It is not writeen in FCOM about this. In CRZ,just passed T/D,and FAM sys MCP SPED and ALT HOLD ,both MCP and FMC alt is FL300.Then ATC required me to desend to FL200.I set mcp alt FL148,then tried to connect VNAV ,but it dosen't work. Then I pished ALT INVT then VNAV it worked.but why??? FCOM only says how to use ALT INVT in VNAV mode,no other information. How many stuations,in which the VNAV can't be engaged?
Location: Above the clouds where the sun is always shining
first of all in my opinion you got strange flightlevels
Basically VNAV is the vertical path that's put in the fmc. If you have any altitude restrictions while you are climbing and the mcp alt is set higher, if you then press alt intervent the altitude restriction is deleted. If you are in cruise and want to descend in vnav you have to put your new altitude in the fmc which you can do manually or by setting the lower altitude in the mcp and pres alt intervent.
I'm not sure anymore if it's the same when you already entered the approach in the fmc. Ours says reset mcp alt when we reach our TOD
You where in ALT HOLD because you passed the VNAV TOD? and then selected VS? you where descending in VS and then tried to connect VNAV? With 10.8A if you where in VNAV and then reset the MCP to a lower altitude the ALT Intervention should descend you in VNAV to the lower Altitude if the lower altitude is in the FMC legs page and after your current waypoint. If the lower altitude is not in the active legs page, as previously mentioned, you need to insert it.
I was CRZ at FL256,FMA says MCP SPD and ALT HOLD.Then I was asked to desend to FL148.SO ,I set FL148 on MCP (while FL256 in FMC).Then I push VNAV butten ,nothing happend. why? But then i put ALT INVT butten and VNAV,it worked,stared VNAV PTH descend.
Hey dude, you do yourself no favour with your haughty reply to flying apple's observation about your " strange flight levels ".....a hint of arrogance at your current ability to fly modern shiny jets in PRC. Secondly, your query betray a complete lack of understanding of VNAV vis-a-vis basic vertical profile management. Some wonder how you manage to garner a job flying this shiny new jet around the PRC. Now you last post clearly takes the cake.
Now I don't fly the B738 but I have previous experience on the B767. When you set FL148 on the MCP without pushing ALT INVT, the FMC cruise altitude remained at FL256 on VNAV cruise page. If you pressed VNAV after being at ALT HOLD, VNAV descent would not occur because there was no defined active descent path from current airplane altitude to the FMC VNAV cruise page. Then by engaging ALT INVT you had either resetted the FMC cruise altitude on the VNAV cruise page to FL148 or activated the VNAV DES function. When FL148 was propagated to the FMC cruise altitude on the VNAV cruise page, there was now a defined cruise descent function which allowed VNAV to engage. If you had passed TD, then the VNAV DES would be activated. In both cases, the AFDS would descend the aircraft; in the first case it would be a cruise descent and in the second instance it would be descent at idle thrust at VNAV SPD trying to achieve VNAV PTH.
I hope you and JT finds this helpful! I fly airbus products now; so my recollection of Boeing VNAV operation may be faulty.
I understand haejungkuk's lament because I have seen a quite a number of newbies who bought rapid type rating training which never prepared them for flying the shiny new jets at all. They then had the miraculous 500 or 1000 flying hours in their logbooks when they show up in the Orient for plum jobs. Your question at the start of this thread is one of the many we have come across. The interesting thing is that the some " knowledgeable " replies to the query will certainly raise eyebrows, if not scare a lot of people.