Hi all,Many (many) moons ago when I was learning about FS, I read a 1001 posts about all things related, including the difference between IAS/TAS and GS.Without exception, fellow flyers would say to use IAS as opposed to TAS (an option in FS) as this more accurately reflects "real life".Being a aviation "nut", I also read many articles on this subject, and all roads pointed to IAS being the desired speed indicator to use.And life was good.I'm sure there are differing points of view on this (really??), but when nearly everyone says the same thing, you can assume it's gospel.So......why does my beloved PMDG 737 600/700 show TAS to the right of GS on the PFD?No, I'm not having a go at PMDG - but this appears to contradict many, many views that IAS is the way to go.As always, comments would be appreciated.CheersAllblackBledisloe Cup:AB's by 12

First things first, please read the PMDG Forum's rules and guidelines, you'll see that you are required to sign your post with your real name.Thanks ;)Now, The reason that TAS and GS are displayed where they are in PMDG Sim is that they are displayed there in reality. That's just what PMDG's mission is, to accuratly reflect the cockpit enviroment.Why is TAS and GS displayed there in reality? I'm sure someone else will have a more concrete explanation,but IAS is displayed on the PFD already. You'll also notice that the TAS and GS is displayed on the ND just above the wind readout, I'm assuming have accurate TAS and GS aswell of wind readings allows for easier computations of speed etc. http://www.airliners.net/open.file/597933/L/John http://homepage.eircom.net/~eamonnmca/images/logo_ba.JPGwww.bavirtual.co.uk Senior Captain Simflight.com Staff Reviewer

Greetings :)Heavily based on my training book ('The Complete Student Pilot Manual' by Val Dyson-Holland c 2000):IAS is linked with dynamic pressure, which is linked again to the aerodynamic forces produced on the aeroplane - if a certain amount of lift is produced at 240 kts at sea level, then the same amount of lift will be produced at 240 kts at FL220. IAS is independent of density.TAS is the actual speed of the aeroplane through the air, or the speed of the relative airflow past the aeroplane. At sea level IAS is the same as TAS, but as density decreases the difference between IAS and TAS increases. In zero wind conditions, TAS is equal to GS.For navigational purposes, though the book says that 'I'll be learning about the importance of TAS from a navigational point of view later in my training', you can calculate the ground speed by adding the average estimated wind component to the TAS.I hope this is the information you were after ;)Geoffrey BaleanCanberra, AU (YSCB)http://www.hifisim.com/images/as2004proudsupporter.jpg http://www.vozvirtual.org/images/pmdgforumsignature.png"Today is the tomorrow you worried about yesterday. All is well!"Pentium 4 2.4GHZ, P4G8X Deluxe with Broadcom NetXtreme Gigabit Ethernet, 512MB RAM, LG Flatron L1710S 17" LCD, GeForce MX440 Millenium Silver, Creative Soundblaster Audigy, Logitech Speakers, CH Yoke & Rudder, Quantum Fireballp AS30.0 - Basically nothing crash hot :)MSFS 2004 acof, PMDG 737 6/7/8/900, Activesky 2004, Flight One Cessna 152, SquawkBox 2.3 w/ guage, ServInfo 2.2, FS AutoStart, FS Real Time, FSUIPC 3.212 (unreg.:()

you can calculate the ground speed by adding the average estimated wind component to the TASSo I was close enough ;)John http://homepage.eircom.net/~eamonnmca/images/logo_ba.JPGwww.bavirtual.co.uk Senior Captain Simflight.com Staff Reviewer

When you are flying prop planes down near the bottom of the ocean of air, there usually isn't much more than a few knots difference between true airspeed and indicated airspeed. If you fly Skyhawks and find that you need those few knots to make the leg calculations work, then you better add a fuel stop in there.When you move up to planes that fly higher, the gap between true and indicated widen to where you see that you can be below 200knots indicated but 460 true. Watch as you descend. When you get around 10,000 they start to be almost equal again. You might also notice that the display switches from IAS to a mach percentage too. So why worry about TAS and groundspeed? The planes IAS indicator doesn't care if you are doing 450 or 350 knots across the ground. It will still read the same thing. Knowing you have a 100 knot headwind can be the difference between being on schedule or even running low on fuel and having to divert. Light GA aircraft have a lot more flexability if they are running short of fuel. Commercial airliners can't just drop into the closest strip and top off the tanks, it annoys the businessmen who expect to get to their destination on time. Bob K.

Why not? As stated by one of the explanations above, the IAS is on the PFD.You could say, IAS is used to controll the airplane.The TAS is on the ND for quick information.TAS is good information for flight planning and flight plan following.With a suspected failure of the PFD's IAS , the ND's GS and TAS information are good cross check.Floyd

"With a suspected failure of the PFD's IAS , the ND's GS and TAS information are good cross check."Not necessarily, Floyd. It might be a mistake to do this. The source/origin of TAS, at least on most Boeing's I'm familiar with, is same as that of the speed indicated on the PDF speed tape ("IAS" for the sake of simplification).The same Air Data Computer is used and the same pitot static sources are used. However, the air data computer processes the raw pitot static (and TAT) data in different ways to produce TAS and IAS.Unfortunately, the complex diagrams of the Air Data Computer found in the Boeing Maintenance Manual is copyrighted (so I can't show you how the ADC processes the information).Perhaps a better cross check would be to use the F/O's and Stby Instruments?Cheers.Ian.

G'day Allblack (Go the Wallabies!!)Pretty straightforward actually.The KISS principle is involved here.TAS is used for navigation.IAS is used for flying the aeroplane.Hence.IAS on Air Speed Indicator.TAS on PFD.Far simpler than many expect.CheersDarren

Possibly the Stby instruments would be ok, but the QRH does not mention it as a cross check. I don't think it would be a good idea to use the other PFD's airspeed indicator to compare because it may be the inaccurate one. If the two PFD airspeed indicators differ by 30 knots, which is the inaccurate one?The best check would be GS and Winds provided by the IRS and FMC displayed on the ND's. This information has nothing to do with any pitot/static systems.Floyd

Basically, TAS = IAS / a compressibility factor or density ratio. It is used in fuel burn calculations, and for computing wind correction angle. TAS reflects the ram pressure a pitot tube experiences on the ground, traveling at say 250kt CAS, and a density altitude of FL350. Under these conditions, TAS ~ 450kt. Similarly, TAS = GS + wind component.

"The best check would be GS and Winds provided by the IRS and FMC displayed on the ND's. This information has nothing to do with any pitot/static systems."Where are you getting your wind information from, Floyd? If it's from the ND or FMC, then it is from the pitot-static system (in combination with the IRS). You can't compute winds without ADC information.This is how Boeing derives windspeed/direction...http://members.ozemail.com.au/~b744er/737NG/WindVector2.gifWindspeed is the resultant of two vectors, TAS and Groundspeed. TAS comes from the ADC (AD part of the ADIRU) and Groundspeed comes from the IRS (IRU part of the ADIRU).I'm not going to tell the pilots how to fly their airplanes, but they must be made aware that a lot of information presented to them comes from a common source.I mentioned Standby Instruments AND F/O's Instruments because the best way to determine if something is amiss is to compare it to two other independent sources of information (that's why nav systems like IRS's are triple mixed).Hope this makes sense.Cheers.Ian.

Excellent Darren. I must remember that for when I get that CFI rating !! I know that some student pilots at our flight school are continually confused about TAS/IAS. And TAS and IAS deviate at about 2% / 1,000', as I recall. :)Bruce.

Ian,Not totally convinced. The Boeing 737NG QRH that says:Airspeed Unreliable - Crosscheck ground speed and winds provided by the IRS and FMC to determine airspeed accurarcy if indicated airpseed is questionable.Why would they want you to do this if the information is derived from a unreliable pitot/static system?I'll research it some more.Floyd

That's a good call Darren. Summed up nicely.Almost as succinct as 16-7. : )Cheersallblack

No worries, Floyd. Let me know what you come up with.BTW, here's what the Boeing Maintenance Manual (Ch 34-21-00) says about it..."Inertial Reference....Gyros and accelerometers supply movement data to the IR processor (IR= Inertial Reference section of the ADIRU). The ADR (Air Data section of the ADIRU) data bus gives altitude, altitude rate, and true airspeed to the IR processor. The IR uses this ADR data as part of its inertial altitude, vertical speed and wind calculations"All you seem to be doing by comparing instrument data with the IRS/FMC data is seeing if the data is being processed to produce the same result. If it isn't, you are still none the wiser.It may of course, go deeper than this. If it doesn't, then all the pilots are getting is a placebo/sugar pill.Cheers.Ian.