Originally Posted by
silvertate
Not so. The final ping-ring gives a multitude of fuel-burn rates and speeds, all of which can reach somewhere along that long arc. Remember that the track-length between ping-rings varies, depending on whether you fly south or fly east - and that will greatly effect the fuel-burn/speed profile of the flight.
So you can have a low-burn/high-speed flight - and end up south and west of Australia. (Yellow track.)
Or you can have a high-burn/low-speed flight - and end up north and west of Australia. (Purple track.)
The first of these options must be a high level flight. (Yellow track.)
The second of these options must be a low level flight. (Purple track.)
But how low?
You misunderstand.
We know (or at least assume we know) the initial track of the aircraft to loss of radar contact
We know the fuel load the aircraft carried and the last miss-ping gives us the out-of-fuel time therefore we know the burn rate.
That burn rate can be achieved in a limited number of ways - assume it is relatively constant from loss of radar contact.
Then use the different levels and speeds that give that burn rate and see which fit the ping rings.
We may of course be having a noisy agreement - but the burn rate is simple - time of flight and known fuel upload adjusted for initial tracked flight. Assumption necessary is that the aircraft maintained a steady cruise.