Some more info on trying to locate debris form flight MH370, emphasis mine:

As Malaysia Expands Search Area for Missing Jet, Wide Speculation Over Its Fate

WASHINGTON — Watching a seat-back display with a plane-shaped icon gliding across the map, it is easy to forget that in true scale, the airplane is very small and the route very large. As the hours and days drag by with no trace of the Malaysia Airlines flight that disappeared over the Gulf of Thailand early Saturday, the world is getting a reminder that if something goes wrong on a jet five miles up in the sky, traveling at 10 miles a minute, it can cover a lot of ground — or water — before it comes down to earth. […]

It is not yet known whether the Malaysian plane deviated from its planned flight path, or how long the pilots could still fly the aircraft after the last reported contact. Assuming that the plane remained in powered flight or a controlled glide, the potential search area would have to be wide and long, covering thousands of square miles. After more than two days of fruitless search, Malaysian officials expanded the search area on Monday.

The rule of thumb for a crew planning a normal descent to an airport is to allow three miles of distance for every thousand feet of altitude. So a jetliner at 30,000 feet that cut its engines to idle would fly another 90 miles or so before reaching a runway near sea level. […]

But extended searches are sometimes needed. When Air France Flight 447 vanished over the Atlantic in June 2009, it took five days to find any wreckage, and almost two years to find the black boxes. Similarly, the cockpit data recorder from a South African Airways Boeing 747 that went down in November 1987 was not located until January 1989. It revealed that the plane crashed because of a fire onboard, not because of an act of terrorism, so no further search was conducted for the flight data recorder, the other black box. […]

nytimes.com