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When an aircraft uses air to air refuelling are there constraints on the maximum altitude this can take place at?

For instance I'm guessing that a Boeing Globemaster when on a long journey would cruise at 8,000m or higher (Wikipedia says service ceiling is 14,000m / 45,000'). Is there any reason why air-to-air refuelling can't take place at that sort of height?

I'm wondering whether there's something about the lack of air pressure that makes it problematic to transfer fuel or the speeds that must be used for the transfer are not viable at very high altitudes.

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The main limitation is that the receiver needs to have enough excess thrust available to be able to precisely maintain formation on the tanker. Near the service ceiling, you typically don't have a lot of excess thrust, so refueling happens somewhat lower than that. Since fighter generally have more excess thrust at a given altitude than a transport will, they can refuel at somewhat higher altitudes.

Other than wanting to have enough excess thrust available to accelerate forward in the event of a "breakaway" call, tankers don't have particular limitations that I'm aware of, though I've not flown those. (I'd assume being right at the limits of IAS / TAS isn't particularly comfortable for a tanker, so at least something below their service ceiling, but I don't know what that limit is.)

Pumps, rather than differential pressure, move the fuel, so that system is unlikely to drive any altitude constraints.

It's common to hear refueling happening in the mid-20's over the U.S. How much that's driven by airspace availability vs by performance, I don't know.

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  • $\begingroup$ Totally ignorant on this but wouldn't an aircraft be easier to control in denser air? This manoeuvre seems involve some precision flying and I would expect ease of control to be an issue. $\endgroup$
    – RoyC
    Dec 22 '20 at 15:31
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    $\begingroup$ @RoyC: Thicker air would give you more control authority for the same deflection of a control surface at the same true airspeed, but IDK if that makes precision formation stuff easier. You want to make really small adjustments to angle to guide the plane onto the refuelling tether; if it takes larger actual control inputs from your hand to do that, it might actually help. $\endgroup$ Dec 22 '20 at 19:14
  • $\begingroup$ @Peter Cordes Makes sense, thanks for clearing that up for me. $\endgroup$
    – RoyC
    Dec 22 '20 at 19:17
  • $\begingroup$ @RoyC: According to @Efe's answer, high altitude refueling would often mean lower indicated airspeed, so the small planes actually are more sluggish. So my assumption of near-constant IAS with altitude apparently wasn't right, and it makes sense that having the plane seem to respond more slowly to control inputs would make things harder for the pilot. Maybe my guess about making larger control inputs isn't how it works. $\endgroup$ Dec 22 '20 at 19:23
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From the NATO website describing the a330mrtt:

The Airbus tankers will be able to refuel a wide range of aircraft such as NATO AWACS surveillance planes; F-35, F-16 and Rafale fighter aircraft; and C-17 transport planes. Refueling can be performed at an altitude up to 35,000 ft while cruising at speeds between 180 knots and 325 knots.

enter image description here

The A330mrtt has a service ceiling of 42,700ft so refueling is constrained to occur below the maximum cruising altitude of the tanker, but not by much.

The service ceilings for the F35, F16 and Rafale are all above 50kft, so fighters are not the constraint. The AWACS service ceiling is 35kft, while the C17 goes to 45kft. The website did not disclose whether each class of aircraft had a different limit.

Fuel is transferred by pump and is independent of speed or altitude.

In response to comments:

The MRTT designation stands for Multi Role Tanker Transport; intended for uses in addition to tankering. It is based on the A330-200 but uses the A340 wing which has reinforcing for two outer engines, which are swapped out for Cobham fuel delivery pods. Standard passenger and cargo decks cover the transport part of the designation; tankering fuel is only carried in the wings unlike the KC10 which also has aux tanks in the cargo hold.

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    $\begingroup$ Why does it have windows? Isn't that where the fuel goes? $\endgroup$ Dec 22 '20 at 2:05
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    $\begingroup$ @DanSheppard dollars to doughnuts it’s a commercial variant, and removing the windows costs more not less. If you want a real answer, ask a real question :) $\endgroup$
    – fectin
    Dec 22 '20 at 3:28
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    $\begingroup$ @DanSheppard Tankers often are also used for troop/supply transport. The passenger deck is still available. See this related answer: aviation.stackexchange.com/questions/15091/… $\endgroup$
    – ceejayoz
    Dec 22 '20 at 11:56
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    $\begingroup$ @DanSheppard the RAF A330MRTTs are operated by AirTanker, which uses them for passenger transport when they aren't being used by the military. en.wikipedia.org/wiki/AirTanker_Services $\endgroup$
    – zymhan
    Dec 22 '20 at 16:36
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    $\begingroup$ @DanSheppard, I've been inside a couple (KC-135 and KC-10) at airshows. The tankage is basically the bottom third to bottom half of the fuselage; the upper part can be used for cargo (self-loading or otherwise). $\endgroup$
    – Mark
    Dec 23 '20 at 2:31
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Airliner-type aircraft fly close to stall speed at cruise altitude as their maximum speed in terms of Mach (Mmo) and their stall speed are within 15-20 knots to each other. Tankers cannot fly any faster (excluding buddy tanking, which generally happens at much lower altitudes and in the vicinity of the carrier), so fighters are also constrained to fly close to their stall speed to stay with the tanker. Fighters (well all aircraft actually) are sluggish to maneuver at that speed and this makes refueling more difficult. Furthermore, the weight added to the receiver during refueling more strongly affects the behavior of the receiver at low airspeeds.

For this reason, refueling is normally done at lower altitudes, to enable the tanker to fly at higher indicated airspeeds to keep the receiver aircraft further away from stall.

The SR-71 used to refuel very close to its stall speed, while the KC-135Q tanker flew at Vmo at an altitude where Mmo was not a problem. Despite this, to stay with the tanker and not stall, the SR-71 had to light one afterburner when its tanks were close to full and refueled with a sideslip.

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    $\begingroup$ At cruise, an airliner may have 15-20 knots between MMO & the low speed buffet limit, but that low speed buffet is significantly above the actual stall speed. $\endgroup$
    – Ralph J
    Dec 22 '20 at 15:03
  • $\begingroup$ True, what I meant is that the tanker has only 15-20 knots to play with. The fact that a tanker won't actually stall doesn't mean that they'll fly it in the buffetting region to help a slow receiver airplane catch up. $\endgroup$
    – Efe Ballı
    Dec 22 '20 at 15:05
  • $\begingroup$ If they fly on the buffet wouldn’t that make it turbulent for the receiver? $\endgroup$
    – Arkhem
    Dec 22 '20 at 19:45
  • $\begingroup$ As long as they've wiped the buffet clean... sorry that was too easy. I don't believe so, the receiver would be further below the tanker's wings so it would most likely be away from wingwash. $\endgroup$
    – Efe Ballı
    Dec 22 '20 at 19:47
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    $\begingroup$ No, ramp open was definitely not SOP. Possible, and makes for cool photos, but doors were closed and we flew high and fast while refueling jets. $\endgroup$ Dec 23 '20 at 19:00
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Some of the answers have hinted at a bit of a problem in the question, but I think it's worth bringing it out a little more clearly: whose cruising altitude do you care about?

When a KC-135Q is refueling an SR-71, it's flying what for it is high and fast--but for the SR-71 is fairly low and slow.

When a (non-Q) KC-135 is refueling an A10, we get pretty much the opposite situation. The A10 is flying high and fast (for an A10)--but the 135 if flying low and really slow (as in: drop not only the flaps, but also the landing gear to add drag).

As others have also alluded, you do need more maneuvering than just normal flying so as a starting point, take the aircraft with the lower cruising altitude, and subtract something like 20% for a starting point on a reasonable altitude for refueling.

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    $\begingroup$ Great point Jerry, same for C-130s. Especially as the performance gets worse the more fuel you onload! :) $\endgroup$
    – Arkhem
    Dec 23 '20 at 11:31

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