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Simulation Key Enabler For Auto ACAS Flight Testing


Designed to save aircraft from collision with just seconds to spare, Auto ACAS is meant to operate in a highly dynamic environment in which time and space become quickly compressed. Long before such a system could be flight tested with real aircraft, it had to be developed, evaluated and proved on the ground in some of the most sophisticated high fidelity flight simulators yet designed.

The technical challenge of testing an air-to-air collision avoidance system was even more complex than that facing the developers of Auto GCAS, a sister safety system designed to protect against ground collisions. “We knew early on we had to come up with a better way of testing Auto ACAS,” says Ed Griffin, Lockheed Martin program manager for the Automatic Collision Avoidance Technologies (ACAT) Fighter Risk Reduction Program. “With Auto GCAS we knew where the ground was most of the time. But for this we had to have advanced simulation capabilities and tools to start validation before we went to flight test.”

Two simulation facilities at Lockheed Martin in Fort Worth, Texas, and at Wright Patterson AFB, Ohio, were used to evaluate the Auto ACAS system and prepare for flight testing. The Lockheed Martin facility includes two wide field-of-view dome simulators and four six-monitor desktop simulators that allow for up to six aircraft to fly collision scenarios against one another.

The site also incorporates a handling qualities fixed base F-16 cockpit mock-up which allows hardware-in-the-loop integrated testing of flight controls and avionics hardware and software. The cockpit includes real F-16 controls, displays, throttle quadrant, sidestick and rudders. The simulation lab is also connected to a real AN/ASQ-T50(V)1 P5 wingtip-mounted combat training pod which houses the Auto ACAS algorithm. The pod’s datalink antennas are disconnected to prevent radio frequency energy from being radiated in the lab, and information is instead transferred via coaxial cables.

Although the HUD symbology is a hard to make out in this video of an Auto ACAS encounter in the dome simulator, it is interesting to see the chevrons (a test feature only) begin to warn of an impending activation at second 33. They almost touch at second 37 as the F-16 passes below. However, as the collision threat recedes, the system does not interfere and no activation occurs.

The Lockheed Martin facility also includes a ground station consisting of four monitors, a stick and throttle. The portable system is connected via a USB-to-MIL-STD 1553 hardware adapter to the combat training pod which enables it to act as a simulated threat to an Auto ACAS test aircraft in flight, providing it is close enough for the two pods to communicate. The ground station is therefore used in the build up to flight testing of one-on-one F-16 Auto ACAS encounters.

The Air Force Research Laboratory (AFRL) at Wright Patterson AFB runs Auto ACAS simulation through its Aerospace Vehicles Technology Assessment and Simulation facility. The site includes four fixed base, reconfigurable "Octonian" cockpit simulators and six station control room.  Three of the Octonians are configured with an eight-channel rear projection visual display system, while the fourth has similar displays in addition to a 24-in. liquid crystal display touchscreen head-down display.  The AFRL site is used to evaluate flight-test procedures and independently verifies the development and test work at the Lockheed Martin facility. It also enables the Auto ACAS system to be demonstrated to other Air Force system program offices.

“In flight tests, it is very dangerous to run aircraft at each other and it is too hard to involve more than two aircraft, so we needed to come up with a simulation that had a high-fidelity model and visuals with enough field of view and resolution that pilots could do a tracking task for air combat and formation flying.  We also needed the ability to do repeatable runs and regression testing,” says Lockheed Martin Auto ACAS simulation lead Richard Lehmann.

The simulators played a key role in helping to develop test techniques that met the best compromise between a thorough system evaluation and safe flight testing. “We beat ourselves against the wall for a year and a half figuring out how we could test this safely,” says Travis Millet, ACAS Technical Lead from the U.S. Air Force 416th Flight Test Sqdn. “The closer you make it look like a midair mishap, the better we can evaluate it, but the less safe it is. The more safety planning we put into it, the less it looks like a real midair.”

Simulations were used to perfect key methods to help get around these issues. During my time in Lockheed Martin’s simulators, I found it surprisingly difficult to collide with the other aircraft, and it took a while to learn how to consistently trigger a collision avoidance maneuver even during one of the easiest air-to-air mishap scenarios, a botched straight-ahead rejoin. To ensure activations, the test team came up with a range and bearing method that included start points and action points.

“You start off at a very specific bearing and distance away from the other aircraft,” says Millet.  “He first accelerates to 425 KCAS and, once he sees a particular range and bearing, he checks 10 deg. right and then rolls wings level. Then at 1,000-ft. distance on a 44-de. relative heading, he will bank 15 deg. left. If he does all these things and the target does his, they should get a very repeatable scenario where they cross the same way and get the same activation. At first we were told this was too much detail, but as we continued to go to the simulators we found it was the only way to get consistent activations.”

See preparations for my F-16 Auto ACAS test flight: Auto ACAS F-16 Trianing Day





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