No matter what Flight plan I use, I just don't seem to be able to hit my waypoint altitudes when descending via VNAV. My first few flights I took using the Tutorial by Ryan and all worked bang on.

 

I generate my flight plans with FsBuild 2.4 and then manually enter my Star and set my GS intercept alt on the MCP from the approach plate, just a few minutes before top of descent, but find myself going into a holding pattern when closing on my LOC intercept heading because I am too high and have to get rid of about 3,000 ft to be at my correct alt.

 

I notice all the way down, that I miss my alt at every waypoint. I am obviously doing something wrong, but not that expert to know what it is.

 

Some advise please.

 

Thanks,

First of all, is the aircraft descending to the point you think it is? On the DES page (where you can select a waypoint for OFFPATH DES), is anything set here? I need to check, but it seems whatever is set here overrides anything else. I found this out when descending into Anchorage. I was too high overhead DEST, which got interesting to say the least due to terrain (MAX ANGLE CLIMB to MSA followed very promptly - I have not had that level of screw-up in quite a long time).

 

I was using the OFFPATH DES function which might have been the reason it bust every altitude on the way down (I had noticed it was doing this but I did not appreciate why until I analyzed the situatiuon later - use of OFFPATH DES is my only conclusion).

 

Best regards,

Robin.

What is your cost index and how much fuel are you landing with? Is VNAV engaged along with LNAV or are you going into other lateral modes? Also, how close to TOD are you entering the STAR? Altering the route will alter your TOD points, sometimes dramatically. I've altered the flight plan before only to find that the T/D point disappeared from the map because according to the flight plan alterations I had already passed it. Changing STARS, runways, transition points, etc. can all change the T/D and DECEL points.

Are you entering the correct wind (or any wind, for that matter) into the descent profile? Note: disregard if flying with zero wind, obviously.

Wind entry is not required! People obsess about entering the winds. The FMC may compute ToD point based on zero wind if you don't enter anything, but the next waypoint is always based on current conditions, because even if you did enter *predicted* winds, it could still be wrong vs. actual winds encountered.

 

I just remembered that an oddity I had when I saw this "problem" at Anchorage (and consider this was my first full flight since getting the 777) is that it always showed on vertical path (which is why I didn't manually correct the flight path). I have screenshots from the flight, but none during the descent in question!

 

What would be interesting to know is whether the OP also saw that it was on profile whilst it was doing this.

 

Best regards,

Robin.

Not sure if relevant but during my first sortie I was too high during decent (no winds) then I notice my thrust levers were not in idle. On pulling them back it got her closer to the correct descent path.

Wind entry is not required! People obsess about entering the winds. The FMC may compute ToD point based on zero wind if you don't enter anything, but the next waypoint is always based on current conditions, because even if you did enter *predicted* winds, it could still be wrong vs. actual winds encountered.

 

Don't think that's correct. Certainly enroute, the 777 uses predicted winds entered into the fmc for waypoints far away and progressively blends them with the winds currently being encountered as it approaches each waypoint in turn. I expect the descent is similar. Given that descent and approach is likely to take at least 15mins, winds could clearly have an effect in that time, resulting in the TOD point moving.

 

At anyrate if wind entry wasn't required to optimise descent, there wouldn't be a descent forecast page.

It is a konown problem described in several previous posts.

The cause is that during the firs part of VNAV PROF DESC the throttle levers are retarted to an intermidiate position then the A/T goes to ARM status. Well above flight idle! This is more pronounced when you start your VNAV desc with the "descent now" function of the FMC.

My procedure is to start the VNAV descent, wait a couple of thousand feet and when on the desc PATH I disengage the A/T and retard the throttles fully to flight idle.

NOT PRETTY!

 

Cheers

 

 

It is a konown problem described in several previous posts.

 

Happy you said that as I was finding the same problem as the OP.

 

 

 

Wind entry is not required! People obsess about entering the winds. The FMC may compute ToD point based on zero wind if you don't enter anything, but the next waypoint is always based on current conditions, because even if you did enter *predicted* winds, it could still be wrong vs. actual winds encountered.

 

You may well be correct, but I'd question what is the point at all of entering forecast winds into the FMC if they are ignored. It would make the excercise futile woudn't it?

 

 

but I'd question what is the point at all of entering forecast winds into the FMC if they are ignored. It would make the excercise futile woudn't it?

It improves VNAV performance, but it's not required.

but I'd question what is the point at all of entering forecast winds into the FMC if they are ignored. It would make the excercise futile woudn't it?

 

Well as I keep saying... it doesn't change what you actually encounter. Fuel burn and flight time will be what they are regardless of the predictions entered. Just hope flight dispatch did their homework when calculating the fuel required - that is when the winds matter.

 

At the end, if you fail to meet the fuel on board required at certain waypoints, you divert. No amount of wind entry into the FMS will change the amount of flammable liquid in the tanks. All that matters is the next waypoint. Accuracy of the ETA at destination that is still 10 hours away is nothing but useless information.

 

This is why I find averages to be sufficient - they tend to over-estimate. Either way, you should end up earlier than scheduled, and with more fuel on board than planned. If you are flying the route regularly then you can tweak the figures (which is what airlines do in reality), but without that data, there is not much else to do.

 

On my 9 hour 9 minute flight the other night (CTP), I was 6 mins late departing due traffic delays. By the time I landed I was 8 minutes behind schedule. 6 of those minutes were due to late takeoff, and 2 were due to ATC delay at destination. Not bad considering I only used the average wind component (40 knots tail).

 

I'm also generaly pretty good calculating fuel burn for a flight - recently I produced a bunch of figures for the B763 and compared them with actual fuel plans for real-world routes. I was consistently within 1200 lbs of fuel required for the route, compared with the real plan. My figures were based off observation and much flying. I produced a spreadsheet to compute all the values. The conclusion from that is that our sims are pretty damn accurate when it comes to overall fuel burn and flight time.

 

Best regards,

Robin.

I must admit, I did not put any numbers into the descent forecast page, however i assume from the answers here (thanks all), it should not cause me to bust my altitudes. This is just a totally different a/c to the NGX. If you have an unrealistic descent path in that a/c, you get a FMC message telling you so.

 

Regards,