2
$\begingroup$

I'm concerned about safety rules for commercial flight in North America. There is supposed to be a fuel reserve of "30 minutes" if I remember correctly. This seems to imply an extra 30 minutes of flying time, but this is not perfectly clear.

Specifically, for example, does this 30 minutes include the worst case scenario of a headwind during the entire flight? If so, what is the max headwind speed used in the calculation?

$\endgroup$
  • $\begingroup$ The "reserve" meant for anything unplanned during the flight is called contingency fuel, which is about 5% of the trip fuel (some exceptions apply). $\endgroup$ – Sami Nov 29 '15 at 8:47
  • 1
    $\begingroup$ Fuel needs include forecast winds. If a headwind is predicted, the pilot needs to plan for that. If a tailwind is predicted, they account for that as well. Wind forecasts are usually quite accurate. $\endgroup$ – abelenky Nov 29 '15 at 16:33
  • 1
    $\begingroup$ The "duplicate of" question does not have the word "wind" in the question nor either answer. I'm mystified how this can be considered a "duplicate" of the other question, when it is CLEARLY asking for something else beyond "all the rules governing required fuel." Simple conceptual question about an aspect that IS NOT addressed in the other set of answers. $\endgroup$ – Ralph J Nov 29 '15 at 18:28
4
$\begingroup$

The fuel "to reach the destination" is computed using winds, head- or tail-wind as appropriate. For "30 minutes of reserve" that is 30 minutes, and it is the same 30 minutes regardless of flying into or with the wind. In 30 minutes of flying with a tailwind you'll fly FARTHER than 30 minutes with a headwind, but 30 minutes is 30 minutes.

-- Edit to answer the question as expanded in a comment:

Wind predictions are actually pretty good now, and "30 minutes" isn't really all that the pilot has to work with.

For FAR Part 91, essentially general aviation (think taking a Cessna on a flight of a few hundred miles as an easy picture), the legal minimum in good weather is 30 minutes. For FAR Part 121, which governs airline flying, it's 45 minutes (plus other categories as explained in the other answers for things like a destination alternate). Beyond that, the plan for the flight will almost always include additional fuel above that to account for the routine things that happen to burn more gas: delays waiting to take off, small changes in routing that add a few miles to the planned path, and so on. That way, even with the routine "stuff" that happens, you don't plan to eat into the 45 minutes reserve. If the wind forecast is off, then that tends to be another of "those things" that changes the plan.

Realistically, how much are the winds off of the forecasts? Typically, the forecasts are pretty close. The computer models that are used now are very detailed, which is possible with all the computing power that's available now as compared to a decade or two ago. Also, these models are constantly updated by data coming in from aircraft that are airborne. Various datalinks can transmit the winds and temperature in real time, and ATC will also sometimes ask for current winds at a particular altitude. All of this combines to give a picture that isn't simply a wind forecast issued in isolation, but a combination of forecast + recent data.

The issue that would arise if an airline were required to plan on the worst-case wind that might be encountered as if it would be present for the entire flight, is that in some cases the flight would significantly under-burn: it would arrive with more fuel than planned. Why is that an issue? Maximum Landing Weight. If your airliner is fully loaded with passengers & cargo, there is a limit to how much fuel it can have on board at landing, driven by the maximum certified landing weight. On my airplane, that tends to be enough fuel for a bit over two hours of flying. (Obviously a LOT of factors influence this on each flight -- this is a ballpark number.) So if I have a full airplane and I want to plan on having as much fuel as possible upon arrival (so that we can hold if necessary, fly the approach, and still have adequate fuel for a missed approach and then a divert, arriving at the alternate with adequate margins), there is a limit to how much I can take off with. And that limit is driven by how much fuel we're expected to burn enroute. If my enroute burn is lower than planned and I do NOT have to hold or take a longer route than planned, I can show up too heavy to legally land.

There are two options when that happens: land anyway (safe, but requires an inspection of the aircraft, and it's generally frowned upon to do this without good reason), or fly circles to burn off the gas. The latter is what is generally done, and is obviously wasteful. So it's preferred that the enroute burn calculations be as accurate as possible, rather than including unrealistic headwind values.

$\endgroup$
  • $\begingroup$ Yes but how can you be sure that the predicted winds will be the same all along the flight path the whole flight? That's 1,000+ miles and 2+ hours for the typical US flight if I'm not mistaken, and that's a lot of predictions for a lot of ground for a lot of time. Shouldn't they be taking into account the worst case scenario? Plus if I recall correctly from researching wind power, wind is the hardest part of the weather to predict, even harder than rain. $\endgroup$ – DrZ214 Nov 29 '15 at 19:04
  • $\begingroup$ Expanded the answer to address the questions you added in the comment. You can edit your question if you'd like to incorporate these questions into the original one. $\endgroup$ – Ralph J Nov 29 '15 at 23:04
  • 1
    $\begingroup$ 'from researching wind power, wind is the hardest part of the weather to predict' High attitude winds are a lot easier to predict than those closer to the ground. That's one of the reasons for considering high altitude wind generators. $\endgroup$ – TomMcW Nov 30 '15 at 1:29
  • $\begingroup$ @DrZ214 High altitude wind predictions 3-4 hours out will usually be quite accurate. Even surface wind predictions are pretty accurate on that time scale. Air has momentum, too. It doesn't change velocity without a reason. $\endgroup$ – reirab Nov 30 '15 at 5:42
  • $\begingroup$ @DrZ214 - high altitude winds are pretty consistent, give me the last year's worth of data and I can probably predict next years with a reasonable degree of accuracy. At the very least, they tend not to be entirely surprising even when they deviate from the norm. $\endgroup$ – Jon Story Nov 30 '15 at 14:52
3
$\begingroup$

The full regulation reads (S121.639)

Fuel supply: All domestic operations.

No person may dispatch or take off an airplane unless it has enough fuel--

(a) To fly to the airport to which it is dispatched;

(b) Thereafter, to fly to and land at the most distant alternate airport (where required) for the airport to which dispatched; and

(c) Thereafter, to fly for 45 minutes at normal cruising fuel consumption or, for certificate holders who are authorized to conduct day VFR operations in their operations specifications and who are operating nontransport category airplanes type certificated after December 31, 1964, to fly for 30 minutes at normal cruising fuel consumption for day VFR operations.

Part a denotes that they must be able to fly to the airport to which they are dispatched. There is an implied assumption that this includes any and all headwinds in the calculations. Likewise part b requires them to land at the most distant alternate (where required) which would also imply headwinds are calculated for.

Interestingly enough the part 91 regulations (non airline) do specifically state that winds must be accounted for,

Fuel requirements for flight in VFR conditions.

(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.

(b) No person may begin a flight in a rotorcraft 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, to fly after that for at least 20 minutes.

$\endgroup$
1
$\begingroup$

The required fuel reserve is for 45 minutes. According to 14 CFR 121 §121.639 Fuel supply: All domestic operations:

No person may dispatch or take off an airplane unless it has enough fuel—

(a) To fly to the airport to which it is dispatched;

(b) Thereafter, to fly to and land at the most distant alternate airport (where required) for the airport to which dispatched; and

(c) Thereafter, to fly for 45 minutes at normal cruising fuel consumption or, for certificate holders who are authorized to conduct day VFR operations in their operations specifications and who are operating nontransport category airplanes type certificated after December 31, 1964, to fly for 30 minutes at normal cruising fuel consumption for day VFR operations.

The factors to be considered in calculating the fuel reserve include the winds. According to 14 CFR 121 §121.647 Factors for computing fuel required.

Each person computing fuel required for the purposes of this subpart shall consider the following:

(a) Wind and other weather conditions forecast.

(b) Anticipated traffic delays.

(c) One instrument approach and possible missed approach at destination.

(d) Any other conditions that may delay landing of the aircraft.

This explicitly states that any headwind experienced now or is expected based on weather forecasts is to be included in calculating the fuel quantity required.

Note that the ICAO annex 6, part I 4.3.6 Fuel and oil supply calls for similar fuel reserves, stating,

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.

In all these cases, headwinds enter into calculation only if the the present weather conditions or forecasts indicate their presence. However, in general, the fuel reserve carried is generally more than the 45 minute reserve, for safety.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.