Originally Posted by
extricate
The crossover altitude is transparent to us, what are the ways to know when Mach has changed to IAS on descent in the Boeings?
The crossover altitudes during climb and descend change on each flight each day. When using a standard CI this transition occurs often at similar altitudes.
Provided VNAV is used to climb and descend:
- CLIMB
- VNAV works out an IAS for climb based on Cost Index, Weight, predicted Headwind Component and ISA Deviation at cruise level (based on the PERF INIT CRZ WIND and ISA DEV entered in preflight, based on track direction 180nm downstream of liftoff point magenta line);
- VNAV works out an initial CRZ Mach number based on (again PERF INIT references are used) initial Flight Level, Weight, Headwind Component and CI;
- VNAV changeover occurs when the CLB IAS equates to initial CRZ Mach;
- Changeover can be observed on CLB page in FMC and above speed tape;
- DESCENT
- VNAV works out a descent IAS based on CI (no other inputs);
- VNAV Changeover occurs when the last used CRZ Mach matches this CI derived IAS;
- Changeover can be observed on DESC page in FMC and above speed tape;
Provided VNAV is NOT used to climb and descend: passing FL260 an automatic changeover from IAS to Mach and vice versa occurs