“NKAWTG!” reads the sign upon entering the 351st Air Refuelling Squadron, 100th Air Refuelling Wing – and it’s true: Nobody kicks ass without Tanker Gas. Michael Buckle and Ben Montgomery joined the crew of “Quid 42” to find out more.

The 100th Air Refuelling Wing & the Boeing KC-135R Stratotanker

The Boeing KC-135R Stratotanker is no stranger to the skies of Europe -with around 15 of the aircraft permanently based at RAF Mildenhall, assigned to the resident 100th Air Refuelling Wing. The 100thARW is the only United States Air Force air refuelling unit stationed in Europe, and as such provides a vital link in the chain required to supply the ability to provide global air power – and has conducted its mission from Mildenhall since being assigned there in 1992. The 100th had been in Europe previously, stationed at Thorpes Abbot Airfield in Norfolk during World War II, as the 100th Bombardment Group (for more history see “The Bloody Hundredth” and “The Century Bombers Return“). Because of their wartime history, the 100thARW are the only unit in the US Air Force with permission to display their wartime markings on their aircraft – the infamous “Box-D”, which adorned the tails of 100thBG B-17s and now has pride of place on the KC-135R.

The ability of the 100th to provide and sustain air power at distance has been tested during many conflicts, most recently the 2011 Libyan Civil War, under the command of Joint Task Force Odyssey Dawn, US Africa Command. Beginning on March 19th 2011, the 100th flew daily missions from Mildenhall down to the Libyan coast, enabling coalition forces to maintain the No-Fly Zone over Libya, put in place as part of the operation to end Muammar Gaddafi’s regime and protect civilians in the country. The 100th, in combination with tankers from other USAF units deployed to Europe for the conflict, topped the tanks of strike aircraft from a wide range of air arms – enabling air assets to stay on station for longer periods of time.

Closer to home, the 100th are often called upon to provide air refuelling for the units stationed throughout Europe. For many Air Forces that utilise aircraft with the Boom/Receptacle refuelling method but who do not have their own refuelling aircraft, the 100th is one of the few units around that can provide the valuable training required to sustain the skill of Air to Air Refuelling (AAR). One of the most frequent users of the services provided by the KC-135 are the 48th Fighter Wing from nearby RAF Lakenheath – and it was with F-15E Strike Eagles from Lakenheath that QUID 42 would rendezvous on July 26th 2012.

The KC-135s in service with the USAF are all of the -135R and -135T variant (with the T model being ex KC-135Q aircraft, used for the special fuel requirements of the SR-71 Blackbird). The 135R and T are equipped with significant upgrades over the original KC-135A model – the most noticeable being the high bypass CFM-56 turbofans. These new engines, coupled with the Pacer CRAG upgrade program (which was awarded to Rockwell Collins in 1995 to modernise the avionics suite of the aircraft, with the final upgraded aircraft being delivered in 2011), means the KC-135R is effectively an entirely different aircraft to the original model. Compared to the KC-135E, the -135R can offload up to 50% more fuel on long duration missions, increases fuel efficiency by 25% whilst reducing maintenance costs by 25%, and is an impressive 96% quieter than the older models (a fact which the towns around RAF Mildenhall will surely appreciate). There is no escaping from the fact that the airframes are approximately 50 years of age (59-1492, the KC-135R flown by the crew of Quid 42 first flew on June 24th 1960), but structural reinforcements (including the complete replacement of the lower wing surface) and rigorous maintenance programs have enabled the KC-135 to keep flying – and the last aircraft are not expected to leave service until 2040. Current airframe hours on the KC-135R fleet are approximately 17,000 hours, with the aircraft structural life estimated to be around 39,000 hours.

KC-135 Factsheet

41.53 Metres
12.7 Metres
39.88 Metres
530 mph at 30,000 ft
50,000 ft
11,015 miles, zero fuel load
86,536 lb Total
Maximum Takeoff Weight
322,500 lb
Maximum Transferrable Fuel Load
200,000 lb
Maximum Cargo Load
83,000 lb
Unit Cost
$39.6 Million (FY98)

Air to Air Refuelling Sortie – “Quid 42”

The first stage of any flight, especially one with the unique nature of a KC-135 refuelling sortie, is the pre-flight briefing. During the briefing the flight crew which consists of a pilot, co-pilot and boom operator (or more formally, the In Flight Refuelling Technician) check the latest weather forecasts, NOTAM’s (Notice to Air Men) and the sortie flight plan for any last minute additions or amendments.

Once their briefing is complete, the crew board the awaiting bus and head out to the aircraft. While the co-pilot begins configuring the aircraft, the pilot carries out his pre-flight walk around. The pilot is checking that all panels have been closed correctly, all of the remove before flight tags have been pulled and that the engines and the general condition of the aircraft is good.

The boom operator and pilot both then carry out internal checks, ensuring all loose objects are secured and that the doors and hatches are firmly closed. The KC-135R is a much louder aircraft inside than the civilian airliners that many will be used to – no extra effort has been taken to blank the noise of the engines and the APU (located inside the cargo compartment just forward of the boom operator’s position), and headphones are required for the majority for the flight.

Departing from RAF Mildenhall from Runway 11 at 09:15L, “Quid 42” (KC-135R 59-1492, one of the newer arrivals to the 100thARW, having been stationed at Mildenhall since January 14th 2012) climbed to FL100 on route to AARA8, before a further climb into the designated block of FL150 to FL170. After departing Mildenhall, Quid 42 coordinated with London Military on the journey up to AARA8, as did the aircraft on their way to refill their tanks from the KC-135. Trade expected for the day was initially expected to be ten F-15Es from the 492nd Fighter Squadron (the “Madhatters”), which dropped to eight once airborne. Two of these eight were unable to make it to the tanker, leaving only six Strike Eagles requiring fuel. The six in question were from SHARK, RAIDER and CLAW flights, all assigned to the 492nd Fighter Squadron.

The receiving aircraft is provided with vectors to the tanker from a local airspace air traffic controller before checking in with the KC-135. Contact is made with the boom operator on a dedicated air-to-air refuelling frequency who will clear them to the pre-contact position, and then to the contact position. The pilots of the receiving aircraft use a set of formation lights on the underside of the KC-135 to align themselves in the correct position to make contact with the boom. There is also a white line across the KC-135 belly centreline, which when lined up with the TACAN antennae to form an inverted T shape puts the receiver in a position on a 30 degree angle – lining up with the boom, which normally sits around 30 degrees. Approaching from directly astern, the F-15Es are hard to spot – appearing from behind the canoe of the extended boom as they approach slowly towards the tanker. “Slowly” is relative, with both tanker and receiver flying around 300 knots – the speed varies depending on the receiver, with aircraft such as the C-130 requiring slower speeds.

The boom is attached to the KC-135 via a gimbal (a pivoted support allowing rotation around one or more axes) attachment, allowing it to move freely with the receiver aircraft. If the boom could not move with the reciever, refuelling would be effectively impossible as the receiver would have to stay in exactly the same position in relation to the tanker to prevent damage to either aircraft. Fuel flows down the boom through a rigid pipe, ending in a poppet valve to prevent fuel from exiting the pipe until a secure connection is made with the receiving aircraft. When this connection is established, toggles in the receptacle hold the fuel nozzle in place to prevent inadvertent disconnection – which could cause aviation fuel to spray across the receiving aircraft, causing a problem especially for those aircraft in which the refuelling point is directly ahead of the cockpit. In a normal disconnect, it is not uncommon for a small amount of fuel to spray out before the poppet valve closes off the fuel flow.

The movement of the boom, and the refuelling process itself, is controlled from the boom operator’s position at the rear of the aircraft. Slightly below the level of the cabin floor, the boom operator lays in a prone position on a flight couch facing rearwards, with an additional position on the right and left hand side (the couch on the right is for an instructor during training). One main window, protected by an external shield which is raised for refuelling, is situated at the interface of the boom and the fuselage, with a small additional window on either side to allow situation awareness – especially helpful when aircraft depart the boom to the wingtip position, or vice versa. The boom is flown by the boom operator using two main control sticks – one on the boom operator’s console (directly below the main window) that controls the retraction of the probe from the boom, and the other at the operator’s right side, which allows the boom to be “flown”. A combination of these two controls, and a skilled operator, allows refuelling to take place. The boom has two ruddervator flying control surfaces, which have around 10 degrees of movement to each side, 10 degrees down and 20 degrees up. Outside of this “safe” envelope (indicated by a green zone on the boom operators dials), the boom and fuel flow will automatically disconnect. Oral commands from the boom operator help the receiver to keep station if they drift too far forward or aft.

Whilst the boom operator does most of the work to ensure the boom stays attached to the receiver, and within the refuelling envelope, some responsibility must also lie with the pilot of the receiver aircraft. For the Strike Eagles refuelling from Quid 42, the pilot maintained eye contact with the visual markers on the underside of the KC-135 for the duration of the refuelling contact, with the Weapons Systems Officer keeping an eye on the connection of the boom to their aircraft. Flying straight and level might sound easy, but keeping position so close to another aircraft and dealing with the small aerodynamic fluctuations of both aircraft is an acquired skill especially considering refuelling can often take place in a turn to remain with the refuelling area.

The KC-135 can also be fitted with Mk.32B wingtip pods, or a hose and drogue unit on the end of the boom, to allow non Boom/Receptacle compatible aircraft to be refuelled. This applies to European combat aircraft, US Navy aircraft and many others. When the drogue unit is attached to the boom, it is the receiving pilot’s task to connect himself to the boom – leaving the boom operator with little work to do during refuelling, as minimal flying of the boom is needed. The fuel flow rate using a drogue unit is much lower than that of the pure boom – down around 1,500-2,000lb/minute as opposed to approximately 6,500lb/minute (at highest capacity, enough fuel flows in 1 minute to operate an average car for 1 year). This isn’t much of an issue for fighter type aircraft, which can only accept fuel at 1,500-3,000lb/min, but for larger aircraft with higher fuel capacities could cause a very long connection time if the hose and drogue system were used.

Fuel is stored in the KC-135’s 22 internal fuel tanks – the majority being integral to the wings, and stored in the lower fuselage lobe. The fuel cell skins are a very thin nylon derivative – a cell weighing 40 kilograms can have a fuel capacity of around 7 tonnes. The fuel used for the majority of missions is JP-8, a common standard for the US Military and many NATO forces. The exception to the rule is the US Navy, who use JP-5 fuel which has a higher flash point, but higher cost.When the receiving aircraft is filled up with fuel, the system will automatically halt fuel flow and disconnect the boom. Manual disconnect can also be accomplished by the KC-135 pilot if the receiver can only be given an allotted amount of fuel, with the boom operator being able to manually disconnect if required. The KC-135 can transfer a maximum fuel load of 200,000lb off the aircraft – the six Eagles tanking with Quid 42 allotted 80,000lb – although with an initial 8-10 aircraft expected, not all fuel was offloaded.

Whilst an aircraft is refuelling, any wingmen take station on the wingtip positions of the KC-135, and slot back and in when it is their turn to refuel. It is not uncommon to see multiple aircraft stacked on the wingtips, but as the flights refuelling from QUID42 were pairs, only 1-2 aircraft were alongside the Stratotanker at a time.

Whilst refuelling, the Stratotanker can be flown manually, or on autopilot, depending on pilot preference (normally autopilot, with manual control being used when autopilot is impossible or for training). The autopilot system can actually be “tripped” when large aircraft such as the C-5 or C-17 are refuelling. The bow wave generated by these aircraft can cause a downwards pitching movement in the tanker, which disconnects the autopilot if the movement is too large to trim out. A slower closure rate is dictated for larger aircraft, to help negate the effects of the large bow wave.

After being on station in AARA8 for over an hour, and having topped up the tanks of all expected receivers, QUID42 returned to RAF Mildenhall, conducting an ILS approach onto Runway 11 around 11:40L. The weather at Mildenhall had been sunny for much of the morning, but the cloudbase in AARA8 was effectively 100%.

Achieving missions such as that flown with QUID42 takes teamwork – and the 100th ARW has a wide range of units assigned to ensure that teamwork can happen, enabling missions to be flown effectively. Whilst being “all in a day’s work” for the men and women at RAF Mildenhall, we would like to sincerely thank them for the privilege of being allowed an insight into their work. Special thanks must go to SSgt Tabitha Lee for liaising with us and organising the flight, and to the crew of Quid 42 for flying us and answering our questions.