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Refueling Realism

An SwRI-developed trainer helps Air Force refueling boom operators gain skills without flying

By Joe D. Wilson

Joe D. Wilson is a principal technical specialist in the Advanced Interactive Simulation Section of SwRI’s Training, Simulation and Performance Improvement Division. His area of technical concentration is in object-oriented development. Over a 23-year career in software engineering he has worked primarily in the telecommunications and digital imaging industries.

Few maneuvers in military aviation are more complex than aerial refueling. The maneuver brings into physical contact two aircraft hurtling through turbulent air at hundreds of miles per hour. One may be carrying thousands of pounds of high-explosive ordnance while the other carries thousands of gallons of highly flammable fuel.

Despite this complexity, military pilots perform aerial refueling numerous times daily, around the world, in all weather and at all hours. Intensive training is crucial to prepare the air refueling crew to ensure the safety of both the refueling and receiving aircraft, as well as to maximize mission success. 

The U.S. Air Force uses an aerial refueling method that involves extending and lowering a telescoping metal boom from a tanker aircraft. Depending on the type of receiver aircraft being refueled, the tanker crewmember who operates the boom (the “boom operator”) uses a unique set of controls to either “fly” the boom’s nozzle into a refueling receptacle on the receiver aircraft or to extend a boom drogue adapter to receive a refueling probe. Developing the skills needed to operate the boom safely and reliably requires repetitive practice, much of which is currently gained during multiple training flights. 

A team of engineers at Southwest Research Institute, in conjunction with Trainer Development at Randolph Air Force Base, Texas, is developing a high-fidelity training device for KC-135 boom operators that will significantly enhance training capabilities, better preparing boom operators for actual missions and potentially saving millions of dollars in operational costs associated with training flights required to certify boom operators. The three-year, $10 million project will result in the delivery of two Boom Operator Weapon System Trainers (BOWSTs). Both will be installed at Altus Air Force Base, Oklahoma. 

Refueling Environment

A KC-135 boom operator works in a small, rear-facing compartment in the aircraft’s belly. Lying prone on a small cot, the operator uses a chin rest to maintain a steady gaze out a small ventral window while directing the boom toward the approaching receiver aircraft. This is done using hand-operated controls to manipulate a pair of wing-like aerodynamic surfaces on the boom.

There is inherent danger to the operation, such as a punctured or disabled fueling receptacle, a cracked windshield or canopy, or a damaged boom nozzle. The high-fidelity training provided by the BOWST device will prepare boom operators to avoid these incidents and potentially save lives and reduce costs related to accidents and damage.

A simulated C-17 cargo plane approaches the tanker aircraft in this view as seen through the refueling boom operator’s window.

The BOWST Training Device

Operation of the boom during a refueling mission presents several challenges to the operator which must be reproduced in the simulation to provide effective training. The boom operator must coordinate with pilots of diverse skill levels and a variety of receiver aircraft in a wide range of environmental conditions, while simultaneously processing visual, aural and physical cues to safely complete the refueling mission. To reproduce these conditions in the training device, a high-fidelity simulation must be provided to accurately model the aircraft dynamics for both the tanker and receiver aircraft, generate and display highly detailed real-time images of the refueling environment, produce simulated forces the boom operator senses on the controls, and create appropriate aural cues associated with the aircraft and operation of the boom systems. Detailed simulation of the refueling system and controls are also required, including the associated malfunctions, to allow the boom operator to practice not only critical motor skills, but also the decision-making skills necessary to work around problems which may occur in-flight.

To address these challenges, SwRI and Trainer Development are creating highly flexible and capable simulations using PC-based technologies. The BOWST will consist of a boom compartment and its related structures; a simulation system including various aircraft systems; aural and aerodynamic models; a visual system with screens, projectors, image generators, and visual aircraft and terrain models; instructor and operator systems including scenario generation, monitoring, control and debrief components; and software support centers. Trainer Development is largely responsible for designing and developing the BOWST system hardware including the boom compartment structure and simulated instrumentation and controls. SwRI is responsible for the full life-cycle development of the simulation software as well as the design, procurement, installation and test of the device subsystems, including visual, aural and electrical control loading components. 

The Boom Compartment 

The BOWST will include a high-fidelity representation of the KC-135 boom compartment, including all controls necessary for aerial refueling. This compartment will be constructed from a combination of actual aircraft components and simulated controls and panels. Simulated controls and panels will be indistinguishable from actual aircraft components and will include a full complement of switches, gauges, controls and circuit breakers modeling the boom operator’s working environment and responding appropriately to simulated events and malfunctions. More than 60 functioning controls and gauges will be installed in the trainer. Controls critical to the operation of the boom will include highly accurate control loading cues simulating the “feel” of major boom controls during a refueling scenario. Forces applied to these controls are critical in simulating the operation of the boom in various conditions. Control loading models will be developed to simulate the mechanical and aerodynamic characteristics of the boom as well as the aerodynamic effects of the bow wave created by the receiver aircraft.

Simulation System

The simulation system, which will run on a PC-based host computer, will simulate the aircraft and boom dynamics and systems and the associated responses according to student inputs and instructor-defined and controlled events and environmental conditions.

One of the key components in the simulation system is the dynamic modeling of the various aircraft and the boom components. Simulation of the dynamics of these components is highly complex, as the motion is coupled. Of particular importance are the aerodynamic effects of the receiver aircraft on the motion of the boom as the receiver aircraft approaches the tanker. Accurate representation of this interaction is critical to providing realistic training. Typically, flight data is used to generate the aerodynamic models necessary to support this level of training. Because limited data is available regarding the aerodynamic characteristics of the boom in the presence of various receiver aircraft, SwRI will generate simulated flight data using a computational fluid dynamics approach. The computer-generated data will serve as a basis for developing and validating the aerodynamic models. This approach will provide a significant cost savings over methods that involve the collection of aerodynamic data by means of a series of flight tests.

Another complication to simulating the refueling environment is modeling the effects of purposeful or accidental contact of the boom or drogue with the receiver aircraft. If performed correctly, contact will result in successful connection for refueling, and if not, it may result in potential damage to the boom and/or receiver aircraft. In either case, the effects on the boom and receiver aircraft must be simulated and visually displayed to provide the appropriate training cues. An advanced collision detection system will be developed to represent collisions between the boom and the aircraft receptacle and other areas of interest, such as canopies, windows and antennas that can be damaged by contact. The system will be so accurate that boom nozzle contact with the aircraft will actually show scratching as the nozzle moves across the aircraft surface. 

There is a significant human element which must also be simulated; specifically, the behavior of the tanker and receiver pilots. During actual refueling missions, pilots will approach the tanker and maintain position during refueling based on skill level and aggression. The BOWST will support this capability by allowing the instructor to define and control pilot skill level, aggressiveness and fatigue through a set of interactive controls on a user interface.

To better prepare the boom operator to adjust to system failures during a mission, SwRI will develop a sophisticated malfunction simulation system. This system will support more than 90 different malfunctions associated with electrical, mechanical, hydraulic, lighting and instrumentation failures. This malfunction system will differ from previous boom operator simulations in that malfunctions will be based on root causes and may be triggered by a series of instructor-defined events. This increased fidelity will give the student a more realistic capability to address the causes of malfunctions and the procedures for handling them.

The SwRI-developed boom operator trainer uses part of the aft section of a KC-135 aircraft fuselage to provide realistic surroundings for trainees.

Visual System

Perhaps the most important aspect to training boom operators is the presentation of visual cues necessary to insert the boom nozzle into the receiver receptacle at a distance of approximately 30 feet. Boom operators use several visual cues to accomplish this, so it is critical that these cues be reproduced in the simulation to provide the most effective training possible. This requires the generation of very high-resolution imagery representing various lighting conditions across a wide field of view. To accomplish this, a visual system is being developed that projects real-time computer-generated imagery on a 12-foot radius screen with a 200-degree horizontal by 54-degree vertical field of view. Ten single-chip digital light processor projectors, synchronized with sophisticated software, will be used to generate these images. Real-time images will include detailed graphical models of the boom and 22 receiver aircraft types, accounting for virtually every aircraft the KC-135 would be expected to refuel. These models will include highly detailed receptacle areas as well as key visual components including articulated control surfaces, lights, antennas and pilots within the receiver aircraft. These visual models will be near-cinematic quality with up to 30,000 polygons. Simulated weather conditions as well as time-of-day lighting specific to a selected training location and date will be supported. Weather conditions will include instructor-defined cloud decks, scud and turbulence. A detailed terrain database based on satellite imagery, accurate to 10 meters, will be included to support training anywhere over the continental United States.

Aural System

Realistic aural cues are critical to support effective training. The BOWST aural system will deliver 10 different dynamic aural cues such as engine noise, fuel flow noise, boom extension sounds and air traffic control chatter. An interphone communication system will also be provided to allow communication among the operator, instructor and student and will also allow simulation of inter- and intra-plane chatter. Sounds for these cues will be collected during actual refueling missions and will be replayed in the simulation based on the current training scenario.

Instructor/Operator Systems

A sophisticated set of instructor and operator components is being developed allowing the instructor to define, control, monitor and replay training scenarios. A graphical user interface will be developed to enable the instructor to predefine many of the events in a training session including the type of aircraft and the associated formation, along with environmental conditions and malfunctions. This will greatly reduce the workload on the instructor and will allow greater focus on training as opposed to operating the system. Two versions of the instructor and operator system will be provided. The main system, which will reside outside the boom compartment, will provide multiple displays providing extensive monitoring and control capabilities. A simplified version will also be provided to allow an instructor inside the boom compartment to have direct access to the controls necessary to customize the training scenario on-the-fly to support a particular training need based on the performance of the student. In either case, the instructor and operator station software will include the ability to dynamically insert malfunctions and to control receiver aircraft using a sophisticated pseudo-piloting interface to introduce situations to which the trainee must respond. 

The BOWST will also provide a recording and playback system that allows a training exercise to be recorded for use during a debrief session. This system automatically records the state of the simulation at all times and allows the instructor to input mark points and comments that can be used to mark particular points of interest during the training. This system will support playback in two modes, either inside the boom compartment itself or outside in debrief conference rooms. Playback inside the compartment will consist of a physical playback of the simulation. This means that the instructor will be able to stop the playback at any point and allow the trainee to restart the training from that point. Playback outside the compartment will involve the playback of two video channels, simulation graphics, audio and data.

A boom operator, lying prone in the belly of the tanker aircraft, watches the receiving aircraft through a small, rear-facing window and uses hand-held controls to maneuver the boom into the refueling receptacle.


The BOWST system will provide simulation-based training which greatly exceeds current capabilities. Through the use of high-fidelity modeling of dynamic, aural and visual effects, trainees will gain experience that is currently only possible during training flights. Training in the BOWST will better prepare boom operators to maximize the effectiveness of training flights and better prepare them to successfully complete refueling missions. Initially, the BOWST will support training of the boom operator in a stand-alone mode. With further extensions, this training capability can be increased to support distributed training missions in which multiple training devices for tanker and receiver aircraft pilots could participate with the BOWST in joint exercises, further enhancing the training experience and further reducing operational costs associated with training flights. 

Comments about this article? Contact Joe Fohn.

Published in the Summer 2005 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

Summer 2005 Technology Today
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