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Kevin666

True airspeed

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Hi guys, I'm just looking for answers for these rookie questions...

 

Do modern aircraft (like the 737NG) have an instrument for true airspeed calculation?

 

Is this information really needed for current flights? if so, in which way?

 

Thanks in advance.


John Connor

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Are you really John Connor or a T1000? I need to know before I can answer.


Rob Prest

 

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Hi, since I'm at work and on my iPhone, I'll try to explain in brief as I can't recall the equations to calculate true airspeed, calibrated airspeed etc.

 

The dynamic air pressure (ram air) into the pitot tube minus the static pressure from the static port gives the indicated airspeed. As you climb higher, the air pressure decreases therefore indicating lower readings on the instruments than the aircraft is actually travelling at. For GA pilots ou can dig out the old wizz-wheel or electronic calculator to work out true airspeed, but modern day computer systems in commercial aircraft, and I guess GA aircraft too, now calculate true airspeed automatically. On the airspeed tape will be the IAS and on the navigation display will be the TAS. Another thing to keep in mind is, as an airliner climbs higher and faster, the speed regime will change from IAS to Mach number.

 

I hope this helps a bit, I'll try to remember to return here later with some calculations.


Alaister Kay

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Hi, since I'm at work and on my iPhone, I'll try to explain in brief as I can't recall the equations to calculate true airspeed, calibrated airspeed etc.

 

The dynamic air pressure (ram air) into the pitot tube minus the static pressure from the static port gives the indicated airspeed. As you climb higher, the air pressure decreases therefore indicating lower readings on the instruments than the aircraft is actually travelling at. For GA pilots ou can dig out the old wizz-wheel or electronic calculator to work out true airspeed, but modern day computer systems in commercial aircraft, and I guess GA aircraft too, now calculate true airspeed automatically. On the airspeed tape will be the IAS and on the navigation display will be the TAS. Another thing to keep in mind is, as an airliner climbs higher and faster, the speed regime will change from IAS to Mach number.

 

I hope this helps a bit, I'll try to remember to return here later with some calculations.

 

Thanks a lot now you mention it I remembered it's showed in the ND

 

 

Are you really John Connor or a T1000? I need to know before I can answer.

 

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John Connor

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Dude you made my day :LMAO:


Rob Prest

 

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This is really not pertinent to your question as worded, but there is an old GA "Rule of Thumb" that was used before transistors and printed circuits showed up:

 

Just add 2% to your IAS for each 1000 feet of Altitude above sea level to get TAS. The result will be close enough for most purposes.

 

Otherwise calculating it (forFlight Sim anyway) is somewhat of a pain, since it requires outside temperature, Calibrated Air Speed, and having an ISA Standard Day chart available plus a hand calculator unless you remember how to take square roots.

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1. Do modern aircraft (like the 737NG) have an instrument for true airspeed calculation?

 

2. Is this information really needed for current flights? if so, in which way?

 

 

.

 

1. Yes. It's on the ND (Navigational Display), top left corner. Here it displays ground speed and True airspeed.

 

True airspeed is calibrated airspeed corrected for altitude and nonstandard temperature. Because air density decreases with an increase in altitude, an aircraft has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure

 

2. Yes, it's the actual speed the airplane is traveling through the air and the speed filed on an ATC IFR flight plan.

.


John Floyd

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On most airraft, you can dig into the FMS and find the TAS (usually on the PROG page). TAS is of course important, but not someting you'll use every day in a modern airline cockpit. ATC instuctions are given in IAS or Mach, and grondspeed is calculated continiously, so under NORMAL conditions, you wouldn't worry too much about it- unless you have reason to doubt the system.

 

Paul

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On most airraft, you can dig into the FMS and find the TAS (usually on the PROG page). TAS is of course important, but not someting you'll use every day in a modern airline cockpit. ATC instuctions are given in IAS or Mach, and grondspeed is calculated continiously, so under NORMAL conditions, you wouldn't worry too much about it- unless you have reason to doubt the system.

 

That's what I thought, thanks!


John Connor

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TAS is of course important, but not someting you'll use every day in a modern airline cockpit. ATC instuctions are given in IAS or Mach, and grondspeed is calculated continiously, so under NORMAL conditions, you wouldn't worry too much about it- unless you have reason to doubt the system.

 

Don't really agree with you here.

 

Knowing your TAS is important!


John Floyd

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I figured I'd catch some flak for that. I never said it wasn't important. There are of course times when you absolutely must know your TAS. However in a modern cockpit in normal ops, the technology available to pilots is such that we needn't check TAS very frequently. Instead, you can easily find a direct measure of things like groundspeed and track.

 

I only really use TAS as a sort of gross error check on a daily basis. I know that at cruise, I should have a reasonable combiation of power, altitude, fuel flow and TAS.

 

Paul

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TAS figures in some calculations like for dynamic pressure or lift force... actually in almost all aerodynamic calculation where factor is speed, you use TAS

 

For example db7735d03f8de6082982164856a0d8ba.png

 

In flight, u usually need GS for navigation and IAS/mach for flight


[color=#a9a9a9][size=1][size=4][img]http://forum.avsim.net/public/style_images/flags/rs.png[/img][/size] Lj. Prodanovic[/size][/color]

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