By CNN National Security Producer Jennifer Rizzo
When the highly classified RQ-170 Sentinel drone crashed in Iran recently, there were many questions about how this could happen. Then, a few days later, a Predator drone crashed on the island nation of Seychelles. Are drones falling out of the sky?
These recent high-profile crashes of U.S. drones raise questions about the reliability of the crucial unmanned aircraft.
Unmanned aerial vehicles, or UAVs, have about the same mishap rate as the F-16 manned fighter jet did at a comparable stage of development, according to retired Lt. Gen David Deptula, who ran the Air Force drone program until he left the service in 2010.
An Air Force chart obtained by CNN confirms Deptula's assessment, showing as flight time increases, mishap rates for drones drop, just like they do for the piloted F-16 and F-22 fighter jets.
When looking at the total number of flight hours, the mishap rates for "Predators (MQ-1s) are a bit higher," said Deptula. "Reapers (MQ-9s) and Global Hawks (RQ-4) are a bit lower."
Predator drones fly at medium to high altitudes and were initially designed for gathering intelligence. They have since been modified to carry munitions such as Hellfire missiles.
Reapers are similar to Predators and carry Hellfire missiles, but they are larger and more powerful. Up to three times faster than the Predator, Reapers are designed to go after time-sensitive targets, according to the Air Force.
The Global Hawk is a high-flying, unmanned aircraft that provides intelligence, surveillance and reconnaissance. Its long endurance allows it to stay in flight longer than any manned or unmanned aircraft. The unarmed Global Hawk collects pictures and video of the ground, and some versions have the ability to do electronic eavesdropping.
All drones are not created equal. Some have different crash rates, just as manned systems do, according to drone expert Peter Singer. And, like manned craft, crash rates diminish as the technology advances.
"The crash rates tend to go down the longer that they've been in service," said Singer. "The newer the system the more technical problems, the more we don't know about its particulars, the more kinks that have to be worked out."
And after almost a decade of war in Iraq and Afghanistan, the United States has gotten an enormous amount of drone flight experience, improving their reliability, according to weapons expert John Pike.
"They've gotten a lot better," said Pike. "For a long time drones were halfway between aircraft and ammo. Aircraft usually comes back and ammo never comes back."
Drones are also often flown in circumstances that a piloted aircraft would not be sent - into hostile territory for example, or in bad weather.
"We have used them in some ways that we wouldn't use manned systems, so to compare them wouldn't be a perfect fit," said Singer.
In certain instances U.S. forces may even intentionally down a drone, according to Singer, who has served as a consultant for the Defense Department.
Singer wouldn't elaborate further but was able to cite examples: flying a drone until it runs out of gas to provide an eye over soldiers on missions, or turning it into a kamikaze-type weapon to hit a high-value target.
"You sometimes put them into situations that you might not put a manned system in," said Singer. "Or in certain situations, you decide to crash them, lose them, destroy them in a way that wouldn't be on the table with a manned system."
A deliberate loss of a UAV would count in the accident rates.
In fiscal year 2010, the last year with available data, there were eight major "Class A" drone crashes, according to the Air Force Safety Center. The Air Force refers to "Class A" mishaps as accidents that result in fatality or total permanent disability, loss of an aircraft, or property damage of $2 million or more.
Some of these crashes involved major technical glitches.
In September 2010, a Predator crashed in Afghanistan's mountains after an oil system malfunction caused a catastrophic leak leading to engine failure, according to an Air Force accident report.
In November 2009, a "catastrophic" electrical system failure in a Predator drone flying in Afghanistan caused the aircraft to lose contact with the flight crew and eventually crash.
But despite the high-tech nature of the aircraft, not all crashes are technical mishaps. Early versions of the Predator drones were fair-weather fliers and were prone to crashing in icy conditions. Now, de-icing systems have been added to the UAVs.
"Many of the (crash) cases have nothing to do with the fact that it's high technology. It has to do with the fact that it's new technology and they have to work out the kinks," Singer said.
Then there's the human factor. In 2010 a Predator drone crashed northeast of Kandahar Air Base in Afghanistan after the remote pilot pushed an incorrect button.
In August 2010 a Reaper crashed in California as part of a training exercise. The investigation found that the drone's pilot improperly controlled the aircraft's speed, causing it to stall. The same month, a Predator crashed shortly after takeoff near Joint Base Balad in Iraq when the drone's pilot forgot to turn on a system that keeps the aircraft stable.
Drones can "fly longer than any single human operator can take," said Singer.
The Air Force made reference to this challenge in a recent study that looked into the psychological attributes a drone pilot needs to be successful on the job.
"It is also important to note many MQ-1 Predator and MQ-9 Reaper squadrons are engaged in demanding shift work that can lead to mental fatigue, affecting cognitive performance," the study said.
In October 2009 a Predator drone met its demise after a crew succumbed to the pressure of a war zone. The drone was on a reconnaissance mission in Afghanistan when the crew received directions to provide close air support to U.S. and Afghan ground forces that were under heavy attack. The drone crews were "consumed with a high-degree of urgency," according to an Air Force report, and did not pay enough attention to the mountainous terrain. Not flying the drone high enough caused the Predator to crash.
So what happened to the highly sophisticated RQ-170 Sentinel that crashed in Iran?
Flying at altitudes up to 50,000 feet and designed to evade sophisticated air defenses, the RQ-170 is "impossible to see," one former intelligence official told CNN. The official discounted Iranian claims that it had been brought down by some form of electronic countermeasures. Other analysts have agreed that a cyber attack would have been unlikely.
House Intelligence Committee Chairman Mike Rogers said emphatically that Iran had nothing to do with the fate of the aircraft, but that there was "a technical problem that was our problem, nobody else's problem."