Vnav approach error.

[Pltskiracer 1]

There is an error in the approach logic when flying a VNAV approach. Normal procedure is to set the MCP missed approach altitude when aircraft altitude is minimum 300 feet below missed approach altitude and the FMC has transitioned to "on approach" mode.

 

The FMC transitions to "on approach" mode for ANY of the following conditions:

- the descent phase is active and flaps are out of up, or

- the aircraft has sequenced to the first waypoint of the active approach, or

- the aircraft is on a direct-to or intercept course-to the active waypoint and the distance to go is less than 12 NM, or

- the missed approach point is the active waypoint and the distance to go is less than 25 NM.

 

When the FMC is "on approach" the following features will be available:

- the IAS/MACH window can be opened and the command speed can be set while Vnav remains in Vnav Path descent.

- the MCP altitude can be set above the airplane altitude for the missed approach. When the MCP altitude setting is at least 300 feet above the current airplane altitude, Vnav continues to command the set speed.

(this feature does not work on the PMDG B777). The PMDG B77 aircraft goes from Vnav path to Vnav ALT. Not correct and would be great to see a fix for this.

 

 

Regards,

Christian

 

[777simmer 117]

There is an error in the approach logic when flying a VNAV approach. Normal procedure is to set the MCP missed approach altitude when aircraft altitude is minimum 300 feet below missed approach altitude and the FMC has transitioned to "on approach" mode.

 

The FMC transitions to "on approach" mode for ANY of the following conditions:

- the descent phase is active and flaps are out of up, or

- the aircraft has sequenced to the first waypoint of the active approach, or

- the aircraft is on a direct-to or intercept course-to the active waypoint and the distance to go is less than 12 NM, or

- the missed approach point is the active waypoint and the distance to go is less than 25 NM.

 

When the FMC is "on approach" the following features will be available:

- the IAS/MACH window can be opened and the command speed can be set while Vnav remains in Vnav Path descent.

- the MCP altitude can be set above the airplane altitude for the missed approach. When the MCP altitude setting is at least 300 feet above the current airplane altitude, Vnav continues to command the set speed.

(this feature does not work on the PMDG B777). The PMDG B77 aircraft goes from Vnav path to Vnav ALT. Not correct and would be great to see a fix for this.

 

 

Regards,

Christian

This works fine for me. Setting the missed approach altitude does not engage Vnav Alt here!

But I think you have to pass through your current level rather quick with the MCP altitude selector to prevent Vnav Alt engaging. I have two keyboard keys linked to increase/decrease altitude so I can change my MCP altitude very fast!

 

On the other hand, I do have the level off problem when I fly a VS or FPA approach. In that case, setting the missed approach altitude does cause a level off.

No matter how fast I wind up my MCP missed approach altitude.

 

I have informed PMDG about this a while back and they said they would fix it in SP1.

[Pltskiracer 1]

It should not matter how slow or fast you wind up the altitude selector on the MCP. If in Vnav path and on approach and 300 feet below missed approach altitude, the logic will never allow vnav alt to engage.

I have flown several vnav approaches and every time i set the Missed approach altitude it captures Vnav alt. Again this is not correct.

If you fly the approach in basic mode; VS or FPA you set the missed approach altitude approx 300 feet above MDA. ALT HLD should not capture at any time. At MDA both F/D's off and PNF's FD back on.

 

Christian

[777simmer 117]

It should not matter how slow or fast you wind up the altitude selector on the MCP. If in Vnav path and on approach and 300 feet below missed approach altitude, the logic will never allow vnav alt to engage.

Yes I know that. I offered my way of linking keyboard keys to the MCP Altitude selector as an option. To try and see if that works for you. I had the up and down arrow linked to the MCP altitude selector from day one. Because I find it easier then trying to wind that MCP selector in a moving cockpit. No because of Vnav!

I never tried setting the missed approach altitude on a Vnav Approach with the MCP selector, I have always used my up/down arrow key. You can either give it a try or not.

Up to you.

 

But as it stands, all is working fine (in Vnav) here but not for you. So you are doing something wrong unless you show PMDG that you only have the problem when using the MCP selector but not when using keyboard keys. In that case you can send them a ticket.

[777simmer 117]

the logic will never allow vnav alt to engage.

 

That is not quite right. Or at least not in case of the real 777s I am familiar with.

 

Examle:

you start the Vnav Approach at 3000ft and set the MDA to 700ft for example.

Now assume your missed approach altitude is 2000ft.

If you set those 2000ft upon descending through 2700ft.....The airplane will level off in 2000ft. (I checked to make sure, and found proof of that in the FCTM).

 

So take that one step further and imagine that the missed approach altitude is now 4000ft.

You set the MDA of 700ft again and start the Vnav approach.

Then at 2700ft you start dialing in the missed approach altitude of 4000ft.

So you wind the Altitude selector up from 700 to 4000ft.

And as you do that, at some point you will move that alt selector right through your own present altitude. lets say that happens at 2600ft.

You are at 2600ft as you dial the altitude selector through 2600ft.

Ok?!

Now how does the plane know you dont want to level off in 2600ft?

I am betting that if you would do this real slow in the real aircraft (as in a second of finger trouble (turbulence) as you dial through 2600ft) the plane will level off just as in the example of setting the 2000ft missed approach too early.

I am betting there is some kind of time delay here in the real and in the PMDG777 that decides between level off or missed approach finction.

I dont know, I might be able to dig something up in the tech manual if you insist, but it sounds logical to me.

 

So if you are too slow (or if the PMDG timing is not long enough) than I could see how that causes a problem for you with the selector and not for me with the keysboard keys.

I have flown several vnav approaches and every time i set the Missed approach altitude it captures Vnav alt. Again this is not correct.

No it is not, you are right. But try dialing faster with a keyboard key and see if that works for you as well.

If you fly the approach in basic mode; VS or FPA you set the missed approach altitude approx 300 feet above MDA. ALT HLD should not capture at any time. At MDA both F/D's off and PNF's FD back on.

Not quite right either. If you leave the altitude at your MDA, then Alt Hold will capture (like for a circling approach).

If you are setting the missed approach, then it should not. In this case PMDG have fogotten to set the time delay function I spoke of above. And as I said in my first post, they know about this and they wrote me that this error will be corrected.

 

Now, since setting the Vnav missed approach works fine here, I obviously did not mention to them that Vnav has this problem as well.

So, if you still have this problem after using keyboard keys to set the missed approach altitude, or if you find you just cant dial fast enough with the selector to prevent a level off, then you need to submit a ticket.

 

If you dont submit a ticket, the problem might still be there even with SP1!

[Pltskiracer 1]

I also happen to be relatively familiar with the real 777's as I have over 7000 hours flying them. About 4000 hours as a Captain. I think I have a pretty good understanding of the way the airplane is flying.

 

I am just informing you that when in "approach mode" and with Vnav Path engaged and when 300 feet BELOW the published missed approach altitude. Vnav Alt will not capture. Its a pretty smart airplane. When you are on approach with Vnav Path and minima set followed by missed approach alt set in the MCP, you can run the altitude selector up and down, slow or fast all day as much as you want. It is "locked" in vnav path and will not level off until you get to minima. The FD's will then command level flight if AP is not disconnected.

 

Example:

 

Start approach at 3500 feet. Missed approach altitude is 3000'.  Approx two miles prior to  the FAF and with Vnav Path or Vnav Alt engaged you set the minima. (The altitude selected and displayed in the MCP window is tied to the minima selected on the PFD) and you open SPD Intervention. This is the only time you can open SPD Intervention and still have Vnav Path engaged. At 2700 feet with Vnav PTH engaged you set missed approach altitude.

 

Christian

[777simmer 117]

When you are on approach with Vnav Path and minima set followed by missed approach alt set in the MCP, you can run the altitude selector up and down, slow or fast all day as much as you want. It is "locked" in vnav path and will not level off until you get to minima. The FD's will then command level flight if AP is not disconnected.

 

It is not locked. If it were locked then setting the missed approach altitude has no use.

(The altitude selected and displayed in the MCP window is tied to the minima selected on the PFD) and you open SPD Intervention. This is the only time you can open SPD Intervention and still have Vnav Path engaged. At 2700 feet with Vnav PTH engaged you set missed approach altitude.

 

Christian

Ah!

So not at 3500 - 300 = 3200 ft but you wait untill reaching 2700ft to set the missed approach altitude of 3000ft

 

Why?

 

Because The MCP altitude and the minima selected on the PFD are NOT tied to each other!

Vnav Alt would engage in 3000ft when you set that BEFORE passing through 3000ft.

 

The only way the set MDA on the PFD is linked to the MCP is that it allows you to dial in the MDA exactly (rounded up to 10ft) rather than rounded up to 100s of feet!

If you dont set MDA on PFD to 710ft, then you can only dial in 700 or 800ft with the altitude selector.

[777simmer 117]

I also happen to be relatively familiar with the real 777's as I have over 7000 hours flying them. About 4000 hours as a Captain. I think I have a pretty good understanding of the way the airplane is flying.

 

I am a real world 777 FO. 8000hrs on type.

Captains can be wrong.

Ofcourse not in real life ;-)

But here on the Forums they can be wrong haha :-)

 

Or I can be wrong this time, and I will take everything back and appologize if that prooves to be the case.

 

But I am betting that if you dial in your missed approach altitude on the real 777 and stop to dial for a second, 100ft before reaching your current flight altitude, Vnav Alt WILL engage.

I have never tried this ofcourse, why would one other than to proove a point in a discussion like this.

But I cant see any other way of making the 777 know that you are setting the missed approach altitude, and not an intermediated altitude restriction, than by a time delay.

(many switches on the 777 also work with a time delay....think about all those non normal checklist that have "press and hold for one second" in them.)

 

Even of I am wrong, if you would try my keyboard key linked to Alt Selector, you will not have this problem anymore in the PMDG. (realistic or not, it solves the problem!)

[777simmer 117]

This is from page 5.40 (Vnav Approaches) of the PMDG777 FCTM

 

"The flight crew must wait until the airplane is at least 300 feet below the missed approach altitude before setting the missed approach altitude in the MCP to avoid level off from occurring during the final approach descent."

 

Which prooves you cant sit there all day and dial in whatever you want.

As long as you are still dialing back and forth nothing will happen (I am sticking to my time delay theory)....but leave that Alt Sel somwehere below your current flight altitude and the aircraft WILL level off

[Pltskiracer 1]

Im a bit surprised you have not done this on a ppc or seen it when flown a vnav approach. Vnav Alt will never engage when " on approach" mode is active and you are at least 300' below the fmc missed approach altitude. It is system logic. Vnav path will follow its computed path to at DA. Airplane will even level off, fly straight to follow the computed path/ altitude restrictions. Irrespectful of what you have in the MCP Altitude window. Try it next time you are doing a vnav approach or in the sim.

 

Thanks for the tip regarding the keyboard key. I might give that a try.

 

Christian

[777simmer 117]

I said this:

"But I am betting that if you dial in your missed approach altitude on the real 777 and stop to dial for a second, 100ft before reaching your current flight altitude, Vnav Alt WILL engage.

I have never tried this ofcourse, why would one other than to proove a point in a discussion like this."

 

Why would I dial up to the missed aroach altitude and then stop dialing for one second 100ft before my actual altitude?

Nobody does that.

Vnav Alt will never engage when " on approach" mode is active and you are at least 300' below the fmc missed approach altitude.

Christian

I know what you mean to say.

 

That once your current flight altitude is 300ft or more below the missed approach altitude, you can set it with the MCP altitude selector and Vnav Alt will not engage.

 

But, what I am trying to explain is that the logic we are talking about here, does not know what the published missed approach altitude is.

 

So if your published missed approach altitude is 3000ft and you start winding the MCP altitude selector up towards it, but then stop/set at 2000ft....the airplane will descend untill 2000ft and at 2000ft Vnav Alt will engage.

How could it not?

That would be like busting the MDA of 2000ft!

I hope you agree with me on that.

 

Why does it level off in 2000ft?

I say because of the time delay feature.

if you stop to dial, then the logic assumes the MDA is reached (MDA seems to fit better than intermediate approach altitude here) and complies with the alt constraint by leveling off.

And the same is true if you dial passed your current altitude so slow (as in pause) that the logic assumes it has reached the set MDA.

 

It is really not any different than having the aorcraft level off at the MDA for a circling approach.

 

The real 777 does not engage in Vnav Alt even if you you wind up towards your missed approach altitude real slow. I agree with you here.

But if you were to pause just before your current altitude, then it will.

And the PMDG777 seems to level of even if you just dial up too slow (my theory).

So the only thing that I see that could be wrong with the PMDG is that the time delay is not long enough.

 

I might do a Vnav approach later to confirm.

[Pltskiracer 1]

The aircraft knows exactly what the missed approach altitude is. This altitude is already pre-programmed in the fmc database. Just look on the legs page. I really do not understand why you are having such a difficult time understanding this. Also, we never fly an Vnav approach to an MDA. We only fly a Vnav approach to a DA if it meets the validation criteria and if the Rnav or Rnav GPS minimums box on the chart (We use lido charts) includes the text "VNAV'. Most approaches do these days and it always validates. Its is the same principle for RNP AR approaches also.

 

A circling approach is a different animal all together. It is recommended to fly an ILS to a circle using V/S. If not it is a bit difficult to change from G/S to ALT. Vnav Path works good. This automatically changes to Vnav ALT at the MDA. You then fly TRK SEL followed by TRK HLD etc during the circling phase. AT MDA set missed approach altitude when Vnav ALT or ALT is annunciated.

 

Christian

[777simmer 117]

The aircraft knows exactly what the missed approach altitude is. This altitude is already pre-programmed in the fmc database. Just look on the legs page. I really do not understand why you are having such a difficult time understanding this.

On our aircraft the missed approach altitude in NOT pre programmed. We have to enter it in the legs page ourselves if we want it to be in there. I am not sure why you are saying I have difficult time with this because it is the first time the legs page come up!

 

Now, whatever you put in the legs page....that has nothing to do with the aircraft knowing anything!

I can choose to set NO missed approach altitude in the legs page at all, and I can still fly a Vnav approach AND set the missed approach without the aircraft leveling off!!

They are not locked or linked or whatever!

 

Or I can choose to write a bogus missed approach altitude in the legs page of 200ft or 10.000ft or FL350, and I can still fly a Vnav Approach and I can still set the missed approach altitude without the aircraft leveling of in Vnav Alt as I dial in that bogus altitude.

No lock or link or whatever!

 

That missed approach altitude in the legs page is for info only.

You fly a missed approach altitude in TOGA, so it does not even level off there, regardless of what I type in the legs page.

Only when I re-engage Vnav after the Go Around and clean up, does the legs page missed approach altitude have an effect.

 

On top of that, you can do the same for a VS and FPA approach (talking real 777 here).

Here too you can dial up to the missed approach altitude without causing a level off.

Or I can use VS/FPA without anything in the legs page at all.

Dont even need the approach in there if I fly the thing raw data, and I can STILL dial up to the missed approach altitude and not cause a level off.

It is not a Vnav specific function!

Also, we never fly an Vnav approach to an MDA. We only fly a Vnav approach to a DA if it meets the validation criteria and if the Rnav or Rnav GPS minimums box on the chart (We use lido charts) includes the text "VNAV'. Most approaches do these days and it always validates. Its is the same principle for RNP AR approaches also.

 

We are not allowed to use the Vnav minimums nor are we RNP approved. We have to use the minimumbox that sais LNAV!

[scandinavian13 5,192]

 

 

I also happen to be relatively familiar with the real 777's as I have over 7000 hours flying them. About 4000 hours as a Captain. I think I have a pretty good understanding of the way the airplane is flying.

 

...of the way your airplane is flying.

 

The Cessna 172 I flew last Monday had a G/A button in it (no joke).  Would it be correct of me to assert that every Cessna 172 had this button?  No.

 

See where I'm going?

 

I have a feeling this is dependent on software version, and the planes you fly have that version.  PMDG's does not.  It's worth submitting a ticket, though.

[777simmer 117]

Anyway, I just did a couple of Vnav approaches.

 

The timing theory goes right out the window (as far as the PMDG777 goes, so could very well be the same for the real 777 as well).

 

If I use right mouse clicks on the altitude selector to the set the missed approach altitude, and halt just before my current flight altitude, then Vnav Alt will engage.

 

Just like you said!

(I am saying the real 777 does the same, but it doesnt matter for now).

 

If I then (after Vnav Alt has engaged) continue to right mouse click to increase the MCP altitude, then Vnav Alt changes again to Vnav PTH once I have selected an MCP altitude that is 300ft higher than where Vnav Alt engaged.

 

As soon as you dial away from your current altitude Vnav PTH re-engages and the approach continues as it should.(one should not wait to long here because in the mean time you are in level flight)

I can even dial in a lower than current altitude in the MCP and that too will re-engage Vnav PTH (untill it levels off at the lower altitude ofcourse). It does not matter, up/down, just dial on.

 

If I wait too long with re-engaging Vnav PTH then the altitude deviation becomes quite large of course.

So large that if I dial the Altitude selector up as before, Vnav SPD engages (instead of Vnav PTH) and it stays like that untill you help the aircraft to dive down (speedbrakes), capture its path again and then Vnav PTH engages again.

 

So it is not timing on the PMDG777.

As soon as you set an MCP altitude that is 300ft away from where Vnav Alt engaged, you will be in Vnav PTH again.

 

Does that work the same at your end?

...of the way your airplane is flying.

 

The Cessna 172 I flew last Monday had a G/A button in it (no joke).  Would it be correct of me to assert that every Cessna 172 had this button?  No.

 

See where I'm going?

 

I have a feeling this is dependent on software version, and the planes you fly have that version.  PMDG's does not.  It's worth submitting a ticket, though.

Hi Kyle, long time no see!

 

Been flying with SP1 I hope ;-)

 

 

I agree, so many different options between operators, I have made that mistake as well!