B744 FMC VNAV question

[Tonic Please]

Recently, with a bit of spare time from translating, I have been feeding a serious hobby of mine, which is advanced aircraft systems within a simulator. I have the "complete" (i know i know, as best as it can be for the comp) FMS and all that for the 744.. and I have some questions regarding VNAV.
It's not a real aircraft, but the FMS has been created to function as a real one, so these quesions could be of help to others with an interest.

1) I run through the procedure at startup on the INIT page, entering aircraft weight, reserves etc, and to cut a long story short, once flying, LNAV works fine, follows the magenta line, goes to waypoints, sets itself up in the hold via info entered into the HOLD page, but VNAV, goodness me. It only seems to enjoy holding the speed, and not changing the altitude as I have entered on R1/R2 etc.. What could be the cause for this?

2) Must changes to altitude be entered before the waypoint is reached, or may you command it to change altitude whilst still on route to the next one? Oh wait, I think I've just answered my own question.

-- Say I'm flying from Pole Hill to Ockham (forget specifics) and I entered, before reaching Pole Hill, in the OCK line, .790/FL230. On route to Pole Hill, it's at .790/FL290. Why would the altitude not change when it transfers to flying to OCK, and, once en route to OCK, could I change this FL230 to FL170 or must something else be done?

3) How do I make the thing fly from the hold, say OCK, and go to a 15 mile approach to EGLL 27R? I go to DEP/ARR, select the runway, and on R3, I am able to enter a distance in the form XX.XX NM. This seems to take me to a fix, and sometimes it automatically tunes the ILS and sometimes it does not.

Could someone enlighten me?

Answers can be complex since I have done much reading on this, so should understand without too much explination of abbreviations.

I look forward to it !
Dan.

[banana head]

Vnav information entered as a constraint on the legs page will not command an altitude change on it's own - because in the real world we cannot climb/ descend without a clearance to do so!
For example if cruising at FL330 over waypoint ALPHA, and you have a constraint of FL290 at the next waypoint BRAVO, you will need to select 290 or lower in the MCP ALT SEL window when cleared to descend.
If you enter the info BEFORE computed TOD, the aircraft will automatically transition to Vnav Path for descent at the computed TOD.
If not then you will need to push the ALT SEL button to engage Vnav Spd - or you could just push FLCH
To sum up, try entering/ selecting the new altitude in the ALT SEL window on the MCP, then push the alt select knob (click on it I guess in Flight Sim??)

BH

[Tonic Please]

I see. Thank you.

1) So VNAV receives and acknolwedges restraints on the LEGS page?

2) What is the principal behind TOC and TOD?

3) What does the FL CH actually mean?

4) What determines the descent/climb rate when VNAV is in control of altitude changes?

all the best

[BerksFlyer]

TOC = Top of Cruise (where you reach cruise level)
TOD = Top of Descent (where you start descent)

FL CH = Flight Level Change (will descend/climb at the speed designated, so it will control the V/S to keep to the desired speed)

VNAV will try to keep to the constraints, so if at ALPHA you need to be at 8000' and BRAVO you need to be at 2000', it will calculate the V/S in a way that you will lose 6000' in the distance to be covered between ALPHA and BRAVO.

Hope that helps?

[Tonic Please]

Thanks.

I know what the TOC TOD are, as abbreviations, but not quite what their function is.

Why would one press VNAV, and not FL CH, or vice versa? As far as I'm currently concerned, they seem to do the same thing based on what you just said...

Is the FL CH taking data from the LEGS page data on the R buttons? (speed/alt)? Just like VNAV is?

I'd really like to get my head around this, and haven't had much luck finding a website with it all, hence my PPRUNE question.

Perhaps somebody could post a regular route they know from heathrow to perhaps paris/ams/manchester and I could give it a go. I've always been interested and this software is allowing me to discover it "for real".

The VNAV, when I select it, seems to go mad, the descent rate comes out at 5000fmp even if it has a long way to go until the next way point, and also, the T/D green circle moves and I can't work out what makes it move.

Questions questions :confused: but responses appreciated so far.

Thanks again...

[mustafagander]

FLCH attempts to reach alt in alt window (MCP) within 120 sec (?) at speed selected in MCP speed window. It cares not about legs page constraints, that's for you to manage with thrust/drag.

Vnav attempts to satisfy legs page constraints - speed/alt - with a constructed path but will not take you away from/through MCP alt. Vnav will descend at a speed to allow it to follow/intercept then follow the calculated path. It will generate "drag reqd" msg or add thrust as it sees fit.

ToD (green circle) is calculated by constructing a path backwards from the lowest speed/alt in legs page allowing for (entered) f/c winds.

[AtoBsafely]

Regarding the holding pattern,

The only ways to leave a holding pattern are:

To select "Exit Hold" on the HOLD page, in which case the FMC will cycle to the next waypoint after passing the hold fix;

or to manually select direct to another waypoint on the LEGS page.

The relevant ILS should auto tune if the ILS approach has been selected on the DEP/ARR page.

[skiesfull]

FLCH is a basic autothrottle/speed contol with altitude acquire and hold. VNAV is an FMC feature which allows for various speed control, such as economy or MEL requirements via cost index etc.. VNAV also allows for constraints in RNAV SID's and STARS and is usually SOP's for departure right through to approach.

[skiesfull]

Don't forget HGD select as a means to exit the hold - more usual in holding situations such as LHR/LGW.

[fox niner]

Het there tonic please,

It sounds like your VNAV mode is working fine. As to the difference between FLCH and VNAV: they are different levels of automation. In both modes the airplane will do the same thing, it will change its flight level. with VNAV it will follow the flight profile as programmed into the FMC. With FLCH it will not follow any profile, it will simlpy get to the next flight level as soon as possible.

Example:

Suppose you're flying at FL350. and you change the altitude in the MCP to 10000. And then you press FLCH: the throttles will close and you descend to the lower altitude. When you get there, the airplane will level off and the throttles will advance to maintain the flight level.
With VNAV, same situation at FL350. When you select 10000 in the MCP altitude window, nothing will happen! Initially.....But when you reach the TOD mark on the magenta line, the airplane will start its descent according to the FMC profile, and will continue to do so until you reach 10000 feet. This may include several level-offs at for example FL230 if this is necessary to meet altitude constraints during descent. (programmed into the FMC)
The TOD (and TOC) marks are not always on the same point. They are variable and change their position, depending on wind (headwind or tailwind?) airplane weight (a heavy airplane needs more decelleration space) icing conditions, outside air temperature, and more.

VNAV takes all this into account. FLCH does not. it simply gets to the selected level ASAP.

[Tonic Please]

Excellent. All of the responses have given more a much better understanding.

The example pretty much concludes my problem! I suppose the opposite is said for the ToC, right? VNAV climbs to it as required.

I do have one more little question, and that is can you limit the thrust power settings for the climb, and/or the rate of climb? Come to think of it, I guess it just does what it needs to, and if it has time, it will reduce climb rate. Perhaps I'm worried about the engines. Does a 3/4 to full loaded 744 really climb at FL180 upwards with 93% power? Even if I use MCP V/S climb, and set it at 500 fpm, at a speed of 290 knots (an A340 captain told me once they usually climb at 290 until .79 mach then go with that ?!),even so, in the cruise at .81, it's at 92/93. Seems pretty high since thats only 10 ish less than take off power!!

:confused:

[fox niner]

Okay, here we go:
Takeoff thrust is close to maximum. (just forget about derated takeoffs for now) At 1500 feet, the VNAV transitions to CLB thrust, which is a bit less, about 5% reduction. It is very, very economical to get to your cruising altitude as soon as possible, so unless you are told to level off by ATC, you want to climb as quickly as possible. There is no reason to reduce thrust. So, 95% is a very normal setting during climb. VNAV will command as much thrust as possible during climb.
Also, as the air pressure decreases with altitude, the N1 power setting will INCREASE. By the time you reach FL250 or so, the power setting will be around 101%. (depending on many factors)
Flight profiles...
Depending on the inserted cost index (see the perf init page) a certain economical climbout speed/descent speed will be calculated.
Example: 316/M.842
This means that the airplane climbs with 250 kts until FL100, then accelerates to 316 kts, and climbs, climbs climbs....At a certain altitude, 316 kts will equal M.842, usually around FL280 or so. It will then transition to M.842 and continue to climb to cruising altitude with this mach number. The airspeed will then decrease to around 280kts at FL350. 280 kts is still M.842 in this case.
FLCH will do the same. you can climb with FLCH at 310 kts (insert in the MCP) and at a certain altitude it will automatically transition to a mach number. (M.84) the third digit is not selectable on the MCP.

During cruise, a B747 will have about 92% N1 at FL350 or so. This is 10% less than its maximum, but don't worry. It is an airplane and not your car. So running it at 92% of its maximum for 12 hours is not going to end in disaster. The engines are built for this kind of work.
Mind you, I fly B777's so the climbout speed example is typical for a B777. It will work the same for a B747 though. Bt they usually have higher airspeeds, but comparable mach numbers.
Good luck!