Pick 'em up, put 'em down…

Flying Safety, July, 2002 by Sue Keister, David Nelson

Those of us who can remember our first day of basic training probably do not have fond memories of this phrase. Yet this phrase has been commonplace in the KC135 Tanker community for almost 40 years. Each time a KC-135 needs to refuel a probe-equipped receiver aircraft such as all U.S. Navy and most NATO and foreign air service aircraft, the Boom to Drogue Adaptor (BDA) kit must be put up on the air refueling boom.

When the mission is over the BDA must come down. Sounds simple, right? Unfortunately, it takes maintenance crews around two hours to complete the aircraft configuration change each time this has to happen. In addition to this, when the KC135 has the BDA kit installed it cannot refuel USAF aircraft equipped with an Air Refueling Receptacle (ARR). This means if an aircraft has to refuel a probe-equipped aircraft in the morning and a receptacle-equipped aircraft in the afternoon, there is going to be a very busy maintenance crew "putting it up... and taking it down."

There are other disadvantages to using the BDA kit. The kit consists of a hard metal basket attached to a stiff nine-foot hose. This leaves the receiver aircraft with a very small refueling window. In-flight refueling is inherently a potentially dangerous operation, but add turbulence, darkness or a new receiver pilot to this equation, and the danger of the receiver aircraft making contact with the drogue in ways neither the tanker nor receiver ever intended increases greatly. For these reasons, many receiver pilots refer to the BDA as the "Iron Maiden," and most do not like the idea of having to refuel against a KC135 because of it. Most Navy pilots will tell you that, short of a bad weather night aircraft carrier landing, refueling with the BDA is the most difficult thing they do.

Funding was made available in 1996 to modify 20 KC-135R Tankers to a Multi-Point Refueling System (MPRS) configuration by installing a MK32B-753 Air Refueling pod on each wingtip to increase operational capability and safety. The modification of all 20 aircraft is nearing completion, with scheduled delivery of the last aircraft in April 2002. The refueling pod contains a 74-foot hose with a flexible paradrogue attached to the end of the hose. Once the hose and drogue is trailed out of the pod it is well behind the KC-135, and it provides a much larger, and safer, refueling window to the receiver aircraft as well as allowing two aircraft to be refueled at once.

There were some initial problems after the new MPRS system was operationally introduced in 1997. A team of engineers, logistics specialists, contractors and field unit representatives were assembled to analyze and correct the problems. Within the past 18 months the causes of the problems have been determined, and corrective actions are either in place or will be within the next year.

The first problem addressed was the drogue itself. Each drogue has 84 hooks that hold the para-wing portion of the drogue in place. When the system was first put into service these hooks were made from a Dupont plastic called Hytrel[TM]. It was discovered that this plastic was susceptible to ultraviolet degradation that made the hooks brittle, and when they reached the sub-zero temperatures seen at operational altitudes they were easily broken. This was a FOD hazard to the receiver aircraft, and the lost hooks could cause drogue instability. These plastic hooks have been replaced with a metal hook and retaining clip that solved the problem. This one action has greatly enhanced safety and in-flight stability of the drogue.

Another problem the system faced was inadequate hose take-up response. Aerodynamic forces keep the hose and drogue fully trailed behind the pod, but when a receiver aircraft contacts the drogue and pushes it in towards the pod, the pod needs to take up the slack in the hose, and this was not happening properly. If the hose slack is not taken up when the receiver makes contact with the hose, the hose may/will create a large bend that can make contact with the receiver aircraft. The hose can also then be violently whipped around by the KC-135's wake and damage the receiver.

The team determined the best course of action was to beef up the hose take-up system, known as the Tensator System, by adding an additional Tensator and other hardware to keep the system functioning properly. The Tensator System previously used five motor springs to take up the hose slack, so a sixth motor spring was added to increase the take-up force.

The tensators are basically a set of big parallel springs that are put under tension as the refueling hose is unwound. This tension allows the refueling basket/hose to follow receiver movement fore and aft. The tensators actively dampen any slack or hose whips as they develop, thus protecting the receiver and tanker components from damage. There are now six of these springs developing a total of up to 270 pounds of force that is trying to pull the hose back into the pod. So when the receiver aircraft pushes into the drogue and overcomes the aerodynamics load on the drogue, the hose slack is taken in by the Tensator System. Since these changes were made, hose takeup response has been increased by leaps and bounds.