KUALA LUMPUR, March 17 (Reuters) - Whether by accident or design, whoever reached across the dimly lit cockpit of a Malaysia Airlines jet and clicked off a transponder to make Flight MH370 vanish from controllers' radars flew into a navigational and technical black hole.
By choosing one place and time to vanish into radar darkness with 238 others on board, the person - presumed to be a pilot or a passenger with advanced knowledge - may have acted only after meticulous planning, according to aviation experts.
Understanding the sequence that led to the unprecedented plane hunt widening across two vast tracts of territory north and south of the Equator is key to grasping the motives of what Malaysian authorities suspect was hijacking or sabotage.
By signing off from Malaysian airspace at 1.19 a.m. on March 8 with a casual "all right, good night," rather than the crisp radio drill advocated in pilot training, a person now believed to be the co-pilot gave no hint of anything unusual.
Two minutes later, at 1.21 a.m. local time, the transponder - a device identifying jets to ground controllers - was turned off in a move that experts say could reveal a careful sequence.
"Every action taken by the person who was piloting the aircraft appears to be a deliberate one. It is almost like a pilot's checklist," said one senior captain from an Asian carrier with experience of jets including the Boeing 777.
There is so far no indication whether the co-pilot was at fault or had anything to do with turning off the transponder. Pilots say the usual industry convention is that the pilot not directly responsible for flying the plane talks on the radio.
Police have searched the premises of both the captain and co-pilot and are checking the backgrounds of all passengers.
Whoever turned the transponder to "off", whether or not the move was deliberate, did so at a vulnerable point between two airspace sectors when Malaysian and Vietnamese controllers could easily assume the airplane was each others' responsibility.
"The predictable effect was to delay the raising of the alarm by either party," David Learmount, operations and safety editor at Flight International, wrote in an industry blog.
That mirrors delays in noticing something was wrong when an Air France jet disappeared over the Atlantic in 2009 with 228 people on board, a gap blamed on confusion between controllers.
Yet whereas the Rio-Paris disaster was later traced to pilot error, the suspected kidnapping of MH370's passengers and crew was carried out with either skill or bizarre coincidences.
Whether or not pilots knew it, the jet was just then in a technically obscure sweet spot, according to a top radar expert.
Air traffic controllers use secondary radar which works by talking to the transponder. Some air traffic control systems also blend in some primary radar, which uses a simple echo.
But primary radar signals fade faster than secondary ones, meaning even a residual blip would have vanished for controllers and even military radar may have found it difficult to identify the 777 from other ghostly blips, said radar expert Hans Weber.
"Turning off the transponder indicates this person was highly trained," said Weber, of consultancy TECOP International.
The overnight flight to Beijing from Kuala Lumpur is packed year-round with business people, Chinese tourists and students, attracted in part by code-sharing deals, regular travellers say.
The lockdown of MH370 may have begun as early as 40 minutes into the flight at a point when meals are being hurriedly served in time to get trays cleared and lights dimmed for the night.
"It was a red-eye flight. Most people - the passengers and the crew - just want to rest," a Malaysia Airlines stewardess said. "Unless there was a reason to panic, if someone had taken control of the aircraft, they would not have noticed anything."
At some point between 1.07 a.m. and 1.37 a.m., investigators believe someone switched off another system called ACARS designed to transmit maintenance data back to the ground.
While unusual, this may not necessarily raise alarms at the airline and the passengers would not have known that something was amiss, said some of the six pilots contacted by Reuters, none of whom agreed to be identified because of company rules.
"Occasionally, there are gaps in the communications systems and the guys in ground operations may not think much of it initially. It would be a while before they try to find out what was wrong," said one captain with an Asian carrier.
Cutting the datalink would not have been easy. Instructions are not in the Flight Crew Operating Manual, one pilot said.
Whoever did so may have had to climb through a trap door in full view of cabin crew, people familiar with the jet say.
Circuit-breakers used to disable the system are in a bay reached through a hatch in the floor next to the lefthand front exit, close to a galley used to prepare meals.
Most pilots said it would be impossible to turn off ACARS from inside the cockpit, though two people did not rule it out.
Malaysia Airlines said 14 minutes elapsed between the last ACARS message and the transponder shutdown that - in the growing view of officials - confirmed a fully loaded jet was on the run. The ACARS must have been disabled within 16 minutes after that.
In the meantime a voice believed to be that of the co-pilot issued the last words from MH370 and the transponder went dead.
The northeast-bound jet now took a northwestern route from Kota Bahru in eastern Malaysia to Penang Island. It was last detected on military radar around 200 miles northwest of Penang.
Even that act of going off course may not have caused alarm at first if it was handled gradually, pilots said.
"Nobody pays attention to these things unless they are aware of the direction that the aircraft was heading in," said one first officer who has flown with Malaysia Airlines.
The airline said it had reconstructed the event in a simulator to try to figure out how the jet vanished and kept flying for what may have been more than seven hours.
Pilots say whoever was then in control may have kept the radio on in silent mode to hear what was going on around him, but would have avoided restarting the transponder at all costs.
"That would immediately make the aircraft visible ... like a bright light. Your registration, height, altitude and speed would all become visible," said an airline captain.
After casting off its identity, the aircraft set investigators a puzzle that has yet to be solved. It veered either northwards or southwards, within an hour's flying time of arcs stretching from the Caspian to the southern Indian Ocean.
The best way to avoid the attention of military radars would have been to fly at a fixed altitude, on a recognised flight path and at cruising speed without changing course, pilots say.
Malaysian officials dismissed as speculation reports that the jet may have flown at low altitude to avoid detection.
But pilots said the best chance of feeling its way through the well-defended northern route would have been to hide in full view of military radar inside commercial lanes - raising awkward questions over security in several parts of the Asia-Pacific.
"The military radar controllers would have seen him moving on a fixed line, figured that it was a commercial aircraft at a high altitude, and not really a danger especially if he was on a recognised flight path," said one pilot.
"Some countries would ask you to identify yourself, but you are flying through the night and that is the time when the least attention is being paid to unidentified aircraft. As long as the aircraft is not flying towards a military target or point, they may not bother with you."
Although investigators refused on Monday to be drawn into theories, few in the industry believe a 250-tonne passenger jet could run amok globally without expert skills or preparation.
"Whoever did this must have had lots of aircraft knowledge, would have deliberately planned this, had nerves of steel to be confident enough to get through primary radar without being detected and been confident enough to control an aircraft full of people," a veteran airline captain told Reuters.