FMC ISSUE

[Garciamk4 1]

OK I am just learning how to use the FMC, I have gotten as far as programming it understanding the difference between LNAV and VNAV so here we go.

 

I decided to give it a shot I programmed the FMC I took off With LNAV ON once the wheels where airborn i engaged the vNAV to control the climb, then the plane stalled

 

I took over and recovered the stall just about 4000 feet up in the air I've decided to engage the VNAV once again the plane started climbing on its own, so everything started working smothly and the LVNAV was doing its job in directing the plane to its destination and the VNAV was controlling the speed and climb to the preset altitude i programed into the FMC. One FL350 Was reached Vnav did its job and leveled the aircraft

 

I'd DEcided to double checke the FMC and on one of the waypoints I saw that the altitude was set to FL 250 the problem was instead of the VNAV Controlling the decent of the plane to reach its altitude of FL250 I got warning light TCAS off and another warning light FMC/PRST did I forget to do something? PMDG 737/800

[pschlute 37]

If i remember correctly the 737-800 procedure is to "arm" LNAV before take off (you cannot take off with it "on"). After you have reached 400 feet above ground you can switch on A/P and you can select VNAV (you cant arm VNAV on the ground).

 

What height did you turn on the A/P ?

 

Did you complete the FMC pre flight.... no route discontinuities ?

 

What altitude was your MCP set to ?

 

On your descent did you remember to set the MCP altitude? The FMC cannot descend the plane below what this is set to.

 

Peter

[Garciamk4 1]

Ok, Thanks for the tip i dint know you coulnt set the V-NAV On the ground lol

So let me just go over this step by step,

 

(Do not arm LNAV while taking off After climbing 400 feet Engage A/P and VNAV)

 

1.)Right there i found to of my mistakes, LOL

 

a.) I did take off with A/P and LNAV ON

b.) Now i know why the VNAV dint want to kick in haha i was still on the gorund

 

(You must forgive me im still learning the FMC lol)

 

 

2.)OK Lets continue

 

A.1) What height did i Engaged the A/P

 

a.)Id say about 500 FT

 

A.2) Did I complete the FMC pre flight.... no route discontinuities ?

 

a.) Yes, I created the Route Using FSNAV fixed the VOR'S and other Waypoints Converted Using PMDG.DLL And loaded it up into the FMC.

b.) I've completed and checked for any discontinuities (None where found) I cliked on Legs to check the waypoints for discontinuities

c.) I've set the Fuel weight, Reserve,TOC 18000, FL350

d.) I've set the Takeoff (Flaps 15, and Speeds where genirated automaticaly.

e.) Performance i left it on the default setting

 

EXECUTED AND ACTIVATED

Everything went well once i recovered the Stall and RE-Activated the VNAV-LNAV The FMC Worked

 

Then i got close to the decending stage, all i got was 2 warning lights

 

1.) FMC/PRST

2.)TAC OFF

 

As far as the MCP Goes i did not set it lower then what it was (FL350) MPC Setting throughout the entire flight would that cause the TAC Light and FMC/PRST to come on?

 

 

Also one more thing confused about the MCP Should i leave it at FL3500 During the Flight and then Drop it before ths FMC VNAV starts to decend the plane.

 

Reason i ask is cause i read somewhere that the FMC will not Overide the MCP so if im set at FL200 on the FMC and set TO FL3500 on the FMC it wont go past FL200

 

Then i also read that THE FMC cannot go lower then the MCP So if im still set at FL35O when the FMC reaches its decending point it wont decend?

[pschlute 37]

On take off the A/P cannot be engaged below 400 feet radio altitude. When you do engage it, it will follow LNAV if you have armed it on the ground. The climb mode will default to "LVL CHG", so if you want VNAV you must select that.

 

The altitude display on the glareshield/MCP will override any altitude in the FMC. So during climb it needs to be at least at the same altitude as the next waypoint restriction in the FMC. Before descent you need to reduce the altitude displayed until it corresponds with the next altitude restriction in the FMC. If you do this before you reach TOD point the aircraft will descend in VNAV aoutomatically. You will normally get a warning before you are at TOD point to reset the MCP altitude.

 

The two warnings you refer to i have not seen before... are you sure the first one did not say MCP RESET ?

[Gray_Lensman 0]

...

(you cant arm VNAV on the ground).

...

Peter

 

Wrong.

 

You can "arm" VNAV on the ground. In fact Tutorial #1 has you do just that. If conditions are right meaning the 1st waypoint is within just a few degrees of the runway heading you can also "arm" LNAV on the ground. Tutorial #1 also does just that.

 

What might have happened to the OP was that "autothrottle" was/became disengaged for some reason... This has happened to me on more than one occasion and each time results in a stall if not immediately corrected of course.

[pschlute 37]

 

Wrong.

 

You can "arm" VNAV on the ground. In fact Tutorial #1 has you do just that. If conditions are right meaning the 1st waypoint is within just a few degrees of the runway heading you can also "arm" LNAV on the ground. Tutorial #1 also does just that.

 

 

 

Right actually !

 

As the Op is posting in FS9 forum I am assuming his a/c is the FS9 legacy version, not the NGX FSX version from which tutorial you have quoted.

 

The "legacy" FS9 PMDG 737-800/900 does not have the ability to arm VNAV on the ground, as was the situation in real world at the time PMDG created the sim. By the time they made the NGX version the real world FMC allowed VNAV arming on the ground and this is modelled by PMDG in that version. Not in the legacy product however.

[PittsS2B 33]

I don't know what flavor of PMDG 738 the topic starter has, but the version I have (Aerosoft boxed) comes with a terrific tutorial flight between Hamburg and Munich, which is an essential starting point for understanding how an FMC works in conjunction with the VNAV and LNAV functions. There must be a tutorial flight of some kind available in the manual. Trying to wing it without practicing a tutorial is, in my experience, a very frustrating process for FMC-equipped airliners. And trying to move between Airbus and Boeing designs, as you have done, is even more confusing as they have different approaches for accomplishing the same task. I highly recommend picking one airliner and practicing the tutorial until you can do it from memory.

 

My first complex airliner in FS was the Leonardo Maddog, which is very complex and realistic, but the tutorial flight made learning it feasible, after much practice.

[Nandan Patel 10]

Also, you could try youtube for some videos on fmc tutorials and such. That's how i learnt my airbus fmc, i watched an actual real piot program his fmc on his A320.

 

Try it out! The videos help! Once you get the hang of 1 Boeing fmc, you'll basically mastered all Boeing aircraft fmc's.