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B777 CON/CLB Switch

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B777 CON/CLB Switch

Old 24th Apr 2017, 11:20
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B777 CON/CLB Switch

Hi there,

Can someone provide me with more information on how the CON/CLB switch work? In what scenarios do pilot use it and what has it got to do with FMS.

What's the difference between pressing the ENG OUT button on the FMS and the CON/CLB switch?

Thanks for your enlightenment
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Old 24th Apr 2017, 14:12
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I don't know the level you are working at so I'll paraphrase/simplify for now -

FWIW CLB/CON switch inhibited below 400' AAL.

If pressed with 2 engines running changes engine thrust limit to the FMC selected climb thrust.

1 engine out increases thrust limit to MAX continuous.

(In reality if your SOPS involve using VNAV it's not a switch much used since VNAV should take care of the engine handling, even if an engine fails on takeoff, as long as the FMS is working properly).

OTOH the FMS VNAV "ENG OUT" function does a lot more than just control thrust or the indications thereof - it's interfacing with the likes of target speeds, makes info on engine out capability available to the crew, can be used to do a "automated" drift down...obviously the full details are in the FCOM.

Last edited by wiggy; 24th Apr 2017 at 16:54.
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Old 25th Apr 2017, 00:23
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Originally Posted by wiggy
1 engine out increases thrust limit to MAX continuous.
Wouldn't it be more correct to say for engine inoperative operations....

If at takeoff thrust, adjusts the thrust to max continuous, which could be an increase or a decrease in thrust depending on what amount of derate/ATM was used for takeoff but definitely a decrease if a full thrust takeoff was done.

Once at climb thrust or cruise thrust, then an increase to max continuous would happen.

I believe that selecting Eng Out in the FMC changes FD commands to a best climb gradient if using VNAV(as is normally done). Not sure if pressing the CLB/CON switch does this.
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Old 25th Apr 2017, 03:03
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MCT is set manually? If so, how? or by the A/T?
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Old 25th Apr 2017, 08:23
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JS

Wouldn't it be more correct to say for engine inoperative operations....
It would, but in my defence I didn't claim to be giving chapter and verse from the FCOM - I did say in response to the OP (because I don't know his/her knowledge base ) "I'll paraphrase/simplify for now"...

extricate - since it seems you haven't got access to the FCOM are you able to give us a clue as to what level you're working at - e.g. flight simmer, or somebody coming up for the rating?

MCT is set manually? If so, how? or by the A/T?
The 777 is "full time autothrottle", so, and again paraphrasing/simplifying if the autothrottle is servicable and armed it sets the thrust in response to e.g. MCP selections, FMC selections, TOGA switch operation, etc etc..

Last edited by wiggy; 25th Apr 2017 at 09:22.
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Old 25th Apr 2017, 12:38
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Preparing for the type rating in a few months time, I've got the FCOM, but cant seem to find anything in relation to CON CLB in FCOM 2. If you can point me in the correct direction, that would be great.

Thanks
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Old 25th Apr 2017, 13:06
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OK, In our (single volume) the basic logic behind the CLB CON button is in the Mode control panel section, "Autopilot Flight Director IAS/Mach Controls" (section 4.10 in our issue), but there's not much detail.

All the other engine out stuff is covered in "autoflight" or "autothrottle" sections. TBH the CLB CON switch that doesn't get used much in normal ops or even in event of an engine failure. I wouldn't lose too much sleep at this stage over what it does and it's functioning is often better explained/understood in the sim or fixed base trainer than in the FCOM...
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Old 25th Apr 2017, 13:12
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I see. Thanks

One ques on MCT on the B777. Just to simplify things, if there is an engine failure on climb out, one of the steps in the checklist is to verify MCT thrust is set after flaps are retracted. How do I verify? Is it an automatic process? I understand there is an option in the FMC to select CON thrust, is that it? Or by selecting ENG OUT prompt in the fmc, thrust will automatically be set to MCT?

Thanks
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Old 25th Apr 2017, 14:25
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99.9% of the time, don't worry about the CLB/CON switch. I suspect many pilots on type are not sure when it would be used(perhaps myself included) as it really never is used. But the button is there and allows you to immediately select the climb thrust setting that you entered into the FMC(the three options being CLB2, CLB1, or CLB), instead of having to make several button pushes in the FMC to achieve the same thing(or waiting for the FMC to automatically set it as it was programmed to do by the pilot prior to takeoff).

Maybe during the takeoff at a high power setting, climb thrust is set at 3000' AAE in the FMC but for whatever reason decide you want the climb thrust setting set in the FMC earlier, press the button and thrust changes to what you programmed into the FMC on the ground.

Maybe you set up the FMC prior to departure so that the climb thrust is greater than the takeoff thrust due to derates and ATM and for example, a selection of full climb thrust(CLB). You decide soon after becoming airborne that you want more power from the engines right away as you can see an aircraft on the TCAS ahead of you at the same altitude. Push the button and the power comes up. Or maybe the setup in the FMC is the more typical reduction in thrust at the thrust reduction altitude and for some reason you want a reduction right away, perhaps for a new low altitude level off. Press the button and the thrust reduces, maybe a bit or maybe a significant amount depending on the takeoff thrust versus climb thrust difference.

The aircraft is smart and knows that one engine is inop. You have lost an engine at low altitude and you have lots of derate/ATM for the takeoff and want Max Cont right away. Push the button. Or maybe you are at a very high thrust setting and you want the good engine to reduce to Max Cont. Push the button. Or patiently wait for the proper speed for it to automatically happen(as is normally done), or make the appropriate selections in the FMC.

Or maybe you are doing pattern work for touch and go's and you want a reduction in thrust soon after takeoff to climb thrust as you will be leveling off soon. Perhaps at 600 feet, you push the button.

As for your question...it is automatic at Vref+78 and you confirm on the EICAS screen by noting that TO(or TO1 or TO2) indication has changed to CON and you might see the thrust levers move and engine indications change. Or you can manually do it in the FMC or you can press the CLB/CON switch both of which are not normally done.

Last edited by JammedStab; 25th Apr 2017 at 14:37.
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Old 25th Apr 2017, 14:46
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Just a thought but it might help if you approach this in another way and rather than getting bogged down with the CLB CON switch have a good look (if you can) at the autoflight takeoff profiles in the FCOM ( and Flight Crew Training Manual if you have it), especially the thrust modes. They explain in sequence what the system should do automatically and also in sequence what modes you'll get/see annunciated for both a normal take-off and an engine out. If you follow the profiles through you'll see CLB CON hardly (if ever) gets a mention...unless VNAV is inop or not selected - which is almost unheard of.

Otherwise it's as jammed stab describes..it's not now the newest aircraft on the block but it's got a fair degree of automation, even when "hand flying" the autothrottle is used and the system controls thrust/thrust selections in accordance with either what you've loaded in the FMS pre departure (e.g. Thrust reduction altitude) and/or various defaults, provided VNAV and Autothrottle are working normally. So on a departure with VNAV/autothrottle engaged and working if an engine fails the system "sees" the failure and changes it's thrust management to it's engine out schedule, giving you CON power automatically, with CON annunciated (at top of EICAS), once flaps are up and /or you are above the flaps maneuver speed....all without a button being pressed.

I'd also agree with JS that generally the CLB CON button only really gets used if you want to intervene and set thrust at something other than non standard times/altitudes, which in turn leads to his point that:.

I suspect many pilots on type are not sure when it would be used(perhaps myself included) as it really never is used.
Agreed. It gets used in the sim on occasions, but I don't think I have ever used the CLB CON button on the line, that's why I'd recommend not getting bogged down in what it does ( and again - it does make much more sense in the sim).

Last edited by wiggy; 26th Apr 2017 at 07:29.
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Old 27th Apr 2017, 02:12
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Thanks for the detailed explanation. Appreciate them both
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Old 27th Apr 2017, 06:13
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The only time we really use CLB/CON in the real world, at my airline at least, is to set climb thrust prior to initial flap retraction, when kept slow by a SID restriction etc. Of course, this can also be achieved by inputting an altitude in place of the flaps in the FMS – in which case the button is once again redundant!
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Old 27th Apr 2017, 06:37
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For the OP - it's covered in the FMC systems, preflight loading section of our FCOM - look for "takeoff reference" page.
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Old 3rd May 2017, 01:23
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Probably answered before but i'll take a stab at it anyways. The most "common" situation you will use this button is in the sim when you've gone around (missed approach), single engine and the missed altitude was captured fast enough before you had time to select another pitch mode (FLCH or VNAV).

i.e lets say your missed approach altitude is 2000 ft (AGL) and it went straight from TO/GA to ALT while you were cleaning up the flaps or going to your 'planned go around flap setting'. In that situation the verification of CON thrust would require you to go and select it in either the THRUST LIM page or using the CLB/CON button as to not bust the 5 minutes T/O Thrust limitation on the engines.

That's basically the situation you will most likely use this button for.

To answer your second question about the diff between using this and the ENG OUT selection on the VNAV page. It's not so much a difference but rather using ENG OUT on VNAV does absolutely nothing if you're not in a VNAV pitch mode. Sometimes (especially in the SIM) if you're going around on a heading and an altitude (great idea to simplify life when you are dealing with non normals and no terrain/obstacles considerations) and you don't want to mess around with VNAV you will select FLCH or just let it capture with ALT as described above. In the former situation, you will find yourself using this button in lieu of selecting ENG OUT on VNAV. If you use FLCH, it will make the transition from G/A to CON for you.
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Old 3rd May 2017, 05:25
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hello,
i am not a 777 rated pilot. just flying the baby boeings (737s) but i can think of a reason for that switch. i may be absolutely wrong about it though.
recently our recurrent training has an exercise of landing gear will not retract after takeoff. possibly due the landing gear lever/handle selenoid (i guess 777 may not have one) or the air ground sensor still stuck in ground. thats a pretty tricky one as loads of things wont happen even though aircraft is airborne. LNAV/VNAV wont kick in, there wont be automatic thrust reduction, aircraft wont pressurize etc etc, as the aircraft still thinks its on the ground.

maybe in 777s in that situation a single button press can bring the A/T in clb or con mode. who knows maybe it will even let VNAV to kick in also. as in 737s in that situation we are not LNAV/VNAV capable anymore.
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