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MED1473

Descent in VNAV stops

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Hi all! I'm sure this has got a very simple answer and I've just missed it but how do you get the plane to descend when going down on vnav and reaching the altitude set in the mcp?

Once reached this altitude the airplane levels off and even after changing the altitude in the mcp it stays levelled off. I usually go in level change but I'm suspecting this might not be the correct procedure.

Apologies if this has been previously discussed.

Matteo Capocefalo MED1473

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Hi all! I'm sure this has got a very simple answer and I've just missed it but how do you get the plane to descend when going down on vnav and reaching the altitude set in the mcp?

Once reached this altitude the airplane levels off and even after changing the altitude in the mcp it stays levelled off. I usually go in level change but I'm suspecting this might not be the correct procedure.

Apologies if this has been previously discussed.

Matteo Capocefalo MED1473

 

Did you press the little black knob next to (the right) of the mcp altitude setting knob? Altitude intervene? This will override the exihisting altitude in the Legs page. You can do it with FLCH as well.

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What phase of flight? What are the indications on the Flight Mode Annunciator?

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Matteo-

 

That is EXACTLY the correct behavior. The MCP altitude is a HARD RESTRICTION- period. There is only one time the aeroplane will fly away from that altitude in auto-flight. That is when the LOC/GS are captured and you pass the selected ALT in ILS descent. That way you can still set the missed approach altitude. Other that that, the 737 will indeed stop any vertical deviations at the MCP ALT altitude. Once that occurs and you change the MCP ALT window, you will have to action something (LVL CHG or ALT INTV if in VNAV or even VS if you are paying particular attention).

 

Boeing has a fundamental design/logic stipulation that the PILOT has to control the flight path at all times- AFDS can be used to assist, economize, allow for more heads-up flying, but not to do the thinking for you.

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Thank you everybody for the quick responses!

Thanks Carl for the spot on, very useful answer! So the correct behaviour is VNAV descent- reach and level off at MCP ALT- set new MCP ALT- select LVL CHG- select VNAV again? Have I got it right?

Can you explain the difference between executing this action in LVL CHG and doing it in ALT INTV? I'm a bit unsure about the latter, my knowledge stops at it deletes the next alt constraint in the LEGS page but I'm a bit unsure on its other functions..

Thanks!! Mat

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Either continue descent in VNAV (via Alt Intv) OR select LVL CHG (NOT BOTH)...... VNAV maintains FMC speed and multi-dementional flight path computations..... LVL CHG does NOT.

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Ah awesome, got it! ALT INTV deletes the next altitude constraint therefore also the hard restriction on the MCP ALT once it has been acquired and the new MCP ALT has been entered!

Thanks mate! Mat

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Matteo & Carl,

 

You two are not quite on the same page. Carl, he's not talking about flying away from the MCP ALT - re-read the first post.

 

 

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Matteo:

 

Say you were cleared to FL200 by ATC and descend in VNAV. Then, if you have 20000 in the MCP, descent will stop at FL200.

 

If you reach FL200 and level off in VNAV before getting a new clearance to FL100 (an example), you will have to set 10000 in the MCP AND activate either ALT INTERVENE (for VNAV descent), LEVEL CHANGE OR V/S in order for the aircraft to continue down.

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2 options, just reset the next altittude before reaching it, don't care about altittude restrictions in the procedure because the FMS has all the fixes and restrictions( i confirm it before starting descent during the approach brieffing). You can even set the lowest/highest altittude restriction in the MCP and the aircraft respects all the others.

The other option is just recycle the VNAV mode by pushing it 2 times (OFF/ON). and the aircraft continue the descent/climb to the next altittude. I hope it helps you. B)

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Reset MCP altitude below current and re press VNAV to tell the FMC to decend back on Vspeed or Vpath. Works for me.

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Matt- brilliant! I defer to your ascerbic and thoughtful insight into this issue. Perhaps you are correct, I am not on the same page? Do enlighten us with your thoughts, however, I am acutely interested. I wait in hopeful anticipation.....

 

Matteo- I would advise you to disregard my response, apparently it is inaccurate. Do however check back, Matt is on the case.....

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Well again thank you all for all the responses!

As I understand it we've got two main orthodox ways to descend below the MCP ALT once the aircraft has reached and aquired it in a VNAV descent (this is without considerind the VSPEED mode)

1. Carry on in VNAV mode by pressing ALT INTV to clear the constraint after selecting the new altitude on the MCP

2. Descend in LVL CHG and the resume VAV if required.

Correct me if I'm wrong.

And thanks to Daniel for confirming the procedure and thanks to Carl for the exhaustive explanation, you clocked what I was (badly) trying to say straight away and I personally think we're EXACTLY on the same page!

Cheers guys

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Carry on in VNAV mode by pressing ALT INTV to clear the constraint after selecting the new altitude on the MCP

 

Yes, and using my example, the constraint was effectively the altitude set in the MCP (20000).

 

2. Descend in LVL CHG and the resume VAV if required.

 

 

Yes, that also works. ^_^

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Awesome, I finally see the light lol

I like it when that happens!

Ps just one last question, does ALT INTV in the example. clear the actual NEXT alt constraint in the legs page?

(eg. If all this capering happens between waypoints will ALT INTV affect the next waypoint's ALT CONSTRAINT if there's one?)

Mat

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No, in the above example, the current constraint is the FL200 set in the MCP. Pressing ALT INT will only affect that.

 

However, lets's say you've done that and are on your way down to FL100 (for example) with 10000 in the MCP - and there is a waypoint alt constraint of FL150 over XYZZZ. Then, ALT INV will delete that constraint in the FMC.

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Matt- brilliant! I defer to your ascerbic and thoughtful insight into this issue. Perhaps you are correct, I am not on the same page? Do enlighten us with your thoughts, however, I am acutely interested. I wait in hopeful anticipation.....

 

Matteo- I would advise you to disregard my response, apparently it is inaccurate. Do however check back, Matt is on the case.....

 

Clearly you misunderstood the OP and I politely pointed that out. No flames, just facts.

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Ok guys no reason to start bickering about nothing, my qestion has been answered, greatly I say and in depth. I've understood everything I needed and this post has reached its purpose!

I am just a bit puzzled Mat on how has Carl misunderstood my first post? I think everyone here has got what I meant and it seems it is you that perhaps think differently. I was talking precisely about flying away from MCP alt once reached it! On top of that Carl and the others took the time and effort to write an exhaustive, in depth answer that cleared my doubts. That couldn't really have happened if they ALL misunderstood my OP unless I've misunderstood my own post too... a distubing thought...

No offense.

Cheers, Matteo

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I think the question you asked was really about VNAV ALT. That's why I asked about the FMA. Most pilots and instructors will ask, "Well, what did the FMA say?" when trying to answer questions about auto-flight.

 

Carl and many of the other posters gave you good advice - reset the MCP and hit ALT INT.

 

Here's the sections you need:

 

• VNAV ALT – when a conflict occurs between the VNAV profile and

the MCP altitude, the airplane levels at the MCP altitude and the pitch

flight mode annunciation becomes VNAV ALT. VNAV ALT

maintains altitude.

 

During descent, VNAV ALT engages if the airplane levels at an MCP altitude not

in the FMC.

 

Altitude Intervention (ALT INTV) Switch

Allows manual deletion of next FMC altitude constraint via altitude SEL and ALT

INTV switch.

Push – (during VNAV climb)

• lowest FMC altitude constraint below selected MCP altitude is deleted

• if airplane is currently at an FMC altitude constraint, deletion allows

airplane to resume climb. MCP altitude must be set above current altitude

• for each press of switch, one deletion occurs

• if MCP altitude is set above current FMC altitude, FMC cruise altitude

resets to MCP altitude. FMC cruise altitude cannot be decreased using

ALT INTV switch.

Push – (during VNAV cruise)

• if MCP altitude is set above current FMC cruise altitude, FMC resets

cruise altitude to MCP altitude and initiates a cruise climb

• if MCP altitude is set below current FMC cruise altitude, an early descent

is initiated. Lower FMC cruise altitude cannot be entered using ALT

INTV switch.

Push – (during VNAV descent)

• the highest FMC altitude constraint above MCP altitude is deleted

• if airplane is currently at an FMC altitude constraint, deletion allows

airplane to continue descent. MCP altitude must be set below current

altitude

• if all FMC altitude constraints are deleted during VNAV path descent, an

automatic transition to a VNAV speed descent is made.

 

I hope that info adds to the discussion.

 

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