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Pushing MCP ALT knob in VNAV ALT deletes constraint?

Started by Hardy Heinlin, Tue, 26 Jul 2016 20:42

Hardy Heinlin

VNAV CLB phase is active.

I'm flying level at 5000 with VNAV ALT engaged. The next climb constraint is 7000. My MCP ALT is 9000.

When I push the MCP ALT knob, VNAV ALT changes to VNAV SPD.

Should this same single push also delete the 7000 constraint?

Or should this push only delete the constraint if VNAV ALT is not engaged during the push action?
In other words: Is a second push required to delete it?

Experienced pilots will probably say a second push is required. But this may be a misunderstanding because in such a situation the MCP ALT is typically set to the next constraint when pushing the knob to get out of VNAV ALT. And if the MCP ALT is equal to the next constraint, no push will ever delete it.


Regards,

|-|ardy

emerydc8

#1
I hope Peter chimes in too, but I think a second push would be required here. If you are level in VNAV ALT at 5000 and you select 9000 in the MCP with a 7000 restriction on the LEGS page, and you push it once, the first push would release you from 5,000 and get you into THR REF|VNAV SPD. If you didn't push it again, it would level off at 7000 in VNAV ALT PTH. If you pushed it a second time it should delete the 7000 restriction and climb to 9000.

If currently climbing or descending in VNAV, there is no need to push the altitude selector to initiate a climb or descent. Pushing the altitude selector during a climb or descent will delete altitude constraints on the LEGS page. Only constraints between your current altitude and the MCP altitude can be deleted. Pushing the altitude selector will delete altitude constraints even while in FLCH or V/S modes.

QuoteDeleting altitude constraints.
 Altitude constraints are displayed on the LEGS page. Altitude constraints cannot be deleted in the CRZ mode. Altitude constraints between your current altitude and the altitude on the MCP may be deleted. There are 4 methods to delete altitude constraints:
Pushing the altitude selector during a climb or descent.
 Selecting CLB DIR> or DES DIR> on the VNAV pages.
 Manually deleting on the VNAV page 1R. Once deleted, the next altitude constraint appears.
 Manually deleting on the LEGS page.
 If an inadvertent deletion of a speed/altitude constraint occurs and there is no time to re-enter the constraint:
 Select the altitude constraint into the MCP altitude selector.
 Cross the fix at the originally cleared to constraint altitude.
 Select the final cleared to altitude in the MCP and push the altitude selector.

cavaricooper

Hardy-

ALT INTV removes ONE constraint for each push, so yes, it would delete the 7000.  If there were constraints at 7000 and 8000 it would not delete the 8000 constraint, requiring 2 pushes to do so.  That said, I more than qualify as an inexperienced pilot on the 744 ;)

Best- C

PS- I'd wait for Peter or Jon before changing anything... my wife believes I'm daft about our fat girl...
PPS- Whoops, Jon beat me to it and as I mentioned... go with Jon... I only try to decipher & digest manuals...
Carl Avari-Cooper, KTPA

Hardy Heinlin

Carl, thanks, that's been clear to me since day 1. My question is whether VNAV ALT should inhibit this effect :-)

Thanks Jon, my question is just the one I posted above re VNAV ALT :-)


|-|

cavaricooper

Carl Avari-Cooper, KTPA

emerydc8

Hi Hardy,

Not that you don't already have enough to do, but since you brought it up, I did try a situ to see if PSX would allow you to delete a constraint while level in VNAV ALT. Here's the situ out of SFO on the PORTE 3 Departure. https://drive.google.com/file/d/0B-WRh0Hf7VdZRXQ3VFp3T3J3OWc/view?usp=sharing


To set this up, I set a hard 9000' at PORTE and leveled at 7000 with the MCP. When the situ opens, roll in 10,000 on the MCP and altitude intervene. Presuming you should not be able to delete a constraint while level in VNVA ALT (and I don't think you should be able to), it should retain the 9000 constraint at PORTE. Thoughts?

Jon D.

Hardy Heinlin

Hi Jon,

I know. This is intentionally coded like this in PSX because other manuals don't state that VNAV ALT prevents this.

That's why I started this thread :-)

If someone can prove the real FMC won't do this, I'll modify it accordingly.


|-|ardy

emerydc8

I figured you did and I'm glad you raised this. I'd still like to get Peter's thoughts on it. I found this in the Bulfer book:

Quote
Altitude Intervention does 5 things:

In climb, if LEGS page wpt altitude constraints exist between the airplane and the MCP altitude, each push deletes the next constraint, one at a time. A constraint equal to the MCP is not deleted. The same operation occurs in descent - one constraint deleted for each push.

When they say "climb," I don't think they are referring to climb mode but, again, input from Peter would be welcome.


Hardy Heinlin

See my comment in the first post in blue font.

Imagine this: You are at 5000 in VNAV ALT and the next constraint is 7000. ATC clears you to 9000. So you set 9000 on the MCP and push it. You'll get VNAV SPD and a direct path to 9000 by a single push. No need for a second push. That's pretty ergonomic, I would say.

Imagine this: You are at 5000 in VNAV ALT and the next constraint is 7000. ATC clears you to 7000. So you set 7000 on the MCP and push it. You'll get VNAV SPD and a direct path to 7000 by a single push -- and because the MCP ALT is not higher than 7000, the constraint remains intact. That's ergonomic too.

That's how PSX is programmed.

emerydc8

In your first example, I think it will stop at 7000 unless you push it again. If you push CLB DIR, I think it will go to 9000.

Hardy Heinlin

QuoteIn your first example, I think it will stop at 7000 unless you push it again.

Maybe. But why would you want to keep the 7000 constraint if you're cleared for 9000?

You will never wind the MCP ALT farther away than your clearance. So when the MCP ALT is equal to the next constraint, the push will not delete the constraint. It remains intact as desired.

emerydc8

#11
Because on complex departure procedures, you want VNAV to keep you from getting an altitude violation. Other than the US and maybe the UK, if they clear you to 9000 and don't say the magic words like, "delete altitude restrictions," or "climb unrestricted," or "climb now," you are still expected to comply with those altitudes. It's been a long battle between the Europeans and the US/UK on this. I don't know what the status is lately. The point is that unless you wanted to specifically delete those restrictions on a SID -- and there could be many -- I think you would want them to stay.

I don't know if this is still good law, but this does explain a bit of how the UK procedure split from the ICAO Doc. 4444 procedure back in 2007. http://publicapps.caa.co.uk/docs/33/CAP493SupplementaryInstruction201004.pdf


emerydc8

The official Honeywell manual is pretty useless, but sometimes there's a piece of the puzzle that helps you figure it out. Would you agree that the constraint deletion logic in this context should work the same way on a descent as it does on a climb?

Quote3.5.5.1 Constraint Deletion
If the aircraft is actively descending, the pilot may dial the MCP altitude window to an altitude below the current altitude and delete descent constraints. Each time the MCP altitude knob is pushed the next descent constraint below the current altitude and above the MCP altitude is deleted.

Hardy Heinlin

Quote from: emerydc8 on Wed, 27 Jul 2016 02:39
Because on complex departure procedures, you want VNAV to keep you from getting an altitude violation. Other than the US and maybe the UK, if they clear you to 9000 and don't say the magic words like, "delete altitude restrictions," or "climb unrestricted," or "climb now," you are still expected to comply with those altitudes. It's been a long battle between the Europeans and the US/UK on this. I don't know what the status is lately. The point is that unless you wanted to specifically delete those restrictions on a SID -- and there could be many -- I think you would want them to stay.

I see. Yes, that makes sense.


QuoteThe official Honeywell manual is pretty useless, but sometimes there's a piece of the puzzle that helps you figure it out. Would you agree that the constraint deletion logic in this context should work the same way on a descent as it does on a climb?

Sure.

emerydc8

Just to be sure, though, I think we need to hear from Peter ;D

Hardy Heinlin

The one-push one-deletion logic is completely clear. I just wanted to be sure about the VNAV ALT effect on this.

Your point above makes sense; I'll change the PSX code.

emerydc8

Thanks, Hardy. I'm glad you raised the issue, sua sponte. It caused me to break out the books!

Jon D.

Britjet

I think this is all well understood. It is correct in PSX, I am sure, and there is a demo of this in the VNAV section of my videos.
Peter.

emerydc8

#18
Hi Peter,

Maybe the version you used to do your video was before it was changed? When I checked it yesterday I was able to delete an altitude constraint while level in VNAV ALT. Or maybe you were climbing or descending when you tried it.

Jon D.

[ADDENDUM]: I see you were climbing when you altitude intervened. I think it only works when the aircraft is actively climbing or descending. It shouldn't work while level.  https://youtu.be/nUf8bYL7V1U?t=4m28s


Britjet

Sorry, I didn't read Hardy's first post well enough.
In his example I am pretty sure the first push would go to VNAV SPD, and it would subsequently level in VNAV PTH at 7000, without deleting the intermediate constraint. A second push (now in the climb) would delete the 7000 constraint.
Peter.