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kmw510

Transitioning between FMC and MCP auto flight

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I am having trouble transitioning between vnav and using the mcp to make altitude transitions. I have stayed away from vatsim flights until now due to my lack of confidence in this area. While flying under lnav/vnav I can easily make heading changes and speed changes due to 

ATC direction and then get back into lnav/vnav, however, when using level change and V/S to make altitude changes at times I have trouble getting back into VNAV flight. My understudying is that while in vnav engaging mcp vertical path modes would disengage vnav and then when wanting to disengage the mcp vertical path mode simply engaging vnav to cancel the mcp mode. There are times (more often than not) that I am unable to re-engage vnav and don't know if there is a sim issue or some other condition in the plane preventing vnav engagement.

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When you are finished using level change to the altitude you've been directed to, enter your new current altitude on the cruise page of the fmc. When it finishes calculating the page, you should be able to hit vnav again and it will follow those new calculations.

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There are times (more often than not) that I am unable to re-engage vnav and don't know if there is a sim issue or some other condition in the plane preventing vnav engagement.

 

I'm willing to bet that it's just due to user error somehow.  Don't think of it as FMC and MCP modes.  It's all just the MCP.  The FMC just gives the MCP VNAV mode data to work with.

 

Here's a video on the use of the other modes (which may or may not help, because I don't even touch VNAV at all in it), but it's worth knowing:

 

I honestly can't think of a single reason why VNAV wouldn't re-engage after you temporarily went into another mode.  What were you doing in the cases that it wouldn't re-engage?

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Maybe the A/P was already capturing the new altitude? Does re-engaging VNAV work for you, when still 1 - 2.000 ft away from your target ALT?

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Thank you William, however, I do not want to program the FMC to a new cruise altitude, just follow atc direction until they direct me to resume my plan as filed

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I'm willing to bet that it's just due to user error somehow.  Don't think of it as FMC and MCP modes.  It's all just the MCP.  The FMC just gives the MCP VNAV mode data to work with.

 

Here's a video on the use of the other modes (which may or may not help, because I don't even touch VNAV at all in it), but it's worth knowing:

 

I honestly can't think of a single reason why VNAV wouldn't re-engage after you temporarily went into another mode.  What were you doing in the cases that it wouldn't re-engage?

Ok after 3 experimental flights here is what happened. I first flew the flight entirely on lnav/vnav to baseline the fmc programming and verify all aspects of the flight were error free. The flight flew perfectly. The 2nd flight I did the following:

 

At cruise FL315 the pfd showed "FMC SPD  LNAV  VNAV PATH". I dialed into the mcp window the new lower altitude of FL305 and pressed LVL CHG. The aircraft descended to the new lower level and settled into on the pfd "MCP SPD  LNAV  ALT HOLD".  The FMC still showed FL315 as the cruise altitude. To resume at FL315 and get back into vnav I dialed in FL350 into the mcp alt window. Upon hitting vnav nothing happened and the fmc message window showed "check alt tgt". Looking at what that message means, the aircraft was not between the mcp altitude and the vnav target. The fmc still showed FL315 as the cruise level, the mcp window was set to FL315 and the plane was flying currently at FL305. 

 

I then used lvl chg to ascend to the FL315 and settled into a cruise at FL315 with the pfd state as "MCP SPD   LNAV  ALT HOLD". Vnav can now be engaged with pfd showing "FMC SPD   LNAV  VNAV ALT" and an fmc message of "reset mcp altitude"  The problem now is I am in VNAV ALT and can't get back into VNAV PATH where I should be. 

 

I recalled what William had to say and tried setting the cruise altitude to FL315 in the FMC even though the FMC already showed the cruise altitude as FL315. Upon execution of the cruise altitude on the crz page the plane entered into the VNAV PATH mode again and everything was back to normal. This makes no sense to me and appears counterintuitive and doesn't match the FCOM literature as far as I can tell. Don't know if this is the way it works or the pmdg implementation has a bug

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Maybe the A/P was already capturing the new altitude? Does re-engaging VNAV work for you, when still 1 - 2.000 ft away from your target ALT?

 

 

I mixed up something here; sorry for that.

 

I checked the VNAV on the T7 again and it would re-engage every time I tried.

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I recalled what William had to say and tried setting the cruise altitude to FL315 in the FMC even though the FMC already showed the cruise altitude as FL315. Upon execution of the cruise altitude on the crz page the plane entered into the VNAV PATH mode again and everything was back to normal. This makes no sense to me and appears counterintuitive and doesn't match the FCOM literature as far as I can tell. Don't know if this is the way it works or the pmdg implementation has a bug

 

The short version is that the plane is programmed to be monotonic: you ascend, and then you descend.  If you descend, you're not going to ascend again.  If you do, you're going to have to do some work to get it to "reset" itself into thinking it's cruising again, and to behave normally in a descent again.

 

Also, cruise levels are in thousands, not 500s: FL310, FL320, FL330, and so on.  That shouldn't have an effect on the plane's ability to work in VNAV, but I figured I'd point it out.  500 offsets are for VFR cruise alts, which, when flying in Class A airspace, are not allowed.

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There are two Boeing options that might be a bit confusing: There's the VNAV ALT option OFF or ON.

 

The PMDG is a VNAV ALT plane. Take a look at the FCOM for how that's handled. I would suggest using the ALT INT button on the MCP rather than LVL CHG, if that fits your scenario. VNAV ALT is still VNAV, but I forget how it transitions to VNAV PATH.

 

It's be best to make sure your CRZ ALT and MCP match up when you're in cruise. If you need to change cruise altitudes for turbulence or step climb, make sure you keep everything aligned.

 

Don't be surprised if you don't operate it as intended you get bad results. It's not an iPhone - they don't try to make it idiot proof - you're not supposed to be in a cockpit if you're an idiot. Imbeciles and morons - yes. Idiots - no. I should know. :Drooling:

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Don't be surprised if you don't operate it as intended you get bad results. It's not an iPhone - they don't try to make it idiot proof - you're not supposed to be in a cockpit if you're an idiot. Imbeciles and morons - yes. Idiots - no. I should know.

 

Exactly.

 

Has Alaska implemented the Dog option yet?  I've heard the Dog Monitoring option really helps to keep the Moronic Crewmember issues to a minimum...

Some airlines are getting excessive calls to scheduling for absence due to doctors appointments for bites, however.

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It's a tazer system - Alaska being cutting edge and all.

 

Thanks for that.  Reminds me of singing this after that new story about that college kid getting tazed back when I was in college, myself...haha:

youtube.com/watch?v=Xzkd_m4ivmc (partial user discretion: mild character peril)

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Kyle/Matt/Oliver- First of all thanks for your replies. I appreciate the time you have spent trying to help.

 

Kyle- After considering your comment about getting the FMC to think it is cruising again and remembering that I received the "chk alt tgt" when I hit VNAV in an attempt to ascend to new altitude after having been in Alt Hold, I scoured the FMOC and determined that when you descend and are left in either ALT HOLD or VNAV ALT the FMC thinks that the new lower altitude is the cruise altitude although the FMC itself stills says the cruise altitude is the original higher cruise altitude. In this case that is why the "chk alt tgt" message appeared since the FMC thinks its cruise altitude is the current altitude leaving the plane between the FMC altitude and MCP altitude. Given this I determined that to properly get back into VNAV after having ascended or descended using a mode that leaves you in ALT HOD or VNAV ALT you must go back into the FMC and set the cruise altitude the the value you want. After doing this you engage VNAV and be left in VNAV Path.

 

Matt-I never try to operate the AFDS in a manner not intended. The problem is the FCOM provides simple case statements relating how each button and mode works but doesn't tell you how to go about performing required maneuvers. The problem is further exacerbated by the fact that no where in the FMOC does it tell you that upon a descent and subsequent altitude hold that the FMC thinks the new cruise altitude is the current hold altitude in contradiction of what the FMC itself displays. And it is a good thing that the flight computers aren't like iPhones otherwise planes would be lost and crashing all the time.

 

 

In any case the method I have settled on for making ATC directed altitude changes is as follows:

 

To ascend to new altitude-Dial new higher altitude into the mcp alt window then hit ALT INTV. This will reset the FMC cruise altitude to the new altitude dialed in and the plan will ascend accordingly leaving you in VNAV Path at the end.

 

To descend to new altitude-Dial new lower altitude into the mcp alt window then hit ALT INTV. The plane descends and holds at the new lower altitude and enters the VNAV ALT mode. To transition to VNAV PATH set the FMC CRZ level to the new lower altitude thereby convincing the FMC this is the new cruise altitude. VNAV ALT transitions to VNAV Path.

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To ascend to new altitude-Dial new higher altitude into the mcp alt window then hit ALT INTV. This will reset the FMC cruise altitude to the new altitude dialed in and the plan will ascend accordingly leaving you in VNAV Path at the end.
 
To descend to new altitude-Dial new lower altitude into the mcp alt window then hit ALT INTV. The plane descends and holds at the new lower altitude and enters the VNAV ALT mode. To transition to VNAV PATH set the FMC CRZ level to the new lower altitude thereby convincing the FMC this is the new cruise altitude. VNAV ALT transitions to VNAV Path.

 

Sounds like you're making this a lot more difficult than it needs to be...

 

To climb, you should climb on your VNAV PATH in VNAV.  Just set the controller-assigned altitude in the MCP ALT window.  Once cleared higher, roll it up to the next altitude and press ALT INTV.

 

To descend, you - again - should descend on your VNAV PATH in VNAV.  Set the assigned altitude in the MCP ALT window and let the plane descend to it at T/D automatically (or if they say "descend and maintain" use the DES NOW function on the VNAV DES page).  If you stop at that intermediate altitude, you don't need to make the FMC think you're cruising at that lower altitude, just roll the selector to the next altitude down and press ALT INTV.

 

This is explained in the tutorials.  I'd recommend running them a few more times until you're pretty familiar with them.  The extra steps you're adding are neither necessary, nor proper procedure.

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Sounds like you're making this a lot more difficult than it needs to be...

 

To climb, you should climb on your VNAV PATH in VNAV.  Just set the controller-assigned altitude in the MCP ALT window.  Once cleared higher, roll it up to the next altitude and press ALT INTV.

 

To descend, you - again - should descend on your VNAV PATH in VNAV.  Set the assigned altitude in the MCP ALT window and let the plane descend to it at T/D automatically (or if they say "descend and maintain" use the DES NOW function on the VNAV DES page).  If you stop at that intermediate altitude, you don't need to make the FMC think you're cruising at that lower altitude, just roll the selector to the next altitude down and press ALT INTV.

 

This is explained in the tutorials.  I'd recommend running them a few more times until you're pretty familiar with them.  The extra steps you're adding are neither necessary, nor proper procedure.

Hello Kyle,

I am guilty of not explaining my issue correctly in the first place. I have no problem with ascent and descent using VNAV with pauses for altitude restrictions. My issue is making altitude changes between top of climb and top of descent when in cruise. At that point it is necessary to make altitude changes per ATC direction or to try to avoid turbulence or head winds. I have gone through the tutorials and many of the Angle of Attack training videos and haven't seen a consistent method of doing this. I have also seen in some videos the attempt at using alt intv with no response from the plane and a bewildered pilot. I won't bother you again except to ask where the proper procedures are documented. I have looked through the FCOM and FCTM for this and have yet to find it. Thanks again for your time and help

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So I would suggest this procedure:

 

For a lower cruise altitude, reset the MCP then reset the cruise.

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The PMDG is a VNAV ALT plane. Take a look at the FCOM for how that's handled. I would suggest using the ALT INT button on the MCP rather than LVL CHG, if that fits your scenario. VNAV ALT is still VNAV, but I forget how it transitions to VNAV PATH.

 

It depends on where you are in relation to the path.  Climbing is easy, set the higher altitude in the MCP and press the ALT/INV button.  It will change from VNAV ALT to VNAV SPD until you reach the cruise altitude set in the FMC, where it will change to VNAV PATH.

 

Descending is trickier.  If you are above the path in VNAV ALT setting a lower altitude will put you in VNAV SPD with idle thrust and it will either revert to VNAV PATH upon intercepting the path from above or level off in VNAV ALT if it reaches the MCP altitude prior to intercepting the path.  If you are below the path and set a lower altitude in the MCP and push the ALT/INV button it will stay in VNAV PATH.  What happens then depends on if the MCP speed window is open or not.  If the speed window is closed the airplane will descend at 1000' FPM until intercepting the path and descend on the path at idle thrust.  If the speed window is open it will go into VNAV SPD and the A/T retards to idle.  With the speed window open it behaves just like level change.

 

Clear as mud.

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