757/767 FMS Fuel / ETA Disappear

[NW3]

Hi All,

Sorry to ask a question without a huge amount of detail, but I was wondering if anyone has come across the following.

A couple of times, when entering data into the 757/767 FMC (CRZ parameters, or perhaps the arrival, or DES parameters or something), I make a change which causes the destination ETA and Fuel Arr to go missing on PROG 1 (in the way they do when it's having a think about a RTE mod). On further inspection it seems that at some point along the route - normally at a seemingly random waypoint - the ETA and Fuel calcs stop, and this carries forward to the end of the route.

I can't remember exactly what I do to get it back to normal (the answer is basically 'fiddle') but things like changing some of the hard heights / speeds / routes towards the end normally do the trick.

My assumption is that it's something to do with the particular descent I have put in - some combination of height and speed at a particular waypoint, or some level / speed being outside a range - but I would just be interested to know if anyone had a concrete reason.

Many thanks in advance for any thoughts,

NW3

[No_Speed_Restriction]

Anything to do with a discontinuity you may inadvertantly have put in?

[md-100]

well it happened to me (not on a B757/767) but the point is as follows..

when you enter an altitude in a waypoint which is not logical to the subsequence wayponts the "Fuel / ETA" Disappear.

E.G:

you enter a STAR whose defautl values are (speed/altitude):

WP A 250/8000
WP B 250/5000
WP C 180/2500
.
.

and you enter in the scratchpad

WP B 250/15000

then the FMS blanks the ETA/FUEL

I think thats the problem
try it and tell me

[NW3]

Sounds good - will give it a go next time LHR starts getting busy :)

Thanks!

NW3

PS N_S_R - I thought of that, and it didn't seem to be the problem. Great minds, and all that, though.

[steve757]

It can sometimes occur when the FMC thinks it cannot make the programmed FL. Try putting in a lower cruise ALT, or ,preferrably, stick in a higher cruise mach no ( 0.82 ) and then speed intervene until the weight reduces. The Fuel and ETA should magically reappear.

[misd-agin]

I'm with Stevie...put in data, eg too high a Mach for the wt/alt, or too high a cruise alt, and it can't compute for parameters outside of the performance envelope.

[Atlantis]

Steve757, you're dead right. But you can type in normal cruise mach (.8 or .81) without selecting anything in the speed window and you'll still get your estimated fuel and ETA for destination.

[757_Driver]

On a similar vein - if you are a long way above or below Optimum FL, I've seen the fuel and ETA's blank - same sort of thing I guess- too far outside its calculation performance envelope.

[Check Airman]

On a similar vein - if you are a long way above or below Optimum FL, I've seen the fuel and ETA's blank - same sort of thing I guess- too far outside its calculation performance envelope.

Does that mean you've run out of fuel?:)

[757_Driver]

Does that mean you've run out of fuel?:)

Yeah, and I'm guessing that's why the ETA's dissapear aswell - if we've got no fuel, then we're gonna be up there forever and never get to the next waypoint!

[misd-agin]

Here's another scenario, if you have a 'vectors' segment, at least on the arrival, it doesn't compute fuel/ETA after the 'vectors' segment.

[misd-agin]

Saw this today - no fuel/ETA's after a 'vectors' segment on the arrival.
Deleted the 'vectors' from the LEGS page and the fuel/ETA appeared.

[757jetjockey]

I have seen this a few times on the 767 going across the pond and flying way above optimum due to ATC etc..the fuel and eta's disappear.

This is due as several peple have suggested already to it beng outside of its performance parameters and usually only occurs with a variable cruise speed set in the fmc ie ECON or LRC. if you intervene at your desired econ/lrc speed as commanded and enter that speed in the fmc it will recalculate everything as it now has a constant speed to work from.

It does not ean however you are going to run out of fuel.

As for the descent profile situation. I hven't come across that one yet, come t thik of it i cant think when i have ever wanted to be 10000' above a star hard height either..