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foggygyro
11th Jul 2023, 18:52
`I asked this question some years ago, and thought I understood the answers. But in retrospect I remain unsure of the answer, or indeed there is a definfive answer.
To be clear, my interest is based on my membership in the IG (Independent Group) a group of persons with various expertise who have been still seeking the remains of MH370. As you may know, several extensive searches have failed to locate the wreckage, except for debris that was carried by the winds and currents to the west coast of Africa. For a number of years, the analytic efforts have been focused on a extensive analysis of the possible drift paths so as to provide another estimate of the point of impact. Recent results have clearly confirmed that the debris did originate in the area of the Southern Indian Ocean (SIO) which was identified in the original flight path analyses. But the area to be searched is still large and there is still some uncertainty in the exact area to be searched.

In a detailed flight paths simulations, most of us agree on the events up till the flight disappeared from radar near Indonesia, but is not sufficient data to clearly define the path thereafter, which has to be inferred from some Imarsat technical data.

The technical question:
For many of us, the navigation question to be resolved is. "Assuming that the path is navigated by waypoints and the final leg is from IGOGU to ISBIX and there are no further waypoints entered, now does the plane navigate at the ISBIX discontinuity?"

The heading of the plane at ISBIX is 186T. The apparent choices (made without pilot input) would be TT, TH, MH, (others?). Most pilot responses seem to be that it defaults to "heading". But this begs the question of how are winds treated? Is there any compensation for cross-winds?
Some people answer that they use True Track, over oceanic regions either to check their path or to navigate. Since most We are taling about a 777 in 2014 and one imagines that all navigation is done through the FMC. So, does the pilot switch from MH to TT once clear of local ATC.

I have found airline ops manuals that called for TT over oceanic areas, and others that do not or are silent, so there may be a number of "correct" answers.
More particularly what is done over the SIO and by Malaysia Airlines in particular?

For those interested in more details, just ask.

Thanks,
Sid

FullWings
11th Jul 2023, 21:13
The heading of the plane at ISBIX is 186T. The apparent choices (made without pilot input) would be TT, TH, MH, (others?). Most pilot responses seem to be that it defaults to "heading". But this begs the question of how are winds treated? Is there any compensation for cross-winds?
When you pass the last waypoint on the plan, or a route discontinuity is next, LNAV maintains the current heading (M or T). Itís a heading so the ground track will vary according to the sum of the aeroplane TAS+heading and the wind vector being experienced, so unlikely to be a straight line over more than a short distance. If youíre flying a MH, then the TT will change as the magnetic variation changes, driven by the FMC database.

Some people answer that they use True Track, over oceanic regions either to check their path or to navigate. Since most We are taling about a 777 in 2014 and one imagines that all navigation is done through the FMC. So, does the pilot switch from MH to TT once clear of local ATC.
Depends on airline SOP. We stay in MH (although the FMC will automatically switch to TH in polar regions); others may go to true in all oceanic airspace, YMMV.