Murphy's Law: Hosing Down Hot UAVs


August 26, 2007: U.S. Air Force RQ-4 Global Hawk UAVs operating in the Middle East have a unique heat problem. Being relatively small, unmanned, aircraft, they do not have industrial strength air conditioning for their electronic systems. There are a lot of electronics on this bird, most of them spy satellite grade sensors. These things are built to operate a high altitudes where it is very cold, very much below freezing. But on the ground in the Persian Gulf, just taxiing from an air-conditioned hanger to the runway and takeoff, and even doing this at night, can allow the electronics to heat up enough for some of fail. Take-offs have been aborted because within a space of ten minutes or so, the hot outside temperature, and all those electronics took the internal temperature of the aircraft to over 40 degrees Celsius. One key component fails, and the UAV aborts the take off.

So the ground crews came up with, not one solution, but a bunch of them. First, they ran the takeoffs as a race (against overheating.) Another maintainer came up with the idea of having a fire truck hosing down the wings of the RQ-4, once it was outside the hanger, because it was the fuel in those wings that was circulated to help cool the electronics. Other maintainers found that if you get all the seals, gaskets and air hoses inside the UAV in good order (repair worn items, tighten loose ones) you saved a few degrees. Using the cool air generating ground equipment as efficiently as possible also made a difference. Using all these improvisations, the number of "overheating" aborts for takeoffs just about disappeared.

In the last six year, RQ4s have flown 15,135 hours, 71 percent of that combat missions, and many of them from Persian Gulf bases. Some of the more recent models have been able to fly 20 hour missions, land for refueling and maintenance, and be off in four hours for another twenty hours in the sky. Once airborne, heat is not a problem. But while servicing the RQ-4, you have to watch out for heat problems.

The RQ-4 has been very reliable, with aircraft being ready for action 95 percent of the time. Earlier this year, the U.S. Air Force has ordered five more Global Hawks, at a cost of $58 million each. This is the B version, which is larger (wingspan is 15 feet larger, at 131 feet, and it's four feet longer at 48 feet) than the A model, and can carry an additional two tons of equipment. To support that, there's a new generator that produces 150 percent more electrical power.

The first three RQ-4Bs entered service in 2006. At 13 tons, the Global Hawk is the size of a commuter airliner (like the Embraer ERJ 145), but costs nearly twice as much. Global Hawk can be equipped with much more powerful, and expensive, sensors, than other UAVs. These more the double the cost of the aircraft. These spy satellite quality sensors (especially AESA radar) are usually worth the expense, because they enable the UAV, flying at over 60,000 feet, to get a sharp picture of all the territory it can see from that altitude. However, these high tech electronics generate a lot of heat. The B version is a lot more reliable, at least once all the cooling tricks were developed. Early A models tended to fail and crash at the rate of once every thousand flight hours, mostly because of design flaws, not cooling problems.




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