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VNAV questions

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Hi folks

Although I feel I am making progress with the FMC, I have a number of VNAV questions that I am unsure about:-

 

1) When should I receive the 'Reset MCP' altitude warning message?

I am at my cruise level i.e FL300 and just before TOD I receive the 'Reset MCP altitude' warning message. How far before TOD should I see that message?

 

2) The a/c has descended to meet an altitude restriction in the FMC. If there is a subsequent altitude restriction at a waypoint further on in the route I assume I should receive a 'Reset MCP altitude' warning message again?

 

3) I am still unsure what the different font sizes mean on the ND (FMC). As far as I can see there are 3 fonts for altitude/speeds including the magenta ones.

Can someone explain the significance of them please?

 

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Hi Colin,

 1) Unsure of the question because the answer is obviouse, ie you receive the message before TOD when you see it appear.  Are you not seeing the message?

 

2) No further messages. For example, descent on the LENDY into JFK may have a descent restriction at FL190 by ATC, you set your MCP ALT to 19000 and keep it there until ATC clears you lower. Conversely, descent has a crossing restricting of AT OR ABOVE FL190 but you are cleared by ATC to 12000, you can set your MCP to 12000 but be sure to monitor the descent to ensure you cross the restriction as published.

 

3) All the symbols and fonts/colors for the ND are defined in the FCOM beginning at 10.41.2.  Tip:  To find stuff in the FCOM the first table of contents in v1 provides volume and chapter. Then find the table of contents for that chapter and the usually gets you to within a few pages of your goal. Also get used to using the Ctrl+F Find feature in  the adobe reader if you have a specific term you are looking for.

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When should I receive the 'Reset MCP' altitude warning message?

15 miles from T/D.

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Hi Colin,

 

You may be interested in watching the 20+ MCP training videos I made for the 737NGX. Michael Cubine (above post) was one of my technical consultants. This series on the 737NGX MCP is the most exhaustive that was ever done on the MCP...anywhere...and its all free!!

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1) When should I receive the 'Reset MCP' altitude warning message?

I am at my cruise level i.e FL300 and just before TOD I receive the 'Reset MCP altitude' warning message. How far before TOD should I see that message?

 

Remember: That's not a command, that's a reminder. Normally, you shouldn't be seeing that message. ATC usually knows where you'll be needing to begin your descent. If you're approaching your T/D and ATC hasn't given you a descent clearance, ask for it.

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To the OP, you should prepare for your descent well before that message should appear. Have your frequencies set, check your altitudes in the CDU in accordance with the STAR, set your landing altitude, your minimums etc. and your MCP.

 

Setting your MCP altitude as the others have said to your next altitude restriction either by ATC or as per the STAR. Remember an ATC clearance to descend to a certain level still has you to comply with the descent path, unless the controller specifically tells you to cross a certain waypoint at a specific altitude and or airspeed.

 

When in cruise before the descent, you will be in altitude hold or VNAV ALT I believe, nothing will happen if you set a different altitude on the MCP altitude window unless you:

 

1) Press the altitude intervene button then you will descend at 1,000 fpm to the MCP altitude.

2) Hit the V/S button and even then your Vertical speed will light up at 0 and you will have to set our descent rate.

3) Hit the level change button your autothrottle will change to retard then arm and conduct a descent at idle to the MCP ALT.

 

Don´t be afraid to set your next altitude in the MCP ahead of time before that FMC yellow reminder comes up. Think of it as merely giving your FMC permission to descend to that altitude as per the VNAV PATH.

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Hi guys

Thanks for all your replies. I have learnt or confirmed a few things from each reply.

If I understand correctly the large white fonts are speeds and altitudes that the a/c will hit in VNAV PTH and the small white fonts are only predicted speeds and altitudes.

 

I am still a little confused by the magenta waypoints and speed or altitude constraints. If a waypoint is in magenta what does that signify? Sometimes a waypoint is magenta without any magenta speed and altitude constraints. Other times there may be magenta speed contraints with white large font altitude constraints with it. Other times there are white large font speed constraints and magenta altitude constraints with it. That is confusing me. Sorry if these are obvious questions.

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If you have a waypoint that is MAGENTA (M) - ABCDE that means it is the active waypoint on your route plan from the FMC.

 

If its WHITE (W) this means its modified and needs to be activated from pressing EXEC key on the CDU.

 

If you have an altitude or speed in big MAGENTA letters this means its a constraint. Example in big MAGENTA letters - 280/17000

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When in cruise before the descent, you will be in altitude hold or VNAV ALT I believe, nothing will happen if you set a different altitude on the MCP altitude window unless you:

 

In cruise, you would normally be in VNAV PATH or in ALT HOLD (if you press ALT HLD button).

In the first case, in VNAV PATH, if you set a lower altitude on the MCP, the aircraft will start descending on herself when reaching the T/D.

So you should ask for the descent clearance well in advance before the T/D to avoid to have either the aircraft starting to descend without clearance or starting the descent late which would force you to use the speedbrake to stay on the path.

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Thank you for all your replies, much appreciated.

I have another question.

 

I am practicing the KEPEC3 STAR into KLAS. I have LNAV and VNAV activated. I have just passed KIMME at 8000ft with an airpeed of 210K and am approaching CHIPZ which is showing in magenta

CHIPZ 170/8000. I have set a hard altitude constraint of 7100ft at POKRR. I was expecting the a/c to slow up for the 170kts speed constraint at CHIPZ but it sails past doing 210kts and I have to reduce speed manually. Am I doing something wrong?

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Was it in VNAV SPD or FMC SPD on the FMA?

Remember you are the pilot that is controlling the plane not the other way about. You must be ready at all times and monitor flight progress and step in to intervine for things like this that pop up. This could of easily been sorted out if you didn't as much rely on the automatics as you did.

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Thank you for all your replies, much appreciated.

I have another question.

 

I am practicing the KEPEC3 STAR into KLAS. I have LNAV and VNAV activated. I have just passed KIMME at 8000ft with an airpeed of 210K and am approaching CHIPZ which is showing in magenta

CHIPZ 170/8000. I have set a hard altitude constraint of 7100ft at POKRR. I was expecting the a/c to slow up for the 170kts speed constraint at CHIPZ but it sails past doing 210kts and I have to reduce speed manually. Am I doing something wrong?

 

If you want to decelerate to 170 knots you need to extend the flaps and set 170 in MCP window manually. 210 knots is roughly the flaps up maneuvering speed. You don't want to fly slower than that speed without flaps extended (and VNAV won't do that either).

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Don't quite agree with David, you do need flaps to see the programmed speed reduction but the NGX will do it without having to set speed manually.  The 777 requires manual speed intervention but not the NGX in this case.

 

Pay attention to the small circles on the path shown in the ND that signify the deceleration points, and watch what happens as soon as you drop Flaps-1, the speed command bug will move down the tape automatically as flaps are extended.

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Was it in VNAV SPD or FMC SPD on the FMA?

Remember you are the pilot that is controlling the plane not the other way about. You must be ready at all times and monitor flight progress and step in to intervine for things like this that pop up. This could of easily been sorted out if you didn't as much rely on the automatics as you did.

I actually spotted it straight away and reduced speed manually but it still surprised me :-(

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Looking at the STAR. I would be at flaps 5 and hold my speed at 170 with SPD INTV. And from PRINO (IF) you still have a 10nm final till the Descent point. So let VNAV PTH guide you down to 4900 platform altitude for the GP. Here in the Europe i've never had a RNAV STAR or such that levels you off at specific altitude. Its all CDA over here.

 

Once on 255 QDM and shortly before SHAND reduce to flaps 5 speed.

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I've never been there on VATSIM, but flying at -500 fpm for like 10 minutes is not an exciting experience ... 

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You really have to watch the speed bug in the PFD to see what the commanded speed is as when in VNAV the MCP speed is blacked out, unless you intervene the speed and you are now responsible for it instead of the FMC.

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