* Regional air defences missed wandering airliner * Indian radar may have been turned off at night * Remote oceans barely covered by surveillance By Peter Apps and Frank Jack Daniel LONDON/NEW DELHI (Reuters) - Whatever truly happened to missing Malaysian Airlines flight MH370, its apparently unchallenged wanderings through Asian skies point to major gaps in regional - and perhaps wider - air defences. More than a decade after al Qaeda hijackers turned airliners into weapons on September 11, 2001, a large commercial aircraft completely devoid of stealth features appeared to vanish with relative ease. On Saturday, Malaysia's Prime Minister Najib Razak said authorities now believed the Boeing 777 flew for nearly seven hours after disappearing early on March 8. Either its crew or someone else on the plane disabled the on-board transponder civilian air traffic radar used to track it, investigators believe. It appears to have first flown back across the South China Sea - an area of considerable geopolitical tension and military activity - before overflying northern Malaysia and then heading out towards India without any alarm being raised. The reality, analysts and officials say, is that much of the airspace over water - and in many cases over land - lacks sophisticated or properly monitored radar coverage. Analysts say the gaps in Southeast Asia's air defences are likely to be mirrored in other parts of the developing world, and may be much greater in areas with considerably lower geopolitical tensions. "Several nations will be embarrassed by how easy it is to trespass their airspace," said Air Vice Marshal Michael Harwood, a retired British Royal Air Force pilot and ex-defence attaché to Washington DC. "Too many movies and Predator (unmanned military drone) feeds from Afghanistan have suckered people into thinking we know everything and see everything. You get what you pay for. And the world, by and large, does not pay." "TOO EXPENSIVE" Air traffic systems rely almost entirely on on-board transponders to detect and monitor aircraft. In this case, those systems appear to have been deactivated around the time the aircraft crossed from Malaysian to Vietnamese responsibility. At the very least, the incident looks set to spark calls to make it impossible for those on board an aircraft to turn off its transponders and disappear. Military systems, meanwhile, are often limited in their own coverage or just ignore aircraft they believe are on regular commercial flights. In some cases, they are simply switched off except during training and when a threat is expected. That, one senior Indian official said, might explain why the Boeing 777 was not detected by installations on India's Andaman and Nicobar Islands, an archipelago which its planes were searching on Friday and Saturday, or elsewhere. "We have many radar systems operating in this area, but nothing was picked up," Rear Admiral Sudhir Pillai, chief of staff of India's Andamans and Nicobar Command, told Reuters. "It's possible that the military radars were switched off as we operate on an 'as required' basis." Separately, a defence source said that India did not keep its radar facilities operational at all times because of cost. Asked what the reason was, the source said: "Too expensive." "SOMEONE ELSE'S PROBLEM" Worries over revealing defence capabilities, some believe, may have slowed cooperation in the search for flight MH370, particularly between Malaysia and China. Beijing has poured military resources into the search, announcing it was deploying 10 surveillance satellites and multiple ships and aircraft. It has been critical of Malaysia's response. While Malaysian military radar does appear to have detected the aircraft, there appear to have been no attempts to challenge it - or, indeed, any realisation anything was amiss. That apparent oversight, current and former officials and analysts say, is surprising. But the incident, they say, points to the relatively large gaps in global air surveillance and the limits of some military radar systems. "It's hard to tell exactly why they did not notice it," says Elizabeth Quintana, senior research fellow for air power at the Royal United Services Institute in London. "It may have been that the aircraft was flying at low level or that the military operators were looking for other threats such as fast jets and felt that airliners were someone else's problem." Current and former officials say that - hopefully, at least - such an incident would be detected much faster in North American or European airspace. There, military and civilian controllers monitor radar continuously on alert for possible hijacks or intruders. The sudden failure of a transponder, they say, would itself prove a likely and dramatic cause for concern. "I can't think of many situations in which one would actually need to switch them off," said one former Western official on condition of anonymity. U.S. and NATO jets periodically scramble to intercept unidentified aircraft approaching their airspace, including a growing number of Russian long-range bombers. In some other areas, it is simply not seen as worth maintaining a high level of alert - or radar coverage itself may not even exist. "NOTHING MUCH HAPPENS AT NIGHT" Investigators now say they believe MH370 may have turned either towards India and Central Asia or - perhaps more likely, given the lack of detection - taken a southern course towards the Antarctic. That would have been an effectively suicidal flight, the aircraft eventually running out of fuel and crashing. The waters of the southern Indian Ocean and northern Southern Ocean are among the most remote on the planet, used by few ships and overflown by few aircraft. Australian civilian radar extends only some 200 km (125 miles) from its coast, an Australian official said on condition of anonymity, although its air defence radar extends much further. Australia's military could not be reached for comment on Saturday and if it did detect a transponder-less aircraft heading south, there is no suggestion any alarm was raised. U.S. military satellites monitor much of the globe, including some of the remotest oceans, looking primarily for early warning of any ballistic missile launch from a submarine or other vessel. After the aircraft's initial disappearance a week ago, U.S. officials said their satellites had detected no signs of a mid-air explosion. It is unclear if such systems would have detected a crash landing in the southern Indian Ocean. On India's Andaman Islands, a defence official told reporters he saw nothing unusual or out of place in the lack of permanent radar coverage. The threat in the area, he said, was much lower than on India's border with Pakistan where sophisticated radars are manned and online continuously. At night in particular, he said, "nothing much happens". "We have our radars, we use them, we train with them, but it's not a place where we have (much) to watch out for," he said. "My take is that this is a pretty peaceful place."