During the ongoing War in Ukraine, one can regularly spot planes from Royal and US Air Force flying over the Eastern parts of Poland, see screenshot. They often fly in circles and return after several hours to their base. As a newbie in aviation, this particular aircraft looks like a transport to me and not like a reconnaissance craft. I'd therefore expected it to land somewhere at some point and unload. Why do they instead often fly in circles for a longer period of time?
1$\begingroup$ Didn't know military planes could be traced by FR24 $\endgroup$– Déjà vuMar 22, 2022 at 10:15
76$\begingroup$ To quote my 12yo nephew: because flying in squares is hard ;) $\endgroup$– MSaltersMar 22, 2022 at 10:33
12$\begingroup$ @Déjàvu of course, not all of them fly always with their transponders on. But it profoundly simplifies a lot of peoples' lives when a military plane flies over a friendly teritory and in somewhat busy airspace. $\endgroup$– fraxinusMar 22, 2022 at 11:45
6$\begingroup$ @fraxinus And sometimes it says "we're here, keeping an eye on you" to a geopolitical opponent. $\endgroup$– ceejayozMar 22, 2022 at 18:44
2$\begingroup$ @Déjàvu FR24 shows the planes with their transponders turned on. Military planes can decide to be traced. It is a bit different :) . For example, sometimes the drones can be traced: flightradar24.com/FORTE11/2b3cda55 and there is surely a reason why the drone operator wants its drones to be seen $\endgroup$– EarlGreyMar 23, 2022 at 10:28
Generally speaking, Air Force (and military in general), aircraft will "orbit" (fly in circles around a given point) when the mission calls for them to be "on station" (nearby and ready to perform other parts of their mission). Ground support craft (A-10, Su-25) may orbit on station awaiting a call for them to intervene in ground combat. Fighters may orbit near an area where interceptions are expected to be needed.
In this specific case, the KC-10A is a mid-air refueling aircraft. Basically a flying gas station. It was probably holding on station to service aircraft needing refueling (thus the holding pattern).
This video on YouTube is probably not dissimilar from that KC-10's actual mission, refueling the E-3 AWACS, which have been maintaining situational awareness for NATO.
8$\begingroup$ To further add: The aircraft's mission calls for it to be in a specified location over the ground. But the aircraft needs to generate lift in order to remain in the air, and lift is generated by air moving over the wings, so the aircraft needs to fly forward continuously... and flying forward moves them away from their desired location. Hence the circles. $\endgroup$ Mar 21, 2022 at 20:56
26$\begingroup$ @randomhead, I think the question was more "why don't these transport category airplanes go from point A to point B and offload cargo?" rather than "why can't they just magically stay in one place in the sky?" (But then I could be wrong...) $\endgroup$ Mar 22, 2022 at 1:20
9$\begingroup$ Note also that depending on the configuration, a tanker can transfer fuel from its auxiliary tanks (those meant for fueling other aircraft) to its main tanks (those meant for fueling the tanker itself). This means that they can remain airborne for much longer than most planes their size depending on how many other planes need to refuel. (Not always true, since some tankers carry a different type of fuel for other planes than what they use themselves - I'm not sure which is the case in this example.) Source: I programmed the fuel system on a training sim for this exact model aircraft. $\endgroup$ Mar 22, 2022 at 13:28
$\begingroup$ If you're interested in tanker operation I'd also suggest this video: youtube.com/watch?v=w5GoJ_ecMQs A400M as tanker refueling Panavia Tornados. Has some really nice footage from both on board the A400M and the Tornados $\endgroup$ Mar 23, 2022 at 13:24
2$\begingroup$ The last link is from 2014. $\endgroup$ Mar 23, 2022 at 13:45
That is a tanker. Flying gas station. It will orbit for several hours, specified time and track. Various other aircraft (fighters, AWACS, JSTARS) will come up and refuel as needed.
While you see the tankers in flightradar24, you do NOT see the dozens of other aircraft that are also on the border. F-16, F-35, Typhoon, F-16, etc, etc.
There are generally 2-3-4 tankers orbiting near the border between NATO and Belarus/Ukraine. USAF, RAF, RNLAF.
Taking off and climbing to altitude takes time. Having planes already in the air facilitates defense from a hypothetical attack and may deter that attack altogether when the attacker anticipates a faster defense. The term "military readiness" is relevant.
Also, having planes in the air all the time in a tense situation facilitates attack, as planes taking off become routine and so the enemy won't be immediately alerted when the attacking ones take off.
Because of that, keeping planes in the air just being ready may be worth it despite being expensive.
Unlike a helicopter, an airplane cannot hover in place, it needs to move forward to stay flying. That's inconvenient when you want it in the same place. A common mitigation is to turn it slightly to one direction, so it makes a circle, staying more or less in the same place. The smaller the circle, the tighter the turn, the lower the hovering efficiency and loitering time.
6$\begingroup$ Even a helicopter will use less fuel when it's flying around in circles than when it's hovering. When an airplane or helicopter is flying, the wings or rotor will be constantly encountering air that is relatively motionless. By contrast, when a helicopter is hovering it will constantly be sitting in a downdraft of its own creation. $\endgroup$– supercatMar 23, 2022 at 21:53
This particular type of plane, a KC-10A tanker is used for refuling other military aircraft with aerial-refuling-capabilitiy, such as for example German EFA Typhoons or F-16s and AWACS airborne radar aircraft.
The track they're flying is called an orbit. They literally orbit a waypoint which other planes can request and fly to, to refuel. Also other type of military aircraft are using this type of flight-path as it fits to their assigned mission. Examples are recconaissance-aircraft, groundattack-aircraft on standby, etc.
Right now, there are - give or take - about 3 tankers regularly flying close the ukrainian border to support allied military aircraft on patrols and reccon missions in relatively close airspace.