PERF/VNAV unavailable Boing 777 prof

[tobbe490]

Suddenly my Boing 777 doesn´t work like before. In the FMC I get the message "PERF/VNAV unavailable" and "Not on intercept heading". Can somebody help me?

Edited February 17, 2016 by tobbe490

[mmerelles]

42k kgs of fuel is given for a +4hs of trip, your cruising ALT FL200 doesn't make sense at all.

can you share me your entire route? sid, trasition, rouute, star, trasition landing runway?

something does not match on your parameters and the FMC is judging it will not make it. all of your waypoints have no vnav information populated (speed/alt), the fmc can not calculate a valid profile

 

a few side tips:

-you have to set the HDG knob to runway heading prior to departure

-if you do not have fuel on the center tanks do not activate center fuel pumps on the overhead panel, you have an alarm trying to suction fuel from a dry tank (fuel low center)

-the speed dial should be set to V2 speed given by the fmc when you set takeoff parameters prior to departure. This is the safe climb out speed in case something goes wrong.

-edit: also your doors are not set to auto, you have an alarm too

 

Edited February 17, 2016 by mmerelles

[tobbe490]

FL 200 was only because I wanted to experiment with different values (I also got the message "cannot reach cruise alt" or something like that) the intended alt was FL280.

The problem for me was I have flown with the exact values before and with no FMC messages. I can give you all the requested information but I dont think it will help.

I will try tomorrow.

Thank you for your concern!

[mmerelles]

My fear is if you tried FL280 and the FMC said "unable cruiz alt" this is most likely due to a short flight, but a very fuel loaded (weight) bird, the FMC was complaining unable to make a valid VNAV profile for such a trip.  You are also entering a low fuel conservative CI which means restricted thrust (aircraft takes longer to climb)

if you share your complete route i can double check instead of guessing and speculating what it could be.

[tobbe490]

Hallo

I wanted to fly from ESSA (Stockholm) to EFHK (Helsingfors) 

Route: NTL - UXETI - LAKUT (airways not in database) Dep. runway 19L, SID NTL2E, Arr. Runway 22L, STAR LAKU2A. FL280. This time I had ca 39k of fuel and it says not enough fuel?

[tobbe490]

I know it is too much fuel but why do I get this message?

[mgeiss]

The reason is that you chose the wrong instance of NTL, which can easily be seen by looking at the distances. You route would have a total lenght of nearly 10.000nm

When there are multiple entries for fixes, navaids or airports with the same name (NTL in this case), the FMC will prompt you with a list, and you need to be careful to choose the right one.

In this case you seem to have chosen the NATAL (NTL) VOR in Brazil instead of the NORTEL (NTL) VOR in Sweden.

Edited February 18, 2016 by mgeiss

[tobbe490]

Oh, I feel embarrassed!

Can this be the reason for every other FMC messages?

Thank you for your help!

Edited February 18, 2016 by tobbe490

[mmerelles]

this time you was entering a wrong route which leads into a 10000 NM trip and insufficient fuel arises as MGEISS pointed out.

 

3 hours ago, tobbe490 said:

Can this be the reason for every other FMC messages?

 

No, now you shared your route i see your trip ESSA to EFHK is 221 NM only. See it here https://skyvector.com/?ll=60.097992270051215,21.298645022262377&chart=304&zoom=4&fpl=M082%20ESSA%20NTL%20UXETI%20LAKUT%20EFHK

 

this is an extremely short flight for this bird meant for 4000 NM to 6000 NM trips average.

if you load the FMC with a 221 NM trip and you put fuel for a 4000 NM trip the aircraft will be SO HEAVY that it will take your entire trip distance just to reach cruising altitude, so the FMC is unable to compute a valid VANV profile. Putting also a CI (cost index) of 20 which is very restrictive on engine thrust will make things even worst.

so, if you are trying to make this flight happen, you should enter 12k kg of fuel aprox and a cruising alt of 270 should work. I am not at the sim know to give you a tested profile.

 

Edited February 18, 2016 by mmerelles

[tobbe490]

Hallo,

Now it works! It was very satisfying. The only FMC message I got was "V Speed unavailable", but I solved it by looking it up on the Internet and found a solution.

I even tried to change heading and altitude and it worked fine too!

Every time I ask something I will learn something new. Maybe someday I can answer questions also.

Thank you for your help!

Edited February 18, 2016 by tobbe490

[mmerelles]

glad to hear you got it up on the skies nicely

 

unable vref speeds background:

yes, i forgot to mention  my fault.

because this aircraft was meant for very loooong trips and heavy loads (pax, fuel, cargo)  the engines are extremely powerful. When you configure the bird for a very short trip you need to derate (remove thrust) the engines considerably so the FMC can computate a safe takeoff. To derate your engines you have pre-programmd profiles under the takeoff fmc page. derate as much as required. This will limit engine thrust during takeoff. You can also derate the engines by cheating the FMC introducing an extremely hot temp above current conditions.