Originally Posted by
737Andi
I fear I did not get this point
My understanding of climb, cruise and descent phase with active VNAV was this.....
Climb phase is the phase from takeoff to the set cruise level.
As soon as I have reached the T/C (cruise level) the plane automatically switches to cruise phase and stays in this phase until it reaches the T/D where it automatically descends when a lower altitude is set on the MCP. If Des Now is pushed the plane starts the descent earlier with a moderate vertical speed until it reaches the planned descent path of the FMC. As soon as the plane has started to descend from cruise level to any lower altitude the plane is automatically in descent phase. How can the plane still stay in cruise phase when going down from cruise level to any lower altitude?? I understand the procedure of step climb to higher altitudes but not to lower ones? Would be great if someone can explain this to me. Thank you!
Imagine that you have entered a single cruise level in the FMC RTE LEGS and VNAV cruise page. The MCP altitude is also set to this level.
Your weight allows a higher level and of course you could also cruise at a lower level, if desired.
To cruise at a higher level, you enter could enter the new level in the MCP altitude and the FMC VNAV cruise page and execute; however, if your 737 has “Altitude Intervention”, you can simply set the new altitude in the MCP and push the selector. The new altitude will be sent to the FMC VNAV cruise page as well and the page title will change to ACT ECON CRZ CLB.
To cruise at a lower level, the same technique can be used if you are more than 50nm before T/D. The page title will be ACT ECON CRZ DES.
If you are within 50nm of T/D this action will result in an “early descent” and the page title will be ACT ECON DES. The VNAV function will start a descent at about 1000fpm until intercepting the profile.
Step climbs are normally used for two main reasons:
1. to maintain recommended altitude as weight reduces; and
2. to comply with different cruise level systems in different parts of the world, e.g. China.
The recommended altitude will change as a function of weight, cost index and predicted winds. Normally, the recommended altitude increases as weight reduces but adverse winds could mean that a descent is recommended at a certain point. Adjusting to a metric flight level may also involve a climb or a descent. For these reasons, you can pre-load the RTE LEGS page with step climbs or descents against certain waypoints. The autopilot will not initiate the level change until the MCP altitude selector is also changed.
All of the above is based on the 787, so I’m not sure how much is applicable to the 737NG. I have about 5000hrs on the 737-300 and -400 and they had very similar functionality, so I think that the NG will have features broadly the same as I have described.
Hope this helps!