Can anyone explain why my TAS changes i thought this never changed?

 

Did you make changes in OAT, IAS/Mach number or cruising FL and or tropopause altitude? The TAS is still a variable speed it changes while you fly through a dynamic air mass (air pressure and temperatur ). Furthermore it is still a corrected air speed. On the other hand you are using an app that is used for sim world only. It is still a very complex app but may contain errors. In your example you can see the GS is incorrect TAS 454 - HWC 27 GS should be 427. In real world you fly a Mach number as this speed will only change with temperature. 

Vernon, 

 

As Kevin explains you are now seeing why, when flightplanning, you tend to assume a fixed TAS for calculation purposes. It's precisely because the TAS variation it is defined as the `speed used for flightplanning, before considering the effects of wind.`

 

Your flight will carry you through multiple changes in wind strength and direction, so are you going to calculate and recalculate for every change? Of course not. You're there to fly the plane.

So you have calculated just two based on a single change in parameters to wind strength. But what if there were fifty two on that flight? And they aren't just 180 degrees apart with a slight variation, but also include crosswind components? Not feasible.

 

PFPX is recalculating a TAS, based on wind variation. Change the wind, and the calculation changes. But you wouldn't flight plan like that, would you? You flight plan will based on the best estimates for the actual wind en route. You stick with one number, and keep that for the duration of that flightplan. So your flightplan would begin by looking like one or other of the options above, not both - unless you wanted comparisons for comparisons sake. I'm going to stop short of stating the PFPX is wrong, because it isn't. But that isn't how TAS will normally be used for FP purposes. 

TAS is either a consequence of calculation or an assumed constant for the purposes of calculation. Never both.

 

Let's put this into the real world: I habitually do a best case/worst case scenario for my FP's based on HW and TW component - because I do my preflight planning on a PC before I even leave the house this allows me to `contain` a VRP within a time window set around the average (e.g +5mins/ -4mins), which I cross-refer to both GPS, weather reports and map, against what I am actually seeing out the window. So I might start looking for that VRP or ground feature 5 mins before I expect to see it, and stop looking 4 mins after.

TIME is my only constant.

If my FP matches my estimates (the weather is doing exactly as forecast, the aircraft is performing precisely according to specs) then I actually see the point at +/- 0 mins. Almost never happens!

 

Therefore I can make an assessment based on the true time of that ground feature whether I am ahead or behind on my flightplan, and potentially adjust accordingly - On my kneeboard I will normally write time difference against that feature as a carryover to the next. If that deviation is constantly growing or decreasing then I know my flightplan weather assumption is wrong - it might be that the wind at my flying altitude is different, it might indicate a change to prevailing conditions I need to be aware of. Tailwinds are not always a good thing.

 

The GPS tells me the TAS averaged for the journey so far, and extrapolates against the journey still to come. It is that one compares against the nominal TAS of the aircraft to cross check against the flightplan. Massive changes mean something is going on that wasn't planned for. And thats where pilotage comes in: It's never what the numbers ARE, it's always what the numbers MEAN.

 

In conclusion, don't get hung up on TAS or air distance. Time is the most important constant for aviators.

TTG is far more important to you than DTG

IAS more important than TAS for all phases of maneuvering flight 

And in cruise, Ground Speed is  more important than TAS

Only thing i done was swap wind direction. Thanks for that explanation of effects to TAS

Right, and all you are seeing is the effect from spending a longer time at cruise speed on your average tas for the version with the headwind.

If I may say, your fixation on TAS and Air Distance v. Ground Distance is commendable, but misplaced.

 

Focus on `Ground Speed` and `TTG` as these are readily available in the GPS and take TAS and `distance` as a given. Your GPS flightplan in FS does adapt to changing conditions and will change its enroute timings correctly.An airline pilot has a slot time for arrival, same as departure, so you might slow down a bit with an unplanned tailwind, to get your arrival time back into the ballpark. But its IAS/MACH speed you will monitor to achieve it in the Flight Levels.

 

The fun part is that you are `Weather God` and can set whatever and change whatever weather you like with a press on the keyboard while in the sim, so play with that to see what actual effects are important...

One of the best threads I have read here.

Vernon, 

 

As Kevin explains you are now seeing why, when flightplanning, you tend to assume a fixed TAS for calculation purposes. It's precisely because the TAS variation it is defined as the `speed used for flightplanning, before considering the effects of wind.`

 

Your flight will carry you through multiple changes in wind strength and direction, so are you going to calculate and recalculate for every change? Of course not. You're there to fly the plane.

So you have calculated just two based on a single change in parameters to wind strength. But what if there were fifty two on that flight? And they aren't just 180 degrees apart with a slight variation, but also include crosswind components? Not feasible.

 

PFPX is recalculating a TAS, based on wind variation. Change the wind, and the calculation changes. But you wouldn't flight plan like that, would you? You flight plan will based on the best estimates for the actual wind en route. You stick with one number, and keep that for the duration of that flightplan. So your flightplan would begin by looking like one or other of the options above, not both - unless you wanted comparisons for comparisons sake. I'm going to stop short of stating the PFPX is wrong, because it isn't. But that isn't how TAS will normally be used for FP purposes. 

TAS is either a consequence of calculation or an assumed constant for the purposes of calculation. Never both.

 

Let's put this into the real world: I habitually do a best case/worst case scenario for my FP's based on HW and TW component - because I do my preflight planning on a PC before I even leave the house this allows me to `contain` a VRP within a time window set around the average (e.g +5mins/ -4mins), which I cross-refer to both GPS, weather reports and map, against what I am actually seeing out the window. So I might start looking for that VRP or ground feature 5 mins before I expect to see it, and stop looking 4 mins after.

TIME is my only constant.

If my FP matches my estimates (the weather is doing exactly as forecast, the aircraft is performing precisely according to specs) then I actually see the point at +/- 0 mins. Almost never happens!

 

Therefore I can make an assessment based on the true time of that ground feature whether I am ahead or behind on my flightplan, and potentially adjust accordingly - On my kneeboard I will normally write time difference against that feature as a carryover to the next. If that deviation is constantly growing or decreasing then I know my flightplan weather assumption is wrong - it might be that the wind at my flying altitude is different, it might indicate a change to prevailing conditions I need to be aware of. Tailwinds are not always a good thing.

 

The GPS tells me the TAS averaged for the journey so far, and extrapolates against the journey still to come. It is that one compares against the nominal TAS of the aircraft to cross check against the flightplan. Massive changes mean something is going on that wasn't planned for. And thats where pilotage comes in: It's never what the numbers ARE, it's always what the numbers MEAN.

 

In conclusion, don't get hung up on TAS or air distance. Time is the most important constant for aviators.

TTG is far more important to you than DTG

IAS more important than TAS for all phases of maneuvering flight 

And in cruise, Ground Speed is  more important than TAS

 

Very nicely done, seems like you fly in the RW? You mention time is the most important so on my OFP after starting the timer on takeoff roll, i should check my elapsed time against my ETA at each waypoint...

Those airspeed and groundspeed values are averages for the route that were backed into after establishing the time enroute. When fighting a headwind, the longer time enroute means you will have spent more time at cruise altitude at cruise speed than if being helped by a tailwind. And since the time spent in climb and descent, which is done at slower speeds, is the same for both hw or tw, the average tas will work out higher if fighting a headwind and lower if being helped by a tailwind.

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Cheers kevin! So the obvious is having a TW will reduce the TAS as the TW is pushing along with the acft...

Check this website out guys

 

https://www.grc.nasa.gov/www/k-12/airplane/Animation/airrel/anrel.html

 

Great animations

 

Also this

 

https://www.grc.nasa.gov/www/k-12/airplane/move.html

Cheers kevin! So the obvious is having a TW will reduce the TAS as the TW is pushing along with the acft...

A tailwind reduces your -average- tas due to the lesser portion of time spent at cruise speed.

Hi Vernon, Remember actual distances to fly are increased over ground distances with altitude, since the radius of the Earth plus altitude means larger circumference and further to fly. 

Good  one ,  now  more  confused  :smile:, so going by your  theory the closer  to the ground  we fly  the quicker we get here

..the closer  to the ground  we fly  the quicker we get here..

quickens the pulse too...  

Well the extrapolation is ultimately quite true: You will certainly get there quicker when you fly into the ground. And for any pilot that is likely to be too soon!  :excl:  

 

See, it's about time, not distance... 

Well the extrapolation is ultimately quite true: You will certainly get there quicker when you fly into the ground

 

Not if  you got  crash  detection off