One of the many baffling aspects of the MH370 disappearance is the absence of radar data after the plane left Malaysian primary coverage at 18:22 UTC. If, as is generally presumed, the plane took a sharp left turn shortly afterward and flew into the southern ocean, it should have remained visible to Indonesian radar for more than an hour. Yet Indonesian military officials insist that nothing appeared on their radar screens.

This is an issue that has gotten less attention than it deserves, and for understandable reasons. Indonesia is a developing country, and a sprawling one at that. With a land area the size of Western Europe, it spans the same east-to-west distance as the continental United States. So one might reasonably assume that the country lacks the ability to comprehensively monitor its airspace. Why shouldn’t MH370 have passed through without a trace?

In fact, though, Indonesia has quite a capable air defense radar system, and one which it utilizes quite aggressively. In the last month alone, its air force has intercepted three civilian planes which wandered into the national airspace without first getting the proper permission.

The westernmost part of the country is covered by the Indonesian Air Force’s Radar Unit 231 at Lhokseumawe in Aceh, Northern Sumatera. The unit is equipped with a Thomson-CSF TRS-2215 radar, the type pictured above. The system, Indonesian military officials say, is capable of detecting aircraft up to 240 nautical miles away.

It demonstrated its capability last year, when the unit detected a Dornier 328 twin turboprop entering Indonesian airspace from the west. The plane was ordered to land at Sultan Iskandar Muda airport in Banda Aceh. It turned out to be a US Air Force Special Operations plane carrying five crewmembers; the pilot claimed he had been running low on fuel and didn’t realize that his paperwork to enter the country had expired. After being impounded for a few days the plane and its crew were allowed to leave.

Based on its specs alone, Radar Unit 231 (so much easier to spell than Lhokseumawe!) should easily have been able to detect MH370. The radar track released by the Australian Transport Safety Board (ATSB) in June shows that the plane came within 60 nautical miles of the installation before it disappeared from Malaysian and/or Thai military radar. Afterwards, according to the consensus view, the plane’s track should have stayed within the radar’s viewing range as it headed west, made a turn to the south, and proceeded into the southern Indian Ocean. (See map after the jump)

So how could the Indonesians have seen nothing?

For insight, I turned to someone with long experience with military radar systems. Steve Pearson is a former RAF navigator who later worked as an avionics and mission systems engineer for the Royal Air Force Warfare Center. Today he works as an engineer for the defence consulting firm Qinetiq.

First, I asked Pearson if a 777 would make a big, clearly visible radar target.

“Yes, it should do,” Pearson said, “because it’s got lots of corner effects on it, if it was flying in a straight line across a radar screen I can’t think why it shouldn’t show up. I can’t think of anything that would prevent it. “

Of course, he added, there is always the human factor to consider. “There’s humans in the loop, isn’t there?” he said. “You’ve got a human operator looking at a display, quite easy for people to miss things. We’re all humans. We make errors.”

Especially in the middle of the night. It’s likely significant that, of all the interceptions that I mentioned at the top of this piece, all took place in the middle of the day. It’s not hard to imagine a sleepy radar operator, in the middle of the night, expecting nothing to happen, overlooking the passage of a blip.

Was Indonesia’s failure to detect MH370 related to its peculiar flight path? After its initial diversion, MH370 passed mostly along the boundaries between Flight Information Regions, or FIRs. These are aerial territories under the jurisdiction of different air traffic controllers. When the plane disappeared from secondary radar near IGARI, it was on the boundary between Singapore and Ho Chi Minh FIRs. It did a 180, then flew along the border between the Kuala Lumpur and Bangkok FIR. After crossing the Malayan peninsula, it stayed close to the boundary between Kuala Lumpur and Jakarta FIRs.

“That’s quite clever,” Pearson says, “because if you fly down the FIR boundary, the controller on each side might assume the other was controlling you. Usually, a civilian air traffic controller would call his counterpart to check. Military, not so much. They might think, ‘Oh, that must be the other country’s aircraft, it’s not my problem, I won’t worry about it.’ And the other country thinks, ‘Oh that’s their problem, I won’t worry about it.’”

Back in his days as a military navigator, Pearson would take advantage of this dynamic to slip through airspace where he wasn’t supposed to be. “When we used to go to other parts of the world, which I won’t mention on the phone, you could fly down FIR boundaries, and each side thought you were in the other one’s control. You could fly right down the boundary and no one would talk to you. It’s something we didn’t do very often, I must admit, but it’s something you can do.”

If MH370 was being steered in such a way as to escape detection, the plan worked. Both Malaysian and Thai military radars apparently picked up the plane, but no human noticed until the radar recordings were inspected in the aftermath, by which point the plane was long gone.

“If it’s flying along the boundary very precisely, that points to someone knowing exactly what they were doing, doesn’t it?” says Pearson. “It wasn’t random.”

The confounding thing is that the Indonesians say that nothing turned up in their recordings. This is seems hard to comprehend. Some observers have speculated that the Indonesians are lying. Others suspect that the plane didn’t pass thorugh the Malacca Strait at all. It remains possible, Pearson says, that the the plane was picked up by the radar unit, but that the recording device either failed or was turned off at the time. But, he says, “it all seems a bit coincidental, doesn’t it?”

At the end of the day, Pearson suspects that if the Indonesians say they have no radar data, it’s because they don’t.

“It’s probably not a cover-up,” he says. “They probably just haven’t got it.”