Selected by: SAAQ
In the mid-seventies America faced a new and rising crisis, with US commercial jets being hijacked for geopolitical reasons. To tackle this problem two American multinationals collaborated with the Defense Advanced Projects Agency (DARPA) on a project designed to facilitate the remote recovery of hijacked American aircraft. Brilliant both in concept and operation, "Home Run" (not its real code name) allowed specialist ground controllers to listen in to cockpit conversations on the target aircraft, and then take absolute control of its computerized flight control system by remote means.
In order to understand about Home Run, we need to look at the ways in which an aircraft is normally controlled by its pilot.
To raise or lower the nose of the aircraft, the pilot pulls or pushes on the control joystick, which in turn raises or lowers the elevators on the horizontal tail plane. To bank the aircraft left or right, the pilot moves the control joystick to the left or right, which in turn operates the ailerons on the outer wings. The control joystick and rudder pedals were connected to the various flight control surfaces by thin cables, meaning the pilot had direct physical control over every movement the aircraft made.
However, by the late fifties we were well into the age of hydraulics, just like the power steering on your automobile, hydraulic rams were placed in line between the pilot's control cables and each individual control surface. Now when the pilot moved the control joystick, the cables activated sensors, which in turn activated one or more hydraulic rams, which in turn moved one or more control surfaces.
When the multinationals and DARPA finally came on the scene in the mid-seventies, aircraft systems were even more advanced, with computers controlling onboard autopilots, which in turn were capable of controlling all of the onboard hydraulics. In combination these multiple different functions were now known as the "Flight Control System" or FCS, through which they could automatically land the aircraft in zero visibility conditions. In summary, by the mid-seventies most of the large jets were capable of effectively navigating hundreds of miles and then making automatic landings at a selected airport in zero-zero fog conditions.
In order to make Home Run truly effective, it had to be completely integrated with all onboard systems, and this could only be accomplished with a new aircraft design. Under extreme secrecy, the multinationals and DARPA went ahead on this basis and built "back doors" into the new computer designs. There were two very obvious hard requirements at this stage, the first a primary control channel for use in taking over the flight control system and flying the aircraft back to an airfield of choice, and secondly a covert audio channel for monitoring flight deck conversations. Once the primary channel was activated, all aircraft functions came under direct ground control, permanently removing the hijackers and pilots from the control loop.
The signals sent provide a unique "identity" for each aircraft, essential in crowded airspace to avoid mid-air accidents, and equally essential for Home Run controllers trying to lock onto the correct aircraft. Once it has located the correct aircraft, Home Run sends a data transmission onto the transponder (combination of radio transmitter and receiver) channel and takes direct control from the ground!