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160knots
15th Dec 2009, 07:40
For North Atlantic Ops. Item 15 of the Flight Plan require:

1. Cruising TAS.
2. Oceanic Entry Point and Mach No.
3. Oceanic Landfall and cruising TAS

What is the reason for wanting to know 3 lots of speeds. Why can't you insert just the Mach No. for the crossing? Why a TAS and a Mach No?

Jumbo Driver
15th Dec 2009, 08:52
My instinctive reaction is to say that the two speed measurements are used for slightly different purposes - TAS for airways flying and Mach No for the procedural Oceanic portion of the flight.

TAS would be used in conjunction with the known or forecast wind to provide a Ground Speed for the calculation of ETAs, elapsed time between waypoints and so forth, whereas Mach No is more useful tactically for procedural purposes, for example with successive aircraft on the same track to preserve longitudinal separations. Ensuring that Mach Nos do not differ significantly where aircraft are longitudinally close on the same track will maintain separation quite independently of the actual wind experienced, because all aircraft will be subject to the same headwind or tailwind effect. Also, at the planning stage when entry clearance is initially requested, it would be good practice to clear aircraft with similar planned Mach Nos on the same track in order to maximise overall traffic density.


JD
:)

DCDriver
15th Dec 2009, 22:59
Jumbo Driver is correct. In a nutshell, speeds within MNPS are referenced to Mach because "Mach No Technique" is employed for longitudinal separation on both Random Routes and the OTS.
So for ATC flight plan purposes, before entering / after leaving MNPS (the so-called "domestic" routings) speed is filed in knots TAS, even though one is in all probability still going to fly the same cruise Mach No as one would within Oceanic airspace.

Hope this helps