Here's what happens when you lose a plane — and how you find one in the aftermath of a watery crash.
Radar isn't infallible
When flying over bodies of water where setting up a radar tower would be impractical, air traffic controllers establish pre-set flight routes and rely on periodic radio check-ins to determine a pilot's progress. This is partly why a pilot's final messages before a crash are so important, and why there's often uncertainty about what those messages mean: A lot can happen in the minutes between the last transmission and when things start to go wrong.
Transponders, ADS-B and ACARS
When a plane truly goes missing, it's often the result of a cascading failure that takes out multiple systems. While it's unclear what equipment may have malfunctioned on the missing AirAsia flight — a point that may be moot now that investigators suspect the plane is already at the bottom of the ocean — the case of MH370 had officials racing to piece together the plane's location based on a number of technologies.
One key way of finding a plane is with its transponder — a device that sends back information about the flight in response to a ping from a control station. Transponders are meant to work with radar, and if they're turned off or out of range, that limits their usefulness. To get around this problem, the aviation industry is shifting toward transponders that use a technology called ADS-B. It's a newer, satellite-based system that provides much greater coverage than traditional radar. Unfortunately, not every plane supports ADS-B yet -- and it's unclear whether the AirAsia had it either. In the United States, for example, only about a third of aircraft have it. The Federal Aviation Administration is currently working on rolling out a new air traffic control system called NextGen that would make ADS-B the standard.
Beyond the technologies that are specifically designed to help pilots communicate with the ground, circumstantial evidence can help investigators infer what might have been happening on board before a crash. Planes are increasingly being outfitted with equipment known as Aircraft Communications Addressing and Reporting System, or ACARS. Depending on which ACARS package is installed, a plane can send diagnostic data about fuel levels and engine status, among other details. These transmissions not only tell officials that a plane is still operational when other means of communication have failed, they also offer clues about the state of the plane.
Although the ACARS service had been disabled on MH370, investigators still found an ingenious way to narrow the search field with it. ACARS relays information back to earth using a satellite link, which hadn't been turned off. It was still telling the satellite, "Hey, I'm here." So scientists used that periodic "handshake" to get a rough picture of MH370's trajectory.
An airplane's "black box" is often the best means of finding out what really went wrong. These flight recorders have historically been rather easy to find (more on that in a bit), but the MH370 disappearance prompted calls for better ones. There are definite limits to these devices; the ultrasonic ping it sends out can only travel a few miles underwater, and once the battery is dead, you can forget about finding it. Folks have suggested upgrading flight recorders with GPS devices, but as anyone who's used a smartphone indoors knows, obstructions that prevent you from getting a clear line of sight to a satellite can easily frustrate these chips.
Speaking of cellphones…
Experts generally agree that cellphones aren't a reliable way of getting a plane's location, for the same reasons just mentioned. Wireless location tends to work only when you have cell service, meaning you have to be close enough to a tower on the ground to get it. Even then, say network architects, passengers are likely flying too fast to get a good fix. Europe is beginning to open up the skies to mobile data, which means cellphones may soon get service even when they're not close to the ground. But it's unclear what that means for location data — and besides, it's up to individual airlines to decide whether to even offer the service.
Drones and microphones
When all else fails and it's time to find the black box, investigators turn to a decidedly old-school technique: Hunting by hand. Over the years, the tech we use to locate flight recorders has gotten more advanced. We're now using underwater drones and microphones we tow along the sea floor. The Navy's TPL-25 listening device can cover 150 square miles of ocean per day, listening for a flight recorder's ultrasonic ping. In the case of Air Asia QZ8501, this'll be an easier task as the waters there are more shallow (and the search radius smaller) than the patch of ocean where MH 370 is thought to have crashed. But when it comes down to it, we're still relying on our most basic senses — sight and sound — to recover a memory device the size of a breadbox.