Hey everyone! First time poster longtime lurker/NGX flyer. Not so much a technical question but mainly curious on the procedure of arming LNAV/VNAV prior to departure. When is it common practice to do so? Is this mainly an airline SOP, personal pilots preference, or a procedure that is dependent on the type of SID being flown? Me personally only arm the modes when flying a RNAV SID. Just curious on what other pilots on the forum do.

 

Jeff Cowles

This is VERY operator specific.  Some will not engage either until fully cleaned up and pointed in the right direction.  Some use LNAV for the departure.  Some action both (with later FMC revisions).  The thing to remember is that even if you take away all the automation, she still flies beautifully.  It's all part of your took-kit, but you decide what and when to use.

I'm not a real pilot but I never arm LNAV nor VNAV prior to takeoff simply because a lot of times I can press the buttons but the lights on the buttons won't go on.

I'm not a real pilot but I never arm LNAV nor VNAV prior to takeoff simply because a lot of times I can press the buttons but the lights on the buttons won't go on.

 

That's because the conditions in which they would become armed haven't been met, so the system isn't allowing them to be armed.

 

I typically arm them prior to departure when the SID I'm flying has very quick turns or altitude restrictions immediately after departure, otherwise I usually get runway heading from ATC and just use LVL CHG.  You can always just arm VNAV, so that you'll get climb speeds and Acceleration Altitude per the FD, and then just manually adjust heading based on ATC instruction.

I usually activate LNAV and VNAV when I'm 

 

a ) No longer above the airport

 

b ) 500 feet above the terrain I'm flying over

 

Both restrictions have to be met before I arm either of the two.

That's because the conditions in which they would become armed haven't been met, so the system isn't allowing them to be armed.

 

I typically arm them prior to departure when the SID I'm flying has very quick turns or altitude restrictions immediately after departure, otherwise I usually get runway heading from ATC and just use LVL CHG.  You can always just arm VNAV, so that you'll get climb speeds and Acceleration Altitude per the FD, and then just manually adjust heading based on ATC instruction.

 

Thanks for letting me know. Do you know what those conditions are? Sometimes there is a turn almost instantly after I got airborne according to the SID I've selected but still, LNAV won't arm.

Thanks for letting me know. Do you know what those conditions are? Sometimes there is a turn almost instantly after I got airborne according to the SID I've selected but still, LNAV won't arm.

 

LNAV will only arm when you are within 20 or 30 degrees of your first waypoint I think. I don't know the actual number, but if you aren't within that number of degrees in terms of heading away it's not going to arm.

Guys, this is Intro Manual fodder in the 737.  Seriously.  Take a few minutes and read it.  Sure, it's 131 pages, but you'll learn a ton from even spending 10 minutes in it just browsing it.

 

Not surprisingly, one of the sections in there is a bunch of notes about restrictions and quirks they had a feeling people would always ask about...like this one:

 

I couldn’t get LNAV to arm/engage: LNAV has some specific parameters that must be met in order for it to arm/engage. These are: 

• To arm LNAV while on the ground: The first waypoint in your flight plan must be within 5 degrees of the departure runway course.

 

 

 

Guys, this is Intro Manual fodder in the 737.  Seriously.  Take a few minutes and read it.  Sure, it's 131 pages, but you'll learn a ton from even spending 10 minutes in it just browsing it.

 

Not surprisingly, one of the sections in there is a bunch of notes about restrictions and quirks they had a feeling people would always ask about...like this one:

 

 

 

 

OH so it's 5 degrees away, sorry for my mistake everyone.

Yep - 5 degrees. VNAV will always arm if there's valid performance data entered. On the NGX the most common reason for it not arming is failing to EXEC the PERF INIT page.

I used to arm VNAV and LNAV, but now I am using FS2Crew, the F/O does not let me select it. Or he will switch it off if I try and switch those modes on.

I used to arm VNAV and LNAV, but now I am using FS2Crew, the F/O does not let me select it. Or he will switch it off if I try and switch those modes on.

hmm, that is odd. I also use FS2Crew and am still able to arm LNAV/VNAV. Once airborne and when the those modes become available, the F/O calls them out.

 

Jeff Cowles

 

 

I can press the buttons but the lights on the buttons won't go on.

 

Forget about the lights already - those mean something completely different. Always look at the PFD for active and armed modes.

Hi Jeff,

 

 

Not so much a technical question but mainly curious on the procedure of arming LNAV/VNAV prior to departure. When is it common practice to do so? Is this mainly an airline SOP, personal pilots preference, or a procedure that is dependent on the type of SID being flown?

The use of the LNAV/VNAV arm is not SOP at my company. However, generally, once we have received the clearance for an SID LNAV and VNAV are armed. If we receive a clearance for a heading, then only VNAV is armed. Every now and then I will choose not to arm VNAV/LNAV and do it the 'old fashioned' way.

I always arm VNAV and LNAV before takeoff in whatever I am flying if possible, because most of my flying is in Europe and you're on the SID as soon as you liftoff the runway. That's not to say I don't hand fly a good section of the departure, but it means I have the FD cue to assist me and I can engage the AP without further hassle when I want to.