What else can make the FMC change from DES to CLB phase?

[Hardy Heinlin]

Good evening,

as we all know, when the FMC is in ACT DES, and the E/D point is sequenced or the TO/GA switch has been pressed or the aircraft has landed, the FMC changes to ACT CLB.

Does anybody know whether there are further (undocumented) methods to reactivate the CLB phase?

For example ...

Can the CLB phase reactivate when a new destination airport is entered on the first RTE page?

If so, there must be a rule based on CRZ ALT and certain distances that determines whether the currently active CLB, CRZ, or DES phase has to be reset.

E.g. if you are in ACT DES at FL200, and your CRZ ALT is FL310, and you're approaching London Heathrow, I think the DES phase should remain active when you change the destination to London Gatwick.

But --- e.g. if you are in ACT DES at FL200, with the CRZ ALT at FL310, approaching London, and then change the destination to Moscow, it might be helpful if the CLB phase automatically reactivates.

Or -- what if you activate the other RTE that contains a different destination and a different origin, including a SID? What if you are halfway between Bangkok and Singapour, on an active route from Perth to Tokyo, and then activate the other route that starts with a Bangkok SID and ends with a Singapour STAR? How can the FMC tell whether you want to fly towards Bangkok or towards Singapour?


Regards,

|-|ardy

[Britjet]

Hi Hardy,

Thanks for following up on this - for the benefit of others, and as you know, Gary and I found what may be an issue when we attempted a diversion yesterday. The diversion took place from final approach when in DES mode on ROUTE 1, but without pressing TOGA.
We entered and activated a diversion ROUTE 2. This showed the VNAV mode to still be an ACT DES mode - CLB and CRZ did not show ACT CLB or ACT CRZ, just CLB or CRZ. We were only able to climb on the diversion therefore using basic modes such as FLCH.
You suggested that not pushing TOGA was the cause of this - and so it would seem, but obviously we should be able to get a vertical profile without resorting to this?
We eventually managed to get an active profile after filling in the complete route with landing runway, and succeeded in getting to ACT CRZ with VNAV PTH...
Peter.

[Britjet]

Well, with a lot of these scenarios, the situation would be fixed because you would have to do a DIR TO on the legs page after activating..
Peter

[Hardy Heinlin]

How about this rule?

If in ACT DES, and if the DEST airport has changed by a manual ICAO entry or by activating the other RTE that contains a different DEST, then the FMC will immediately test the following (not waiting for any RTE MODs):

If the direct* distance to the new DEST is such that the current CRZ ALT can be reached within the direct* halfway distance, minus 100 nm, the FMC will go to ACT CLB and will, as always, use the TRANS ALT and SPD TRANS points currently displayed on the CLB page; as usual, the CLB page refers to the ORIGIN airport. Therefore, the TRANS ALT and SPD TRANS points refer to the ORIGIN. If the CLB phase has not been activated by the normal methods (E/D overflight, TOGA, or touchdown), the ORIGIN remains unchanged, or -- if the other RTE has been activated -- the other route's ORIGIN is now used. In either case, the ORIGIN may be hundreds or thousands of miles away from the current CLB restart point. That's not nice.

* "Direct" because in the moment when DEST has changed, the FMC cannot know what the crew wants to modify on the LEGS page. And once the whole stuff is reset, and the crew has made a direct-to to a certain waypoint, the route, luckily, can't get shorter than the direct distance which the FMC already assumed, i.e. the FMC is already on the safe side. Should an unexpected, extreme tailwind occur that shrinks the gap between T/C and T/D, the FMC will just show the usual UNABLE CRZ ALT message.


Regards,

|-|ardy

[Britjet]

I really wouldn't know. It would be useful to just "do it" in the sim and watch carefully.
At the moment there is a good chance that Gary, Triple7 and I will be using the big sim at the end of the month for a personal session so we might be able to check this out..
Peter.

[Will]

Please bring along your GoPro Heros and stick them artfully around the cockpit... ;-)

[Jeroen Hoppenbrouwers]

And outside.... I want to see the crash.  ☺