It's the latest mystery in the hunt for Malaysia Airlines Flight 370: Was a key communications system on board the plane disabled before or after the co-pilot calmly bid air traffic controllers goodnight?

Malaysian authorities said on Sunday that the system, known as ACARS, was disabled before that final radio transmission, which would seem to bolster theories that the pilots were somehow involved in the plane's disappearance. But further digging provides some reason to doubt that account. Here's why.

First things first. What is ACARS?

ACARS stands for Aircraft Communications Addressing and Reporting System. It's a way for planes to relay information about their engines and other systems back to ground stations so that repairs can be made more efficiently. This happens mostly automatically, though the system can be set up to support manual text messaging between pilots and other aircraft, or between pilots and the ground.

Okay. And Malaysian officials say ACARS failed before the co-pilot's last words?

That part isn't so clear. We do know that ACARS sent an update containing data about MH370's systems at 1:07 a.m., shortly before the co-pilot last spoke. Then, half an hour later, ACARS was supposed to transmit another update, but never did, according to Malaysia Airlines's CEO. So we know that ACARS failed sometime between 1:07 a.m. and 1:37 a.m. — for context, the flight's transponder fell silent at 1:21 a.m. — but it's hard to say with any greater precision than that.

I've heard that a satellite detected MH370 after the plane went dark. If the flight's communications were down, what was talking to the satellite?

The short answer? ACARS.

But you said ACARS had been disabled.

ACARS was disabled. But the satellite equipment it uses hadn't been. It had been responding to "pings" from a satellite that, at a minimum, tells investigators the plane was still up and running. Others have called this activity a digital "handshake."

"You have to see these two things at two different layers," said David Cenciotti, an aviation writer based in Rome. "At the higher one you have ACARS with all the series of messages that can be exchanged between the plane and the receiving station on VHF, HF or SATCOM. At the lower layer you have the network that is used to deliver these messages: pings are used to check the status of the underlying network."

No information gets exchanged in a ping — it's simply a way for one entity to make sure that the other is there.

If somebody had turned ACARS off, why didn't the pings stop then as well?

Disabling the transmission of engine performance data and other information may be as simple as flipping a switch. But to stop the pings from occurring, you'd have to dig around in the guts of the plane itself, said one ACARS expert who asked for anonymity because he wasn't authorized to speak to the public.

"The antenna is on top of the aircraft, so it can't be reached," the expert said. "And secondly — think of it like a fuse box. The fuse box for the power — to disable that you'd have to open up the floor, go down and find the correct switches and cables and disable that. For a lot of pilots, they don't even know that that's there."

Could the plane have crashed someplace and kept responding to pings anyway?

Probably not. The pings tell us two things: 1) that the plane has power; and 2) that because it had power, it was likely intact. That still doesn't answer the biggest question of all, however: What happened after the pings stopped?