Investigators probing the disappearance of Malaysia Airlines flight MH370 have reiterated that there was no evidence of unusual air traffic control communications before the aircraft vanished.

But the Malaysian ministry of transport, which has released the air-ground communication transcript, says it remains convinced that the Boeing 777's movements after it lost contact are “consistent with deliberate action” by someone on board.

“There is no indication of anything abnormal in the transcript,” the ministry states.

It shows that the aircraft requested a cruise altitude of 35,000ft for the 8 March service to Beijing. The flight was assigned the transponder squawk code 2157 and cleared for the PIBOS Alpha departure.

From Kuala Lumpur’s departure runway 32R this normally involves flying a straight course until 9nm distant from the airport’s VOR, then turning right at about 1,200ft.

The aircraft would then track north-east, in the direction of the PIBOS waypoint, in order to intercept the R208 air route towards Kuala Trengganu, on the coast, and the IGARI waypoint over the Gulf of Thailand.

But the transcript shows that, after the aircraft took off from Kuala Lumpur, it was instead cleared by departure control to climb to 18,000ft and take a shorter route by turning right immediately and flying directly to IGARI.

Flight MH370 was then handed off to the en route centre and cleared to climb to 25,000ft. Just over 3min later it was cleared to its requested cruise altitude of 35,000ft.

Eleven minutes after acknowledging this clearance, at just after 01:01, the flight informed the en route centre that it was maintaining 35,000ft. It transmitted a near-identical message a few minutes later, shortly before 01:08. Both were acknowledged by air traffic control.

There was no further communication until 01:19 when air traffic control instructed MH370 to contact Vietnam’s Ho Chi Minh centre and gave the radio frequency as 120.9MHz. The response from the aircraft did not read back the frequency, but simply stated: “Good night Malaysian three seven zero.”

While the sign-off is not unusual in itself, it differs from the version – “all right, good night” – previously suggested by government officials as early as 12 March. In spite of scrutiny over the phraseology, this version had remained uncorrected even after an unverified transcript, published in Chinese media on 16 March, clearly indicated a different sign-off, one which matched the newly-disclosed final communication.

Investigators have yet to determine the source of the sign-off. Malaysia Airlines had previously indicated that it believed the first officer transmitted the message, and the transport ministry says the inquiry is “working to confirm this belief”.

About 2min after this last voice message, all normal contact with the 777 was lost in the vicinity of the IGARI waypoint. No further voice, transponder or ACARS transmissions were received. Military radar indicates that the aircraft deviated from its course, turning west and heading out over the Indian Ocean. Analysis of rudimentary satcom signals suggests the aircraft then turned south and eventually crashed into the sea west of Australia.

Source: Cirium Dashboard