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Thread: autothrottle problem

  1. Default autothrottle problem

    i'm having a problem with the autothrottle on the default 737, i'm flying normally at FL 370 at 250 knots when all of suden the autothrottle sets the speed to 400 knots

  2. Default

    forget it boys i figured out the problem, the honeywell FMC VNAV had a extremely high speed set on it so i just turned it off

  3. #3

    Default

    Quote Originally Posted by Hunonymous View Post
    i'm having a problem with the autothrottle on the default 737, i'm flying normally at FL 370 at 250 knots when all of suden the autothrottle sets the speed to 400 knots
    Er, why are you flying at FL370 using IAS/Knots..?
    At heights above about FL240 you should be in MACH speed mode...

  4. Default

    Quote Originally Posted by mallcott View Post
    Er, why are you flying at FL370 using IAS/Knots..?
    At heights above about FL240 you should be in MACH speed mode...
    Sorry for late answer but the default 737 mach speed is glitchy see, if i set it mach 0.78 or climb or descend the speed wil become incorrect that's why i just use KIAS

  5. #5

    Default

    Quote Originally Posted by Hunonymous View Post
    Sorry for late answer but the default 737 mach speed is glitchy see, if i set it mach 0.78 or climb or descend the speed wil become incorrect that's why i just use KIAS
    the 737 hasn't been wrong for me in about 7 versions of FSX in about the last 10 years... you are doing something wrong.

  6. #6
    Join Date
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    Default

    Classic "I want 350 KIAS, no matter what altitude I'm at."
    Sadly, many don't realize the KIAS above about 18000' MSL is never the same. Temperature, pressure, humidity, there are a ton of factors that affect KIAS. THAT's why you use Mach above 18000', not KIAS. That is a more useful, accurate, measure of how much air is flowing over the aircraft.
    Plane requires M0.80, to fly properly, and efficiently, then it requires M0.80, not 289 KIAS, or 1,050 KIAS or whatever. M0.80 is M0.80 regardless of altitude, humidity, or whatever. KIAS changes with a change in any factor involved.
    That's why modern aircraft use Mach above 18000' MSL, not KIAS. As long as the plane maintains the correct Mach number, it will fly correctly.

    Probably a really lousy explanation, but the best I can do this time of day...
    Pat☺

    Had a thought...then there was the smell of something burning, and sparks, and then a big fire, and then the lights went out! I guess I better not do that again!
    Sgt, USMC, 10 years proud service, Inactive reserve now

  7. #7
    Join Date
    Mar 2005
    Location
    Westminster, CO
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    6,925

    Default

    Sorry for late answer but the default 737 mach speed is glitchy see, if i set it mach 0.78 or climb or descend the speed wil become incorrect that's why i just use KIAS
    Perhaps you could describe what you see that makes "mach speed" seem glitchy. If it's the fact that your airspeed indicator shows variation as you climb and descend, perhaps even as you go from one weather system to another, then read what Pat said above. It's not NEARLY as cut and dried as you seem to think.

    But note that KIAS (Knots Indicated AirSpeed) is nothing but the measure of the pressure that comes into the pitot tube, and at a given ACTUAL speed through the air (True Airspeed) that pressure varies (thus so does the Indicated AirSpeed, or IAS) with temperature, barometer, etc. (as Pat indicates above), thus you are getting the wrong impression about what the airplane is doing. In other words, KIAS have little to do with showing you the True speed. Real airplanes do just as you're seeing and real world flight uses mach, rather than IAS, at the jet altitudes. Things are different at those altitudes.

    And DON'T use ground speed for anything except figuring how long to get somewhere at the current rate, since it bears no relationship to what the airplane is doing, unless you figure in winds and True AirSpeed, unneeded calculations.

    Perhaps if you read this article in Wikipedia you'll clarify a few things for yourself. Or you could go to the Real Aviation Tutorials & FAQs section of the forum below and look at the section titled Speeds and Altitudes for some good info on the subject.

    Larry N.

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

  8. Default

    Quote Originally Posted by PhantomTweak View Post
    Classic "I want 350 KIAS, no matter what altitude I'm at."
    Sadly, many don't realize the KIAS above about 18000' MSL is never the same. Temperature, pressure, humidity, there are a ton of factors that affect KIAS. THAT's why you use Mach above 18000', not KIAS. That is a more useful, accurate, measure of how much air is flowing over the aircraft.
    Plane requires M0.80, to fly properly, and efficiently, then it requires M0.80, not 289 KIAS, or 1,050 KIAS or whatever. M0.80 is M0.80 regardless of altitude, humidity, or whatever. KIAS changes with a change in any factor involved.
    That's why modern aircraft use Mach above 18000' MSL, not KIAS. As long as the plane maintains the correct Mach number, it will fly correctly.

    Probably a really lousy explanation, but the best I can do this time of day...
    Pat☺
    Sir i'm no newbie i know what is IAS, GS or TAS the problem is the default 737 if i set mach 0.8 at an altitude where my IAS is 280 knots and i climb for a higher altitude i'll end with my speed at mach 0.82 at 280 knots, it's just MS was too lazy to make it realistic or fix its issues

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