The 14  Americans who died in Afghanistan on Monday were a reminder that U.S. troops who die in Afghanistan are twice as likely to be killed in helicopter crashes as are their counterparts in Iraq.

And the reasons for that discrepancy are not to be found in the country’s skies, but on the ground — the Taliban’s growing footprint has forced the U.S. to be far more reliant on moving troops and supplies by air.

And the rugged terrain often makes helicopters the only option, even as the altitudes involved greatly increase the risks.

Afghanistan’s few roads are now increasingly monitored — and mined — by insurgents, meaning that many of the 180 U.S. outposts spread across the country can now only be reached by helicopters.

"We don’t have freedom of movement on the ground,” a senior Army logistics officer says. "We’re resupplying between 30% and 40% of our forward operating bases by air because we just can’t get to them on the ground.”

That forces the U.S. military to rely on helicopters, not only to reach remote outposts, but also to carry out dangerous combat missions that thinly spread troops couldn’t do without the helicopter’s ability to hopscotch hundreds of miles.

It was precisely such an antidrug mission that a twin-rotor Army MH-47 Chinook was flying when it went down in western Afghanistan, killing 10 Americans including three civilians with the Drug Enforcement Administration.

Earlier in the day, a Marine UH-1 Huey troop helicopter collided in midair with an A-1 Cobra helicopter gunship over southern Helmand province, killing four. U.S. officials said they don’t believe hostile fire caused either crash.

The death toll could rise because some of the 28 people left injured by the crashes are in critical condition.

"Helicopters are not shot down in battle very much in either place [Iraq or Afghanistan],” says Brookings Institution defense analyst Michael O’Hanlon.

He and his colleagues are keeping running tallies of U.S. fatalities in both theaters. While 5% of U.S. deaths in Iraq have been caused by helicopter crashes — 216 out of 4,348 — the total is 12% in Afghanistan — 101 of 866 — even before Monday’s losses.

"The main issues [responsible for the higher rate of helicopter-crash casualties in Afghanistan] have to do with terrain, weather and of course frequency of use,” O’Hanlon says.

The U.S. has over the past year doubled its number of helicopters based in Afghanistan to about 225, but troop numbers have risen even faster, making for a more acute chopper shortage. Helicopters are swift but delicate machines.

The physics of flight make them inherently unstable, and therefore less reliable, than fixed-wing aircraft which generate their lift from stationary wings instead of egg-beater-like rotor blades. More critically, chopper pilots are commonly expected to fly in hot weather at high altitudes, where less-dense air offers them less control over their aircraft.

Air Force Captain Matthew Miller wrote about the challenges of flying in Afghanistan after returning from a four-month deployment there in 2007.

His medevac unit, from Georgia’s Moody Air Force Base, had lost three helicopters and seven crew members in the two wars. Enemy fire had been a factor in none of the Afghan crashes.

"In Iraq, helicopter pilots face a greater prospect of being shot at by ground fire,” Miller wrote. "In Afghanistan, the greatest threat is the terrain.”

He described flying in Afghanistan as "’graduate level’ piloting more challenging than cruising over the flatlands of Iraq. "It didn’t take long to feel the perils of mountainous flying in Afghanistan,” he added.

"Between Iraq and Afghanistan, most helicopter pilots I’ve spoken to consider Afghanistan the more dangerous place to fly.”

TIME