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wilco77
26th Jun 2006, 15:44
Hi all

Could Someone explain to me the exact explanation of the mode FLCH on the mode control panel of the B777.. and what does it do when you engage during climb or decent with respect to vertical speed and performance..

Thanks Alot

Wilco

Hand Solo
26th Jun 2006, 16:42
If it works like the 747 then it will climb or descend at your commanded airspeed to attempt to reach the selected altitude in two minutes. In the climb it will give you power up to your selected thrust limit (CON/CLB/CLB1/CLB2). In the descent it will reduce power to achieve the target in two minutes or will bring it all the way back to idle and just go for it.

Old Smokey
26th Jun 2006, 18:08
In simple terms, it's the same as "Speed Hold" on earlier types, with some refinements.

It will maintain the selected Airspeed or Mach Number, set the appropriate Climb Thrust if the selected Level is above you, set Idle thrust if the selected Level is below you, and level off at the selected level upon reaching.

There's not a lot of difference to VNAV during the Climb, both functions are trying to achieve pretty much the same thing, with some small differences. During the Descent however, VNAV PATH will adjust thrust and speed to seek the desired flight path. FLCH during descent, however, only maintains the pilot selected speed and Idle Thrust, irrespective of the Flight Path.

I don't know about reaching the selected altitude in two minutes Hand Solo! Does that mean that if I select FLCH at 1000 feet after Takeoff with a set Altitude target of 35000 feet, that it will climb at 17,000 feet per minute!:confused: Perhaps you meant that it will achieve the selected speed within 2 minutes? That sounds more reasonable.:ok:

Regards,

Old Smokey

78deg
26th Jun 2006, 20:19
If its like the B767 and B757
FLCH will set ref N1/EPR if the new level cannot be achieved in 125 secs going up, and IDLE if new level cannot be achieved in 125 going down. If can be achieved in less than 125 secs, sets appropriate power to achieve new level in 125 sec

I think the is what the 2 mins are on the B747

mbcxharm
27th Jun 2006, 04:13
...and it's amazing how many 757 pilots I fly with that don't know about the 2 minute thing. We get high in the descent, they wind the speed up and I start to work out how to tell them that the thrust levers haven't completely closed...

BuzzBox
27th Jun 2006, 06:20
I don't know about reaching the selected altitude in two minutes Hand Solo! Does that mean that if I select FLCH at 1000 feet after Takeoff with a set Altitude target of 35000 feet, that it will climb at 17,000 feet per minute!

I believe Hand Solo is correct. For small altitude changes the FLCH logic aims to provide enough thrust to achieve the new altitude in two minutes at the selected speed. That's why the annunciation on the FMA is always "THR" rather than "THR REF" (in a climb) or "IDLE" (in a descent). Obviously, once the thrust reaches the thrust limit in a climb (or idle in descent) it can't do any better so the two minutes goes out the window.

esreverlluf
27th Jun 2006, 09:25
BuzzBox - you and Hand Solo are indeed correct.

Cornish Jack
27th Jun 2006, 20:10
Buzz Box
Were the FMAs you quote for the 777?
Worth noting re. the 47 that you do NOT get an 'IDLE' FMA during a FL CH descent. The sequence is THR, until the levers are fully closed, then HOLD.
'IDLE' is a specific computed thrust level which the FMC can compute - FL CH can't do this.
The 2 minute function is part of the computation for FL CH which is most useful in providing for a non-overshoot capture of selected altitude. As the altitude is approached the Pitch/Power input will be progressively modified to allow this.

mbcxharm
27th Jun 2006, 21:25
I think he means (THR) rather than (THR REF or IDLE) as opposed to (THR rather than THR REF) or (IDLE), if you see what I mean...

On the 757, the autothrottle mode is FLCH then THR HLD, with of course the pitch mode remaining SPD throughout.

BuzzBox
28th Jun 2006, 04:59
Thanks mbcxharm,

I did not mean to imply that IDLE is annunciated during a FLCH descent, on the 744 or the 777.

As I said, 'the annunciation on the FMA is always "THR"...'. As Cornish Jack stated, if the thrust comes back to idle during a FLCH descent, the FMA will change from "THR" to "HOLD".