Domestic fuel requirements under 14 CFR 121.639
The three-part fuel test for a domestic Part 121 dispatch, how the alternate leg enters the calculation, and why forecast wind is part of the regulation rather than an afterthought.
Video scheduled
The lesson video for this topic is in production on The Flight Dispatch Room.
Written lesson
The lesson
Fuel questions look like arithmetic. They are mostly reading comprehension. If you can state the three legs of 121.639 in order and say when the second leg disappears, most of the exam set answers itself.
The regulation
Under 14 CFR 121.639, no person may dispatch or take off an airplane in domestic operations unless it has enough fuel, considering wind and other weather conditions forecast, to fly to the airport to which it is dispatched; thereafter to fly to and land at the most distant alternate airport specified in the dispatch release, where an alternate is required; and thereafter to fly for 45 minutes at normal cruising fuel consumption.
- Fuel to fly to the destination airport.
- Fuel to then fly to and land at the most distant alternate listed, where an alternate is required.
- Fuel to then fly for 45 minutes at normal cruising fuel consumption.
Most distant, not nearest
If more than one alternate is listed on the release, the fuel calculation uses the most distant one. The logic is that the release authorises the flight to proceed to any listed alternate, so the aircraft must carry enough fuel for the worst case among them. Listing a second, further alternate for flexibility therefore increases the required fuel load.
Wind is inside the regulation
The phrase considering wind and other weather conditions forecast sits in the regulation itself. Required fuel is not a still-air figure. A forecast headwind that increases trip burn increases the regulatory minimum, and a release built on still-air numbers is not compliant regardless of how much fuel is physically on board.
The 45 minutes is a time, not a distance
Reserve is expressed as 45 minutes at normal cruising fuel consumption. It is converted to a quantity using cruise burn rate, not by planning 45 minutes of holding at low altitude, and not by converting a distance. Questions that give you a holding burn rate and ask for the domestic reserve are testing this distinction.
Examination focus
Key exam concepts
- Domestic required fuel = destination + most distant alternate (if required) + 45 minutes at normal cruising consumption.
- The alternate leg only applies when an alternate is required under 121.619.
- Where multiple alternates are listed, the most distant one drives the calculation.
- Forecast wind and weather are part of the regulatory calculation, not an operational extra.
- The 45 minute reserve is computed at normal cruising fuel consumption.
Reference
Important formulas
Domestic minimum dispatch fuel
Fuel = Trip fuel to destination + Fuel to most distant alternate + (0.75 h x normal cruise burn)
The alternate term is zero when no alternate is required under 121.619.
Reserve quantity
Reserve = normal cruise burn rate (per hour) x 0.75
45 minutes expressed as a decimal fraction of an hour.
Method
Worked examples
Minimum dispatch fuel with an alternate required
Given
- Trip fuel to destination, corrected for forecast wind: 11,400 lb.
- Two alternates listed. Fuel to the nearer: 1,900 lb. Fuel to the most distant: 2,600 lb.
- Normal cruise fuel consumption: 4,800 lb per hour.
Steps
- 1Leg one: fuel to the destination is 11,400 lb.
- 2Leg two: an alternate is required and two are listed, so use the most distant, 2,600 lb. Do not use the nearer alternate.
- 3Leg three: 45 minutes at normal cruise consumption is 4,800 x 0.75, which is 3,600 lb.
- 4Add the three legs: 11,400 + 2,600 + 3,600.
Answer
17,600 lb is the regulatory minimum. Using the nearer alternate would give 16,900 lb, which is the most common wrong answer in this question type.
Minimum dispatch fuel with no alternate required
Given
- The destination qualifies for the 121.619 exemption, so no alternate is listed.
- Trip fuel to destination, corrected for forecast wind: 8,200 lb.
- Normal cruise fuel consumption: 3,600 lb per hour.
Steps
- 1Leg one: 8,200 lb to the destination.
- 2Leg two: no alternate is required and none is listed, so this term is zero.
- 3Leg three: 3,600 x 0.75 = 2,700 lb.
- 4Add: 8,200 + 0 + 2,700.
Answer
10,900 lb. The 45 minute reserve still applies in full even though no alternate is required.
Watch out
Common exam traps
- Using the nearest listed alternate instead of the most distant.
- Dropping the 45 minute reserve when no alternate is required. Only the alternate leg disappears.
- Computing the reserve at a holding or approach burn rate rather than normal cruise consumption.
- Ignoring forecast wind and planning still-air trip fuel.
- Applying the domestic 45 minute reserve to a flag operations question governed by 121.645.
Test yourself
Practice questions
Choose an answer to reveal the discussion. Every option is explained, including why the wrong ones are attractive.
- Q01
A domestic Part 121 flight lists two alternates. Fuel to the nearer alternate is 1,500 lb and to the more distant is 2,400 lb. Which figure is used to satisfy 14 CFR 121.639?
- Q02
A domestic flight qualifies for the 121.619 exemption and no alternate is listed. Normal cruise consumption is 4,000 lb per hour and trip fuel is 9,000 lb. What is the minimum fuel required by 121.639?
- Q03
The phrase considering wind and other weather conditions forecast in 121.639 means that:
Verify
Official FAA references
- 14 CFR 121.639Fuel supply: Turbine-engine powered airplanes, other than turbo-propeller: Domestic operations.
- 14 CFR 121.645Fuel supply: Turbine-engine powered airplanes, other than turbo propeller: Flag and supplemental operations.
- 14 CFR 121.619Alternate airport for destination: Domestic operations.
- FAA-G-ADXAircraft Dispatcher Airman Certification Standards.
Current regulation text is published on eCFR. Always confirm against the current version before relying on any citation.
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