737-800 Target Speed on CDU ECON CRZ Page

[Skyjob]

• 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