KIAS vs. Mach

[Hawkeye313]

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?  

[Mobius]

I would assume it because as you increase altitude, Mach 1 changes and there are FAA rules about exceding Mach 1, not to mention the fact that your engines would stop working at Mach 1.  Your KIAS would decrease becuase there are fewer particles of air for the pitot tube to measure so ram airpressure decreases and it would be difficult to convert from KIAS to Mach while flying, so it is just read as Mach.

I don't know though, this is just speculation on my part, so anyone who actually does know, please correct me or tell me I'm stupid and wrong.  Thanks

[Eskimo]

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]

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

I'd say that is a faulty argument.

[beefhole]

Nexus preaches the truth, as usual

KIAS is altitude, temperature, wind dependent.

[Saratoga]

Pilots climb using KIAS (250 to 10,000, 280 after) then transition to Mach around 30,000 or so. Usually it's just the fact that around 30,000, 280 knots is easing up into the high seventies of the Mach, so to avoid going past your crusing, speed, you flip over to Mach hold.

[Eskimo]

[quote]

I'd say that is a faulty argument.

[beefhole]

I thought climb speed was aircraft dependent (280 for 737, something like 300 for 777, etc.)

note: directed at saratogas comment.

[Nexus]

I thought climb speed was aircraft dependent (280 for 737, something like 300 for 777, etc.)

note: directed at saratogas comment.

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.

[Saratoga]

Nexus, dunno your reference, our climb speeds don't change unless of course ATC requests they change.

Sorry beef, I stand corrected. Everything I have flown (except the turbos obviously) has had a standard suggested climb speed of 280 knots or as required due to the height and Mach number. I can't speak of the 777 because I ain't a triple 7 pilot. I can imagine them doing it until about 25,000, but for any flight where a triple-7 is required, a few minutes quicker in climb isn't a big difference. Personally, I would enjoy the longer climb, but that's just me. Time is money.

[Nexus]

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?

[beefhole]

767 is close enough, far as I'm concerned

I'm pretty sure the more (or the more powerful) engines it has the faster the climb speed.  eg. the MD-11 and 747 will most likely climb faster than 280 knots.

[Issflareman]

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

[SilverFox441]

Drag increases exponentially as you near limiting Mach numbers...very good reason for commercial planes to stay away from that limit.

Of course..if we built an airliner that was a wave-rider like the B-70 we would have airlines complaining about being forced below critical Mach.

[t_alexander21]

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...

our climb speeds don't change unless of course ATC requests they change.

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.