Malaysia Airlines Flight MH370, with 239 people aboard, lost contact early Saturday with the airline shortly into the flight from Kuala Lumpur to Beijing. Government and airline officials said they weren't ready to speculate about what caused the Boeing 777-200 to disappear from radar. Here are five things to know about the flight and air safety. 

#1: What happened to Malaysia Airlines Flight MH370? 

With 239 people aboard, the plane lost contact with air-traffic controllers shortly into the flight early Saturday. Cruising at roughly 35,000 feet, the plane was about a third of the way into its trip and traversing the South China Sea when Malaysian controllers lost touch and the pilots failed to report to Vietnamese controllers along their anticipated path, according to data provided by the carrier and authorities from both countries. 

cruising：巡航        anticipated：预期的，期望的         

#2: Has anything like this happened before? 

Air-safety experts have likened the sudden loss of contact and some other elements to the 2009 crash of an Air France jet into the Atlantic Ocean en route from Brazil to France; all 228 people on board were killed. The pilots on that plane also failed to issue any emergency calls and the wreckage wasn't recovered until two years later, demonstrating the challenge of search and rescue operations. 

wreckage：残骸

#3: Why is it so difficult to locate lost planes? 

Today's airliners have sturdy emergency-locator devices designed to transmit signals in the event of a crash, as do so-called 'black boxes' that contain digital flight data and cockpit recordings. But without knowing the trajectory of a plane as it went down--or fully understanding wind and wave conditions if it crashed into water--searchers sometimes can end up crisscrossing huge areas looking for relatively small pieces of wreckage. Particularly deep water or rugged terrain can seriously hinder investigations. 

cockpit：驾驶员座舱         rugged terrain：崎岖地带 

#4: What technology do airlines use to track flights? 

Carriers typically have dispatchers and operational-control centers tracking aircraft, often communicating with pilots via data links or even voice communications over satellite connections. In the event of a major mechanical problem or other onboard events that don't immediately require an emergency landing , cockpit crews can call on those experts for advice. On many planes, certain maintenance and operational data are automatically transmitted to ground facilities. 

#5: What's the safety record of the 777? 

The long-range Boeing 777 has had an exceptional safety record since entering service in May 1995. Malaysia Airlines was an early operator of an extended-range version and got its first in May 1997. There have been only two earlier 777 incidents during a flight that resulted in the loss of the aircraft. The first was in January 2008 when British Airways Flight 38 suffered icing inside its fuel lines after a long midwinter flight from Beijing crash-landed just short of London's Heathrow Airport, injuring 47. The second was Asiana Airlines Flight 214, which struck a sea wall on final approach to San Francisco International Airport in July 2013; three passengers were killed.