Mach vs. IAS
02-05-2016, 09:00 AM
#1
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Mach vs. IAS
Hi. Studying for an interview w/ no jet experience. Can anyone please help me understand this question...
If cruising at M.76 and told to descend while maintaining mach until 330KIAS. What would be different if you were told to maintain mach until 250KIAS instead? Will the transition happen sooner or later?
Isn't mach .76 at FL300 something like 450 KIAS? It's even higher at 10,000. So I am confused.
Thank you!
If cruising at M.76 and told to descend while maintaining mach until 330KIAS. What would be different if you were told to maintain mach until 250KIAS instead? Will the transition happen sooner or later?
Isn't mach .76 at FL300 something like 450 KIAS? It's even higher at 10,000. So I am confused.
Thank you!
02-05-2016, 09:09 AM
#2
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No, it is NOT 450 IAS....it's about 450 TRUE.
As a ballpark wag, 0.76 at FL300 would only be about 240 indicated....but you are going 450.
Simple illustration, in a jet that could do it: Mach 1 at Sea Level would be roughly 600 Indicated.
At FL400, it would be about 325 Indicated.
So, for your question, if they had you transition to 250 indicated, it would happen much sooner from starting your descent.
As a ballpark wag, 0.76 at FL300 would only be about 240 indicated....but you are going 450.
Simple illustration, in a jet that could do it: Mach 1 at Sea Level would be roughly 600 Indicated.
At FL400, it would be about 325 Indicated.
So, for your question, if they had you transition to 250 indicated, it would happen much sooner from starting your descent.
02-05-2016, 12:50 PM
#4
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A decent "WAG" to figure out kts IAS from TAS is TAS - (FLXXX / 2). Ex: 450 - (FL300 / 2 = 150) = 300 IAS. Going the other way, kts TAS = (FLXXX / 2) + IAS. Ex: (FL300 / 2 = 150) + 300 = 450 kts TAS. It's pretty darn accurate unless you've got some extreme ISA deviations. .76 at FL300 is actually about 447 kts TAS and 287 kts CAS at ISA.
Last edited by TankerDriver; 02-05-2016 at 01:01 PM.
02-07-2016, 06:05 AM
#7
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Thank you gents. VERY helpful. Esp. the "wags".
So if I am understanding all correctly....
If you hold a constant Mach during climbout, what happens to TAS, IAS, and
AOA as you approach your level-off altitude?
a) TAS increases, IAS decreases, AOA increases
b) TAS increases, IAS decreases, AOA decreases
c) TAS decreases, IAS decreases, AOA increases
d) TAS decreases, IAS decreases, AOA increases
B is the correct answer
?
So if I am understanding all correctly....
If you hold a constant Mach during climbout, what happens to TAS, IAS, and
AOA as you approach your level-off altitude?
a) TAS increases, IAS decreases, AOA increases
b) TAS increases, IAS decreases, AOA decreases
c) TAS decreases, IAS decreases, AOA increases
d) TAS decreases, IAS decreases, AOA increases
B is the correct answer
?
02-07-2016, 06:24 AM
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I would say "D." Um, wait. I just noticed that "C" and "D" have the same answers.
The speed of sound at sea level is 661 kts (std day). At FL300, it is 589.
So, if you climbed at Mach 1, you would indicate 661 on the deck, and 589 at level-off.
Let's use a more realistic 280 knts on the deck, so about 0.42 Mach. (And 280 True, assuming a mythical plane with no correction factors)
By the time you level off, 0.42 Mach x 589 = 247 True.
BUT: the rare-air at FL300 would mean you are only indicating 154 knts.
Since the wing only "feels" the lift of indicated speed, it would obviously be working harder at 154 indicated, instead of 280.
So: TAS would decrease (a little), Indicated would decrease (a lot), and AoA would increase (a lot).
The speed of sound at sea level is 661 kts (std day). At FL300, it is 589.
So, if you climbed at Mach 1, you would indicate 661 on the deck, and 589 at level-off.
Let's use a more realistic 280 knts on the deck, so about 0.42 Mach. (And 280 True, assuming a mythical plane with no correction factors)
By the time you level off, 0.42 Mach x 589 = 247 True.
BUT: the rare-air at FL300 would mean you are only indicating 154 knts.
Since the wing only "feels" the lift of indicated speed, it would obviously be working harder at 154 indicated, instead of 280.
So: TAS would decrease (a little), Indicated would decrease (a lot), and AoA would increase (a lot).
Last edited by UAL T38 Phlyer; 02-07-2016 at 06:43 AM.
02-07-2016, 10:40 AM
#9
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I'm confused. Nothing like aviation to make you feel like a dummy! 
I'm keying in on the "constant mach" portion of the question and assuming one doesn't have a 1:1 power-to-weight ratio (or greater) to work with, such as a fighter jet. Assuming that is true and we're talking about an airliner, as one gains altitude, one would have to lower the AoA in order to hold a constant mach, wouldn't they??
Make answer D) TAS decreases, IAS decreases, AOA decreases
Sorry for being so slow on the uptake.
I'm keying in on the "constant mach" portion of the question and assuming one doesn't have a 1:1 power-to-weight ratio (or greater) to work with, such as a fighter jet. Assuming that is true and we're talking about an airliner, as one gains altitude, one would have to lower the AoA in order to hold a constant mach, wouldn't they??
Make answer D) TAS decreases, IAS decreases, AOA decreases
Sorry for being so slow on the uptake.
02-07-2016, 11:10 AM
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Has nothing to do with T:W.
LIFT is a function of Indicated airspeed. At high IAS, it doesn't take much AoA to creat 1-g of lift.
At lower IAS, it takes MORE AoA to get 1-g of lift.
Do not confuse AoA with aircraft pitch. Yes, in general, as you climb, the pitch will lower.
The classic definition of AoA is "the angle between the mean chord-line and the relative wind." No one can imagine that.
But if I said "It's the difference between where your nose is pointed, and where your butt is going," it makes more sense.
(Works for yaw angle, too).
Note in your example, answers C and D are identical. Real answer to the question: in a constant Mach climb, TAS decreases, IAS decreases, AoA Increases.
LIFT is a function of Indicated airspeed. At high IAS, it doesn't take much AoA to creat 1-g of lift.
At lower IAS, it takes MORE AoA to get 1-g of lift.
Do not confuse AoA with aircraft pitch. Yes, in general, as you climb, the pitch will lower.
The classic definition of AoA is "the angle between the mean chord-line and the relative wind." No one can imagine that.
But if I said "It's the difference between where your nose is pointed, and where your butt is going," it makes more sense.
(Works for yaw angle, too).
Note in your example, answers C and D are identical. Real answer to the question: in a constant Mach climb, TAS decreases, IAS decreases, AoA Increases.
Last edited by UAL T38 Phlyer; 02-07-2016 at 11:30 AM.
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