Simmers,A general question. Maybe a real life pilot can give an answer. 1. What's the difference between GS (Ground Speed), TAS (True Air Speed) and IAS (Indicated Air Speed)?2. How do they relate to each other (how may IAS = TAS and reverse)?3. What do you use in FS, TAS or IAS?Regards,Jeroen van SchoneveldVelserbroekThe Netherlandshttp://www.precisionmanuals.com/images/forum/800driver.jpgASUS A8N-SLI, AMD Athlon 64 4000+ (2,4 Ghz) San Diego processor, 2Gb DDR 400 Mhz Corsair Mem, ASUS V6800 PCIE Ultra 256 MB video

Donny AKA ShalomarFly 2 ROCKS!!!I am not a real world pilot, but I play one one the internet.Indicated airspeed is what you usually see on the airspeed indicator (unless you select "true air speed" in your FS9 settings). The plane usually stalls at the same indicated airspeed, and it's also helpfull when attempting to mantain a certain speed for performance in some cases. (best angle/rate of climb, glide etc...)True airspeed is indicated corrected for atmospheric variations, barometric pressure drops 2 percent for every 1,000 feet above sea level and you usually lose 2 degrees of temperature and 2 knots of indicated air speed. If there is no wind, True air speed will always be equal to ground speed, your actual speed from point A to point B over the earth.Ground speed is the one pax want to know about usually, it is also important for fuel planning- If you have an unexpected headwind, you should doublecheck expected time enroute/distance to alternate against fuel on board and all that good stuff. Always a good thing to do anyway.There is also a fourth speed, Mach. Percentage of the speed of sound.It becomes important in higher performance aircraft, even some turboprops. In many aircraft you climb at an certain airspeed till a certain Mach target, from then on you use the Mach guage to set speed till you slow below your target Mach during descent.Best Regards, Donny:-wave

Ground Speed is exactly that, speed over the ground. Let's say you are heading directly North at 200 Knots and you are in a wind blowing directly south at 15 knots. Your ground speed would be 185 knots. True Airspeed is the actual speed that you are going in the air you are flying in. Indicated airpseed, is the speed that your airspeed indicator is reading, and this is derived from the air pressure on the pitot tube. As you go higher in altitude, the indicated airspeed will drop since the air is less dense at higher altitudes. Your true airspeed, or actual speed though the air will not change. We usually set the airspeed indicator in FS9 to read Indicated airspeed.

Hmmm, deep breathe :-)IAS - If you were to travel 300 KIAS at sea level with no wind your true speed would be 300 knots as would your ground speed.TAS - At higher altitudes the atmosphere is less dense therefore at 300 KIAS your TAS is going to be higher (less resistance) the higher you go. The simplest formula for calculating the increase in your true speed above 240 knots is IAS + FL divided by 2. So at FL100 and IAS of 300 TAS = 300 + 100 / 2 = 300 + 50 = 350.In a nutshell less resistance means a higher True Airspeed. If you know what your True Airspeed is it gives a better estimation of ETA at your destination, estimate time between two navpoints etc. Of course next comes the effect of the wind!Groundspeed - "Simply" take TAS and allow for the wind. A 10 knot headwind would mean 340 instead of 350 a tailwind would mean 360 etc. Obviously more needed than that due to crosswind calculations etc, there is probably a long complicated formula but I don't know it so stick with either GPS or FMC if you have either.In the sim - Use IAS for checking V1, VR, V2 etc. Also your speed below 10,000ft or 18,000ft as well of course must be no higher than 250 KIAS.Use TAS to calculate ETA somewhere unless you know what the approx head/tailwind effect will be in which case GS would be the one to use for the ETA at destination.Sorry best I can do. Just dragged that out from reading a bit on the web and I am no expert so ....Andrew BrownRoaring Thirtieshttp://www.gordiusfs.pwp.blueyonder.co.uk/

Donny AKA ShalomarFly 2 ROCKS!!!We must all have been typing at the same time!!!:-beerchug

>>Donny AKA Shalomar>Fly 2 ROCKS!!!>>We must all have been typing at the same time!!!:-beerchug Not hard. Kind of like writing War and Peace in Latin :-)Andrew BrownRoaring Thirtieshttp://www.gordiusfs.pwp.blueyonder.co.uk/

If you are still confused about TAS and IAS. Think IAS as an air molecule counter. The ASI (Airspeed Indicator) is calibrated at the factory for a "standard ISA" atmosphere. The air is thinner up high (lower density = less molecules per metre^2). Imagine a still wind (as Donney says) at 15,000', you are doing 300kts GS and 300kts TAS, in a normal (or standard) atmosphere you would be seeing an indicated 240kts. This is because only 240kts worth of airmolecules were 'collected' by the pitot in a set amount of time, whereas at seal level you would collect 300kts worth.A pilot's mental arithmetic rule of thumb to work out TAS for a shown IAS is the rule of the squares. You basically calculate the square and that is the % increase of TAS over IAS:5000' = 3^210000' = 4^215000 = 5^220000 = 6^2etcetc40000 = 10^2So 300kts IAS at 5000' = a TAS of 300*((3^2/100)+1) or 300 + 9%. Say 10% to make it easier...a TAS of 330kts.But at 40,000' your TAS is double your IAS!!This is why aeroplanes like to fly high, you get there faster for the same thrust setting (within limits).==========================================Just to complicate matters (as I do!). Lookup CAS (Calibrated Airspeed) and EAS (Equivelent Airspeed) on the web. CAS is the airspeed displayed after it has been corrected for certain known and quantifiable errors. All EFIS (glass) displays show CAS not IAS. EAS is of relevance to aeroplanes that travel in excess of about 400kts TAS. It is CAS corrected for air compressability and is the speed the aeroplane thinks it is flying at and is the same or less than CAS. The difference between EAS and CAS is usually small, but compressability was discovered when prototype P38 Lightnings started to crash in WWII :(

Good Questions! All the answers,with explanations are in every ,book,manual,tape,that is dedicated to becoming a Pvt. Pilot.Ground school study materials are to be found in many areas.Those "approved" explanations will reflect the test questions you may be asked to respond too.I won't attempt to list any of them for the list would be too long. Good Luck Jeroen! VIN

Hi,Thanx very much. I now know how everything is related in real world and FS. And so many replies!! I'm overwhelmed by it.Regards,Jeroen van SchoneveldVelserbroekThe Netherlandshttp://www.precisionmanuals.com/images/forum/800driver.jpgASUS A8N-SLI, AMD Athlon 64 4000+ (2,4 Ghz) San Diego processor, 2Gb DDR 400 Mhz Corsair Mem, ASUS V6800 PCIE Ultra 256 MB video

>>Good Questions! All the answers,with explanations are in every >>,book,manual,tape,that is dedicated to becoming a Pvt. PilotYou won't find references to CAS, RAS and EAS in many private pilot texts.

Hi, Some texts do,even in an old manuals, The explanation of speeds are explained.Indicated,calibrated,and true air speeds are essential for the pvt. pilots. Of course you can also refer to the commercial,instrument,and ATP ground school manuals. However it starts in the Pvt. Pilot ground school texts.I guess other speeds may be in the operating manuals for different aircraft. But,starting from the the begining, can only help. THANX VIN www.aviationboom.com/terms/vspeeds.shtml good to know also!

Fair doos, I have found a non indexed reference to CAS in my Thom book, but it was literally that, a passing reference to the extent 'don't worry about it'. With the exception of RAS (which, by definition, is manually determined although it is often confused with CAS simply because they generate near enought the same result), the first time I came upon it was in the ATPL ground school. There certainly arn't too many piston a/c with air data computers capable of calculating CAS and certainly nothing where Mach figures without being in the super rich league and without a type rating.