Flight Level Change 777

[evyjet]

Hey Guys,

I had a discussion with the instructor on my last sim regarding the logic used for FLCH on the B777.

I was under the impression that FLCH attempted to get you to your selected level in 125 seconds, up to either full climb thrust in a climb, or idle in a descent.

I have since searched all the manuals available to me in my current company, but cannot find any reference.

Can anyone confirm this information? I just can't remember where I initially got my info?

The instructor told me I was wrong, but just said there was an "inbuilt logic", however, he didn't know what that logic was. Just curious?

[oz in dxb]

evyjet, that's the understanding I have too.
FLCH is meant to give you a time rated change for climbs or descents.
The figure of 125 secs sounds about right.

[gondi]

When you push the FLCH mode switch on the MCP, the
airspeed window unblanks to show current airspeed. The
TMCF uses the difference between the present altitude
and the MCP altitude to calculate a vertical speed to
capture the MCP altitude. Airspeed does not change. The
A/T control laws command thrust which produces a
vertical speed to complete the altitude change in 125
seconds. The A/T command can not exceed the thrust
limit mode.

From my engineers training notes

[Spooky 2]

Please help me understand this logic because I don't think I have ever heard that figure. If ATC slows you to 240kts and asks you to descend from say 14,000 to 6,000, are you telling me that the aircraft will attempt to do this in 125 seconds. Not likely IMO and doing the opposite going from say 6,000 to 14,000 at an IAS in the window of say 250kts I'm no so sure about that either.

Maybe I'm missing something here, but I don't get the logic nor have I seen any reference to the 125 sec. Please expand a little so I can get onboard.

Thanks

[BuzzBox]

Think of it like this: In FLCH, the system will ATTEMPT to make the level change within 125 secs, at the speed set in the MCP speed window. It will vary the thrust in an ATTEMPT to achieve that. Once the thrust reaches the limit (whether it be IDLE for a descent or CLB/CON for a climb) it can't do any better.

It's designed to reduce the thrust change required for small climbs or descents (ie a couple of thousand feet or so).

[Spooky 2]

Okay, fine. Where do you see this in the FCOM or FCTM? I don't doubt your word at this time but I would like to see a point of reference.

[maui]

Buzz Box. You left out the most important part.

In FLCH it will honor the selected speed. Most important when working in a high density environment with speed control.

VNAV will honor path in preference to speed.

[evyjet]

OK, so my memory hasn't failed me yet. Well, not on this topic at least !!haha

Thanks for all the info guys!

EVY

[SOPS]

I dont doubt you guys..but I fly the 777..and cant find a reference to the 125 seconds thing in any manual...having said that I have trouble finding lots of things

[EMIT]

Refs

Info like this perhaps can be found in the CBT (Computer Based Training) that you did you did for your initial qualification.

[Spooky 2]

Well I have looked high and low through out the current Boeing manuals and can find no reference to this number. It may have been there at some point in time but if so, it's not there now. The most recent CBT have no reference to it either. I have been around the aircraft for 10 years now and do not recall ever seeing it but that means nothing since there are so many things these days I can't recall.

[Willit Run]

I know the 744 is similar, and FLCH has a 2 minute deal. It TRYS to reach the new altitude with in 2 minutes, but will honor what speed is in the airspeed window by function of the elevator. For the love of God, I can't find the figure in my notes either! WTF? It must be a worldwide conspiracy.

[HelloKitty330]

The specific paramaters are not mentioned in detail in the pilots manuals, but are referenced in the engineers manuals

[Spooky 2]

Just talked to one of the more knowledgeable autoflight gurus at Boeing and yes there is a reference to this number in the AFM or some other engineering data that the line pilot is not likely to see in his/her life time.
Simply a number that FLCH looks at as a point of reference for either going down or up and has no real operational application to our routine flight conduct.

[nitpicker330]

Yes it has no real application in the real world

except it will explain why the thrust hasn't gone to full climb thrust or idle during smaller level changes. Those that don't know the logic behind the system will be left wondering what the hell is going wrong?

[bauduin_alex]

You ll find all the answers in AMM 22. In a nutshell, small amount of thrust will be added / removed to reach (interpolation) within 125 seconds selected FL. Help from elevator command will be mixed to thrust command to achieve this. Of course when target cannot be reached within 125 seconds we're either HOLD or THR and back to speed on elevator mode.

[Haroon]

Are you guys talking about this:

AFDS attempts to reach the MCP selected altitude within two minutes if able with available thrust. Otherwise, A/T uses IDLE or CLB thrust to reach the MCP selected altitude.

FCOM vol 2 [4.10.9]

[SMOC]

I was tought this on the 744 back in '98 it's certainly not new and figured any Boeing with FLCH had it.

It's in the 744 FCOM 1.

Autoflight > Controls & Indicators > Mode Control Panel (MCP) > Flight Level Change (FLCH) Switch.

AFDS attempts to reach MCP altitude within 2 minutes using available thrust. Otherwise the A/T uses CLB or IDLE thrust to reach the MCP selected altitude.

And it's in exactly the same place in the 777 FCOM.

[wiggy]

SMOC

Like you I was taught this on the 744 back in*** in my case, but my 777 FCOM
2, page 4.10.9 is "silent" on the issue of timing. Guess our "lot" decided it was something else we didn't need to know and did an in house edit .

[ajd1]

It's also 4.10.9 in the 787 FCOM.