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Let's say I'm on a long-range flight at my requested altitude of FL250, and I am handed off into a very busy sector where the controller tells me "Descend and maintain FL210, for traffic."

At FL250, my fuel burn and capacity is sufficient to make the flight with better than IFR reserves.

But if I have to descend to a lower altitude, my fuel burn will increase and my true airspeed decrease to the point where I would have to stop en route for fuel.

How can I describe my situation to ATC in such as way as they will allow me to stay at 250? Controllers do have some obligation to get you to your intended point of landing, and if giving you an altitude, airspeed, or vector that will adversely affect your flight, shouldn't there be some way to describe that?

What about saying, "if you give us 210, we're going to have to declare minimum fuel"

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    $\begingroup$ Descend as directed and start to figure out where to have your refueling stop? $\endgroup$
    – hmakholm left over Monica
    Jan 16, 2015 at 5:53
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    $\begingroup$ @rbp Are we talking that with 4.000ft lower, you are having such a significantly lower groundspeed that you will spend your holding fuel, alternate fuel and contingency fuel and risk cutting into your reserve? $\endgroup$
    – SentryRaven
    Jan 16, 2015 at 6:37
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    $\begingroup$ What are you really asking @rbp? You can request a different action but at the end of the day if the controller tells you to descend you do it. If that means you need to stop and refuel then the answer is obvious. $\endgroup$
    – GdD
    Jan 16, 2015 at 12:56
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    $\begingroup$ I guess I'm asking how negotiate for what the altitude I want. This isn't any different than, say, getting a 90 degree off-course vector, except in a different dimension. Does ATC have any requirement to accommodate me for a non-safety related request that will adversely impact the flight? $\endgroup$
    – rbp
    Jan 16, 2015 at 17:17
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    $\begingroup$ @rbp In my experience, the willingness of ATC to accommodate pilots varies widely from country to country. If I had a good reason for not wanting to descend, the first thing I might do is ask if they can give me a higher altitude (assuming I'm able to climb). I'd also want to know the nature of the traffic. If there's someone at the same flight level that I'm overtaking, can I just slow down. If there's someone overtaking me, can I speed up (if that's possible, but that would also burn more fuel). $\endgroup$
    – Terry
    Jan 16, 2015 at 19:13

2 Answers 2

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As I said in my comment, I don't think there's any single answer to this question because there are just too many variables and the scenario itself seems a little artificial.

But, the first thing you can do is negotiate with ATC. Accept the descent instruction but request a return to FL250 as soon as possible. Offer to fly at another altitude (including above FL250 if possible), or on an alternative route, or with a 10-degree course deviation or anything else that gives ATC options for keeping you at the altitude you want. Of course, there's still no guarantee that the controller will be able to allow it.

The next thing would be fuel management: depending on the aircraft and your current power settings, you may be able to sacrifice speed in order to gain range. But that might not help either if the end result is still that you won't have the reserves you need.

If none of those those things are possible then there's no good option except to plan a fuel stop. Declaring an emergency would be an abuse of the system and could lead to an investigation: you have plenty of fuel, just not enough to get where you want to go, and there's no immediate danger to the aircraft. (There's an NTSB ruling where a pilot declared minimum fuel, apparently in order to keep on a direct route. The FAA grounded him, he appealed to the NTSB and lost.)

In fact, you hopefully already planned for a possible fuel stop because if the winds are so different at FL210 and FL250 that it makes the difference between making it to your destination or not, that's something you should have found out during flight planning. But even if you didn't, or if the wind just changed suddenly, a diversion for fuel shouldn't be a big deal.

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    $\begingroup$ And don't forget that pilot planning to fly at FL250 should have some plan for what to do if forced down to FL100 by pressurization failure. $\endgroup$
    – Jan Hudec
    Jan 16, 2015 at 17:52
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    $\begingroup$ yes, that's an emergency, however. the scenario described is not. $\endgroup$
    – rbp
    Jan 18, 2015 at 16:42
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    $\begingroup$ If you encounter adverse conditions and you have to stop for fuel in a turboprop, its really no problem. $\endgroup$
    – rbp
    Jan 18, 2015 at 16:49
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Ask if the traffic is ahead or crossing. If it's ahead, you can negotiate to remain at the current level by slowing down to 'his' speed/Mach no. If this is not an option, then I'd ask if I'd ask for a radar vector to avoid the traffic (possibly even a direct to YXZ) for separation - the controller will be able to see on her/his radar screen.

If still no joy, then you have no choice but to descend. Once down at the new level, ask when can you expect further climb - that way you can plan on whether you need to recompute your fuel or merely cruise for the next 20 mins (or whatever the case may be).

Finally, worst case scenario - lets say that you are stuck at your lower FL or altitude for the remainder of your trip knowing that you will not be allowed to climb, then your options are to request direct to ABC (short cut) and possibly make up some fuel at expense of arriving later due to your lower TAS/Mach no. or, get out the performance manual and calculate at that particular level/FL, what is your best endurance fuel flow.

Taking it further....now lets say you have exhausted all possibilities and ATC is still not accommodating you - now you can get an update of your destination's weather and assess whether at your revised ETA if the weather is still AT OR ABOVE landing minimums FOR THE APPROACH IN USE AT THE TIME OF ARRIVAL. If the weather is landing minimum or better, then you can legally continue to your destination using your alternate fuel as extra fuel. The legality is to land with reserves, not alternate plus reserves.

You declare minimum fuel to ATC if at any stage of the flight, you anticipate that any other changes to the present flight route/conditions will cause you to land with less than reserve fuel. (note: this implies you now still have enough fuel to land with reserves intact. If you don't, then it is not minimum fuel you declare, but a fuel emergency)

So, if the weather at your destination is CAVOK and you filed IFR, since you are carrying an alternate, you are legal to use your alternate fuel after checking the weather (as previously mentioned).

If your destination weather is marginal....lets say it's RIGHT AT MINIMUMS you might find the weather whilst you are on the instrument approach reduces to below landing minimum as you pass the final approach fix...lets say you descend to 100 feet above the runway and cannot identify threshold markings, runway lights, edge lights - the landing environment [first 3000 feet (914 metres) of the runway], then you must perform the missed approach and assess whether you have enough fuel for another approach or merely fly to your alternate. This is an example whereby you would do what you can to preserve your alternate fuel as you may actually use it in the latter part of your flight.

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