Search the archive:
 YaBB - Yet another Bulletin Board
 
   
 
  Simviation Forum › Current Flight Simulator Series › Flight School › KIAS vs. Mach
(Moderators: Mitch., Fly2e, ozzy72, beaky, Clipper, JBaymore, Bob70, BigTruck)
‹ Previous Topic | Next Topic ›
Pages: 1 2 
Send Topic Print
KIAS vs. Mach (Read 1873 times)
Mar 30th, 2005 at 10:00pm

Hawkeye313   Offline
Colonel
Georgia

Gender: male
 Posts: 103
 *****
 
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?   ???
 
Antec Dragon, Asus P4C800 Deluxe, P4 3.2E,Swiftech Water Cooling, 1Gig Mushkin Lv 2 V. 2 PC3200 + 512mb Corsair TwinX LL PC3200, 2 WD Raptors on RAID 0, 1 Maxtor 120GB 7200rpm, Audigy 2, eVGA 6800 GT, Logitech Z-680 speakers, Tracker IR3 Pro, 19
Back to top 
IP Logged
 
Reply #1 - Mar 30th, 2005 at 10:46pm

Mobius   Offline
Colonel
Highest Point in the Lightning
Storm
Wisconsin

Posts: 4369
 *****
 
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 Grin Wink
 
...
Back to top 
IP Logged
 
Reply #2 - Apr 1st, 2005 at 4:41am

Eskimo   Offline
Colonel
I'm not speeding officer
-- I'm just flying low.
YBAF - Archerfield, Brisbane

Gender: male
 Posts: 287
 *****
 
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.
 
Back to top 
IP Logged
 
Reply #3 - Apr 1st, 2005 at 6:19am

Nexus   Offline
Colonel
The greater of two evils...

Gender: male
 Posts: 3282
 *****
 
Quote:
Actually, KAIS is the same no matter what height youare, the true AIS is what changes.


I'd say that is a faulty argument.  ???
It's no secret that KIAS descreases with altitude
"Climb speed: IAS 80kt minus 1kt/2000ft" it says on the Vy diagram (Vy = Best rate of climb speed) for my aircraft.

And airliners use mach number at high altitudes simply because it becomes the aircrafts limiting speed
 
Back to top 
IP Logged
 
Reply #4 - Apr 2nd, 2005 at 1:26pm

beefhole   Offline
Colonel
common' yigs!
Philadelphia

Gender: male
 Posts: 4466
 *****
 
Nexus preaches the truth, as usual Smiley

KIAS is altitude, temperature, wind dependent.
 
Back to top 
IP Logged
 
Reply #5 - Apr 3rd, 2005 at 1:51pm

Saratoga   Offline
Colonel
757/767 Captain   Major,
USAF
Dallas-Ft. Worth Intl. (KDFW)

Gender: male
 Posts: 571
 *****
 
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.
 
Pilot for a major US airline certified in the: EMB-120, CRJ, 727, 737, 757, 767, and A-320 and military, T-38, C-130, C-141, and C-5 along with misc. other small airplanes. Any questions, I'm here for you.
Back to top 
IP Logged
 
Reply #6 - Apr 3rd, 2005 at 9:58pm

Eskimo   Offline
Colonel
I'm not speeding officer
-- I'm just flying low.
YBAF - Archerfield, Brisbane

Gender: male
 Posts: 287
 *****
 
Quote:
I'd say that is a faulty argument.  ???
It's no secret that KIAS descreases with altitude
"Climb speed: IAS 80kt minus 1kt/2000ft" it says on the Vy diagram (Vy = Best rate of climb speed) for my aircraft.

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.
 
Back to top 
IP Logged
 
Reply #7 - Apr 4th, 2005 at 4:41pm

beefhole   Offline
Colonel
common' yigs!
Philadelphia

Gender: male
 Posts: 4466
 *****
 
I thought climb speed was aircraft dependent (280 for 737, something like 300 for 777, etc.)

note: directed at saratogas comment.
 
Back to top 
IP Logged
 
Reply #8 - Apr 4th, 2005 at 5:28pm

Nexus   Offline
Colonel
The greater of two evils...

Gender: male
 Posts: 3282
 *****
 
Quote:
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  Smiley

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.
 
Back to top 
IP Logged
 
Reply #9 - Apr 4th, 2005 at 6:47pm

Saratoga   Offline
Colonel
757/767 Captain   Major,
USAF
Dallas-Ft. Worth Intl. (KDFW)

Gender: male
 Posts: 571
 *****
 
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. Wink
 
Pilot for a major US airline certified in the: EMB-120, CRJ, 727, 737, 757, 767, and A-320 and military, T-38, C-130, C-141, and C-5 along with misc. other small airplanes. Any questions, I'm here for you.
Back to top 
IP Logged
 
Reply #10 - Apr 4th, 2005 at 7:13pm

Nexus   Offline
Colonel
The greater of two evils...

Gender: male
 Posts: 3282
 *****
 
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  Smiley
 
Back to top 
IP Logged
 
Reply #11 - Apr 4th, 2005 at 8:35pm

beefhole   Offline
Colonel
common' yigs!
Philadelphia

Gender: male
 Posts: 4466
 *****
 
767 is close enough, far as I'm concerned Smiley

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.
 
Back to top 
IP Logged
 
Reply #12 - Apr 12th, 2005 at 6:50pm

Issflareman   Offline
Colonel
Trumbull, CT

Gender: male
 Posts: 302
 *****
 
I stick to KIAS with the heavies, but i use mach for all my fighter jets.
 
FSX Deluxe/FS 2004 User
&&
*Dell Dimension 8250*--2.40GHz Pentium 4--GeForce4 MX 420--1 gig RAM--&&--Thrustmaster Top Gun Afterburner II--Logitech Wingman Force 3D--
Back to top 
IP Logged
 
Reply #13 - Apr 13th, 2005 at 2:31am

SilverFox441   Offline
Colonel
Now What?
Mississauga, Ontario, Canada

Gender: male
 Posts: 1467
 *****
 
Drag increases exponentially as you near limiting Mach numbers...very good reason for commercial planes to stay away from that limit. Smiley

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. Cheesy
 
Steve (Silver Fox) Daly
&&
Back to top 
IP Logged
 
Reply #14 - Jul 11th, 2005 at 10:50pm

t_alexander21   Offline
Colonel
I'd rather be flying a
bus
Chicago

Gender: male
 Posts: 104
 *****
 
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...

Quote:
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.
 
Back to top 
IP Logged
 
Pages: 1 2 
Send Topic Print
‹ Previous Topic | Next Topic ›
  « Home ‹ Board Top of this page

Simviation Forum » Powered by YaBB 2.5 AE!
YaBB Forum Software © 2000-2010. All Rights Reserved.

Valid RSS Valid XHTML Valid CSS Powered by Perl Source Forge