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Thread: max IAS gets lower when at high altitudes

  1. Default max IAS gets lower when at high altitudes

    while flying my 738 at FL340 i could go to 300knots without reaching overspeeding, but when i increased my altitude to FL410 the max speed i could reach without reaching, the overspeed markers was 240knots?? shouldn't it be higher because of less drag? why does this happens?
    Last edited by Hunonymous; 06-15-2019 at 10:15 AM.

  2. Default

    The aircraft don't just have a max Ias, they also have a max Mach number.

  3. #3
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    Quote Originally Posted by Hunonymous View Post
    while flying my 738 at FL340 i could go to 300knots without reaching overspeeding, but when i increased my altitude to FL410 the max speed i could reach without reaching, the overspeed markers was 240knots?? shouldn't it be higher because of less drag? why does this happens?
    The air gets thinner and colder at higher altitudes and this decreases the indicated airspeed. There should be a TAS (True Air Speed) readout on the MFD or PFD screen. If not you can calculate your true airspeed with this online calculator: http://indoavis.co.id/main/tas.html
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    The airspeed indicator is nothing but a pressure instrument calibrated in knots at sea level under standard conditions (59ºF, 29.92 Hg"), which is the only conditions under which it is accurate. As you go higher (and to a lesser extent, as it gets warmer) the air becomes less dense. At 18,000 feet the air pressure is about half that at sea level.

    Since the airspeed indicator is a pressure instrument (measuring ram air pressure), as the air pressure is reduced, the instrument reads lower. Therefore what you are seeing is normal.

    shouldn't it be higher because of less drag?
    Nope! At higher altitudes, with less air pressure, engines (yes, jets too) produce less power (and burn less fuel), so with that reduced drag less power is needed for the same speed, that is, for TRUE AIR SPEED. TAS is calculated roughly as IAS corrected for temperature and altitude. So at about 7500 feet an airspeed indicator calibrated in mph reads close to the TRUE airspeed in knots, under standard conditions, that is, sea level minus 3½ degrees F per thousand feet of altitude. There is somewhere around a 15% difference at 7500 feet if you maintain the same temp as at sea level. Standard temp at 7500 feet is about 32º F. So if your IAS is 100 mph at 7500 feet and 59º F, the TAS is 115 mph, or 100 kts., a 15% difference. Higher altitudes, of course, make a larger difference.

    So at 34000 feet and -35º F, with altimeter setting at 29.92 Hg", a 250 kt IAS yields a TAS (actual speed through the air) of 455 kts. Let me caution you -- I've seen a lot of people post something about TAS being equivalent to speed over the ground (GroundSpeed), which is true ONLY with calm winds, otherwise it is affected by wind strength and direction at your altitude (not surface wind).

    So you'll rarely see an IAS much over 250-300 kts or so at, say, 35000 feet. The FS flight lessons have something on this, but check Wikipedia for IAS, TAS, etc. Or it might be easier to look below at the Real Aviation Tutorials & FAQs section of the forum, under the Speeds and Altitudes heading, where a lot of this is explained.

    Another note: TAS, in and of itself is useless information for flying the airplane. Ground speed (you must take winds into consideration) is useful for navigation, helping figure arrival times, time enroute, fuel burn for the trip, etc. TAS does make a difference in calculating things such as VNE (never exceed speed), which is set by the maximum pressure from it the aircraft is designed to take.

    IAS is VERY useful to the pilot, since stall speed, best rate of climb, best angle of climb, approach speed and many other limitations are in IAS (actually CAS but I won't go into that here). The airplane only "knows" about IAS, from a flying and handling standpoint.

    In other words, it's a lot more complex than most people think.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  5. Default

    Speed is relative this is why we have KIAS vrs Mach Vrs MPH Vrs True KIAS

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    Quote Originally Posted by f18driver View Post
    Speed is relative this is why we have KIAS vrs Mach Vrs MPH Vrs True KIAS
    I'm not sure what this means. Speed is relative, true, but has nothing to do with the other pieces you list. They are different ways of measuring the SAME speed. We have KIAS (actually IAS, whether knots or mph or kph or...) and (not vrs) Mach and MPH and TAS (there's no such thing as True KIAS, which would mean True Indicated AirSpeed).

    So you might have (without actually calculating mach, since it varies with temperature, these are example only) 300 Knots Indicated AirSpeed as 470 Knots True AirSpeed and as mach .7 and as 540 MPH True AirSpeed and as 345 MPH Indicated AirSpeed and as 552 Kilometers Per Hour Indicated AirSpeed, and more, all referring to the speed measured at the Same instant on the Same aircraft. They all say the same thing. Please don't confuse them.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  7. Default

    Quote Originally Posted by lnuss View Post

    Since the airspeed indicator is a pressure instrument (measuring ram air pressure), as the air pressure is reduced, the instrument reads lower. Therefore what you are seeing is normal.


    .
    makes sense, thats why when i set speed to mach, when i start to get higher it will still be on mach 0.80 but if i set speed to knots it will be lower

  8. Default

    In the B737_800 in FSX the max speeds are:
    max_mach = 0.82
    max_indicated_speed = 340

    In both examples you gave you got an overspeed warning because you went above the max mach speed.

    At FL340 you could go to 300knots.
    You were not over the max IAS.
    You got overspeed warning, so you were at max Mach of 0.82

    At L410 the max speed you could reach without reaching, the overspeed markers was 240knots.
    Again you must have been at the max Mach at that time of 0.82
    (and, not at the max IAS speed.)

    You should not have been looking at IAS at all there. You should use Mach as speed indication at all times when you are at altitudes of 24.000 ft and over.
    Look at the autopilot and use the Mach speed setting there.
    Have a close look at the MFD display. The current speed is not only displayed in Knots there. It's also shown in Mach. Use that above 24.000 ft.

    Below 24.000 ft you should always use IAS. In the display when you reed your curent airspeed, and in the Autopilot window when you set the speed there.

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