Despite the demand for Air Force drones in today’s military, in 2015 a record number of unmanned aircraft crashed or simply fell out of the sky.

The drones that are crashing and burning the most are the Air Force’s most cutting edge models–the Reaper, an advanced “hunter-killer” drone that can conduct both surveillance and airstrikes against the Islamic State in Syria. Thanks to sudden electrical failures, Reaper drones are losing power mid-flight and dropping like flies. While investigators have narrows down the cause of this problem to a “faulty starter generator,” they are still scratching their heads as to why this keeps happening.

“Once the battery’s gone, the airplane goes stupid and you lose it,” Col. Brandon Baker said. “Quite frankly, we don’t have the root cause ironed out just yet.”

According to the Washington Post, 20 Air Force drones were destroyed or damaged over the course of 2015. They also sustained or caused a whopping $20 million in damage.

Ten Reapers were badly damaged or destroyed in 2015, at least twice as many as in any previous year, according to Air Force safety data.

The Reaper’s mishap rate — the number of major crashes per 100,000 hours flown — more than doubled compared with 2014. The aircraft, when fully equipped, cost about $14 million each to replace.

The Air Force’s other primary drone model, the Predator, also suffered heavy casualties.

An older and less capable version of the Reaper, the Predator was involved in 10 major accidents last year. That’s the most since 2011, when the U.S. military was simultaneously surging troops into Afghanistan and withdrawing ground forces from Iraq.

The Washington Post noted that while it is Pentagon policy to disclose all aircraft mishaps, it didn’t report any of the 20 drone crashes. Reaper drones started getting outfitted with a backup generator in July to prevent more power failures.

Meanwhile, domestic drones in the United States have also wreaked havoc by colliding with military aircraft or forcing first responders to take evasive maneuvers. Domestic drones have interfered with military aircraft 35 times and with first responders more than 1,000 times.

[The Washington Post]