The Reaper has been bedeviled by a rash of sudden electrical failures that have caused the 21/ 2 -ton drone to lose power and drop from the sky, according to accident-investigation documents obtained under the Freedom of Information Act. Investigators have traced the problem to a faulty starter-generator, but have been unable to pinpoint why it goes haywire or devise a permanent fix.

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All told, 20 large Air Force drones were destroyed or sustained at least $2 million in damage in accidents last year, the worst annual toll ever, according to a Washington Post investigation. The Pentagon has shrouded the extent of the problem and kept details of most of the crashes a secret.

The aircraft losses pose another challenge for the Air Force as it struggles to provide sufficient drone coverage for counterterrorism operations in Iraq, Syria, Afghanistan, Somalia, Yemen, Libya, Mali and Cameroon, among other countries.

Despite a surge in requests from field commanders, the Air Force last year had to curtail its drone combat missions by 8 percent because of an acute shortage of pilots for the remote-controlled aircraft. Things have gotten so bad that the Air Force is offering retention bonuses of up to $125,000 to its drone pilots, who have long complained of overwork.

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The Air Force also has contracted out more drone missions to private companies to meet what one general called “a virtually insatiable appetite” from military commanders for airborne surveillance.

While Air Force leaders have publicly bemoaned a lack of personnel and resources, they have said little about the high number of drone crashes, a long-standing vulnerability that worsened substantially last year.

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.

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

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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.

Although the Defense Department has a policy to disclose all major aircraft mishaps, it did not publicly report half of the 20 Reaper and Predator accidents last year.

In five other cases, U.S. military officials provided confirmation only after local authorities reported the crashes or enemy fighters posted photos of the wreckage on social media.

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According to the military, only one drone was downed by hostile forces: a Predator that was hit by Syrian air defenses near Latakia on March 17.

All but one of the 20 Air Force drone accidents last year occurred overseas. Six drones crashed in Afghanistan. Four crashed in the Horn of Africa, near a U.S. military base in Djibouti. Three crashed in Iraq. There were also crashes in Kuwait, Turkey, Syria and Libya.

In two cases, Air Force officials would not identify the country where the mishaps occurred.

In addition to the Air Force, the Army operates its own drone fleet. It is preparing to expand the number of combat missions it flies to help compensate for the Air Force’s cutbacks.

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Last year, the Army reported four major drone crashes, each involving the Gray Eagle — a model identical to the Predator. Three of the Army’s accidents occurred in Afghanistan. One happened in Iraq.

Although the military’s drone programs are largely unclassified, the Obama administration rarely discusses details of the key role they fill in its counterterrorism strategy. The CIA runs its own drone operations on a covert basis, and the secrecy surrounding those missions often seeps into the Pentagon.

Lt. Gen. Robert P. Otto, the Air Force’s deputy chief of staff for intelligence and surveillance programs, acknowledged in an interview that there has been a spike in Reaper accidents.

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Many cases remain under investigation, but Otto and other Air Force officials blamed the Reaper’s flawed starter-generator for causing at least six major crashes since December 2014.

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“We’re looking closely at that to determine what is the core issue there,” Otto said.

Although the drone pilot shortage has compelled the Air Force to reduce the number of combat missions, Otto said the aircraft mishaps have not forced additional cuts. The Air Force has enough replacement drones on hand, he said, and already had orders in place to buy dozens more Reapers over the next few years.

“Any impact to operations has been negligible to barely noticeable,” he said.

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Field commanders, however, have long complained of a drone deficit. In March, the four-star commanders of U.S. forces in the Middle East and Africa both told Congress that the Pentagon has provided less than one-quarter of the drones, other aircraft and satellites that they need for reconnaissance and surveillance missions.

“The Predator has been our most effective weapon in our campaign against the global jihadists,” said Michael G. Vickers, the Pentagon’s former top civilian intelligence official, at a House Armed Services Committee hearing Jan. 12. But he cautioned that the size of the drone fleet “will remain a critical limiting factor in the conduct of our campaign.”

Drones going ‘stupid’

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Military drones have been dogged by persistent safety and reliability problems since the first Predator was deployed to the Balkans on a combat mission two decades ago.

Of the 269 Predators purchased by the Air Force since then, about half have been destroyed or badly damaged in accidents, records show.

Air Force officials describe the Predator as an experimental aircraft that was rushed into war zones, particularly after the U.S.-led invasion of Afghanistan in 2001. They say it has lasted much longer than expected and that, at a cost of$4 million apiece, is relatively expendable in the event of a crash.

The Air Force has about 140 Predators left and plans to retire them all by 2018. They are gradually being replaced by the Reaper.

Introduced in 2007, the Reaper can fly twice as far as the Predator and carry more bombs and missiles. Until recently, it also had a much better safety record.

Over the past three years, however, some production models of the Reaper have been hobbled by an outbreak of electrical failures.

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Investigators and engineers have traced the problem to the starter-generator. It powers the drone but is prone to conking out, for reasons that remain unclear.

The Reaper carries an emergency battery backup system. But the batteries last only for about one hour. If a malfunctioning drone needs more time than that to reach an airfield, it is in trouble.

In such emergencies, the drone pilot usually has no choice but to intentionally crash the aircraft in a remote area, such as a mountainside or a waterway, to avoid striking people on the ground. No one has died in a military drone accident, though many catastrophes have been narrowly averted, documents show.

“Once the battery’s gone, the airplane goes stupid and you lose it,” said Col. Brandon Baker, chief of the Air Force’s remotely piloted aircraft capabilities division. “Quite frankly, we don’t have the root cause ironed out just yet.”

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The Reaper and the Predator are both manufactured by General Atomics Aeronautical Systems, a San Diego-based defense contractor. In addition to the Air Force, other customers who have purchased the Reaper include the Department of Homeland Security, NASA, and the British, French and Italian armed forces. The CIA also flies Reapers.

General Atomics officials declined requests for an interview or to provide data on the Reaper’s history of starter-generator failures.

In an emailed statement, General Atomics spokeswoman Kimberly Kasitz said the firm “stands behind the proven reliability” of the Reaper. She added that Reapers have recorded more than 2.2 million flight hours and have “been very effective for multiple customers.”

The Reaper’s starter-generator is built by Skurka Aerospace of Camarillo, Calif.

Skurka executives referred requests for comment to their parent corporation, Transdigm Group of Cleveland. A Transdigm spokeswoman did not respond to phone calls or emails.

Averting disaster

Government agencies other than the Pentagon have also run into problems with their Reapers.

Shortly before midnight Jan. 27, 2014, an unarmed Reaper was flying a surveillance mission near San Diego for U.S. Customs and Border Protection. Suddenly, an alarm sounded, signaling that the starter-generator had stopped working.

The crew flying the drone from a remote-control ground station in Corpus Christi, Tex., inputted commands to restart the generator, but their attempt failed. The pilot made quick calculations and concluded that the Reaper lacked enough battery power to make it back to its launch point, at Fort Huachuca, Ariz., according to an aircraft accident report by Customs and Border Protection.

Worried that the Reaper might otherwise crash into a heavily populated part of Southern California, the pilot commanded the drone to head out to sea, where it was ditched about 23 miles west of Point Loma, Calif.

The drone sank about 4,200 feet to the ocean floor. Ten days later, most of the wreckage, including the intact starter-generator, was recovered from the depths by a Navy salvage team.

According to the accident investigation report, it was the 18th time in nine months that a starter-generator had failed on a Reaper. Disaster was averted in most cases, but in three of the incidents, the drone crashed.

Working with engineers from General Atomics, investigators identified three parts of the starter-generator that were susceptible to breakdowns. But they couldn’t figure out why they were failing.

No pattern was apparent. Older units had failed, but so had brand-new ones. There was no correlation with operating locations or conditions. The Customs and Border Protection investigation blamed an “unknown factor” that was “likely external.”

The report noted that, unlike most aircraft, the Reaper lacked a backup, or redundant, power supply system. Calling it a “design weakness,” the report recommended that Reapers be equipped with a permanent backup electrical supply.

Two days after the crash near San Diego, General Atomics issued an alert bulletin to its customers, advising them to limit “non-essential” Reaper operations to keep the drones within one hour’s flight of an air base in case of an emergency.

The bulletin, however, did not apply to combat missions.

Crashes pile up

General Atomics engineers made little headway in identifying the mechanical gremlin that was plaguing the starter-generators. Meanwhile, Reapers kept crashing.

On Dec. 12, 2014, a Reaper armed with missiles and bombs experienced a starter-generator failure about 90 minutes after it took off from Kandahar Air Base in Afghanistan.

As the batteries drained, the crew intentionally flew the drone into a mountain. The wreckage was not recovered.

“I thought it was a very prudent place to ditch it, onto a high mountain top,” the unidentified mission crew commander told Air Force investigators, according to the accident-investigation report. “Our deal is we try to do it into high mountain tops.”

Less than two months later, on Feb. 4, 2015, an Air Force Reaper had to cut short a surveillance mission over Somalia when its starter-generator died.

The flight crew tried to rush the drone back to its base in Djibouti. But with about 30 miles to go, the battery ran out and the Reaper was ditched in the sea, according to the Air Force’s accident investigation report.

In an appendix to the report, General Atomics noted that it had completed the development of a “more robust” starter-generator in response to the string of mishaps. The appendix, which was heavily redacted, did not give further details.

In March, the Air Force’s program manager for its Reaper fleet filed a report with the Pentagon noting that there had been “a dramatic increase” in starter-generator failures since 2013.

Col. William S. Leister informed Pentagon officials that investigators from the Air Force, General Atomics and Skurka had investigated the problem for more than a year. The team, he said, had identified “numerous manufacturing quality issues” yet had been unable to determine the exact cause of the failures.

“But, I am pleased to report that we may have light at the end of this dark tunnel,” he added, promising unspecified “corrective actions in the very near term.” He declined to comment further for this article.

Other Air Force officials said the service began installing a secondary generator on its Reapers in July that can provide up to 10 extra hours of electricity in case the first one fails.

The Air Force determined that 60 Reapers in its fleet were carrying the buggy starter-generators. So far, the new backup part has been installed on 47 of those aircraft, according to Baker, the colonel in charge of the drone capabilities division.