VNAV: Flip-flop after T/D

[Hardy Heinlin]

Good morning,

let's apply the information in the manuals:

a) When overflying the T/D, the FMC switches from CRZ to DES mode.

b) In DES mode, when the aircraft climbs to the cruise altitude, the FMC switches back from DES to CRZ mode.

(The latter can happen during descent when the pilot enters a cruise altitude lower than the current aircraft altitude.)

And here's the flip-flop. It occurs if there are no special conditions preventing it.

Scenario:

1. Cruise at FL430. Weather, ATC or an engine failure force us to do a cruise descent.

2. We reset our CRZ ALT to FL310. CRZ DES is the new mode.

3. It's a long descent, we're still high above FL310. But we're overflying the T/D now.

4. T/D is passed, DES mode becomes active.

5. Flip: We're still above FL310, so CRZ becomes active.

6. Flop: T/D is passed, DES mode becomes active.

7. Flip: We're still above FL310, CRZ becomes active.

8. Flop: T/D is passed, DES mode becomes active.

9. Flip: We're still above FL310, CRZ becomes active.




A special condition that could prevent the flip-flop might be this: Automatic mode change from CRZ to DES is inhibited when the aircraft is above the CRZ ALT. -- But this is just my theory.


Cheers,

|-|ardy

[Jeroen Hoppenbrouwers]

H'bout:

When going to DES, disarm trigger to go back to CRZ (descend straight through whatever cruise level happens to be in the FMS, cruise level is  now irrelevant).

While in DES, if cruise alt setting is changed, arm the trigger to go to CRZ.

Think in events, not in logic.


Jeroen

[Hardy Heinlin]

Sounds good.

I also considered certain side effects when starting a go-around. But this is no problem anyway: This first changes from DES to CLB (and resets the CRZ ALT to missed approach altitude), then from CLB to CRZ, i.e. DES mode is inactive during missed approach cruise altitude capture.


|-|

[Hardy Heinlin]

Works good, Jeroen.

This new special inhibit status is now also stored in situ files and synchronized in the network.


|-|

[Michel Vandaele]

Hi Hardy,
I have shown a friend flying the B744F for a major cargo airlines, your question and this is what he comes up with.  

Quote
Since you are programmimg the FMC and using this for the vertical navigation I presume you will fly it in VNAV. If I understand the scenario correctly, I would say the following happens:

Cruising at FL430 initiating the descent to FL310 before T/D for FL430 The FMS switches to THR/LNAV/VNAV SPD (LNAV or HDG) and will give you a RoD of approx 1250 ft/min untill FL310 and then switches to SPD/LNAV/VNAV PTH and then recalculate a new T/D.

During the descent the glidepath from FL430 is still shown/used and aircraft will level off at FL310 or intercept this path when the 1250ft/min is not sufficient. In that case FMS switches to IDLE/LNAV/VNAV SPD then HOLD/LNAV/VNAV SPD and the aircraft will accelerate and decelerate with preprogrammed limits to maintain the vertical path untill reaching FL310.

Since this vertical path was already calculated from FL430, you should not end up in a "too high/too low" scenario. In practice it does happen that you'll end up too high or low but then pilot interaction may be needed if required.

I just realize all this is only valid if within 50NM of T/D.

If more then 50NM
"The idle descent path may not be captured since the target airspeed is economy cruise speed and the descent path is based on idle thrust and economy descent airspeed." In this example VNAV will level of at FL310. I had to copy this from the manual since I really have no clue how to put it in words. :-) I could not find it back in the manuals but I was told once that the system will try to reach the next level within approx two minutes. But I doubt that a RoD of 6000 will be applied by the system.

If you overfly the T/D at FL430 the FMS switches to THR/LNAV/VNAV ALT and maintains the altitude. If you dial in a lower altitude (FL310) push the altitude selector the FMS switches to IDLE/LNAV/VNAV SPD then HOLD/LNAV/VNAV SPD and the again he system will try to intercept the descent path by flying faster then the descent speed entered in the system (their is a maximun speed programmed, but I have no idea what that is) when the path is intercepted again FMS changes to SPD/LNAV/VNAV PTH and continues on the descent path. Because the the max speed is capped it might happen that pilot action is needed (higher speed selection or speedbrakes)

Hope this satisfies the question, if not let me know... :-)

Unquote.

I hope this helps

B. Rgds
Michel

[Hardy Heinlin]

Thank you, Michel and your friend.

During the descent the glidepath from FL430 is still shown/used and aircraft will level off at FL310 or intercept this path when the 1250ft/min is not sufficient.

That was the point where my flip-flop started. The idle path could be intercepted, but when 1250ft/min was not sufficient and the aircraft was still above FL310, the FMC switched back to CRZ mode.

Anyhow, the flip-flop is inhibited now. Everything your friend described can be reproduced in the model.

If more then 50NM
"The idle descent path may not be captured since the target airspeed is economy cruise speed and the descent path is based on idle thrust and economy descent airspeed." In this example VNAV will level of at FL310. I had to copy this from the manual since I really have no clue how to put it in words. :-)

Perhaps the quote tries to say that during the idle path capture process the aircraft will overshoot due to the unpredicted high airspeed.


Cheers,

|-|ardy

[IefCooreman]

If I understand the situation correctly, this happens frequently over central Europe for flights with destinations in this region.

ATC rant mode ON. UK, Germany, or France, they will all have you descend early to lower cruising levels if your destination is below Maastricht control, because it simplifies life of Maastricht who otherwise would have to handle you for 5 minutes before you initiate your descend. They are too busy with "overflying" traffic only... even at 3 o'clock in the middle of the night (my record is an exact 1000fpm descend from initial cruising level to glide intercept... very fuel consumption friendly thanks to ATC).

ATC ranting mode off. Consequences: you are asked to descend to FL250/240/180 very early and suppose you initially use cruise descents (more than 50NM from TOD). If you keep your old cruise level in the FMC, the descent path will show up from calculated TOD. If however you change your cruise FL in the FMC to the lower one, a new TOD is calculated, however, there is no automatic transfer to descent fase. The danger is you overfly the new TOD without knowing it. I have to verify it again, because I changed my working method since the 737 because of this, but I'm pretty sure there is no automatic transfer to descend mode in this case. You need to force it yourself.

[Jeroen Hoppenbrouwers]

If however you change your cruise FL in the FMC to the lower one, a new TOD is calculated, however, there is no automatic transfer to descent fase.

To be sure: do you mean

"If you are still cruise descending to your new lower cruise level while you cross your new ToD, you won't get out of cruise descend mode and miss your cue to go to idle descent" ?

If you did reach your lower cruise level in time, the FMS would reset itself and calculate the new ToD properly, right?


Jeroen

[Hardy Heinlin]

In my model I have an E/D donut for the end of the CRZ DES (a T/C for the end of a CRZ CLB), -- a T/D for idle path capture is always computed whenever the FMC has a cruise alitude (and some arrival data), no matter whether the aircraft is in a CRZ DES or not; and when the E/D (of the CRZ DES) is further away than the T/D, I know that I will capture the idle path before reaching that lower cruise altitude.

Ief, is there no green donut on the ND showing the end of the CRZ DES? For a CRZ CLB there is certainly a T/C anyway.


|-|ardy

[IefCooreman]

Hardy, I guess there is, but I hardly ever notice them. My mind just seems to eliminate them. So I would have to verify. The problem i tried to adress is when your newly calculated TOD for your target cruise level is already behind you. I will look for it on my next flight (end of the month).

Jeroen, yes, that's what I meant.

Now that I think about it, what we do a lot as well is an initial descent in VNAV DES mode, then if delay is expected we reset the cruise FL in the FMC during the initial descent. The reason is the difference in speed the FMC uses for CRZ and DES. Depending on the sitution you can have the same problem as mentioned (overflying your TOD of the cruise FL in the FMC without knowing).

[Hardy Heinlin]

We're now talking about the 777 only, right?

Ief, so you don't see a T/D during a CRZ CLB or CRZ DES. The field where the T/D is normally displayed (CRZ page and PROGRESS 1/3 page) is replaced by T/C (CRZ CLB) or by E/D (CRZ DES) or by "LEVEL AT" (E/O DD level off altitude).

In all other flight modes, even before take-off, the T/D is visible, correct? If not correct, we could say that only one phantom waypoint (donut) can be displayed at a time. E.g. the first deceleration donut would be shown only when the T/D donut is passed; the missed approach T/C is displayed only when the arrival E/D is passed, etc.


Cheers,

|-|ardy

[IefCooreman]

I'll try to do some tests when I fly back home (flew today but wasn't PF so couldn't really try stuff out). I'm comparing my 777 manual with a basic 747 fcom that you probably have as well, trying to figure out what we have "extra". I try to be as carefull as possible, because I'm finding some very intriguing 777 FMC behaviour as well. So I'm working on two fronts for the moment :-).

On the MAP display, T/D is always displayed, as well as start and end of deceleration. What is shown on the cruise page is what bugs me. 777 has the same rule that T/D is displayed within 200NM from T/D (and there is an extra "or within 500NM from destination" rule).

[IefCooreman]

In my model I have an E/D donut for the end of the CRZ DES (a T/C for the end of a CRZ CLB), -- a T/D for idle path capture is always computed whenever the FMC has a cruise alitude (and some arrival data), no matter whether the aircraft is in a CRZ DES or not; and when the E/D (of the CRZ DES) is further away than the T/D, I know that I will capture the idle path before reaching that lower cruise altitude.

Ief, is there no green donut on the ND showing the end of the CRZ DES? For a CRZ CLB there is certainly a T/C anyway.


|-|ardy

The green donuts:
T/C: top of climb
S/C: start step climb, either calculated (VNAV page shows "AVAILABLE AT..." for calculated climbs, or "AT -wp name-"). The MAP does not display an aditional T/C at the point where the step climb would end. (I can check next time if the donut would appear when you execute the step as a CRZ CLB, I forgot that case).
T/D: as you mentioned, VNAV calculated descent from CRZ FL starts here, always displayed.
E/D: I can't get E/D donuts displayed for a CRZ DES on our FMC's. The way I understand it is that the E/D donut is only used for the point the FMC uses to start the VNAV PATH calculation (upwards to CRZ FL) from. This point can change depending on the approach stored and selected by the pilot and the donut is a visual representation on the MAP to aid pilots. I could be wrong about the "background" though... but on our MAP the end of a CRZ DES is only shown through the green banana (alt range arc).

We did a CRZ DES, couldn't really ask for one more than 200NM from T/D (that would've been a waste of fuel :-), however ATC got me an unrequested one at 115NM from T/D. The FMC showed "TO T/D" on VNAV and progress pages. I can't show with proof, but I'm pretty sure this "TO T/D" will change to "TO E/D" once you pass your T/D, even in a CRZ DES.

Another quite strange thing I got: when inserting an altitude constraint, ie FL240 at a certain waypoint way ahead of the initial T/D, the FMC will recalculate the path and show a continuous gradual descent from that constraint. Not sure if this is also on the 747, maybe you know more about this? The interesting thing as well, was that the deceleration donuts disappeared from the MAP display... or maybe this is normal and I'm missing something here.

PS: I've compared 747 and 777 manuals, explanation is almost identical, MAP display explanation on donuts is exactly the same, even page postion, lettering,...etc (also the explanation of the fix predicted alt/ETA points that use the same MAP symbol)

[Hardy Heinlin]

The MAP does not display an aditional T/C at the point where the step climb would end. (I can check next time if the donut would appear when you execute the step as a CRZ CLB, I forgot that case).

In my current model the CRZ CLB T/C donut is displayed only when the CRZ CLB has started. Same for the CRZ DES E/D donut: Only displayed when the CRZ DES has started. It's just my theory. I don't know if it's correct. Do you think I should remove them?

My assumption was this: Whatever is indicated in the "TO ..." line on the VNAV CRZ and PROGRESS 1/3 pages, that same TO point is also represented as a donut. E.g. for a E/O DD there is a, say, "TO FL270" in the 2R label and the associated ETA/distance in the 2R line. So my model has to compute the donut position in any case (to derive ETA/distance). Whether or not there is also a donut on the ND, is just a matter of on/off switching in my code.

Another quite strange thing I got: when inserting an altitude constraint, ie FL240 at a certain waypoint way ahead of the initial T/D, the FMC will recalculate the path and show a continuous gradual descent from that constraint. Not sure if this is also on the 747, maybe you know more about this? The interesting thing as well, was that the deceleration donuts disappeared from the MAP display... or maybe this is normal and I'm missing something here.

I've never heard of this feature. Was the descent to FL240 at -500 or at -1250 fpm? Perhaps the path was so shallow -- or the tailwind so nice (777 FMC is smarter than the 747 FMC re winds) -- that a level off at FL240 wasn't economic enough.

Re missing deceleration donuts: I think they are displayed on idle path legs only, i.e. where HOLD | | VNAV PTH is planned. On non-idle descent legs, i.e. in SPD | | VNAV PTH or HOLD | | VNAV SPD (-1250 fpm with some power), they are not displayed. But I may be wrong.


Cheers,

|-|ardy

[IefCooreman]

Tested in flight: no T/C nor E/D dots are shown for CRZ CLB/DES, also not after being initiated. I was flying with a former 747 captain, he seemed to think for a cruise climb the T/C would appear but he was convinced it was the same as on a 777 which turned out to be wrong. Only green bananas so he started doubting.

Asked a 747 F/O and he stated: only green bananas, no 'end dots' for CRZ CLB or DES.

I'll keep hassling 74 people whenever I can :-)

[Hardy Heinlin]

Thank you! :-)


Cheers,

|-|ardy

[Michel Vandaele]

Hello Harry.

I got some additional info regarding the matter from my friend flying the Queens with a major cargo airline.

He tried out and checked the following.
If you descend rather earlier than your T/D in VNAV to a lower level, the system uses around 1250 ft/m to reach it. In FLCH the system try to reach it in 2 minutes.

Hope this info helps

B. Rgds
Michel

[Hardy Heinlin]

If you descend rather earlier than your T/D in VNAV to a lower level, the system uses around 1250 ft/m to reach it. In FLCH the system try to reach it in 2 minutes.

Yes, that's clear.

Thanks anyway! :-)


Cheers,

|-|ardy