Cirrus EIS

You’re pointed away from the destination airport on some controller’s vector and you are sweating the near-empty fuel gauges. You can’t be certain when you’re going to be turned toward the airport and how long it will take to get there.

As a last resort you tell the controller you are minimum fuel and need priority to the runway. Did you violate FAR 91.167, the rule that sets the requirements for minimum fuel when flying under IFR?

Fuel gauges

Does this violate the FARs?

The only certain answer is “maybe.” The reason is that the FAR describes required flight planning, not the actual amount of fuel in the tanks. If you made a credible flight plan and fueled the airplane accordingly, but some unforeseen event caused you to run short of fuel, you may not have violated the rules.

Even more confusing is the title of FAR 91.167, which is “Fuel requirements for flight in IFR conditions.” Look at FAR 1.1 definitions and it says “IFR conditions” are weather conditions below VFR minimums. If you’re flying on an IFR clearance in VFR conditions do the minimum IFR fuel requirements apply? Or is it the lesser minimums for VFR fuel under rule 91.151? I don’t know. And that’s why if you make it to a runway without losing power from fuel exhaustion you may not have violated any FAR.

Another potentially confusing component of the minimum fuel requirements in the FARs is the definition of the quantity of fuel required. For both VFR and IFR flight planning the reserve fuel is expressed enough fuel to fly for either 30 or 45 minutes at “normal cruising speed.”

What the heck is “normal cruising speed?” Most airplane flight manuals or pilot operating handbooks show true airspeed and fuel flow per hour for three cruising conditions—normal, long range, and high speed.

Sounds simple enough. Look at the chart labeled normal cruise, multiply the hourly fuel flow by .75, and that’s the FAR required reserve fuel amount. But that chart will show cruise and fuel burn at a range of altitudes across the airplane’s operating envelope. And the airspeeds and fuel burns will be dramatically different at various altitudes. That’s particularly true for turbine-powered airplanes where fuel flow can easily be double, triple, and more when flying at 3,000 feet instead of a typical cruise altitude far up in the flight levels.

If you want to fly by the rules you could legally plan a flight with a reserve that represents just 45 minutes of fuel burn at an optimum normal cruise altitude as reserve. Legal, yes, but that makes no sense.

The intent of reserve fuel is to accommodate the unexpected. A wrong forecast for winds aloft en route can make hash of any flight plan. So can an unexpected and longer routing clearance from ATC. Unfavorable altitude assignments, particularly in turbine flying, can also burn up extra fuel at a furious pace.

Then there is always the possibility of air traffic and airport system failures. If an airplane slides to the edge of the runway at your destination and wipes out the gear it’s going to take a long time to clear it. This time of the year over much of the country snow removal activity can close an airport for an extended and impossible to predict time. And there is always the possibility of equipment outages in the ATC system that create considerable delay.

That happened to me not long ago on a trip into Bozeman, Montana. While we were still about a half hour out, Big Sky Approach Control lost its radar. That meant airplanes had to be separated “manually” in the terminal area. So we sat in a hold at an initial approach fix for nearly 40 minutes, waiting our turn for the approach. The weather was well above minimums, but until the tower had the arriving airplane in sight, the next airplane couldn’t be cleared for the approach. That would have been a real crisis if only the legal 45 minutes of fuel had remained at the destination.

The best news is that the computerized flight planning services—supported by really terrific improvement in winds aloft forecast accuracy over the last 20 or 30 years—has made precise fuel planning a snap.

Cirrus EIS

What is “normal cruising speed” down low?

My favorite flight planner is fltplan.com. Many swear by ForeFlight. Over the years flying a variety of airplanes I’ve found fltplan.com to be uncanny in its precision. It is a big surprise if fltplan misses time en route by five minutes on a three hour trip. And fuel burn predictions are reliably within a few pounds.

But fltplan plans by the rules. It calculates fuel burn based on route, cruise altitude and winds forecast. It does the same to plan fuel requirement to fly to the destination, and then the fuel needed to fly to the filed alternate airport. And then it adds in the amount of taxi fuel you have specified and calculates 45 minutes of fuel burn at your selected cruise altitude and power as the FAR-required reserve fuel amount at the destination or alternate airport.

Fltplan shows this fuel total as “minimum dispatch fuel,” and the emphasis is on minimum. It’s the least legal fuel you can depart with based on the considerations of winds and routing. And that’s the starting point.

For example, in the King Air 350i that I fly, fltplan calculated a reserve of 512 pounds for one trip, and 441 pounds for another leg on the same day. The difference was created by planning a higher cruise altitude on the second leg, where fuel flow would be lower at normal cruise. So fltplan did the “legal” calculation and included 45 minutes of fuel at the higher altitude where I planned “normal cruise.”

The real fuel load I want includes a fixed reserve, not the “legal” amount. My real reserve is one hour of fuel at a realistic cruise fuel burn. In the King Air 350 that’s 750 pounds of fuel. And even that number is not truly fixed in my planning because doubts about weather forecasts, or bizarre routing that you can get in busy airspace, or unfavorable altitudes along many routes, add to my reserve.

It’s tempting to think the FARs found in Part 91 are conservative, even safe, but they are really bare minimums in many instances. And fuel reserve planning is one of those.

Latest posts by Mac McClellan (see all)
24 replies
  1. Kirk
    Kirk says:

    Mac, I always enjoy your articles. Fuel exhaustion accidents have been declining for several years thanks to improved planning resources as well as onboard fuel calculation and consumption equipment. You provide good guidance on the use of these resources.

    This was a good analysis of legal vs planned fuel. I do want to clarify something your statement, “ As a last resort you tell the controller you are minimum fuel and need priority to the runway”, brings up.

    Minimum Fuel Advisory to ATC does not provide you priority handling from ATC. AIM 5-5-15 addresses this stipulating that this advisory is used when you can not accept any undue delay due to fuel state. You will not be provided priority handling but will be advised if a delay is expected. Usually, ATC will provide you with an estimated time or distance in miles for the planned arrival and approach at the time you advise them.

    Emergency declaration due to fuel state is addressed in AIM 6-1-2. An emergency can be either a distress or urgency condition. An emergency declaration should include the words “Declaring Emergency due to low fuel” or “Mayday, Mayday, Mayday” followed by statement due to low fuel.

    Aircraft will only be provided priority handling with an emergency declaration. It is best not to use the phrase “Minimum Fuel” if declaring an emergency or needing priority handling to avoid confusion and make it clear that priority handling is required.

    Thanks for the years of great articles! Keep up the good work.

    Reply
    • Mac MCCLELLAN
      Mac MCCLELLAN says:

      Thanks for the kind words, Kirk. And you are correct. The only way to move to the head of the line with a fuel shortage is to declare an emergency.
      But, before you get to that state, telling the controller you’re “min fuel” may help. No guarantees. Another technique I’ve used with success a couple times, and heard other pilots use, is tell them you can’t wait and you want to divert to XYZ.
      Maybe that reinforces the seriousness of your fuel situation, or maybe it makes more work for the controller to pass you off, I don’t know. But it can work and get you pointed to your destination. And if that fails, you’re still cleared to a place you can land for more gas.
      Mac Mc

      Reply
      • Marc
        Marc says:

        We just had a tragic accident in the last couple of weeks in Puget Sound where a pilot was flying from Ketchikan to Port Angeles and ditched ten miles short in the Straits due to fuel exhaustion in a C-170. He knew he was short of fuel for the final hour of flying – but BC has COVID restrictions on operations. We are all wondering if that factored into his passing by Victoria airport. Would declaring a fuel emergency get your past the restriction on COVID travel and did that factor into his decision not to stop and top off. The NTSB just issued the first findings on the accident and it’s a tragic tale. He was texting his mother the whole trip down. Enough fuel to almost get there.

        Reply
  2. Chris
    Chris says:

    I also enjoy Mac’s writing and perspective. But in this case its easy to overthink things. This is really an easy call for me. If fuel is getting tight and a controller has me pointed in the wrong direction without an immediate expectation of “direct back to the airport” I declair an emergency, get back to the airport and back on the ground and then consider what other options might have been and deal with any consequences. Job one is ALWAYS live to fly another day.

    Reply
  3. Stan
    Stan says:

    Newbie question here- why not always top off your tanks before all flights? When I get a rental car it comes full and I need to return it full, why not the same with planes?

    Reply
    • Tony
      Tony says:

      Stan, really good question, and it makes me think that every flight a pilot should ask that same question. The reasons for not departing with full fuel may include weight. A heavier plane will not climb as well, which may matter if you are departing on a shorter runway over obstacles, or if you are concerned about losing one of your two engines just as you lift off. Also, as you add passengers and bags, you may exceed the maximum allowed weight of the aircraft unless you take off with tanks that are less than full. Lesser reasons may include cost or convenience; you may not wish to buy fuel at the airport you are transiting. Finally, even with full tanks you need to asses if that is enough gas. Fly safe!

      Reply
    • Mac McClellan
      Mac McClellan says:

      Hi Stan,
      You’re right. In basic singles topping off for every flight most often makes sense. With topped tanks, fuel you can see through the open cap, gives you positive assurance of fuel onboard.
      But in larger airplanes fuel, payload and performance are most often trade offs. Carrying unneeded fuel adds weight that limits performance. The fuel weight also cuts into payload. So fuel weight planning is as essential as any other aspect of flight planning to achieve maximum efficiency with essential safety.

      Reply
  4. John Krug
    John Krug says:

    Fltplan.com does not include required fuel for the two approaches- destination and alternate.

    The FAA has held, through a General Counsel Letter of Interpretation, that the fuel required is to fly to the destination, execute an approach, fly to the alternate and execute an approach there as well. Those two approaches, especially if non radar can eat up a lot of fuel.

    Reply
  5. Stephen Phoenix
    Stephen Phoenix says:

    The only thing missing here is, “ok, how would you write the rule better”?
    I had that discussion with an FAA person once on a certification issue and I had to admit to not having a better idea. Rats!

    Reply
  6. Ed Wischmeyer
    Ed Wischmeyer says:

    I think this article misses the point. Fuel reserves should not just be about legalities. Rather, “adequate” fuel reserves in my mind mean that, in addition to legalities, adequate fuel quantity (1) does not force the pilot into unplanned, risky situations and (2) does not cause any additional inflight stress.
    Flying VFR cross country, my reserves are two hours. Why? Because if I land somewhere and gas is not available as advertised, I can still fly to another airport, an hour away, and land with an hour’s reserves.

    Reply
  7. Bo Warren
    Bo Warren says:

    I’m 71 and been out of the cockpit since 2002 but do the terms ” N1234 declares Minimum Fuel” and “N1234 declares Emergency Fuel” still work with ATC? The min fuel declaration means ” I can not accept any futher delays for landing clearance” and the term Emergency Fuel means ” I need to proceed direct to the airport and land now or I will be out of fuel”. Declaring an emergency can mean a thousand things but the terms “Minimum Fuel” and “Emergency Fuel” are specific and to the point. Your thoughts?

    Reply
  8. John
    John says:

    I appreciate the thoughtfulness behind these discussions, but it gets too anal. My seat time limit is 3 hours . Most airplanes go well beyond that. Just do 3 hour legs. Nuf. Said. As far terminology with ATC (I’m a former controller)on minimum fuel vs emergency . Don’t worry about getting all official. Just tell them that things could get tight for you if you don’t head towards the airport relatively soon or something similar. It’ll do the job.

    Reply
  9. Eric LeVeque
    Eric LeVeque says:

    Good article. The minimums in the FARs are just that. Minimums. Not something to aim for. A line has to be drawn somewhere. If you can’t at least meet these minimums you’ll need to plan a stop. I’d like to add that it’s imperative to verify the accuracy of your fuel gauges from time to time.

    Reply
  10. Cesar Orbegoso
    Cesar Orbegoso says:

    In the last two/three days this month, Feb 2021, I read an article on the web regarding communication between ATC and pilots. The article cites terrible accidents abroad and in the USA. It mentions English being the standard communication language mode between pilots and ATC. I think this article was written immediately following the lost of an engine by United aircraft. The article indicates the use of standards codes and phrases
    in pronunciation for not native English speaking as well as code words to use in case of emergency. The article challenges to listen to ATC in Mexico and detect some unfamiliar meanings in the communication between pilots and ATC. Can you help me. Thank You.

    Reply
    • Mac MCCLELLAN
      Mac MCCLELLAN says:

      Hi Cesar,
      Your concerns are real. In 1990 an Avianca Boeing 707 ran out of fuel on its second approach to JFK and crashed on Long Island. The NTSB found that communication difficulties, lack of standardized terminology, and even cultural differences, prevented the Avianca crew from successfully relaying how critical their fuel status was to controllers.
      Mac Mc

      Reply
  11. Mike Sheetz
    Mike Sheetz says:

    Good stuff here. Too often pilots have a tendency to stretch fuel, and there really isn’t such a thing. You either have enough or you don’t. Best to have a bigger margin than not. The price of fuel becomes very unimportant when you are staring at the very possibility of running out and being challenged to complete a flight favorably. Some other comments are spot on.

    Reply
  12. Marc
    Marc says:

    Fuel vs. Altitude: Since the danger comes at lower altitudes and approach patterns, wouldn’t it make sense to use a 3000′ AGL normal cruise for your minimum fuel calc? You need the fuel then, not at a flight level XXX?

    Reply
    • Mac MCCLELLAN
      Mac MCCLELLAN says:

      Hi Marc,
      In turbine airplanes 45 minutes worth of flying at 3,000 feet would be a very fat reserve, indeed. But when you’re headed into places like Teterboro it’s not a bad idea. Pilots new to the New York area will be surprised to find themselves down to 3,000 feet, or something close to that, 50 or more miles from the airport.
      And that’s a lot of gas to have in reserve to ease your worries.
      Mac Mc

      Reply
  13. Kirk
    Kirk says:

    Part of your contingency fuel planning should include what delay might you encounter at your alternate? Particularly at busy terminals chances are other flight planning software programs have selected the same alternate for other flights as well. Follow the guidance and everyone shows up at a less capable airport expecting to be vectored just outside the FAF, slide down the G/S, and on to the parking spot with just a tad more than emergency fuel.

    Reply
  14. Charles Lloyd
    Charles Lloyd says:

    Flew a swept wing mid-sized jet where the low level lights were on before you got to VFR reserves. No way did I want to see those lights.

    Earlier SE Cessnas indicated E with 45 minutes on each tank. Alway wanted at least 1 hour VFR and it depended on conditions IFR.

    Great article Mac. Keep’em coming

    Reply
  15. Steve Green
    Steve Green says:

    A Part 121 operator such as mine always carries enough fuel to reach the nearest suitable airport other than the destination, regardless of arrival weather. This is in addition to the required 45 minute reserve at “normal cruising fuel consumption; it is precisely aimed at the air traffic and airport failures that Mac mentioned. If an alternate is required, we have a variety of ways to plan the fuel, depending on the destination. Bear in mind, however, that all of our fuel planning is based on a huge database of statistical delays and possible routings.

    In any case, fuel to the alternate will always contain the necessary fuel to:

    (a) Perform a missed approach at the destination airport;

    (b) Climb to the expected cruising altitude;
    
(c) Fly the expected routing;
    
(d) Descend to the point where the expected approach is initiated; and
    (e) Conduct the approach and landing at the destination alternate airport.

    Regarding the amount of fuel required by turbine powered airplanes at low altitudes, it is obviously significant, as Mac has said. Nonetheless, the basic requirement for international flight planning is to carry 10% of the total enroute fuel burn as a reserve, plus enough fuel to fly for 30 minutes at an altitude of 1500 feet above the destination. These numbers have been massaged through various op specs, and it does not apply to domestic operations, but this may give GA pilots an idea of the degree of conservatism built into our fuel planning.

    The rub comes when the required fuel cuts into the available payload. I have, on occasion, had to kick passengers or cargo off my airplane because of fuel requirements. That is rare in the transport world; it gets more problematic in smaller aircraft that are more often marginal in their useful load capabilities. That is what can drive pilots to cut the margins a bit thin.

    You are legally obligated to arrive at the alternate with 45 minutes of reserve fuel remaining. That is what you will use when the wheels don’t come down over the marker for the approach at the alternate. If the wind forecast falls apart and you are burning more than planned, that means you need to land short and gas up. We have done that in the scheduled world many times. It used to be pretty common on westbound flights coming back from Europe. Moreover, it is not the controller’s job to protect your fuel reserve; that’s your job. If it looks as though you’re going to get jerked around by ATC, you need to suppress get-home-itis and divert. I have also done that many times, and I hate having to do it, because it screws up the rest of today’s schedule and probably tomorrow’s as well, and virtually guarantees some kind of goat-rope for the next couple of hours. But we are simply not going to mess around with low fuel.

    One final caution: when you decide to divert, divert. Don’t get suckered into aborting your divert and coming back for another try, and don’t fall for any wishful thinking on the controller’s part. If you have reached the fuel level at which a diversion is necessary, do it, and stick to the plan, unless it too becomes unsafe. There will be enough additional problems arising from the diversion; you don’t need to compound them.

    Reply
    • Larryo
      Larryo says:

      Steve,

      Good points, and good fuel planning ideas and it’s nice to have a data base of what the trip might look at for planning purposes from 1000s that have done it before. One thing about airline vs GA is that the GA operation has a LOT more flexibility, whereas the airline is usually a scheduled operation and the passengers expect you to deliver them to a specific airport at a specific time. The GA guy can adjust his time frame for better fuel planning and weather and often has a choice of airports close to destination that could be used as an alternate.

      As for the “requirement” to arrive at the alternate with 45 min of fuel, that’s probably a company policy which is a bit unusual. The FAR fuel requirements are for dispatch only. So once you leave the ground with the requirement, all bets are off for the remainder of the flight. But, prudent choices should prevent ever landing with that little fuel. My personal minimum on any flight, part 91, 135 or 121 is wheels on the ground with one hour of fuel, and if that is in question, I’ll declare min fuel.

      As for the decision to divert to the alternate, totally agreed, once decided, there’s no turning back other that a most unusual situation. But had one where on the miss in a foggy situation we had decided to divert, but on climb out saw that the other half of the field was VFR. So we held for a short time (had the fuel) and once the tower called the vis VFR we returned for a visual. (early morning fog that was lifting faster than forecast). That worked well for not inconveniencing a load of pax with a divert.

      I’ve always enjoyed the GA world for the flexibility and capability that the airline world didn’t enjoy. However, there’s still the same level of responsibility and we must always resist the temptation to push fuel. It’s MUCH easier in the GA world to just stop and top off with all the airports available to the GA pilot. No excuse for running out.

      Reply
  16. Larryo
    Larryo says:

    Mac,
    Great article and some good thought for thinking about fuel. A few comments:

    Sure, the FARs do give us some guidance, they give us the min fuel requirements for dispatch, and sometimes that’s plenty. There is no requirement to land with those mins, even in the 121 world.

    And in some cases, one would need way more than the mins.

    A few examples:
    I flew a route from the west coast to the midwest in a 737 several years ago and we always dispatched with just the mins, that’s the best we could do considering the payload, burn, etc., and it worked quite well most of the time. Wx was good and plenty of alternates and can’t remembering ever to use the “min fuel” on that route once.

    Had another shorter run where I declared min fuel on climb out. With our landing weight, and fuel requirements for the flight I had one shot at the destination.. barely and would have to divert. Worked that time, but already diverted twice that night for the same destination. Traffic and wx were not cooperating.

    And, I’ve declared min fuel several times over my career (both pro and private) and ALWAYS got the best of handling, and if they couldn’t accommodate me, they told me upfront so I could divert early. And, yes they would give me priority over and above the “no undue delays”. Never had to declare a fuel emergency.

    My mins for the Baron are the FARs……. most of the time. And that’s because “most of the time” wx is good and my east of the Mississippi flying provides for a lot of alternates. However, I’ve had flights where fuel loads had to be over twice what was needed for the leg, as fog was at mins at departure and below at destination… so enough fuel was required to get where the wx was good.

    With the great fuel measuring we have now, there’s should be really no reason to cut it close. And we always need a plan B. If plan B disappears, then we need to divert now, and that does happen… ATC issues, wind busts, etc., etc.

    Reply
    • Mac MCCLELLAN
      Mac MCCLELLAN says:

      Hi Larryo,
      Your comments about controllers are spot on. I spent most of my career flying out of the northeast U.S. where the controllers talk fast, are often abrupt, and show signs of impatience with pilots who are not paying attention.
      But like nearly all controllers, they are honest. When somebody says they have a fuel concern the controllers respond with help if they can, but if the airspace and airport are saturated and they can’t get you to the runway quicker, they immediately tell you that, too.
      That’s exactly what we want and expect from controllers. They don’t want an emergency anymore than we do, and by telling us immediately upfront what’s possible, nearly all critical situations can be avoided.
      Thanks,
      Mac Mc

      Reply

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