Warplanes: UAVs Saved By The Third Dimension


October 29, 2011: The U.S. Air Force and U.S. Army agreed on new safety procedures to help prevent another collision between army UAVs and air force planes. The army and air force have long argued about such restrictions (about where, and when, army UAVs could fly), but the August 15 collision between an army RQ-7 UAV and an air force C-130 transport gave the air force more clout in this long standing dispute.

The RQ-7 hit a wing of the C-130, between the two engines. The RQ-7 was destroyed, while the C-130 had the skin of the front of that wing torn open and some of the interior spars bent. One of the props on the inboard propeller was destroyed (and that engine had to be turned off). But the C-130 was able to land safely, and parts and technicians were flown in to repair the C-130 where it was.

An RQ-7B Shadow 200 weighs only 159 kg (350 pounds), compared to 70,000 kg for a loaded C-130, so the outcome of this collision is not surprising. But had the Shadow hit in a more vulnerable spot, the C-130 could have been brought down. Shadow is small, being 3.5 meters (11 feet) long with a wingspan of 4.1 meters (12.75 feet). But coming in at over 200 kilometers an hour, at a C-130 travelling over 500 kilometers an hour, the 159 kg Shadow becomes a potent anti-aircraft missile.

Most UAVs in the air over combat zones are even smaller than Shadow. These are usually the tiny two kilogram (4.4 pounds) Raven. Bystanders have seen a few of them destroyed, or simply knocked out of the air by a passing aircraft, usually a helicopter. Raven operators suspect that many of those that were lost for unknown reasons were similarly hit or caught in the backwash of low flying aircraft. A few have been seen getting attacked by birds. There have been very few recorded collisions.

The small, plastic, Raven would not do much noticeable damage to an aircraft. The damage caused by the Shadow collision was understandable because the Shadow is the largest UAV that often operates at low altitude (under 300 meters) and uses military airfields to land and take off.

The army is developing a new radar system (GBSAA, or Ground-Based Sense And Avoid) to increase safety for UAVs. GBSAA is mainly a software system using existing radars to track UAVs and manned aircraft, and alert UAV operators when their UAVs are too close to other aircraft (manned or unmanned). GBSAA can be expanded to use transponders (which commercial aircraft have been using for a long time) and more flexible software. This is done by turning two-dimensional radars (use by most air traffic control systems) into 3-D systems via more efficient use of transponder data. Larger UAVs, like the RQ-7, can easily carry a transponder. Smaller UAVs, like Raven cannot. In any event, Raven flies too low and is too small to be detected by current air traffic control systems.

The basic idea is to insure that UAV operators are no longer “blind” to what is in the air nearby. The military would like to see GBSAA in service as soon as possible, but the system is still undergoing testing. Army officials are certain that had GBSAA been installed, it would have avoided the RQ-7/C-130 collision. It may be a year or more before the military gets to use GBSAA in a combat zone.

GBSAA will likely be more in demand by potential civilian UAV users. Battlefields have much lower safety standards than civilian air space, what with all those artillery and mortar shells, plus the bullets and rockets. But civilian air space has lots of small aircraft and helicopters, so UAVs are generally banned. GBSAA could change that, and make battlefields safer as the UAV traffic becomes denser.


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