So just how often do US military drones "disappear"?

The US military acknowledged Sunday that one of its unmanned aerial vehicles, or UAV’s, had gone missing over western Afghanistan late last week.

“The operators of the UAV lost control of the aircraft and had been working to determine its status,” a statement released by NATO forces in Afghanistan said Sunday.

The news caused an uproar when Iran claimed to have found the UAV – and that it is a highly-secretive RQ-170, the same drone reportedly used in the US Special Operations raid on Osama bin Laden’s compound in May.

It is not often that the Pentagon loses track of its drones for any extended period of time, analysts say. In this particular case, it is possible that military officials know where the drone is, but would rather not acknowledge that it’s in Iranian hands.

This puts US military officials in a tricky position, as demonstrated when Pentagon officials sought Monday to assure reporters that – wherever it might be – the drone definitely was not shot down, as Iran claims.

“The one thing I can tell you is we don’t have any indications that the UAV – that we no longer have – was brought down by hostile activity of any kind,” Captain John Kirby, a Pentagon spokesman, said.

That is not to say that it’s particularly difficult to shoot down the vast majority of drones in the Pentagon's arsenal.

The US military first began using Predators in Europe in 1995, for example, but pulled them out after Serbian air defense gunners shot down two of them.

In 1999, at least 22 drones were felled by enemy fire or crashed due to mechanical failure over Kosovo.

At the end of 2002 – just before the US invasion of Iraq – Predators armed with Stinger missiles flew over Iraq’s no-fly zone in an effort to “bait” the Iraqi Air Force’s supersonic MiG-25 aircraft.

When challenged, one Iraqi MiG promptly shot down at least one American UAV, notes UAV pilot Lt. Col. Matt Martin in his book, "Predator: The Remote-Control Air War Over Iraq and Afghanistan."

Indeed US military officials are quick to acknowledge that UAVs work best in uncontested air space – as has been the case in the wars in Iraq and Afghanistan, where neither surface-to-air missiles nor a robust enemy air force has challenged NATO air dominance since the first hours of invasion.

Even without an enemy threat, however, UAVs can be tricky to fly. That's because, by definition, there is no pilot in the aircraft cockpit, which eliminates the risk of bodily harm to pilots, but presents other difficulties.

Pilots fly UAVs in Iraq or Afghanistan, for example, through a web of fiber-optic cables and satellite dishes. Phone lines carry flight commands from US military ground control to satellite dishes in Europe, which then relay the commands to other small satellite dishes on the Predator, Martin explains.

The string of computer nodes means a two-second lag between the pilot’s command and the aircraft’s response. That’s why pilots based in, say, Kandahar fly the drones during takeoff, then transfer the controls to other Air Force pilots based in Nevada. When it’s time for the drones to land, the Nevada-based pilots then hand back the controls to their counterparts in Kandahar.

But given the complex and varied nature of the communications network that connects the pilots to their drones, links are often lost.

Martin recalls an instance in which his screen froze just has he was preparing to fire one of the missiles on his Predator. “Suddenly, just before I fired, the screen froze. Alarms began sounding and warnings flashed,” he recounts in his book.

“There had been a power outage in the satellite uplink in Europe. I had no further control of the plane.”

In such cases, UAVs are left to their own devices. Though they are generally programmed to fly a pre-determined – and thus trackable – path until pilots are back online, in the past the US military has been forced to shoot down renegade drones. In other instances, they simply crash.

On all of these points, Pentagon officials remain mum. “These are very sensitive reconnaissance missions, and as a rule we don’t talk about the specifics,” Kirby said, “whether it’s air frame, mission intent, or exact route.”

Get daily or weekly updates from CSMonitor.com delivered to your inbox. Sign up today.