FLCH or VNAV (Lufthansa video)

[alistomalibu]

Frankfurt - Los Angeles [1080p] Boeing 747 - landing - YouTube

I have just watched this video and I would like to know if you guys would use FLCH or VNAV for this arrival.

As you can see in this video, the pilots are using FLCH mode and they are modifying the MCP altitud once they pass every fix of the arrival.

They didn´t get a last altitude or speed change request. So, I suppose they have planned to do so in advance.

They got the authorization to "descend via" the arrival.

Thanks

[STBYRUD]

Why not? Havent looked at the video closely, but its possible that the VNAV computed path is undesirable, dont know if the B744 also likes to go bonkers on VNAV when setting flaps like the 737 used to in earlier software revisions ;)

[Intruder]

If the arrival is in the FMS database, and all the limiting altitudes are coded with the fixes, VNAV is the preferable descent mode. VNAV will give as close to a constant descent angle as is possible with the altitude restrictions.

The 744 works just fine with gear and flap changes in VNAV. Opening the Speed window and manually setting speeds makes for smoother transitions.

[gorter]

The video isn't working for me so can't comment on the finer details. However I fly the 757/767 and have been lead to believe that the 747 autopilot/mcp is of a very similar design.

In my experience, VNAV just becomes a bit klunky below 10,000'. It is less able to anticipate ATC instructions (and how can it, when have you last flown the plate?) FLCH just gives a nicer feeling for the pax and greater control for you as pilot.

VNAV is great for climb/cruise/initial descent, but lower down our brains are just better computers. If we could programme the winds exactly and fly the full arrival everytime, VNAV would be perfect, but that's not real life.

[alistomalibu]

I agree. FLCH gives greater control.
They change the MCP altitude between 500 and 700 feet prior to the current MCP altitude to avoid an ALT HOLD.
I find it a good flying technique.

[Intruder]

With proper planning, VNAV works just fine in the 744, especially when flying an RNAV STAR. I fly them quite often, and have found no significant problems. I usually don't use FLCH on an RNAV STAR unless ATC intervenes.

[Pontius]

With this specific LAX approach FLCH seems the preferred option. I say that only because VNAV normally requires a spot of speedbrake to keep you on the path and speed, whereas FLCH doesn't normally need it. I've done it quite happily in VNAV with a reasonably strong headwind and it's managed without speedbrake but most of the time the winds are not strong enough to enable that. Of course, not using speedbrake is purely a pax comfort thing and if I needed it I'd use it but that doesn't seem to be the case in FLCH on this arrival, so why not keep it less rumbly down the back? If you notice on the video at around 1:40 he's at the correct speed but about 1100' high on the VNAV path. To correct that and stay at his assigned speed he'd need a bit of drag but in FLCH he has a nice, constant descent, abides by the altitude and speed restrictions and all without using the speedbrakes. All very nicely done, I reckon, and probably shows that whereas VNAV is perfectly okay to use, FLCH worked better on this particular day.

[alistomalibu]

Thanks Pontius, nice to hear your point of view.

[Dariuszw]

I always had trepidations of flying vertical speed mode during climbs and FLCH mode during descends especially in IMC due to potential for accident in case of engine failure. During climb in vertical speed mode auto pilot will maintain given rate of climb despite luck of power wchich could lead to stall and during descend with FLCH AP will compensate lost power with pitching down in order to maintain assigned speed. Perhaps thats not the case with bigger jets.

[hptaccv]

it is Lufthansa procedure to set the next constraint altitude only in the altitude window - a precaution to avoid altitude busts, vnav or flch.
I personally prefer flch to vnav..

[alistomalibu]

Ok, yes, I was thinking about the idea that maybe it was a procedure of Lufthansa.

I wanna present the following example:


You must overfly point A and B.
Between these two points, you have 20 miles.
The arrival tells you, that point A must be overflown at FL250 and point B at FL220.
After point B you must continue the descend.

So, you have to descend 3000 feet in 20 miles.
Before A, you are flying in ALT HOLD mode maintaining FL250.
Once you pass point A, you set the new altitude in the MCP alt window (FL220) and FLCH is engaged, but you can see, that the green arc of the ND is placed before point B.
In other words, if you continue with FLCH mode, you are going to reach FL220 a few miles before point B.

What do you do?

1- Continue with FLCH until FL220 and then you continue to point B in ALT HOLD mode.

2- You change to VS mode and modify the vertical speed, in order to place the green arc over point B.

[Intruder]

3. Change to VNAV and let the FMS calculate it all for you, arriving at point B exactly at FL220 and transitioning seamlessly to the next descent segment (if you dial down the altitude window).

With a properly set up CDU, I have not had any problems with RNAV arrivals done in VNAV. I don't know why others have so many problems.

FLCH and V/S are valuable when on vectors and when VNAV "falls behind" due to a shortened route. Use the proper mode for the situation.

[OD100]

Pretty much puts to rest the need for the 'turn off PED' briefing!