The southern Indian Ocean, where investigators suspect missing Malaysia Airlines Flight MH370 may have come down, is one place where a commercial airliner can crash without a ship spotting it, a radar plotting it or even a satellite picking it up.
The empty expanse of water is one of the most remote places in the world and also one of the deepest, posing potentially enormous challenges for the international search effort now refocusing on the area, one of several possible crash sites.
Even Australia, which has island territories in the Indian Ocean and sends rescue planes to pluck stricken yachtsmen from the cold, mountainous seas in the south from time to time, has no radar coverage much beyond its Indian Ocean coast.
“In most of Western Australia and almost all of the Indian Ocean, there is almost no radar coverage,” an Australian civil aviation authority source said, requesting anonymity as he was not authorised to speak on the record.
“If anything is more than 100 kilometres offshore, you don’t see it.”
The Indian Ocean, the world’s third largest, has an average depth of more than 12,000 feet, or two miles. That’s deeper than the Atlantic where it took two years to find wreckage on the seabed from an Air France plane that vanished in 2009 even though floating debris quickly pointed to the crash site.
So far, search operations by navies and aircraft from more than a dozen nations have failed to find even a trace of Flight MH370, which went missing a week ago after taking off from Kuala Lumpur for Beijing and diverting from its intended flight path.
The search effort has focused mainly on the South China Sea but is now switching to the Indian Ocean after investigators, having pieced together radar and satellite tracking data, began to suspect the Boeing 777-200ER had been deliberately flown hundreds or possibly thousands of miles off course.
Searchers still face a daunting array of possible last locations for the plane, including the northern end of the Indian Ocean as well as central Asia, though investigators say it is more likely to have flown to the south than through busier airspace to the north where it would likely have been detected.
With an estimated four hours fuel left when last spotted by radar off Malaysia’s northwest coast, the plane could have flown a further 2,200 miles (3,500 km) or so, assuming normal cruising speed and altitude.
Officials think, based on the available data, the aircraft flew south until it ran out of fuel and crashed into the sea, according to a source familiar with data the US government is receiving from the investigation.
In the south, any debris from MH370 would have been widely dispersed by Indian Ocean currents in the week since it disappeared.
The southern Indian Ocean, between Indonesia and Australia, is broken up only by the Australian territories of Christmas Island, home to asylum seeker detention facilities, and the Cocos (Keeling) Islands some 2,000 km (1,240 miles) northwest of Perth. The Cocos Islands have a small airport to serve the islands’ combined population of just 3,000 people.
Further south, the only habitation is the handful of research stations on the scattering of tiny French-run islands including Kerguelen - a group of volcanic outcrops between Africa, Australia and Antarctica. While home to several powerful astronomical scanners and radar, there is no airport and it is seen extremely unlikely the aircraft could have made it that far.
The shipping route from Western Australia north to Asia and Europe is considered relatively quiet in global shipping terms, despite the large amount of iron ore and other resources that are shipped from Australia’s northwest ports.
Ships track north staying close in to the West Australian coastline and then head north through Indonesian waters into the South China Sea or northwest toward the Red Sea.
Australia’s civil aviation radar extends a maximum of just 200 nautical miles (410 km) off the coast, the civil aviation authority source said, and was used only for monitoring scheduled aircraft on approach into the country and subsequent landings.
There are just two primary radars on the west Australian coast, one in Perth and one further north in Paraburdoo, which has even less range and is used to monitor mining traffic heading to the nearby Pilbara region.
Australia’s Civil Aviation Authority relies on aircraft ADSB (automatic dependent surveillance broadcast) to ping information to commercial satellites, such as telecoms firm Optus’ four telecommunications satellites, and back to ground control.
Australia does not have any government satellites.
The Australian military has an over-the-horizon radar network that allows it to observe all air and sea activity north of Australia for up to 3,000 km (1,860 miles). This encompasses all of Java, Papua New Guinea and the Solomon Islands.
While the Jindalee Operational Radar Network (JORN) extends part-way across the northern Indian Ocean, government papers online describe it as a “tripwire” in Australia’s northern surveillance system, helping underpin the defence of the country from any attack originating from the north.
Local media have said its main use recently has been to track illegal immigrants approaching Australia by boat through the region’s largely unguarded northern waters.
The Australian Defence Force was not available for comment on Sunday.
A potential crash site around 1,600 km (1,000 miles) northwest or west of the Australian coast would be well within the search and rescue area of the Australian Maritime Safety Authority (AMSA), one of the largest in the world.
An AMSA spokesman said no request for assistance had been received from Malaysia as of Sunday.
Three pieces of evidence that aviation safety experts say make it clear the missing Malaysia Airlines jet was taken over by someone who was knowledgeable about how the plane worked:
One clue is that the plane’s transponder — a signal system that identifies the plane to radar — was shut off about an hour into the flight.
In order to do that, someone in the cockpit would have to turn a knob with multiple selections to the off position while pressing down at the same time, said John Goglia, a former member of the National Transportation Safety Board. That’s something a pilot would know how to do, but it could also be learned by someone who researched the plane on the Internet, he said.
Another clue is that part of the Boeing 777’s Aircraft Communications Addressing and Reporting System (ACARS) was shut off.
The system, which has two parts, is used to send short messages via a satellite or VHF radio to the airline’s home base. The information part of the system was shut down, but not the transmission part. In most planes, the information part of the system can be shut down by hitting cockpit switches in sequence in order to get to a computer screen where an option must be selected using a keypad, said Goglia, an expert on aircraft maintenance.
That’s also something a pilot would know how to do, but that could also be discovered through research, he said.
But to turn off the other part of the ACARS, it would be necessary to go to an electronics bay beneath the cockpit. That’s something a pilot wouldn’t normally know how to do, Goglia said, and it wasn’t done in the case of the Malaysia plane. Thus, the ACARS transmitter continued to send out blips that were recorded by the Inmarsat satellite once an hour for four to five hours after the transponder was turned off. The blips don’t contain any messages or data, but the satellite can tell in a very broad way what region the blips are coming from and adjusts the angle of its antenna to be ready to receive message in case the ACARS sends them. Investigators are now trying to use data from the satellite to identify the region where the plane was when its last blip was sent.
The third indication is that that after the transponder was turned off and civilian radar lost track of the plane, Malaysian military radar was able to continue to track the plane as it turned west.
The plane was then tracked along a known flight route across the peninsula until it was several hundred miles (kilometres) offshore and beyond the range of military radar. Airliners normally fly from waypoint to waypoint where they can be seen by air traffic controllers who space them out so they don’t collide. These lanes in the sky aren’t straight lines. In order to follow that course, someone had to be guiding the plane, Goglia said.
Goglia said he is very sceptical of reports the plane was flying erratically while it was being tracked by military radar, including steep ascents to very high altitudes and then sudden, rapid descents. Without a transponder signal, the ability to track planes isn’t reliable at very high altitudes or with sudden shifts in altitude, he said.