Q: CLB 1/2 ignored by FLCH?

[rob345 3]

I have a question concerning the operation of FLCH:

Is it correct that if I select a reduced climb thrust CLB 1 or CLB 2 in the FMC, it gets overwritten with full CLB thrust everytime doing a climb/descent with FLCH?

 

You really have to get used to it (besides, it differs from the NGX) escpecially at take-off because full CLB thrust maybe higher than the selected TO thrust, so that engines even spool further up when selecting FLCH change after take-off.

 

Furthermore, there might be a bug with the CON/CLB switch. FCOM states "with two engines operating, changes the engine thrust limit to the FMC selected climb thrust". It doesn't on my side but always selects CLB, although you selected CLB 1/2 in the FMC.

 

Thanks,

Robert

[FLEX1978 373]

The climb button gives you maximum climb thrust or max continuous thrust on a single engine. Also FLCH should give you climb based on whatever derated climb power you have selected, until derated climb thrust washes out.

 

I would normally double check with the FCOM before posting but I'm on my phone. Not 100% sure about the climb button, it may just select the derated climb thrust not CLB

[rob345 3]

Here is another thing I would like to add:

 

When I select FLCH after reaching acceleration/thrust reduction height at take-off (and thrust reduces to pre-selected CLB1 or 2 which is also correctly depicted on the EICAS display), the thrust limit switches to full CLB (so, FLCH still don't consider pre-selected CLB1/2 as descibed), BUT thrust levers do NOT advance for maximum CLB thrust but thrust setting stays more or less the same.

 

Interesting. I would really appreciate if somebody could provide some details to that behaviour.

 

Thanks.

[WYfox22 31]

Just for my own curiosity, why are you using FLCH on departure instead of VNAV? I've never seen that done before, I'm interested.

[FLEX1978 373]

Just for my own curiosity, why are you using FLCH on departure instead of VNAV? I've never seen that done before, I'm interested.

No altitude restrictions, reach cruise altitude and burn the least fuel in the quickest way possible. FLCH uses the maximum rated climb thrust you have chosen to get you to cruise, or IDLE thrust in your descent.

 

VNAV is a great tool in climb to obey restrictions and find you the most efficient climb rate according to what you have punched into the FMC.

 

@ Rob, I am also interested in the 777 climb thrust logic in FLCH, I would have thought CLB1 or 2 would remain If you select FLCH.

[WYfox22 31]

Okay, that makes sense. I can see where that would definitely be advantageous. 

[E6BAV8R 88]

I'll have to test this.  I've actually been having quite a few issues with FLCH and it not working properly - most commonly on descent.  2 out of my last 3 flights I'd punch FLCH and it would begin the descent at the specified airspeed but only descend a margin of the altitude I told it to go and then just level off erroneously.  On my 2nd flight it worked just fine but on the 3rd it messed up again on arrival into SFO.  I haven't figured out why or in what specific parameters it is messing up but I'm going to keep testing it.

[Tabs 2,790]

No altitude restrictions, reach cruise altitude and burn the least fuel in the quickest way possible. FLCH uses the maximum rated climb thrust you have chosen to get you to cruise, or IDLE thrust in your descent.

 

VNAV is a great tool in climb to obey restrictions and find you the most efficient climb rate according to what you have punched into the FMC.

 

@ Rob, I am also interested in the 777 climb thrust logic in FLCH, I would have thought CLB1 or 2 would remain If you select FLCH.

 

FLCH is probably not going to give you "burn the least fuel". FLCH puts the engines to the N1 limit and pitches for the speed in the MCP window. Unless you just happen to set the FMC ECON speed in the window, it's likely not going to be using the optimal climb speed for fuel efficiency. VNAV will always be more efficient because it uses the calculated ECON speed.

I'll have to test this.  I've actually been having quite a few issues with FLCH and it not working properly - most commonly on descent.  2 out of my last 3 flights I'd punch FLCH and it would begin the descent at the specified airspeed but only descend a margin of the altitude I told it to go and then just level off erroneously.  On my 2nd flight it worked just fine but on the 3rd it messed up again on arrival into SFO.  I haven't figured out why or in what specific parameters it is messing up but I'm going to keep testing it.

 

Please give me a specific reproducible example of this if you find one.

[FLEX1978 373]

FLCH is probably not going to give you "burn the least fuel". FLCH puts the engines to the N1 limit and pitches for the speed in the MCP window. Unless you just happen to set the FMC ECON speed in the window, it's likely not going to be using the optimal climb speed for fuel efficiency. VNAV will always be more efficient because it uses the calculated ECON speed.

 

 

Please give me a specific reproducible example of this if you find one.

Hey Ryan, I mean that purely in the sense of reaching cruise altitude as fast as possible whilst ignoring any VNAV restrictions etc