010
ICAO Annex 6 - Operation of Aircraft - Part 2 General Aviation, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part II, July 2008
Standard for Aircraft Fuel Servicing, 2012 Edition, National Fire Protection Association, NFPA 407, NFPA, 1 Batterymarch Park, Quincy, MA 02169-7471