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On 11 March, it was reported that military radar indicated the aircraft turned west away from the intended flight path and continued flying for 70 minutes before disappearing from Malaysian radar near Pulau Perak.[68][69] It was also reported that it had been tracked flying at a lower altitude across Malaysia to the Malacca Strait, approximately 500 kilometres (310 mi) from its last contact with civilian radar.[70] The next day, the Royal Malaysian Air Force chief denied the report.[71][72] A few hours later however, the Vietnamese transport minister claimed that Malaysia had been informed on 8 March by Vietnamese air traffic control personnel, that they had "noticed the flight turned back west".[73][d]
Although Bloomberg News said that analysis of the last satellite "ping" received suggested a last known location approximately 1,000 miles (1,600 km) west of Perth, Western Australia,[76] the Malaysian Prime Minister Najib Razak said on 15 March that the last signal, received at 08:11 Malaysian time, might have originated from as far north as Kazakhstan.[77] Najib explained that the signals could not be more precisely located than to one of two possible loci: a northern locus stretching approximately from the border of Kazakhstan and Turkmenistan to northern Thailand, or a southern locus stretching from Indonesia to the southern Indian Ocean.[78] Many of the countries on a possible northerly flight route – China, Thailand, Kazakhstan, Pakistan, and India – denied the aircraft could have entered their country's airspace, because military radar would have detected it.[79]
It was later confirmed that the last ACARS transmission showed nothing unusual and a normal routing all the way to Beijing,[80] The New York Times reported "senior American officials" saying on 17 March that the scheduled flight path was reprogrammed to unspecified western coordinates through the flight management system before the ACARS stopped functioning,[81] and a new waypoint "far off the path to Beijing" was added.[81] Such a reprogramming would have resulted in a banked turn at a comfortable angle of around 20 degrees that would not have caused undue concern for passengers. The sudden cessation of all on-board communication led to speculation that the aircraft's disappearance may have been due to foul play.
On 6 April, JACC announced that Ocean Shield had also picked up a signal, about 300 nautical miles (560 km; 350 mi) from Haixun 01.[137][138] It was announced the next day that the TPL-25 pinger locator towed by Ocean Shield had picked up a signal twice on 6 April.[139][140] The first was on the morning of 6 April, at approximately 3,000 metres (9,800 ft) depth, and lasted 2 hours and 20 minutes. The second signal reception took place at approximately 300 metres (980 ft) depth and lasted 13 minutes. During the second episode, two distinct pinger returns were audible. Both episodes of recorded signals, which took place at roughly the same position though several kilometres apart, were considered to be consistent with signals expected from an aircraft's flight recorder ULB.[141] The signals received by Ocean Shield were recorded at the north of a newly calculated impact area, which was announced on 7 April, while the Haixun 01 signals had been recorded at its southern edge.[141][142][143] Ocean Shield detected two more signals on 8 April. The first was acquired at 16:27 AWST and held for 5 minutes, 32 seconds and the second was acquired at 22:17 AWST and held for around 7 minutes.[11][144][145] Experts had determined that the earlier signals captured by Ocean Shield were "very stable, distinct, and clear ... at 33.331 kHz and ... consistently pulsed at a 1.106-second interval". These were said to be consistent with the flight recorder ULB.[11] but the frequency of the detections was well outside the manufacturer's specification of 37.5 +/- 1.[146] The later signals were at a frequency of 27 kHz, which raised doubts that they were from a black box.[147] On 10 April, a signal recorded by one of the sonobuoys deployed with a hydrophone at 300 metres depth[148][149] was found unlikely to have originated from Flight 370.[150]
On 14 April, due to the likelihood of the ULBs' acoustic pulses having ceased because their batteries would have run down, the Towed Pinger Locator search gave way to a sea-bed search using side-scan sonar installed in a Bluefin-21 Autonomous Underwater Vehicle.[151] The first day's search was aborted because the sea bed was considerably deeper than the maximum operating depth of Bluefin. Scanning subsequently resumed[152] and after covering 42 square miles in its first four dives, the submersible was reprogrammed to allow it to dive 604 feet lower than its operational limit of 14,800 feet, when the risk of damage was assessed as "acceptable". By this time the search was believed to have cost $100 million (£72m) and had been labelled "the costliest in aviation history".[153]
Bluefin-21 required 16 missions to complete its search of the 314 square kilometre area around the detections made by the Towed Pinger Locator.[154][155] At a news conference in Canberra on 28 April, Tony Abbott said "It is now 52 days since Malaysia Airlines Fight MH370 disappeared and I'm here to inform you that the search will be entering a new phase". Abbott also explained that "a much larger" area of the ocean floor would then be searched; it was "highly unlikely" that any surface wreckage would be found; and that the aerial searches had been suspended.[156][157][158] Mission 17 of Bluefin-21, covering the new, wider search area, was on 30 April.[155]
On 12 May, it was reported that the captain of Ocean Shield said there was increasing doubt that pings #3 and #4 detected on 8 April originated with MH370's black boxes because their frequency of about 27 kHz was too far below the pinger's design frequency of 37.5 kHz. Pings #1 and #2, detected on 5 April at 33 kHz, were still being considered by the search authorities.[159] On 13 May the search was interrupted due to problems with both the transponder mounted on Ocean Shield and that mounted on Bluefin-21.[160] By 22 May those problems had been resolved and the search was resumed, with Ocean Shield being joined by the Chinese survey ship Zhu Kezhen, which was to conduct a bathymetric survey of the area.[161]
On 29 May the Australian Deputy Prime Minister, Warren Truss, announced that no aircraft debris had been found in that part of the ocean where searchers previously had reported 'pings' from the black boxes. The announcement followed a statement by US Navy's Deputy Director of Ocean Engineering that all four pings were no longer believed to have come from the aircraft's flight recorders.[162] Truss informed parliament that, beginning in August, after a new commercial operator for the search effort had been selected, the search would move into a new phase "that could take twelve months".[163] Equipment envisaged to be used would include towed side-scan sonar.[164]
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