Is expanding ocean search area a bad sign?

CNN/ Newswire | 3/13/2014, 4:06 p.m.
Sometimes, the crash site is never found. In 1972 a Pan Alaska Airways flight with one pilot and three passengers ...
BEIJING: "CNN reporter Pauline Chiou & producer Yuli Yang working on the disappearance of Malaysia flight 370 at a hotel near Beijing airport (March 13), where relatives & friends of passengers await news of their loved ones." - CNN's Charlie Miller. Vietnamese searchers reported seeing no sign of debris Thursday (March 13) when they flew over an area of sea that Chinese authorities had flagged as the location of possible remnants of missing Malaysia Airlines Flight 370.

By Sylvia Adcock

(CNN) -- Sometimes, the crash site is never found.

In 1972 a Pan Alaska Airways flight with one pilot and three passengers took off from Anchorage bound for Juneau, planning to fly the route under visual flight rules despite bad weather conditions. After one last contact with air traffic controllers, the Cessna was on its way. The plane never arrived in Anchorage. The flight had two congressmen on board -- Hale Boggs of Louisiana and Nicholas Begich of Alaska.

The search for the missing aircraft was intense, encompassing 325,000 square miles of land and sea, with 3,600 flight hours used to look for the wreckage. But after 39 days, the search was called off. A National Transportation Safety Board report acknowledged that the cause might never be known.

In the case of Malaysia Flight 370, a Boeing 777 missing since Saturday, a search of an area captured by Chinese satellite images that seemed to pinpoint the crash site turned up nothing. Such images are rare, and typically, when a plane goes down in a remote area with no witnesses, one of the most crucial tools available to investigators is radar.

"It's very important," said John Griffin II, an associate professor at Embry-Riddle Aeronautical University who specializes in air traffic control. "There are basically two ways to find a plane -- radar and pilot communication."

Radar can take two forms. One is primary radar, what's sometimes called "skinpaint," that simply reflects off an object. Primary radar doesn't give any information about the object it's tracking. "If you work at an ATC (Air Traffic Control) facility on an East Coast flyway, you will pick up flocks of birds," Griffin said. Primary radar can also pick up debris that might be raining down from an in-flight explosion, and in such cases a target that appears as one object suddenly appears as several objects.

Secondary radar, on the other hand, relies on plane's transponder, a device located in the nose of the plane. The transponder broadcasts a signal to air traffic controllers that identifies the flight and its altitude. Secondary radar is more than just a blip on a radar screen; it tells the controllers the valuable information about the specific aircraft.

Malaysia Flight 370's transponder stopped broadcasting about 45 minutes into the flight. At that point, air traffic controllers in Kuala Lampur had no more contact with the aircraft. At first, the search area focused on the plane's intended flight path, as officials went on the assumption that the plane did not change course. "Then they started exploring other possibilities," Griffin said. Military radar tracked unidentified targets that could have been Flight 370 heading west toward the Strait of Malacca and possibly beyond.

Military radar can cover areas not covered by civilian air traffic control. But without the plane's transponder working, that target tracked by radar is only "skinpaint," or the reflection of a dense object. It can't identify the plane or give an altitude. And even the radar can only provide so much information. It's possible for a plane to literally fly under the radar because coverage usually doesn't go all the way down to the surface. In that case, a plane could continue undetected even in areas covered by radar.