A trompe l'oeil artwork in a school in Manila for those on board MH370. Photograph: Romeo Ranoco/Reuters
The missing Malaysia Airlines plane could have flown as low as 5,000ft (1,500 metres) after diverting from its course, allowing it to avoid detection by radar, according to Malaysian media reports.
Investigators are working to narrow down the last possible observation of flight MH370 after analysis of satellite information revealed it was in one of two vast corridors: a northern area stretching from the border of Kazakhstan and Turkmenistan to northern Thailand; and a southern range stretching from Indonesia to the southern Indian Ocean. Twenty-five countries are now involved in the search for the plane, which officials believe was deliberately diverted from its route to Beijing not long after it took off from Kuala Lumpur just after midnight on 8 March.
Malaysia's New Straits Times reported that investigators were considering the possibility that the Boeing 777 dropped to 5,000ft or potentially even lower to avoid detection.
It suggested that the aircraft might not have roused the suspicions of those watching military radars if it followed commercial routes. It also cited unnamed sources as saying the plane had flown low over the Malay peninsula.
It is unclear where the altitude estimate originated and experts said that if it came from radar data it could well prove incorrect.
Aviation safety expert Sidney Dekker, of the Safety Science Innovation Lab at Griffith University in Brisbane, Australia, noted: "Particularly over oceanic areas, radar coverage is extremely unreliable and partial."
He dismissed the idea that flying at 5,000ft would put extensive strain on the airframe, as some have suggested: "The only really relevant effect is a dramatic increase in fuel use – its range would be very much shortened by that," he said.
Jason Middleton, head of the aviation department at New South Wales University, said avoiding the radar was a well-known technique used by drug runners and others.
He added: "Radar goes in a straight line. If you are in the shadow of a mountain or even the curve of the Earth – if you are under the radar beam – you can't be seen.
"The further [radar beams] go out the weaker they are and the further they need to come back. Radars have dead zones which are low and also which are far away."
However, pilots might also fly low on their way to a landing, if the cabin was depressurised – to prevent crew and passengers suffering oxygen deprivation – or if they were suffering mechanical problems.
Kazakhstan's civil aviation authority told the BBC it was not possible for the plane to have reached its airspace undetected, noting that it would have had to fly over China, India and other countries, while Pakistan's civil aviation authority said checks of its radar recordings found nothing connected to the flight.
Military radar systems can also be limited in their coverage and may not always be in use. Malaysia's briefings have made it clear that its own military did not initially take heed of MH370 when it appeared on radar screens heading westwards after diverting. A senior Indian official also told Reuters this weekend that the Boeing 777 might not have been detected by installations on the Andaman and Nicobar islands because the radars might have been switched off, adding: "We operate on an 'as required' basis."
The Australian prime minister, Tony Abbott, told parliament that the country had taken on responsibility for searching a major section of the southern Indian Ocean at the request of his Malaysian counterpart. He added that Najib Razak had also accepted an offer of additional Australian maritime surveillance resources.
The foreign ministry of China, which has more than 150 citizens on board the flight, said in a statement on Monday that Malaysia must "immediately" expand and clarify the search.
The English edition of the Global Times – a state-run populist tabloid – ran a scathing commentary on the search effort,
accusing Malaysia of incompetence and suggesting it might need to hand over responsibility for the search operation.
Three French investigators have joined the multinational team in Kuala Lumpur to share their expertise from the 2009 search for Air France flight 447. Investigators from the US, UK and China are already involved, but the New York Times reported that
Malaysia had refused to accept large-scale American assistance, citing unnamed senior officials.
Families of the 239 on board have said that investigators' belief that the plane was diverted deliberately has given them fresh hope that the passengers and crew might have survived.
David Lawton, an Australian man whose brother and sister-in-law are on the missing plane, told Fairfax Media: "While you've got hope, you've got worries too. Because if they're alive, are they being treated well, or what's happening?"
Authorities were also exploring whether anyone on board besides the pilots had aviation experience. Reuters reported that Malaysian police were investigating a Malaysian flight engineer who was among the passengers, 29-year-old Mohd Khairul Amri Selamat.
According to Singapore's Straits Times, his father said his son would have done no wrong and that no officers had been to search their home, adding: "Even if they do, we have nothing to hide."
