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Guest katonad

Ground Speed

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Hi!I have got a problem now, what never before now. I reinstalled my flight simulator and the PSS airbus. I think i did something wrong. Now on the ground the ground speed and the tas is the same. So i dont know the GS now. It isnt a big problem, but i like to know when i taxiing.What i did wrong?Thank youRegardsDavid Katona

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I'm not quite sure what you mean when you say you don't know what the GS is when you are taxiing: it gives the figure at the top of the ND next to where it says GS. I have always thought that TAS and groundspeed are the same while on the ground. It is only when you are in the air that the changing air pressure will result in a different TAS. So in my view you should get the same figure while taxiing and during the takeoff roll and they only start to differ after you rotate. That's what happens with me on the simulator anyway. Rob Elliott, EGPE InvernessPSS Airbus Support andAirbus Fleet Training Captain, British Airways Virtual airbus@speedbirdonline.co.ukhttp://www.speedbirdonline.co.uk/airbus.htmlhttp://www.bavirtual.co.uk

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Rob,With no wind (and in level flight!), TAS will equal ground speed at any altitude. However, ground speed and TAS are not always the same. For example, when flying at 300 knots TAS into a 20 knot headwind, ground speed will only be 280 knots. This also applies to taxiing on the ground with a wind.What changes with altitude (actually air density) is the relationship between TAS and IAS (indicated airspeed). For a constant TAS, IAS decreases with air density, i.e increasing altitude. As you always use IAS as your speed reference when flying it appears that TAS is increasing as you climb.TAS isn't a lot of use for anything except navigation.Hope this helpsKevin

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I am well aware of the principles when airborne, but he was only talking about TAS and groundspeed while taxiing, and I have always found on the PSS Airbus that they are the same and only differ after rotation. This is probably something that needs to be re-programmed should there be a future Airbus release to apply the principles properly.Rob Elliott, EGPE InvernessPSS Airbus Support andAirbus Fleet Training Captain, British Airways Virtual airbus@speedbirdonline.co.ukhttp://www.speedbirdonline.co.uk/airbus.htmlhttp://www.bavirtual.co.uk

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My apologies, I took your email to be a general comment on the relationship between GS and TAS. In which case it could be very misleading. However you did talk about air pressure affecting TAS, which lead me to think you might be confusing it with IAS. I'm sure that was not what you meant.I forgot to allow for the way TAS is actually computed for display in the aircraft, thinking in terms of real parameters, not indications.Regarding the PSS Airbus, I think PSS have it correct as it is now. In the real Airbus, TAS and GS both come from the ADIRS and wind data comes from the FMGS. On the ground, the FMGS cannot derive the windspeed (track and heading will be the same), so TAS will always equal GS. Once in flight, the wind speed can be computed, so TAS and GS will be different.Best regardsKevin

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