Investigations into how and why a Cypriot passenger airliner crashed into a hillside near Athens, killing all 121 people on board, are focusing on an apparent sudden drop in cabin pressure.
The aircraft was similar to this Helios Boeing 737
The BBC News website looks at some of the issues surrounding the crash.
How could the crash have happened?
The Boeing 737 appears to have crashed after suffering a sudden loss in cabin pressure. It appears that pilots reported problems with the plane's air conditioning system to Cypriot air traffic control shortly after taking off from Larnaca.
At some point the aircraft depressurised at altitude very quickly. Fighter jet pilots sent to find the plane at 10,350m (34,000ft) reported seeing the co-pilot slumped in the cockpit, while
the pilot was not visible.
They also reported seeing unused oxygen masks dangling in the cabin. Some time later the plane, which may have been on auto-pilot for a lengthy period, crashed into the Greek hills.
How does the cabin pressure differ from the outside air?
The air inside all large commercial aircraft is pressurised, or "pumped up" to the equivalent of a naturally breathable altitude - typically about 3,050m (10,000ft). Air at that height is thin but breathable. The cabin pressure will be kept at that altitude.
Air at higher altitudes contains significantly lower amounts of oxygen. At 9,144m (30,000ft), humans can only stay conscious without additional oxygen for up to 30 seconds. Aviation experts compare it to standing on the top of Everest.
Did a drop in cabin pressure cause the crash?
At the moment there is no definitive answer. Some tell-tale signs - including the unconscious pilots - point to a loss of pressure. Reports that the passengers' bodies were discovered frozen solid back up theories that they were exposed to a high altitude.
What emergency procedures are in place to deal with cabin pressure problems?
There are several emergency procedures in place on large commercial airliners. Pilots in the cockpit should be alerted to pressurisation problems by the sounding of a loud horn. They will quickly don oxygen masks and these will also drop down in the passenger cabin. Jets are designed to cope with rapid descents in the event of a pressure drop, and pilots regularly practise the manoeuvre.
If there are emergency plans in place, then why did the plane crash?
Navigation systems are independent of pressure controls, and airliners are able to continue flying for a long time after a drop in pressure.
Investigators are examining why the pilots were apparently unable to take on oxygen before losing consciousness. If the drop in pressure was extremely quick, the air inside the plane could have disappeared in an instant, accompanied by an explosive noise.
This form of "explosive decompression" can affect blood pressure and cause lung trauma and unconsciousness. Nevertheless, pilots regularly practice this type of emergency.
Have there been other incidents like this?
Helios Airways previously suffered a drop in cabin pressure in an aircraft coming from Warsaw. The plane made an emergency landing in Larnaca. Three passengers were treated in hospital. It is not clear whether the aircraft involved was the one that crashed in Greece.
Other planes have experienced difficulties with pressure. In 1996 a US Trans Air 727 experienced a rapid drop in cabin pressure while flying across the US mid-west. The captain and flight engineer lost consciousness, but the co-pilot had quickly put on an oxygen mask and flew the plane to safety.
In 1999 a Lear jet carrying US golfer Payne Stewart flew for five hours across the US before crashing, after a pressure drop killed or disabled everyone on board.