Difficulty Forcing FMS out of VNAV CLB

[emerydc8]

I'll see if I can clarify what is meant here. When you level at an intermediate altitude and install the approach, does it go to CRZ or right through to DES when it reaches the T/D? I have a friend in training right now and he is absolutely certain that it goes to DES mode without re-cruising at some point after installing the next approach, even though he never gets to his missed approach altitude for the previous approach. He can't remember whether or when it goes to CRZ, but it definitely goes to DES without having to re-cruise.

Just a follow-up on this issue. A check airman just verified this in the Atlas sim:

On a missed approach, aside from re-cruising or reaching the higher of your missed approach altitude, or your top MCP altitude, you will not get out of CLB unless you install another approach and either extend the course intercept for that approach or bring the first fix on the approach into 1L (like going direct to the CF, FF or extending from the runway threshold). Then, it will go to CRZ and, depending on where you are, it will cycle over to DES from there.

Just installing the new approach alone doesn't get you out of CLB. So, if you installed the new approach and it was, say, on page 2 of the LEGS page, it would stay in CLB. (When a new approach on the DEP/ARR page is executed, it is displayed at the end of the LEGS pages. When a course fix intercept (6R DEP/ARR) is executed, the course fix is displayed in 1L of LEGS page 1.)

[Will]

Peter is quite right about training philosophy. If at the end of the day, the purpose of the training is to produce good pilots, it should be done in a way that optimizes learning and mastery, not mere survival. You can get there with sadistic instructors and check airmen, but you can there faster and more comfortably without the sadists.

emerydc8, do you guys have a union?

[emerydc8]

I agree with both of you -- it is negative training. The union can't really change how training is done. That's a management right. They usually just deal with the aftermath, when there is a termination hearing over a training failure. If you are a captain and you bust a PC, their policy is to send you back to the right seat for a year. The union could eviscerate the company on this policy if they really wanted. Under augmented crew rules, which is every flight for us, every first officer must be a "fully-qualified captain." I think you can see the non-sequitur here.

To summarize the subject of this post on getting out of CLB mode, it looks to me like there is not much difference between a go-around and a return to your departure airport. CLB mode will shift to CRZ (and then DES) under the following conditions:

1. You have reached your cruise altitude. For a missed approach, when you press TOGA, the higher of your missed approach altitude or your MCP altitude will be your new cruise altitude;

2. You re-cruise (put your current altitude in either the CLB or CRZ page); or

3. You install an approach and put a fix on that approach in 1L (CF, FF, RW). When you are on radar vectors, this is normally done by using the DEP/ARR page and extending the CF intercept at 6R. Selecting a TRANS after pressing DEP/ARR (but not executing) will put the FF in 6R where the CF is normally displayed. You can extend from the FF like you would for the CF, but be aware that on-approach logic doesn't start until the FF when you extend from the FF.

If you choose not to extend an intercept course at 6R and you just select the approach and execute on the LEGS page (you should only execute on the LEGS page), the first fix on the approach will go to the end of the LEGS page. This will not get you out of CLB. You will have to put one of the fixes on that approach in 1L before it shifts from CLB to CRZ.

If the ILS is out of service (a mysterious occurrence on this side of the pond when down to 2 engines), and you are in a position where you don't know where you will intercept the final course, but you want to see where you are with relation to a 3-degree glidepath, after selecting an approach (it could even be the ILS), you can bring the runway threshold to 1L and insert the inbound course in 6R. The FMC should draw you a 3-degree path starting from 50' above the threshold outward. This will put you into CRZ and, depending on where you are, DES. Once in DES, the VNAV deviation scale and vertical path pointer can be used for a descent profile reference. If on a downwind, turning your base at 1000' low (fly up) is a good starting point. The on-approach logic will also allow you to engage VNAV (LVSA) as long as you are within 25 miles of the threshold (I cant imagine why you would not be if down to 2 engines in the sim).

Other interesting notes:

1. DES NOW only works in the CRZ phase of flight, even though you can see it during CLB when you go to the DES page (it can't be activated).

2. While in VNAV ALT, you can re-cruise by entering your existing altitude, executing, and pushing in on the altitude selector knob (altitude intervention). Peter said this doesn't always work. As an alternative method, you can take it out of VNAV (use FLCH or ALT HOLD) and reselect VNAV to get it to display VNAV PTH. This always works.

[Hardy Heinlin]

If I recall correctly, I once read in an old engineering book that DES NOW is available in CLB too, and that you just have to reset the MCP altitude below the current altitude to make it work. It's sort of a panic button. I see no reason why it should be displayed but not functional in CLB.

[emerydc8]

Thanks, Hardy. That is interesting. Does it skip CRZ and go right to DES? If you happen to find where you read it, I would love to pass this information along to my check airman-friend who has been trying this stuff in the sim this week. Maybe when he determined that DES NOW was not available in CLB, he was at the same altitude as his MCP. I'll ask him.

Jon

[Hardy Heinlin]

I couldn't find it in that old book, but I found two other sources:


Honeywell's B747 FMS Pilot's Guide, page 3.5-5:

"... DES NOW prompt is displayed on the descent page when the aircraft is not in active descent. Pushing 6R with the DES NOW prompt illuminates the EXEC key. Pressing the EXEC key ... activates the displayed descent profile."


Bulfer's FMC Guide, page 204:

"DES NOW: Displayed when descent is not active ... If pressed during climb will delete climb and cruise constraints."


No source says "not in active descent and not in active climb", they just say "not in active descent".

[emerydc8]

Good find, Hardy. I forgot to look at Bulfer's book. I did experience something similar in PSX when I installed an approach and pressed DES NOW. PSX deleted every altitude on the LEGS page. Unfortunately, the only thing on the LEGS page at that point was the approach and missed approach waypoints.

So, according to Bulfer, we know of at least one thing that pressing DES NOW does while in CLB mode --  It deletes climb and cruise constraints. But can DES NOW be used to get you out of CLB after a missed approach or a return to departure airport for landing? I'll have to see if I can get some friends to check and see how the sim handles this.

Too bad Bulfer didn't take it a step further and explain under what circumstance you would want to press DES NOW while in CLB mode. The fact that he's saying it does anything at all when pressed while in CLB mode tends to support the position that it might be used to get VNAV out of CLB.

[emerydc8]

Honeywell's B747 FMS Pilot's Guide, page 3.5-5:

"... DES NOW prompt is displayed on the descent page when the aircraft is not in active descent. Pushing 6R with the DES NOW prompt illuminates the EXEC key. Pressing the EXEC key ... activates the displayed descent profile."


Bulfer's FMC Guide, page 204:

"DES NOW: Displayed when descent is not active ... If pressed during climb will delete climb and cruise constraints."

No source says "not in active descent and not in active climb", they just say "not in active descent".

Hi Hardy,

I have been searching for an answer on this for a week. One check airman is convinced that DES NOW doesn't work in CLB mode, no matter what. But a reply to my question on PPRUNE was posted by a pilot who has been right before on complex issues like this. What he said makes sense -- DES NOW will not work in CLB mode if VNAV doesn't know where to descent to. So, if there is no approach installed on the active LEGS page, DES NOW will not work in CLB mode.

He said that if you had already installed the approach for your destination airport, or you install an approach for your return airport (it could even be on page 20 of the LEGS page), it will go to DES mode in this case. Given the information you posted, coupled with Peter's posts, and what I'm hearing from PPRUNE, I think DES NOW will work in CLB mode, but you must have an approach installed somewhere in the LEGS page, and the first fix on that approach must be a lower altitude than your present altitude, otherwise VNAV cannot figure out how to compute the T/D.

If Bulfer is right, when you press DES NOW while in CLB, it will delete all climb and cruise constraints and I would bet VNAV is then looking at the altitude for the first fix on whatever approach it runs into on the LEGS page after the cruise segment. That could be 3000 miles away, but it would still put you in DES mode.

If you take the scenario I originally posted, I was level in VNAV ALT while in CLB mode and I was trying to get it to descend to a lower MCP altitude using DES NOW. If my MCP altitude was the same as my actual altitude and I selected and executed DES NOW (presuming an approach is already in the LEGS page and I am above the first fix), in order to get it to actually descend I would also have to set a lower MCP altitude and press in on the altitude knob. My rationale for this comes from the ENG OUT prompt on VNAV. If you are in CRZ and you select ENG OUT and execute without setting a lower altitude, it will stay in VNAV ALT and you will not descend until you release it by pushing in on the altitude knob. But if you select ENG OUT, then roll in a lower MCP altitude, and execute, it will desend without having to press in on the knob.

Thoughts? I hope it doesn't sound like another chaos of theories!

Jon D.

[Will]

One check airman is convinced that DES NOW doesn't work in CLB mode, no matter what. But a reply to my question on PPRUNE was posted by a pilot who has been right before on complex issues like this. What he said makes sense -- DES NOW will not work in CLB mode if VNAV doesn't know where to descent to. So, if there is no approach installed on the active LEGS page, DES NOW will not work in CLB mode.

Is that because VNAV doesn't have waypoint + altitude to aim for?

I'm thinking of how that works in cruise, as well.

1. Load the situ Basic 014 - Cruising at final cruise altitude.situ.
2. Press RTE, then 6L to load RTE 2. Then enter the fix GALBO in 1L. Activate and Execute.
3. Now there are no waypoints with altitudes, no approach, and no destination airport.
4. DESCENT PATH DELETED will appear. Use CLR to remove the message.
5. Dial the MCP down to FL250.
6. On the VNAV DES page, press DES NOW> and execute.
7. CHECK ALT TGT appears. Use CLR to remove the message.

The aircraft stays in VNAV SPD at FL370.

8. Go to the LEGS page and enter the constraint of FL120 at GALBO.
9. Note that the RTE 1 waypoints appear in cyan. (Is this normal?)
10. Execute.

Now the aircraft throttles back and starts a descent in VNAV SPD.

That was in VNAV cruise.

To try something functionally similar, load Basic 006 - Clean after takeoff.situ. Use the MCP to level off at 6000 feet. Then set 4000 feet in the MCP. Press DES NOW, and the same behavior happens as above: the aircraft remains level at 6000 in VNAV SPD, until you enter a waypoint with an altitude constraint, and then the aircraft begins a VNAV SPD descent.

So in PSX anyway, DES NOW changes the mode whether starting in CLB or CRZ, but no descent happens until the FMC has a waypoint with an altitude constraint. I don't know what the real aircraft does, but the logic as modeled in PSX seems consistent and reasonable to me. And consistent with your quote from Bulfer: in CLB, pressing DES NOW activates the DES mode, and deletes future constraints.

[Hardy Heinlin]

9. Note that the RTE 1 waypoints appear in cyan. (Is this normal?)

This is a bug.

[emerydc8]

To try something functionally similar, load Basic 006 - Clean after takeoff.situ. Use the MCP to level off at 6000 feet. Then set 4000 feet in the MCP. Press DES NOW, and the same behavior happens as above: the aircraft remains level at 6000 in VNAV SPD, until you enter a waypoint with an altitude constraint, and then the aircraft begins a VNAV SPD descent.

Are you saying it is normal to be level at 6,000 in VNAV SPD?

The Bulfer book says that if DES NOW is pressed during climb, it will delete climb and cruise constraints, but I'm not sure there is much functionality to this as it relates to returning to your departure airport. If an approach was installed for your destination airport prior to departure, as is often done, DES NOW should not delete any of the altitude constraints on that approach, since they are not climb or cruise constraints.

But if you installed an approach, you probably installed an arrival too. How would it treat an arrival with descent constraints? What if the arrival for your destination had a crossing constraint at 13,000' and you took off and leveled at 5,000 with the intent to return to your departure airport? If DES NOW doesn't delete the destination arrival constraints, then you will likely have altitude constraints that are higher than your present altitude and I don't see how it would descend in VNAV when it is looking through the LEGS page to your destination arrival constraint that is above your present altitude.

Also, if you installed an approach for return to your departure airport and put the first fix in 1L, there is no need to press DES NOW because as long as you are at or above the altitude in 1L, it should have immediately shifted to CRZ when you selected and executed the approach. At that point, setting a lower altitude and pressing the altitude knob would be just like starting down early. It should release you for a descent.

So, for DES NOW to be of any help on an air turnback, there would have to be no arrival or approach installed at your destination with constraints that are above the altitude that you leveled off. Otherwise, how could it descend?

[Will]

So, for DES NOW to be of any help on an air turnback, there would have to be no arrival or approach installed at your destination with constraints that are above the altitude that you leveled off. Otherwise, how could it descend?

I agree. In the second scenario above, the FMC doesn't have any altitude constraints enroute. So I entered a few. Pressing DES NOW, in fact, deleted them. I think this is useful. If I took off and had some sort of problem and needed to return, I would want my enroute altitude constraints to disappear. DES NOW would allow for computation of a vertical profile from whatever the current altitude is, down to when the approach begins, and like I said, that seems useful to me.

When a turnback is needed, the FMC route will get updated with the new arrival and approach, and as long as those waypoints contain at least one altitude constraint that is lower than the current altitude, DES NOW will change the VNAV mode directly from CLB to DES and the aircraft will descent on command. Sounds reasonable.

[emerydc8]

When a turnback is needed, the FMC route will get updated with the new arrival and approach, and as long as those waypoints contain at least one altitude constraint that is lower than the current altitude, DES NOW will change the VNAV mode directly from CLB to DES and the aircraft will descent on command.

I had originally thought this was a simple issue; but it quickly became complicated. If you execute a new approach, there is no need to use DES NOW. As long as you are at or above the altitude of the approach fix that is in 1L, it should go right to CRZ and then DES (when you pass the T/D range). This will allow a VNAV descent.

If you choose not to install an approach right away for your return, and you decide not to re-cruise, then your option of using DES NOW gets complicated. If you already had an arrival in the LEGS page for your destination airport, those altitude constraints will not be deleted by DES NOW and will likely be higher than where you leveled off to return back to the departure airport.

So, VNAV should not be descending at all if the fix it is looking at is higher than your present altitude. Deleting all the altitude constraints on an arrival thousands of miles away, that you never intend to fly, just so you can get VNAV to look at a lower altitude for descent is much more work than just re-cruising or installing an approach for your return.

I would be happy if PSX would go to CRZ mode whenever you install an approach for your return and you are at or above the altitude in 1L for the approach (CF, FF, RW). That's why the check airman at Atlas always makes guys level off at 3,000' after they lose an engine and request to level off at 2,000'. Since many of the approaches at JFK start at 3,000' and, other than a 2-engine approach, it is taught to extend from the CF, they would never get the airplane out of CLB (absent re-cruising) and would not be able to do a VNAV approach or even use the VPP for reference.

[ADDENDUM]: I should add that if you extended the approach from the runway threshold outward, as is normally done for the 2-engine approach, you will always be above the altitude in 1L and you never have to worry about whether on-approach logic will be active when you need it. If, say, you extended from the CF,  but turned your base inside the CF, prior to passing the wayline that would cause the FMC to cycle and activate on-approach logic, you would not be able to use VNAV for the approach, because when you speed intervened it would go to VNAV SPD. So, if you were down to two engines, extending from the RW (within 25 nm of the threshold) takes care of both getting it out of CLB mode as well as making sure you will have the ability to speed intervene on a VNAV approach and stay in VNAV PTH.

[Hardy Heinlin]

Summary -- Getting out of CLB phase by installing an approach


An airline manual says:

"The VNAV CLB phase will change to CRZ phase when an approach is entered and executed."


Peter says:

"What they are discussing is a diversion, where the destination is modified. I think that if you modify your destination, [...] and THEN select an approach, then [...] it is possible [...] that a CRZ phase will be initialised [...]."


I suggest this function that agrees with both statements:

The VNAV CLB phase will change to CRZ phase when an approach is entered ...
... and the pilot selects the intercept fix by pushing 6R on ARRIVALS page or upselects any approach fix to 1L on the LEGS page
... and the runway is within 25 nm
... and the modification is executed.


|-|ardy

[emerydc8]

I suggest this function that agrees with both statements:

The VNAV CLB phase will change to CRZ phase when an approach is entered ...
... and the pilot selects the intercept fix by pushing 6R on ARRIVALS page or upselects any approach fix to 1L on the LEGS page
... and the runway is within 25 nm
... and the modification is executed.

And you are at or above the fix altitude you extended from in 1L. So if you level off at 2000 and you extend from ASALT (3000) it will not go out of CLB. You would have to recruise if you wanted it out of CLB. What would be the reason for the 25-mile condition? 

[Hardy Heinlin]

When I extend from ASALT (3000), will this automatically set the FMC CRZ ALT to 3000? Can the FMC automatically change the CRZ ALT at all?

The reason for the 25-mile condition: I want to stay in CLB mode when I modify an approach that is 500 miles away and that contains the active waypoint.

[emerydc8]

When I extend from ASALT (3000), will this automatically set the FMC CRZ ALT to 3000?

Yes, but if you are still at 2000, you will stay in CLB.

This is from an email I got from an Atlas check airman on this issue:

When we depart on a normal takeoff with a cruise altitude of say FL 220 for example, the vnav mode will only change to cruise if we put in an approach at our return airport and execute it and our current altitude is at or above the highest approach altitude. So for example in JFK, the highest approach altitude is 3000 feet. If I have not obtained 3000 feet, vnav will remain in CLB mode.

This is why I do not allow pilots to level off below 3000' during training after a takeoff from JFK. They usually try to request this after a V1 cut. If we have obtained 3000 feet and put in another approach, VNAV will change to CRZ mode and a T/D should be displayed on the ND. If no T/D is displayed, push the VNAV button on the CDU, you will find that you are now already in the DES mode.

The reason for the 25-mile condition: I want to stay in CLB mode when I modify an approach that is 500 miles away and that contains the active waypoint.

Understood.

[Hardy Heinlin]

Summary - Version 2:

The VNAV CLB phase will change to CRZ phase when an approach is entered ...
... and the pilot selects the intercept fix by pushing 6R on ARRIVALS page or upselects any approach fix to 1L on the LEGS page
... and the aircraft is higher than the first approach fix altitude -50 ft
... and the distance to the first approach fix is less than 150 nm (typical climb distance maximum)
... and the modification is executed.

Upon execution, the first approach fix altitude is copied to the CRZ ALT.

[Hardy Heinlin]

What if you upselect the runway to 1L before you execute, and the runway constraint is only 70 ft?

Will the EXEC process copy 70 ft to your CRZ ALT?

Or will your current aircraft altitude be copied to the CRZ ALT?

I think it makes no sense to copy the first constraint of, say, 3000 to the CRZ ALT when your aircraft is at, say, FL190 (while the active CRZ ALT is, say, FL310). This would trigger a CRZ DES to 3000. I guess it will copy FL190 to the CRZ ALT and you will get a T/D at FL190 if it isn't already passed.

[emerydc8]

I guess it will copy FL190 to the CRZ ALT and you will get a T/D at FL190 if it isn't already passed.

Good point. I think you are right. If you are above the altitude of the runway waypoint when you extend from it, I guess your current altitude would be your new cruise altitude; so it will go from CLB to CRZ, and when you get within the T/D range, it should shift to DES automatically. Does that sound right?