Probe and Drogue Explained

You fly the probe, not the tanker. In a probe and drogue contact, the tanker trails a flexible hose behind and below, and you guide a small rigid probe on your nose or fuselage into a funnel shaped basket. If you understand the hose, the basket, the drum and the lights, the whole maneuver starts to make sense.

How a Hose Replaces a Rigid Pipe

The two main refueling systems work from opposite ideas. The flying boom uses a rigid telescoping tube that an operator steers into your receptacle, which offers faster fuel transfer but requires a dedicated operator station. Probe and drogue is simpler to adapt to existing aircraft. The tanker can be specially designed for the task, although refueling pods may be fitted to existing designs in this system. That flexibility explains why the US Navy, the Marine Corps and some US Army aircraft refuel with hose and drogue, as do many aircraft flown by western European forces. The Soviet Union used a hose and drogue system dubbed UPAZ, so later Russian aircraft may carry probe and drogue equipment. The page notes Chinese Xian H-6 bombers modified for aerial refueling and plans to add Russian Ilyushin Il-78 tankers. To see the contrast with boom hardware, read how the rigid boom works.

What the Basket Actually Does

The basket is also called a drogue or para drogue. Think of a shuttlecock. The narrow end, like the cork nose, carries a valve and joins the flexible hose. The wide end stabilizes the hose in flight and forms a funnel to help you insert the probe. The receiver probe is a rigid protruding arm or a pivoted retractable arm on the nose or fuselage. Modern probes are usually retractable for reduced drag on high speed aircraft. At the tip of the probe a valve stays closed until it mates with the forward internal receptacle of the drogue, then it opens and fuel can pass from tanker to receiver. The valves most widely used follow a NATO standard first developed by Flight Refuelling Limited in the United Kingdom and deployed in the late 1940s and 1950s. That common standard lets drogue tankers from many nations refuel probe equipped aircraft from other nations.

Where Does the Hose Go When You Are Done

The hose connects to a hose drum unit, called the HDU. When the system is not in use, the hose and drogue reel completely into the unit. In operation the tanker flies straight and level and extends the hose and drogue, which trails out behind and below under normal aerodynamic forces. After contact you push the hose forward a certain distance, typically a few feet, and the hose reels slowly back onto its drum inside the HDU. That take up opens the main refueling valve and lets fuel flow to the drogue under the appropriate pressure, assuming the tanker crew has energized the pump. A motor in the HDU keeps tension aerodynamically balanced, so as you move fore and aft the hose retracts and extends. That control prevents bends in the hose that would place side loads on the probe. The detailed flexible hose coupling explained shows how the drum, hose and basket work as one assembly.

How Do You Plug a Jet Into a Basket

You extend the probe if it is retractable, then you use normal flight controls to fly the probe directly into the basket. Aim from behind and below, not level with the drogue. That path matters because the drogue is relatively light, typically soft canvas webbing, and it reacts to aerodynamic forces. It can be pushed around by the bow wave of your approaching aircraft, which makes engagement harder even in smooth air. Use a closure rate of about two knots, about walking speed, to push the hose several feet into the HDU and make a solid couple. Too little closure gives an incomplete connection and no fuel flow, or occasionally leaking fuel. Too much closure is dangerous because it can set up a strong transverse oscillation in the hose and sever the probe tip. After initial contact, let the drum take up the slack and hold steady while pressure builds.

Why Do Lights Tell You When Fuel Flows

You do not guess at flow. Fuel flow is typically shown by a green light near the HDU. If the hose is pushed in too far or not far enough, a cutoff switch inhibits fuel flow, typically with an amber light. Disengagement is commanded by the tanker pilot with a red light. Those three colors define your position keeping. Stay in the band that holds the green light, correct gently when amber appears, and back out cleanly when you see red or when your planned offload is complete. The NATO standard probe adds one more protection if your flying gets rough. Shear rivets attach the refueling valve to the end of the probe. If a large side or vertical load develops in contact, the rivets shear and the fuel valve breaks off instead of passing structural damage into the probe or airframe. Pilots call that a broken probe, although it is actually a broken fuel valve caused by poor technique or turbulence. Sometimes the valve stays in the tanker drogue and blocks further refueling from that drogue until ground crews remove it.

Why Does a Steel Basket Earn a Harsh Nickname

USAF KC-135 and French Air Force KC-135FR boom tankers can be field converted to probe and drogue with a special adapter unit. The tanker keeps its articulated boom but carries a hose and drogue at the end instead of the usual nozzle. The boom operator holds the boom still while you fly the probe into the basket. The difference is feel. Most drogue systems use a soft canvas basket that guides a slightly off center probe into the receptacle. Adapter units use a steel basket that naval aviators call the iron maiden because it does not forgive. Contacted even slightly off center, the metal drogue can pivot out of place and slap the fuselage and cause damage. The hose behavior is also different. On contact it does not retract into an HDU. It bends depending on how far you push toward the boom. Push too far and it can loop around the probe or nose, damage the windscreen, or bring you into contact with the rigid boom. Do not push far enough and the probe disengages and fueling stops. The position keeping tolerance is much smaller, so holding contact on an adapter is considerably more difficult than holding contact on a traditional hose and drogue. When fueling is complete, back off carefully until the probe valve disconnects from the basket valve. Off center disengagements can prang the probe or strike the fuselage just as engagements can.

Can One Tanker Feed Two Fighters at Once

Yes, with multipoint hose and drogue installations. Tankers can carry two or more hose points, which lets crews refuel two or more aircraft at the same time and cut time spent refueling by as much as 75 percent for a four aircraft strike package. Some tankers combine systems. The USAF KC-10 carries both a flying boom and a separate hose and drogue system built by Cobham. Both sit on the aircraft centerline at the tail, so only one can be used at a time, but the combination lets one mission serve both receptacle and probe aircraft without landing to install an adapter. Many KC-135s carry dual under wing attachments known as Multi point Refueling System units, while some KC-10s and A330 MRTTs carry similar under wing pods, called Wing Air Refueling Pods on the KC-10. Probe and drogue is not compatible with flying boom equipment, which complicates planning when mixed forces fly together. Planners answer that with drogue adapters and with tankers equipped with both drogue and boom units that can serve both types in the same flight, such as the Boeing MPRS KC-135, KC-10, KC-46 and Airbus A330 MRTT. For small formations without a large tanker, see when fighters tank fighters.

Early probe and drogue work set the pattern you still fly today. After the looped hose, Flight Refuelling Limited developed the improved probe and drogue system now used as one of the two systems chosen by air forces, the other being the flying boom. Post war trials used a modified Lancaster tanker and a modified Gloster Meteor F.3 jet, serial EE397, with a nose mounted probe. On 7 August 1949 the Meteor flown by FRL test pilot Pat Hornidge left Tarrant Rushton and stayed airborne for 12 hours and 3 minutes, taking 2,352 imperial gallons of fuel in ten refuelings from the Lancaster over an overall distance of 3,600 miles for a new jet endurance record. The first combat use followed in Korea, with F-84 fighter bombers flying from Japanese airfields after Chinese North Korean forces overran many jet bases in South Korea, refueling from converted B-29s with the probe in one wing tip fuel tank. Next time you watch a contact video, track the approach from below, listen for the two knot closure, and watch the green light that tells you the drum has taken up the hose and fuel is moving.