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737-800 Target Speed on CDU ECON CRZ Page

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737-800 Target Speed on CDU ECON CRZ Page

Old 29th Nov 2019, 07:12
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737-800 Target Speed on CDU ECON CRZ Page

Hello all,

When I fly using VNAV, the target speed on CDU ECON CRZ page is always shown in Mach speed.
But I think it should be displayed in IAS below 26,000ft.
Because MCP speed window is automatic changeover at approximately FL260.

I hope you will be able to provide the information.
ANAJASJAL008 is offline  
Old 29th Nov 2019, 07:59
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FMC will transition from IAS to Mach when ECON IAS equals ECON Mach, unlike the MCP which has a changeover at approximately FL260.
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Old 29th Nov 2019, 10:13
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I've seen the IAS change to mach at FL320, I believe the temperature is also taken into account.
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Old 29th Nov 2019, 12:10
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Originally Posted by ANAJASJAL008 View Post
Hello all,

When I fly using VNAV, the target speed on CDU ECON CRZ page is always shown in Mach speed.
But I think it should be displayed in IAS below 26,000ft.
Because MCP speed window is automatic changeover at approximately FL260.

I hope you will be able to provide the information.
What are you saying, that the ECON CRZ is in Mach when the CRZ ALT is below 260, or when you're below 260?

Why do you think it should be IAS below 260 - just because that's what the MCP does? Conversion isn't an altitude, it happens when your climb Mach reaches your desired climb/cruise Mach - something the FMC calculates. The MCP on the other hand is a dumb mode: if you kept 300kts up to FL390 you'd be supersonic. So to be on the safe side I'm guessing they've given it an arbitrary changover altitude? I'm not a 737 pilot so I might be talking bollocks of course...
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Old 29th Nov 2019, 14:37
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Put in a cruise altitude of FL50. Or FL80. Or FL100. It doesn't display cruise mach.
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Old 29th Nov 2019, 14:54
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Try entering different altitudes with one Cost Index. Note at what altitude the IAS to Mach cruise speed changes. Now repeat with a higher CI. Is the altitude for the IAS>Mach stay the same? Or does it change? Do it on a colder, and hotter, day at the cruise altitudes where the IAS>Mach occurs. Does it change or does the change over altitude remain the same? You should be able to figure some of the inputs into how the FMC computes the change.
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Old 30th Nov 2019, 09:49
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Hello there.
Thank you for your feedback. Based on everyone's opinions, I will verify with a full flight simulator.
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Old 30th Nov 2019, 21:23
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  • Correct:
    FMC will transition from IAS to Mach when ECON IAS equals ECON Mach
    • When flying using a cost index the initial IAS/Mach is calculated based on Weight, CRZ ALT, TOC ISA DEV, TOC H/W component; basing itself of this calculation to a reference point 180nm after departure from AD to determine track direction. Resultant is an initial Mach and IAS which will transition once IAS = Mach. During descent similar applies, but now initial Mach is last used CRZ Mach and IAS is linear derived from the entered CI.
  • Correct:
    Put in a cruise altitude of FL50. Or FL80. Or FL100. It doesn't display cruise mach.
    • As long as FMC has a cruise level below FL 260 is will not predict a CRZ Mach;
  • Incorrect:
    I've seen the IAS change to mach at FL320, I believe the temperature is also taken into account.
    • The temperature only gets used to calculate ISA DEV correction for TOC ISA calculation, minimal effect in normal operating range, larger effect once exceeds ISA+10
  • Correct:
    MCP speed window is automatic changeover at approximately FL260
    , this applies only when the speed window is open, eg you the pilot control the speed, not when FMC controls the speed. Same phenomena can cause possible safety issues when inappropriately using V/S at altitudes ABOVE FL260 but when FMC transition has not yet occurred so is still commanding IAS, the risk of an underspeed scenario is rife
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Old 4th Dec 2019, 10:26
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Put in a cruise altitude of FL50. Or FL80. Or FL100. It doesn't display cruise mach.
  • As long as FMC has a cruise level below FL 260 is will not predict a CRZ Mach;
Strange my B737NG will predict an CLB, CRZ and DES Mach below FL260.

And by the way I think I can even fly IAS at FL 410 in VNAV PATH. (Though not recommended)
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