Missed approach and VNAV

[Hardy Heinlin]

Good morning,

rather a cosmetic problem ...

Scenario:

Shortly before landing, I transit from approach to missed approach, and the VNAV phase changes from DES to CLB. During the CLB I attach a new approach procedure after the last missed approach waypoint, which is a hold.

So far no problem.

When reaching the missed approach altitude, VNAV changes from CLB to CRZ. As the T/D for the new approach is 2 minutes away, I get the RESET MCP ALT message.

I have the feeling that the RESET MCP message shouldn't appear if a hold is to be flown before the T/D.

Has anyone on the real deck ever seen the RESET MCP ALT message if a hold is in the missed approach?

If you have any hint, thank you!


Cheers,

|-|ardy

[Michel Vandaele]

Hi Hardy ,
I just checked it with my friend flying the big cargo's

You sure it isn't a procedureturn ?

He couldn't remember that he ever noticed that message when he flew a missed approach followed by a hold. But it is something you don't do so much in the daily routine, so to be sure it should be re-created in the sim

B. Rgds
Michel

[Hardy Heinlin]

Hi Michel,

not a procedure turn. A hold.

The reason why I think there should be no reset message is this: a hold at a fixed altitude is a hold at a fixed altitude.

And the termination of the hold is manual (in most cases).

The FMC doesn't know when the pilot will terminate the hold, so the FMC shouldn't tell the pilot to reset the altitude for the descent.


Cheers,

|-|ardy

[Michel Vandaele]

Hi Hardy,
Yes I can follow you for 100%.
That was also the opinion of my friend.

See you
Michel

[Balt]

From memory, RESET MCP ALT only appears in the approach and below a certain RA (2500?). If you're in a hold, you're not in the approach phase, hence you should not get that message I think.

Cheers

- Balt

[Jeroen Hoppenbrouwers]

No Balt, definitely not, the most typical appearance of RESET MCP ALT is at cruise altitude about two minutes before FMC calculated ToD. You may be thinking about the memory procedure to wind the target MCP altitude up above the current altitude (through the current altitude) when on the glide slope during final approach and setting up for a possible missed approach. But this does not generate an FMC message.


Hoppie

[IefCooreman]

If  I understand correctly...

When initiating the go-around, the FMC resets the target cruise altitude to the highest altitude in the missed approach procedure. The holding is usually the last point, and the altitude is equal to the final missed approach altitude. A new TOD is calculated based on this holding altitude so there would be no reason to show the reset MCP altitude, because both will be the same.

Now as for the 777: I think if you set a higher MCP altitude, you need to push the selector to force the FMC cruise altitude to change to the higher level. In this case the target holding altitude will change as well, although this needs to be confirmed because I've never done this before. (In reality a missed approach is pretty much never done down to the last line. You either want to try again as quickly as possible, get vectors with lower altitudes, or you divert immediatly and climb all the way up for your diversion.)

Other question that comes to mind: is the holding altitude a restriction? No (I think). As far as I know the holding can be a climbing tool as well (although the continuous turning limits the climb performance of the airplane so it is avoided). In a missed approach, the final holding can have a minimum altitude that is lower than the final missed approach altitude. This allows for the climbing if necessary.

Cheerz!

[Hardy Heinlin]

A new TOD is calculated based on this holding altitude ...

... and based on the attached new approach which provides some more legs and at least one altitude constraint below the CRZ ALT so that the FMC can compute a descent profile from that lowest point on backwards up until intercepting the CRZ ALT where the FMC then puts the T/D for the new approach.

I think, if the HOLD is part of the missed approach and if it must be terminated manually, the RESET MCP ALT message won't occur; it will occur only if the HOLD is part of an approach (before the runway) and the HOLD type is an auto-terminator that exits after one round or at a specified altitude, in that case the time to the T/D can be predicted and the message will make sense.


Cheers,

|-|ardy

[Balt]

Hoppie, you are of course 100% correct. Not sure what I was thinking! It's been too long since I've been in the drivers seat, I should just shut up. Sorry for misleading the pack....

Cheers

- Balt

[Michel Vandaele]

Hi Hardy,
My friend gave me some addional info concerning above matter.
-
Concerning the T/D : When there is a cruise altitude in the VNAV CRZ page it is normal that a T/D is shown. Once passed it and entering a hold VNAV PTH should change into VNAV ALT. similar to overflying a T/D on a normal path. LNAV will track the hold.
-

Hope this helps.

B. Rgds
Michel

[Berni]

Hi,

as far as I can only speak about the 737NG, I would say, this is what we would see in reality.


After 6 years of active flying and about 6-8 Go-arounds so far in reality and many in the Sim I never had the situation, that the missed approach altitude was higher then the hold.. So maybe it´s not modeled correctly by Mr. RR or the 737 NG isn´t as clever as a 744 FMC... or... that is what you get in the real thing!

I will try to remember this for my next sim session in June.. of course 737 and also no VNAV Alt simulated, as we don´t have this feature at X3..

Berni