Inside the RAAF: Boom times
Wedgetail first cab off the rank to be cleared for boom refuelling from the KC-30A
If ever there was a symbolic milestone that denotes the ever deepening of RAAF capabilities, it was the recent trials to clear 2 Squadron’s E-7A Wedgetail to refuel from 33 Squadron’s Airbus KC-30A Multi Role Tanker Transports.
Once operational clearance for the Wedgetail to tank from the KC-30A is received, 2SQN pilots will be able to maintain air-to-air refuelling (AAR) currency and proficiency in both day and night conditions, facilitating not only training but deployed operational taskings, such as Operation Okra.
Illustrating the utility of AAR, a RAAF Wedgetail has been deployed to the Middle East in support of Operation Okra since October 2014, and it has been tanking over Iraq on every mission, receiving fuel from the USAF KC-135s and KC-10s. Tanking from these aircraft has proven crucial to extending the Wedgetail’s time on station, allowing it to sustain an average of 13.5 hours flight time per mission.
Both the boom clearance and the ‘force multiplier’ it affords the Wedgetail are of themselves important milestones for the platforms and the Air Force as a whole, but it also heralds a wider program to clear the boom for use on the C-17 and the forthcoming P-8 and F-35.
On top of its operational success in Iraq, where it has been using its probe and drogue system to refuel RAAF Hornets and allied fighters over Iraq under Operation Okra, the trials mark something of a coming of age for the KC-30. So it was no surprise when, in July, the federal government announced the acquisition of an additional two KC‑30s to take the RAAF fleet of the type to seven.
But the KC-30’s overnight success has been a long time coming. The initial five KC-30s were delivered with their 17m‑long Aerial Refuelling Boom System (ARBS) booms attached but not cleared for use. Following a January 2011 incident where the boom of the RAAF’s first KC-30, which was being operated by Airbus Military, partially detached while refuelling an F-16, Defence deemed the boom’s flight control software as unacceptable.
This sticking point saw the KC-30 acquisition managed under the Defence Materiel Organisation’s Projects of Concern process while software remediation work was undertaken to correct the control laws and expand the envelope of the boom.
Now, some four years after entering service, the boom has been debugged, and the serious business of clearing it for operational use has been entrusted to the test pilots and flight test engineers of the Aircraft Research and Development Unit (ARDU) at RAAF Edinburgh.
Coordinating the project from the side of Air Mobility Group (AMG) is WGCDR Grant Kelly, 86 Wing’s KC‑30A transition manager. His job is to implement the schedule of clearances for receiver aircraft in the ADF, particularly for the boom.
“It’s a complicated procedure,” he said in outlining the organisational challenges posed by clearing the E-7A to tank from the KC-30A. “There’s a number of teams involved, each with a lot of expertise. In terms of flight test, engineering and flying, getting everyone together amongst a small Air Force to conduct activity, which is heavily tasked to do other things, is always a challenge.”
The nub of the exercise is how two aircraft – the tanker and receiver – will perform as they fly in tight formation at 300KIAS at night, day, or in turbulent air, transferring and receiving fuel.
“When you connect the two aircraft up, we call it a ‘coupled entity’. We have a tanker, a flying boom and a receiving aircraft. And you want to be clear and understand how they react together.
That’s the nexus of our clearance activity – understanding the tanker/receiver pair and making sure it’s compatible within the envelope we want.”
Ensuring the safety of the coupled entity takes thorough planning and fastidiousness that are the hallmarks of ARDU flight testing.
“Each receiver clearance is a mini project where we assess what info we have about a tanker and receiver,” WGCDR Kelly said. “It will involve a team from 33SQN and engineering support and analysis from both sides – tanker and receiver – and then for the boom clearances, it’s highly likely it will contain a flight test component – a team from ARDU will provide flight test engineers, test pilots and support people to assess, design and conduct flight testing.”
Clearance trials for the Wedgetail took place in airspace off the coast of northern NSW, midway between RAAF Amberley, home of the KC‑30A and RAAF Williamtown, base of 2SQN’s Wedgetail. Taking place in the first two weeks of June, the E-7A trials saw a total of 124 wet and dry midair contacts in which, for the first time in its operational history, a RAAF KC‑30A passed fuel to another aircraft via its ARBS.
Tanks a lot
SQNLDR Paul Bowes is the ARDU qualified test pilot assigned to fly the E-7A during its receiver clearance trials, and was responsible for designing the test program, executing it and reporting its findings.
“We started with the envelope that we need to test – the Wedgetail’s operators (2SQN) tell us what sort of altitude and airspeed band that they want to tank in, and we take that, because that determines the number of test points we have to hit. In general terms, you want to examine the handling qualities of the receiver and tanker as a coupled entity at the four corners of the altitude/airspeed envelope,” SQNLDR Bowes explains.
“Within that, you need to look at tanker wake and receiver wake and centre of gravity position, particularly in receiver aircraft where the aft CG is more critical for handling qualities. A heavy tanker produces the most downwash and most intense wake.
Also, the effect of having significant downwash is that if you run into performance limitations from a receiver – as the drag increases on the receiver (while behind the tanker), it may run out of engine thrust to fill up on gas at a particular airspeed.”
Other technical obstacles faced by the coupled entity were likewise the result of clashing with the laws of physics.
“The engineers have to look at compatibility of the two aircrafts’ fuel systems, because even though the fuel is delivered at the same pressure as it is on the ground, the flow rate is much higher which means when the tank valves shut off in the receiver aircraft, it creates a water hammer type effect (or hydraulic shock wave) that can burst the pipes in the aircraft if they haven’t been tested or designed properly,” says SQNLDR Bowes.
“Also, the Wedgetail has a lot of transmitters that you’re putting in close proximity to an Airbus A330, which is all fly-by-wire. So, we needed to closely look at the vulnerability of the flight controls of the KC-30A green aircraft and the boom to particular transmitters that we either didn’t want to, or couldn’t, turn off on the Wedgetail when it tanked.”
Despite the Wedgetail’s plethora of excrescences, from its MESA Top Hat radar to ventral strakes and antennae, SQNLDR Bowes found the aircraft behaved well while locked in behind the KC-30.
“The Wedgetail, contrary to what you’d expect, flies quite nicely, despite the MESA radar. Once you put that big vertical surface behind the tanker you can find some unusual aspects. We found a few that weren’t unanticipated, but they didn’t preclude safe tanking by operational squadrons,” says SQNLDR Bowes.
“Certainly the KC-30A also has quite a sizeable boom, compared to a KC-135 or KC‑10, so it throws off a wake of its own which can be felt in the Wedgetail as you move from astern to contact. But then the flipside is that the Airbus wing is a higher aspect ratio than the older Boeing [KC-135]or KC-10 wing, so the intensity of the downwash is less for the same sort of weight.”
Another benefit of the Airbus design is its supercritical wing – tailored to delay the formation of shockwaves just below the speed of sound – resulting in cleaner airflow coming off the wing, affording smoother tanking.
Stakeholders in the Wedgetail booming trials – ARDU, 2SQN and 33SQN – would teleconference on the morning of flight to cover test points and outstanding maintenance issues. After a formal briefing, specific rendezvous times would be decided and the respective aircraft launched, meeting at the prearranged time in the restricted military airspace blocks (R662 and R671) off the coast of Coffs Harbour.
A typical complement on board consisted of flight test engineers from ARDU and Airbus, plus two ex‑USAF contracted test aerial refuelling operators, and two test pilots (including the highly experienced KC-30 test pilot SQNLDR Chet Takalkar, who
participated in the KC‑30A boom clearance trials in Spain).
“We did seven flights but there were no contacts on the first flight, which was a wake survey – assessing the position of the approach line to the tanker at different distances,” says SQNLDR Bowes.
“The second flight looked at the boom envelope in terms of boom elevation and boom lateral roll connecting and disconnecting at different points in that envelope to see if there was any binding between boom and receptacle. That gave us the safe envelope to build the altitude/ airspeed envelope. Starting at 300ft astern we’d manoeuvre up and down in a box‑type pattern, cued by the tanker flying the boom in a box‑type pattern, and we’d look for discontinuities in the wake, we’d look for vortices that were sudden onset and could cause a control problem. All that was done as part of a gradual process.”
The flight characteristics of the KC‑30’s flying boom and the Wedgetail, within touching distance, were explored next.
“We needed to do a wake survey behind the tanker, 1 to 2ft from the boom nozzle. We’d fly a pattern around the extent of the boom control envelope, looking for vortices, non‑linear control responses that would cause an issue for us if we ended up there. Because every operational pilot will always try to make contact on the centre elevation, different environments, as in night, poor lighting, or flying into the sun means they could find themselves off centre. And if there’s any sort of vortex off the tanker that’s highly localised, that could cause an unsafe control problem.”
Needless to say, the effect of flying an aircraft in the highly energised air behind a large tanker like the KC-30 imposes stresses on the receiver aircraft with the resultant fatiguing of its structure. The Wedgetail used in these trials was equipped with a structural load monitoring system, data from which was sent to Boeing at the end of each test flight to determine whether or not the loads encountered during trials exceeded the Wedgetail’s critical design limitations. Thankfully, Boeing was able to report that the data was well within design limits.
Further, explained SQNLDR Bowes: “This new generation of aircraft allows us to tap into the flight data recorder, which is a low cost, slightly less precise flight test instrumentation, but it gave us everything we needed for the test. On the KC-30A side, thanks to some foresight by Airbus and the RAAF project office, all five aircraft are wired up with the FTI (flight test instrumentation) needed to clear new receivers.”
Boom with a view
Contact with the receiver aircraft is managed by the KC-30’s aerial refuelling operator (ARO), who sits in a rear-facing console immediately aft of the flightdeck.
The ARO’s display is fed with images from the aircraft’s 2D/3D high definition/digital enhanced video monitoring system which incorporates stereoscopic vision and laser-based infrared lighting systems. This enables the KC-30A to tank in adverse weather and at night.
The boom is electrically powered, has a fly-by-wire flight control system and can pass fuel at a peak rate of 4,600lt/min.
WOFF Andrew Newman is an ARO instructor at 33SQN. A former flight engineer, in 2007 he was attached with the USAF to learn the art of flying a boom with the 92nd ARW in Fairchild, Washington.
“About 15 minutes prior to hookup, we conduct the preparation to contact checklist, then the rendezvous procedure would be conducted between the tanker and receiver,” he describes of how the KC-30 prepares for a typical contact.
“Once the receiver aircraft is visual, I will call it on visual and the pilots then hand me the radios and responsibility for the receiver’s management. Pilots say ‘ARO, your radios’, I make contact with the receiver, clear him into stern position behind the tanker, and the boom, and when the receiver is ready, I give a ‘clear to contact’ call and a signal light forward and they would proceed to contact position at 1ft per second. When the receiver is stabilised at the contact position we fly the boom and
extend the telescopic beam to make
WOFF Newman says AROs currently undergo five to six months training which involves qualifying as a loadmaster after ground school, simulator training and flight training. Currently, the wing has 12 AROs trained, against a future requirement of roughly 20.
Describing the art of placing the boom safely in the receptacle, binding two aircraft together at altitude, WOFF Newman explained: “It’s a 3D system, so [the ARO has] depth perception and can judge it fairly well. There’s also cues from the shadows on receiver aircraft – it’s a matter of experience. In making a contact, if there’s movement between the two aircraft, it takes some skill to fly the boom and keep it aligned with the receptacle. You want to avoid scratches on the skin of the receiver, especially low observable aircraft and you have to be careful you don’t break small antennas that may be on a fighter. It’s quite precise and takes time to develop the skills.
At night, the KC-30 has IR illuminators; it’s black and white, rather than colour. That said, it’s easy to refuel at night in the KC-30A – the KC-135 has floodlighting which was more difficult.”
The ARO can effectively ‘tell’ the boom the size of the receiver aircraft to be refuelled, according to WOFF Newman, which then dictates the flight control laws of the boom. “There is a refuelling envelope, or cleared box that the boom will refuel in. The geometry between the two aircraft varies – if you look at a B-2 or F-16, they both have their optimum safe clearances determined. With the E-7A, this program determined that. For each size aircraft, there’s a refuelling limitation in elevation, in roll off the boom’s position and airspeed and altitude.”
Built into the software controlling the boom are numerous failsafes, given the potential hazards of air-to‑air refuelling, as demonstrated by the damage suffered by the KC-30A on the test flight in Spain which lost part of its boom to the Portuguese F-16.
“There are automatic disconnects in the system so that if the lower elevation limit of the boom was exceeded, the boom would disconnect before it allowed the receiver to take the boom down.
“If we didn’t you could wind up with nozzles being snapped off which does happen in the AAR world.”
New boom sweeps clean
The boom has clearly come a long way from the 2011 incident.
“Prior to this test program, the only real boom data in terms of its control handling qualities that we had was with respect to the KC-30A, or F-16, as receivers. When we looked at the Wedgetail, it sits roughly halfway between them. It’s a narrowbody aircraft, which weighs more than an F-16 but much less than a KC-30A. So we decided, with a lot of technical input, to try large receiver flight control laws first,” says SQNLDR Bowes.
“What we found was they were exceptional. The two test boom operators had not a bad word to say about the boom. They both said it was precise and easy to control throughout the envelope.”
Having successfully cleared the Wedgetail for boom use, 86WG and ARDU can turn their attention to clearing other receptacle-equipped aircraft operated (or soon to be) by the RAAF: the F-35, P-8 and C-17. There are plans to deploy a KC‑30A to the US to clear the F-35, while the US Navy is already receiving official briefings on the Wedgetail booming clearance effort ahead of P-8 boom trials.
As for AMG’s C-17 air-to-air refuelling trials, WGCDR Kelly states: “In US service, it’s already cleared to tank behind tanker aircraft. And it’s clearance behind the KC-30A is in our program, to be fitted in over the next 18 months where it’s a little bit fluid at the moment, but it will be one of the first things we look to clear.”
No doubt, these efforts will build the corporate knowledge to develop 33SQN’s ability to conduct boom refuelling. Using the boom on operations will require maturity across logistics, maintenance and operational areas. After its early hiccups, the boom is now winning plaudits from the AAR community, and is helping the KC-30A to deliver on its enormous potential and is helping the E-7A to do the same.
According to SQNLDR Bowes, “It was a very aggressive schedule, it relied on a lot of extra hours and a lot of hard work from people not just within ARDU but certain key individuals in 33SQN, 42WG, 2SQN and the aircraft systems program offices. We got assistance in terms of subject matter expert advice from Airbus and the USAF. We got it done quite successfully in the end.”