No de-icing necessary?

U.S. Air Force considers lifting the ban on Global Hawks flying in icing weather

The U.S. Air Force is considering whether to lift the ban on the RQ-4 Global Hawks flying in icing weather. The high-flying unmanned intelligence planes were pressed into service during the wars in Iraq and Afghanistan without de-icing systems, and test flights conducted in April and May now suggest that the planes might not need them.

Global Hawks typically cruise at about 60,000 feet, where the atmosphere is too cold and dry for ice to form. But getting to that altitude requires traversing the 8,000- to 22,000-foot altitude band where icing can be an issue. Though a Global Hawk spends only about five minutes passing through this zone, the Air Force currently mandates that if its onboard ice probes detect icing, the plane must climb or dive until the ice melts, before attempting to penetrate the icing zone again.

Responding to ice is a particularly sensitive issue for unmanned aircraft like the Global Hawks. While some drones are flown by pilots who are at the ready in ground stations to respond to ice, the Global Hawks fly preprogrammed flight paths and are largely autonomous, with their pilots intervening only to issue general instructions to change altitude, course and speed. On the other hand, unlike other drones, Global Hawks have ice sensors that can alert their pilots to icing conditions. The fleet has accumulated about 200,000 flight hours without a single ice-related mishap. That led prime contractor Northrop Grumman to believe that the aircraft could safely pass through icy weather.

“We had anecdotal information, so the purpose of this test was to provide flight test data that unequivocally stated we could fly through known icing conditions,” says Mick Jaggers, who heads Northrop Grumman’s Global Hawk work.

Formal testing would be required to prove it. Northrop Grumman assisted the Air Force with a series of Global Hawk flights at Edwards Air Force Base, California. Rather than waiting for icy weather, researchers brought the clouds to the plane. They fastened nylon shapes — formed by 3-D printers — to the wings and tail of the aircraft to mimic the airflow disruption from ice as the Global Hawk conducted climbs and dives. The aircraft was also flown with medium and heavy fuel loads to determine if this affected its icing survivability.

The results showed that “our flight control algorithms are sufficient to safely maneuver the vehicle through known icing conditions,” Jaggers says. As for Global Hawk’s maximum tolerance for ice, Jaggers says “the final icing limit has not yet been determined, but the Global Hawk was not designed to operate in sustained icing conditions.”

The Air Force has not said how or when it might rule on the ban. ★