Boeing TMC/FMC & AP questions
 

Hi,

Could anyone tell me the difference between THR and THR REF thrust modes on the B777 or any other similar Boeing aircraft please?

Also, what is the difference between selecting FL CH as opposed to selecting a new altitude setting and unselecting VNAV on the MCP?

And finally, what is the difference between an aircraft flying with VNAV & LNAV engaged with a.) AP engaged and b.) AP disconnected?

The Boeing Maintenance Manual notes can be somewhat limited (as can my brain).

Thanks is advance.

Not familiar with the 777, but to answer the other two;

If you want to descend/climb you need to be in a mode that will achieve that. FL CH or LVL CH will do that. So will VNAV, when certain criteria are met. Another mode that will do it is V/S. However, you can't just unselect VNAV and make it do it without engaging a mode that will!

In the first scenario the autopilot will follow the VNAV path and the LNAV track, in the second the pilot will have to do it using the control column. If you still have the Flight Director(s) switched on they will give you guidance to follow, or if turned off it will be up to you to use the instruments to achieve the desired parameters.

Hope this helps.

PP

Cheers Pete,

Of course, the AP will follow the FD.

Regarding point 2, yes, I should have mentioned that I'd need to select V/S. But if I chose to drop out of VNAV with the AP connected and a different ALT on the ATL SEL, would the MCD default to ALT HOLD or......?

Are there any advantages/disadvantages to FL CH vs V/S when changing ALT? I guess that FL CH may provide more info to the FMC for trip calculations.

777 is not exactly my cup of tea, so i would try answering the other two questions.

1. We have 3 ways to change our altitude in a boeing...LVL CHG, V/S and VNAV.

VNAV follows the programmed path in the FMC and will follow all the speed restrictions and, where present, the altitude restrictions. That is, it will automatically slow the airplane at 10000 ft to 250 KIAS and so on

V/S will follow the rate of descent or climb as set in the MCP and will follow the rate, even if it means your speed inc or dec as a result. In V/S, the pilot will keep an eye or rather he better keep an eye on the speed.

LVL CHG is the basic mode so to speak. No complicated requirements.....just max thrust if you want to climb, and idle thrust if you want to descent.....and it flies a pitch attitude which will maintain the selected speed.

Now as far their using is concerned, VNAV is used in almost all stages of flight, as it , once programmed, will automatically do what you tell it to do. V/S is normally used for altitude changes of 1000ft or less. And LVL CHG for altitude changes of more than 1000 ft.

2. Main difference between flg with AP engaged and AP disengaged.....in one case, you have to fly the plane and monitor the flt progress while in the other you only monitor .

PS: Sorry for the extra long post.....my post here you see...i guess i got a bit carried away:E...anyways hope this helps

You may write as long a post as you wish, its always gratefully received.

I'm getting the idea now. Its one thing understanding the relationship between the components of a particular system, but knowing how they're actually used in anger is another.

Would still dearly like to know a.) the difference between THR & THR REF and b.) If I chose to drop out of VNAV with the AP connected and a different ALT on the ATL SEL, would the MCD default to ALT HOLD or......?

Thanks again for the education.

I'm not sure if it is exactly the same system, but on the 747-400 THR REF & THR are explained as follows:-

THR REF Thrust is set to the "selected" limit on the EICAS. (This could be maximum power for takeoff or go-around, or whatever has been selected in the FMC for reduced power for takeoff).

THR Applies thrust necessary to maintain requested vertical speed (on a go-around the THR mode can provide thrust up to the [G/A] thrust limit, to achieve a 2000 FPM climb)

Hope this helps

Thanks William,

Finally the penny drops.

A few flight crew members avoided these questions at my own airline!

Thanks again.

777 stuff
 

THR REF vs THR
Autothrottle is commanded to a limit manually set by you in the FMC vs Autothrottle is commanded to maintain a vertical speed (as in the case of a G/A)

VNAV can be armed and disarmed only when not engaged by pressing VNAV again unless arming the APP

So VNAV can only be disengaged in your case by selection of another pitch mode TO/GA, FLCH SPD, V/S, FPA, ALT or G/S (selected by arming APP)

When the aircraft captures the MCP selected altitude the Pitch FMA will become ALT and the ALT Hold light on the MCP will come on.

Listen to the sound of all those pennies dropping.................

Excellent feedback. Thanks for taking time out to explain.

While all those pennies are dropping, one might like to consider...
Hamilton Sundstrand.
These fine folks developed the very first true FMS for wide-body aeroplanes, circa mid- nineteen seventies, that had full, LNAV/VNAV and engine thrust management, all neatly bundled into one nice neat package.

Installed?
Yes, indeed, on the Lockheed TriStar.

Simply the best...ahhh, Lockheed.:E
Sorry, folks, couldn'r resist.

L1011
 

I witnessed one of your beloved Tristars get munched into a thousand tiny pieces last week.

Taken away in 5 tipper trucks.

There there now................