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Thread: Default 737 speed

  1. #1
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    Default Default 737 speed

    I realize I am not much smarter than a pile of rocks, but I am a bit puzzled. I am trying to fly a pretty simple flight utilizing the autopilot in the default 737. I have dialed in .75 mach, as well as altitude, heading, etc. Normal take-off, pull up the gear and kick on the autopilot. Almost immediately goes into "overspeed" and screams on up past 500 KIAS. Can I get a simple explanation of where I am going wrong here please?

    Thanks,

    Les

  2. #2
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    Below 20,000 feet or so, you should use knots indicated (KIAS), rather than a mach number. At sea level the speed of sound (mach 1) is around 760 mph. You want to usually have IAS in the neighborhood of 250 kts or less below 10,000 and probably not much over 300-350 before switching to mach at high altitudes.

    One thing you may be missing is the difference between Indicated airspeed and True (actual) airspeed. At 250 KIAS at sea level (under standard conditions), you'll be doing about 205 kts. actual airspeed. But since the airspeed indicator is just an instrument that measures air pressure, calibrated in knots, at 35,000 feet that 250 KIAS may get you in the 500 knot range, depending on conditions.

    Others who are more into big jets may have additional info.

    Larry N.

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

  3. #3

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    That covered it well.

    I normally fly the 738 at 240kn below 10000, climb to cruise altitude at 320kn then set .74mach above FL280. The reverse on the way back down.

  4. #4

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    Generally, in real aviation there is a speed limit of 250 knots below 10,000 ft to facilitate traffic flow by ATC.

    The earlier 737's will NOT do 0.75 mach in normal operations. Normal cruise speed will be 0.72-0.74
    The default FSX is the 737-800 and it will cruise at 0.78 at a FL350 and above.

    I cannot think of a profile which would call for 0.75 - too fast for the 300/400/500; too slow for the -800.

    There is no such thing as a `simple` flight in the Flight Levels, (look that up), and you will want to read up on `coffin corner` as at very high altitudes the difference between stalling and overspeed can be little more than 20 knots IAS, so flying the correct mach speed is vitally important.

  5. #5

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    Silly question I suppose, but have you armed Autothrottle and enabled Speed hold?

  6. #6

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    If you have 0.75 Mach selected right after takeoff and the auto-throttle mode is engaged - no wonder you go into overspeed! You need to set it at 250 KIAS initially until passing 10,000 MSL then 300 (ballpark) until your IMN reaches your desired cruise mach, then shift to mach hold and hold that through level off.

    a 737-800 can cruise at 0.80 with good economy at mid weights and 36,000 - 38,000 MSL. 0.75 is kinda slow

  7. #7

    Default Default 737 speed

    He's getting the overspeed because of exceeding Vmo. Not 250kts. The problem is you're too low to fly .75M below Vmo (which is the max allowed airspeed) Check your airspeed indicator. The barberpole or red line is the max and it should never, in any regime of flight, be exceeded. If you can imagine, eventually you will be able to fly .75M at a higher altitude because as your altitude increases your Vmo stays the same until you reach an altitude where Vmo and Mmo (max allowed Mach number) are the same TAS. Then the barberpole or redline starts to decrease as altitude increase because now your limit is Mmo and as altitude increases IAS also decreases for lack of air density however TAS stays approx the same for Mach. It will fluctuate slightly due to temperature changes.

    The 250kts below 10k mentioned is strictly for regulatory/safety reasons and is generally enforced around the world.

    737-800
    Vmo = 340kts IAS
    Mmo = .82M

    Note: just noticed OP mentioned gear up. What about slats and flaps? That also can cause an overstess/overspeed condition if you forget to retract everything and blast well above 250kts. Observe all placarded speeds for flap speed limits (Vfe)


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    Last edited by dspaulding; 11-05-2016 at 10:45 AM.

  8. #8
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    Thanks to all of you. My biggest issue was the use of % of Mach versus KIAS below 10,000 combined with the capabilities of the 737. You have all provided a lesson for this old-timer. (72 years old and learning)

    Thanks again,

    Les

  9. #9
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    I never use the Mach setting, 0.75 is 0.75% the speed of sound. You will get an over speed warning.

    I set the Speed to 215 IAS on the ground and engage the auto speed after about 1,000 feet, after retracking the flaps I then increase it to 250 below 10,000 then to about 280 IAS for cruise climb, then to about 300-310 for cruise flight (depends on the alt). Ground speed of course in most cases will show faster then IAS. The rule is for 250 KIAS under 10,000 in Class B airspace is based on IAS, not ground speed.

    Dont kick on the auto pilot right after wheels up, get established first. Rulse of thumb is at 1,000 feet you may engage the autopilot

    Thats just my 2 cents how I do it
    PC specs: Windows 7 pro 64 bit,Intel core i5 2500 CPU @3.3 Mhz,GFORCE GTX 570
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    Real world CPL pilot with 800 hours logged in single engine / multi engine aircraft

  10. #10

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    Quote Originally Posted by miamimarlins View Post
    I never use the Mach setting, 0.75 is 0.75% the speed of sound. You will get an over speed warning.

    I set the Speed to 215 IAS on the ground and engage the auto speed after about 1,000 feet, after retracking the flaps I then increase it to 250 below 10,000 then to about 280 IAS for cruise climb, then to about 300-310 for cruise flight (depends on the alt). Ground speed of course in most cases will show faster then IAS. The rule is for 250 KIAS under 10,000 in Class B airspace is based on IAS, not ground speed.

    Dont kick on the auto pilot right after wheels up, get established first. Rulse of thumb is at 1,000 feet you may engage the autopilot

    Thats just my 2 cents how I do it
    Well, that's how NOT to do it.
    Mach is a constant, IAS is constantly variable.
    If you can't follow why this impacts on your selection of speed hold criterion as you climb, then time to hit the books.

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