Where Could MH370 Be?

The satellite terminal aboard MH370 exchanged automated handshakes with an Inmarsat satellite for nearly six hours after contact was lost. Each handshake carries two usable measurements: the time the signal took to travel, which gives the aircraft's distance from the satellite, and the Doppler shift in its frequency, which constrains its speed and direction relative to the satellite.

Distance from a satellite defines a circle on the Earth's surface, not a point. The final complete handshake at 08:11 defines the seventh of these — the seventh arc. The aircraft was somewhere on it.

The Doppler analysis, combined with fuel range, ruled out the northern portion of the arc, which crosses Asia. That leaves a southern segment running roughly southwest of Australia, thousands of kilometres long.

The flaperon on Réunion and the pieces found in Mozambique, Tanzania, Madagascar and South Africa are the second line of evidence. Oceanographers ran drift models backwards from where and when each piece was found, using known current and wind data.

Those models point to an origin in the southern Indian Ocean broadly consistent with the arc — not to a precise point, but they exclude large alternatives. Nothing has ever washed up on Australian shores, which is itself informative about where the aircraft is not.

The third line concerns the last minutes. The 08:19 partial transmission is consistent with the satellite terminal rebooting after a power interruption — what happens when engines flame out and the auxiliary power unit starts. That places fuel exhaustion very close to the seventh arc, and an uncontrolled descent shortly after it, which narrows the search corridor either side of the arc rather than along it.

The searched area is enormous, extremely deep — commonly four to six kilometres — and includes underwater mountains, ridges and trenches. Much of it was mapped in useful detail for the first time by the search itself. A debris field from a high-energy impact can be small and can lie in terrain that hides it from sonar.

The Australian-led search covered around 120,000 square kilometres and Ocean Infinity added more in 2018. Both were looking in defensible places. Absence of a find over a defined area narrows where it can be, which is why later searches have shifted along and across the arc rather than starting over.

On the balance of evidence: on the seabed in the southern Indian Ocean, within a corridor of the seventh arc, most probably in an area either not yet searched or searched with equipment that missed it in difficult terrain. That is a large volume of ocean. It is not an unknown one.