737-800 Target Speed on CDU ECON CRZ Page
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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.
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.
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.
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.
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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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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- 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
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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;
And by the way I think I can even fly IAS at FL 410 in VNAV PATH. (Though not recommended)