At present, there are two tanker aircraft (KC135-E and KA6-D) and three receiving aircraft (A4F, Lightning and T38) capable of in-air refuelling. When flying one of these aircraft in the default scenery area, one can locate the tanker aircraft using air-air TACAN and/or radar and then receive a full or partial load of fuel by flying in close formation behind the tanker. Refuelling is also possible between aircraft in a MultiPlayer session. The KC135 is a boom refueller, while the KA6 has a hose. The A4F and Lightning are both fitted with a probe for hose refuelling while the T38 is fitted with a boom receiver. At the moment, either type can refuel from any tanker, but in the future it is likely that the correct type will have to be used.
Like the aircraft carriers, AAR is implemented as an "AI scenario". Selecting these normally requires editing the "preferences.xml" file in the flightgear data directory.
There is a shortcut in this case though; simply selecting the Lightning, A4F or T38 should automatically load a scenario containing a tanker, assuming you haven't changed anything in your preferences.xml file.
Assuming this is the case, choose one of the aforementioned aircraft, make sure that "AI models" are enabled and start at KSFO (the default airport.)
Depending on the scenario, you might see the tanker crossing overhead when the sim starts; if not, don't worry.
In the cockpit
Perhaps the first thing to do after starting the engines if necessary is to select the appropriate TACAN channel if your aircraft is so equipped (the A4F and Lightning both are). For the KC135 (by default used by the Lightning and T37) this is currently "040X", and for the KA6D (used by the A4F) it is "050X". Enter this channel using the relevant dropdown boxes in the "radios" dialogue (from the menus, "equipment/radios" or press control-r).
You should now see the current bearing to the tanker indicated in the nav display of the A4 or the TACAN indicator (green needle) in the Lightning. If the tanker is within range, it will also appear on the radar display of the T38 or Lightning. Take off...
In the Air
Turn to an appropriate heading, guided by the TACAN bearing (you should try a "leading" approach to close in on the tanker) and look for the tanker on the radar or nav. screen. Around 5nm away, you should reduce your speed to around 20kts faster than the tanker (these fly at 280 kts TAS) - a "slow overtake". The KC135 will be visible from about 10nm, the KA6-D, being smaller, just over 1 nm. You should use airbrakes as necessary to keep control of your speed should you find yourself overshooting.
Close to within 50ft of the tanker (don't get too close, or visual artifacts might hide the boom from view). You should see indication in the cockpit that you are receiving fuel - there is a green light in the A4 fuel gauge, and you should see the indicated tank load increase.
Getting to this stage is not necessarily easy - it can take a lot of practice. As with carrier landings, this is not an easy manoeuver in real life either and there are additional complications in the sim; the tanker, being an AI model, is unaffected by the wind and flies TAS (True Air Speed), while you are flying IAS (Indicated Air Speed) and are affected by the environment. As in real life, your aircraft will also steadily increase in weight as the tanks fill which will affect the trim of the aircraft. (You might find it helpful to use the autothrottle to help control your speed - ctrl-s then Page Up/Down to increase and decrease the set speed.)
Once your tanks are full, or you have taken as much fuel as you wish, close the throttle a little, back away from the tanker and continue your intended flight.
More advanced topics
Refuelling is possible within a MultiPlayer session given certain conditions. A basic flyable KC135 model is available - the pilot of this aircraft should use the callsign "MOBIL1", "MOBIL2" or "MOBIL3". Other numbers are acceptable, but only these three have A-A TACAN channels assigned. These are 060X, 061X and 062X respectively.
If the receiving aircraft uses a YASim FDM, there are no further complications. Should the receiving aircraft be JSBSim based, the user must make sure that there are no AI tankers in their configuration. This means disabling (commenting out) all refuelling "scenarios" in the relevant aircraft-set.xml and in preferences.xml.
MP refuelling works in exactly the same way as AI refuelling and is a fun challenge. It is best to ensure that your network connection is as free from interruptions as possible; the MP code does a degree of prediction if there is a "blip" in the stream of packets and this can make close formation flight very difficult or even impossible.
Selecting Different Scenarios
There are several AAR scenarios available in the AI directory. refueling_demo.xml has a KC135 circling near KSFO at 3000ft; refueling_demo_1.xml the KC135 on a North/South towline at 8000ft and refueling_demo_2.xml the KA6D on a similar N/S path but at 8500ft.
These can be selected by several methods; using the --ai-scenario command line option, or by editing preferences.xml.
Command line method
Add the --ai-scenario option to your usual flightgear command line; e.g.
fgfs --aircraft=lightning --ai-scenario=refueling_demo_2
Use your operating system's search facility to locate this if you don't know where it is). Open preferences.xml in a text editor (e.g. notepad if on windows) and search for the <ai> </ai> tags. Place a line like
<scenario>refueling_demo</scenario> somewhere within the <ai> tags; you should see other scenarios already there too, perhaps commented out : i.e. with <! -- -->