A once-in-a-century hailstorm took a heavy toll on the availability of airpower to support troops on the ground in Afghanistan.

Details of the incident last April 23 have only recently begun to come to light now that coalition air forces are starting to return to service aircraft seriously impaired in the storm, which occurred at Kandahar airfield. Golf-ball-sized hailstones peppered the airfield and the hundreds of aircraft based there, causing hundreds of millions of dollars of damage. Conceivably, a large number of helicopters and fixed-wing aircraft will have to be written off.

The storm also caused a number of civilian deaths in nearby Kandahar City.

Details of the storm's affect on the U.K. Royal Air Force began emerging late last year when RAF, Lockheed Martin and Marshall Aerospace officials revealed work on Operation Weatherman, a program devised to return five of the RAF's 24 Lockheed Martin C-130J Hercules aircraft to operations.

Engineers inspecting aircraft in the aftermath of the storm discovered that C-130Js had suffered “unprecedented damage.” The hail had severely impacted wings, empennages, windows and fuselages. Initial checks showed that more than 850 aircraft panels had been compromised, rendering approximately one-fifth of the RAF's fleet of 24 Hercules unavailable for flight operations. As a result, the RAF was prompted to push one of its C-17 Globemasters, usually deployed as strategic transport, into use as an intra-theater airlifter.

“This once-in-hundred-years event was reported as a monstrous storm,” says Group Capt. Nick Cox, Hercules team leader at the U.K. Defense Equipment and Support agency.

Perhaps most significantly, the hail pummeled flight-control surfaces, which had to be replaced in order to return the aircraft to the U.K. for repairs. Engineers from the RAF, Lockheed Martin and Marshall Aerospace, the company that provides depot-level maintenance for RAF Hercules, set about drafting a recovery plan.

A pair of recently retired RAF C-130Ks were cannibalized for ailerons, and engineering work began to determine if these could be fitted to the C-130Js. “J” ailerons feature two actuators, while those on “K” models, feature just one. The fit was deemed safe for the long-haul ferry flight back to the U.K., but the flight envelope was restrictive and use of one mode of autopilot was unavailable, therefore long stretches of manual flying were mandatory. Within two months, four of the five aircraft had returned to the U.K. and went straight into maintenance at Marshall's facilities at Cambridge. The fifth aircraft was OK'd for operations after new ailerons were fitted.

“Replacing ailerons and elevators is no easy task,” says Cox. “There are no real hangar facilities available for use by the U.K. in-theater. The only large hangars on the airfield belong to the U.S. and were already in use, so the work had to be done outdoors. Returning the aircraft to the U.K. was the only option.”

In Cambridge, engineers assessed the 850 impaired panels, taking photographs of the individual locations of the indentations and noting their dimensions. The examinations revealed nearly 50% more damage than noted by the initial in-country inspection. Each aircraft was found to have sustained 1,600-2,000 hits. Four of the five aircraft were returned to operations in September 2013. The fifth, which remained in Afghanistan, has since be sent to the U.K. to undergo scheduled heavy maintenance. Photographs show that the hail-afflicted aircraft still bear the scars, but repairs of the aircraft skins are scheduled to take place during a more convenient scheduled maintenance opportunity.

Other RAF aircraft types that were damaged in the storm include the second of its new BAe 146-200QCs. It had only been in-theater one day, and was about to embark on its first operational mission when the hail struck. It, too, was flown back to the U.K. for repairs; it reentered service in late August. A single storm-ravaged BAe 125, belonging to the RAF 32nd Sqdn., was deemed not airworthy, according to U.S. Marines who assisted in its dismantling. Government ministers report that repairs to the five C-130Js cost £5.9 million ($9.6 million) and are expected to top £10 million. Another £2.9 million has been spent on repairs to other aircraft.

U.S. Central Command did not answer Aviation Week's request for information, and senior officials have said the post-storm assessment remains classified, perhaps indicating U.S. forces have not fully recovered from the event.

The U.S. Army revealed in June that 80 helicopters operated by Task Force Falcon had been affected, but status updates on other types is difficult to obtain. Few hangars are available to coalition forces at Kandahar and, of those that are, most are consigned to aircraft maintenance. Fighters such as Tornado GR4s and Lockheed Martin F-16s are protected under sun shelters; other types, including numerous intelligence, surveillance and reconnaissance-configured King Airs, are parked outdoors in revetments.