Update on Missing Plane: Plane’s Locating Device Went Silent Before Disappearance

WASHINGTON — One of the mysteries surrounding Malaysia Airlines Flight 370 concerns the aircraft’s transponder, which apparently stopped functioning about 40 minutes into the flight, leaving ground controllers in the dark about where the plane was.

A transponder is basically a robotic radio that automatically transmits a response when it receives an inquiry signal. Most airplanes are equipped with them. When an aircraft is under the active supervision of an air traffic control system, it is assigned an identifying code, which the pilot enters into the transponder. The device will then broadcast that code and a bit of additional data, including the aircraft’s current altitude, which civilian ground radar systems generally cannot discern on their own (though military radar often can).

Two kinds of radar are used to keep track of air traffic from the ground. One, called primary radar, sends out radio signals and listens for echoes that bounce back from objects in the sky. It can spot aircraft whether they have transponders or not, but by itself it cannot tell which aircraft it has detected.

The other type, secondary radar, sends signals that are meant to interrogate the transponders, asking electronically, “Who is out there?”

Since the secondary radar system knows which way it was facing when it sent out the question, it knows the direction of the aircraft whose transponder replied, “Here I am.” It also knows how long it took for the response to come back, which tells it the distance to the aircraft. Together, the two pieces of information pinpoint the plane’s location. The information is refreshed every few seconds as the radar repeatedly sweeps the sky and interrogates the transponder. Other planes in flight can also receive the transponder signals, using a system meant to protect against midair collisions.

Many air traffic controllers turn off the primary radar on their displays and just use the secondary radar, because the primary radar sometimes shows birds, clouds and other extraneous objects in the sky as well as aircraft.

All electronic equipment on an airplane can be turned off by pilots, including the transponder. This is a safety feature, so a device that short-circuits or overheats or otherwise malfunctions can be shut down before it does further harm. But hijackers have been known to turn transponders off. (There is a code that pilots can enter into a transponder, if they get the chance, that indicates the plane has been hijacked.)

It is not possible yet to say whether the transponder on Flight 370 was shut off, only that it stopped responding.