Early "descent" after step climb

[killairbus]

Hi I need some help,

I set FL340 as my step climb on the CRZ page and then dialed in the MCP and pressure panel to that new altitude. Then I pressed flight level change before copying the FL340 on the step line in the CRZ page on the FMC, and setting it as my new cruise altitude after leveling off. 

After I pressed execute I seemed to have started an early descent while I am 2 hours from my TOD.  What did I fumble here? 

-Angelo Busato

[ph-cxz]

Can you share the details about your flightplan as that helps with understanding the step climb.

[downscc]

Hi Angelo, hope this helps:

The cabin pressure cruise altitude setting should be set to the highest level you are going to fly and left alone.

A manual step climb only requires that you dial the new MCP ALT and press ALT INTV during VNAV cruise. See FCOM 4.10.10

I think using the LVL CHG in the way you did may have been the fumble.  See also 4.10.7 for a description of that button.  In any event, you have to look at the FMA to see why the aircraft is doing what it is doing.  Remember:  MCP tells the airplane what you want; FCP airplane tells you what it is doing.

[killairbus]

14 hours ago, downscc said:

Hi Angelo, hope this helps:

The cabin pressure cruise altitude setting should be set to the highest level you are going to fly and left alone.

A manual step climb only requires that you dial the new MCP ALT and press ALT INTV during VNAV cruise. See FCOM 4.10.10

I think using the LVL CHG in the way you did may have been the fumble.  See also 4.10.7 for a description of that button.  In any event, you have to look at the FMA to see why the aircraft is doing what it is doing.  Remember:  MCP tells the airplane what you want; FCP airplane tells you what it is doing.

I can't find 4.10.10 in the FCOM. Is it in volume one or two?

-Angelo Busato

[downscc]

9 hours ago, killairbus said:

I can't find 4.10.10 in the FCOM. Is it in volume one or two?

-Angelo Busato

Not very familiar with the FCOM?  Start with FCOMv1 and find table of contents (3d PDF page).  Note Automatic Flight is in Vol 2 Ch 4.

Now open FCOM v2 and look for the beginning of Chapter 4, or use Ctrl+F to search for Automatic Flight.  The table of contents for Ch 4 are at the beginning of the chapter (4.TOC.0.1).  Notice MCP is first section in that chapter beginning on page 4.10.

Hope this helps.

[BlueStar]

On ‎11‎/‎22‎/‎2018 at 11:03 AM, downscc said:

I think using the LVL CHG in the way you did may have been the fumble. 

Dan,

Once I have the new FL in the FMC and the ALT window, I use FLCH to get the A/C started climbing, and once it starts climbing I push VNAV.  It always works for me, but I use FLCH a lot so I may just be "ol' school". 🙂

Grace and Peace, 

[Spin737]

Why not stay in VNAV? No need to go to LVL CHG.

[BlueStar]

9 hours ago, Spin737 said:

Why not stay in VNAV? No need to go to LVL CHG.

I'm ol' school and I don't trust the magic. 🤣

Grace and Peace, 

[killairbus]

10 hours ago, Spin737 said:

Why not stay in VNAV? No need to go to LVL CHG.

Somebody at some point said that lvl chg is smoother for the passengers so I was trying to pretend to be an extra service sensitive pilot. 

-Angelo Busato

[killairbus]

23 hours ago, downscc said:

Not very familiar with the FCOM?  Start with FCOMv1 and find table of contents (3d PDF page).  Note Automatic Flight is in Vol 2 Ch 4.

Now open FCOM v2 and look for the beginning of Chapter 4, or use Ctrl+F to search for Automatic Flight.  The table of contents for Ch 4 are at the beginning of the chapter (4.TOC.0.1).  Notice MCP is first section in that chapter beginning on page 4.10.

Hope this helps.

Thanks, I found it, appreciate the help

 

-Angelo Busato

[Spin737]

10 hours ago, killairbus said:

Somebody at some point said that lvl chg is smoother for the passengers so I was trying to pretend to be an extra service sensitive pilot. 

-Angelo Busato

I don't think there's any difference - the engines will still go to CLB N1 at VNAV SPD or MCP SPD, which would probably be the same. V/S is smoother if you limit the ROC.

[Crossdrop]

Strangely enough, I was experiencing the same issue today. Cruising at FL360, roughly 25 minutes before TOP I needed a step climb to FL370 due to airway restrictions. Pressurization was set at FL370.

Dial 37000, hit ALT INTV, and suddenly the aircraft said "Reset MCP altitude", as for VNAV, I was way past my TOD. The only way to continue with reasonable VNAV was to stay at 360 and dial it in at the CRZ page, which returned the aircraft to its VNAV state from before. Also dialing in 370 in the CRZ page first brought the issue.

To be honest, once in a while I encounter software issues in PMDG aircraft, but very rarely. This was the first time this happened in the 737 in at least 100 or 200 flights.