KIAS vs. Mach.  Is there a reason to use one over the other?  I've recently started trying to fly "heavies" and I remember reading something earlier.  It seems that you start off using KIAS and then move to mach, but I don't remember why.  Would someone be nice enough to explain it to me?   ???

Actually, KAIS is the same no matter what height youare, the true AIS is what changes.

That said, airliners use mach because it's a standard speed at higher altitudes.

Nexus preaches the truth, as usual

KIAS is altitude, temperature, wind dependent.

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Yeah, that's my bad.  I read in my PPL theory book not two hours after posting that that IAS decreases with altitude and true AIS stays the same.

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The climb speed is rather Cost Index dependent 

If you put down a CI of zero in the 737, you will get a modest climb speed of around 240kts/.750 (depending on model)
But if you boost the CI to 500 you will climb at 330kts/.798...alot faster, but will obviously burn more fuel.

Same logic works in every commercial airliner.

That answer is rather odd to me, Saratoga, seeing you are a captain of several commercial airliners.

The Cost Index -simply put - regulates how much fuel you want to spend during the flight. This will also have impact on the different profiles of each phase of the flight

Example: CI of 0 will give you a very steep climb (near max. rate), and shallow descent profile, at slow speeds

CI of 500 will have a shallow climb and a very steep descent, speeds are maximal in all flight phases.

So are you saying you never pay attention to cost index when programming the FMC? What kind of airline uses that philosophy?  ???

Edit: These speeds are the speed which will be flown in VNAV mode 

I stick to KIAS with the heavies, but i use mach for all my fighter jets.

Nexus really hit the nail on this one in terms of the importance of the cost index. The CI determines climb and descent based on the respective airline's budget limitations. I know British Airways uses a CI of 27 in their A319's, 20's, and 21's to keep from "wasting" fuel. Also I believe Air Canada uses a CI of around 30 but I'm not 100% positive on that...

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Saratoga, I mean once you are above 10,000 you can pretty much fly at any speed you want below 1 mach unless ATC is having seperation issues...but maybe I misunderstood your statement.

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the denser the material the quicker the sound travels. However the density of the air doesn't affect the speed of sound as much as you describe.

Airspeed (and altitude) is determined by the pitot static system, it measures the air passing by the aircraft (I think, someone else will be able to describe it bettter)

Because the air is less dense, then as your ground speed stays the same (the speed you are relative to the ground), however your Indicated airspeed(IAS) changes - goes down as you go higher (this is what happens when you select mach speed as your autopilot mode and you change altitude.) (mach speed is about the same as ground speed)

conversely when you select IAS mode, and your IAS stays the same (say 270kts) then when you change altitude your ground speed changes, it gets slower as you go down.

Someone else will probably be able to give a much better explanation than I can.

IAS is what manufacturers use as the basis for determining aircraft performance. IAS does not normally vary with alt or temp as they affect the aerodynamics of the airframe and ASI equally.

You need to know the types of airspeeds to get the idea of Mach. Most pilots use an acronym of ICETG. Indicated, Calibrated, Equivalent, True, Ground. Indicated is what you read on the ASI. Calibrated is Indicated corrected for installation errors...the pitot tube not pointing into the wind at high AOA. Equivalent is Calibrated corrected for adiabetic flow...compression...at airspeeds above 200 KIAS. True is the actual airspeed you fly through undisturbed air. Ground is ground speed...duh.

Mach is the ratio of the A/C TRUE airspeed to the speed of sound. Ex: .85 is 85% of the speed of sound at that temperature. If you want to compute the True airspeed from a conventional ASI, you have to think about air density which is affected by temp and altitude. With a mach indicator, the temperature determines the speed of sound, and provides a more useful index for aircraft.

Don't think about the speed of sound as being the speed that sound travels through a material of a certain density...it's more about what the temp is and what the speed of the A/C is. As you go higher, the air is cooler, therefore, the IAS is slower compared to GS.

Hope that makes it clear as mud.