Question about flight plan routing details

Gaoth Laidir

I'm following flight ETD450 from Abu Dhabi to Sydney and have checked its filed route on Flightaware. I note that at certain waypoints it shows a step-climb and an airspeed/Mach number. My question is: why are some speeds shown in Knots while others are Mach? This occurs well above the transition from IAS to Mach so it can't be due to that.

Here's the filed route, with examples showin in bold.

KANIP N318 TOLDA P570 METIP/N0507F310 P570 POMAN T6 CIA/M083F310 P895 BIKOK/N0503F310 M641 BAXAM/N0498F330 M641 PIBED/M083F330 M641 CC T41 POROR/N0490F350 T41 LM N640 NOPED/N0480F370 N640 BOBET DCT JACKO DCT CDU J149 WHA J21 BORLI UH205 CULIN Y59 TESAT

Foggy43

I maybe completely off track but from memory when aircraft are out of radar control they use Mach air speed and knots when within? It has been a few years since I listened in on Shanwick HF.

martinsvi

some airspaces, such as NATS will require you to fly a constant Mach for separation purposes. Indeed this happens because there's no radar control but also when the track is long enough for separation to become an issue over time

smurfjed

The majority of the route is over ocean, but i would have thought that they would use constant true mach rather than TAS for separation purposes.

Gaoth Laidir

Plus how do the knots and Mach speed compare to eachother? Are they all the same when the different altitudes are taken into account or are there actual changes in the aircraft's speed? It seems to vary from Mach 0.88 (507 kt @ FL310) to 0.83

martinsvi

Mach is TAS divided by the local speed of sound. Local speed of sound depends on the temperature. Therefore for the same TAS you could get different Mach depending on temperature

the effect on altitude is only apparent - as you climb, the temperature drops. The changes in mach/tas relation are to do with the temperature changes rather than altitude.

smurfjed

Thanks to a certain Dennis Slattery over 20 years ago, i remember the formula as M = 38.968 * SQRT Temperature in Kelvin.

As temperature is the prime variable in this, it seems strange that they use TAS for procedural separation.

deaddonkey15

smurfjed wrote: »

Thanks to a certain Dennis Slattery over 20 years ago, i remember the formula as M = 38.968 * SQRT Temperature in Kelvin.

As temperature is the prime variable in this, it seems strange that they use TAS for procedural separation.

How does this account for TAS though?

smurfjed

The answer is the Local Speed of Sound in Knots, which is TAS. As the altitude increases and temperature decreases the LSS changes.

deaddonkey15

smurfjed wrote: »

The answer is the Local Speed of Sound in Knots, which is TAS. As the altitude increases and temperature decreases the LSS changes.

Your notation was misleading. Speed of sound is usually given as 'a' or 'c'.

smurfjed

OK, what does "a" or "c" mean and where are they usually given?

deaddonkey15

smurfjed wrote: »

OK, what does "a" or "c" mean and where are they usually given?

Aerodynamics / fluid mechanics text books.

Gaoth Laidir

M is the Mach number, the ratio of TAS to a or c

deaddonkey15

Gaoth Laidir wrote: »

M is the Mach number, the ratio of TAS to a or c

Correct, which is why I was confused by the above equation.

smurfjed

Thanks, so glad that i only have to fly them and not build them.