LNAV from 50'?

[Paul_Yorks 97]

If I'm taking off without LNAV engaged (say because my first WPT is more than 5deg off rway heading), at what height can I engage LNAV?

 

FCOM2 says 50', but I'm finding that its a good bit higher than that. The issue came up when I realised that choosing LNAV in FS2crew in NADP1 wasn't activating LNAV - but I find that the same issue arises when I manually mouse-click LNAV - it's not activating till a good bit higher.

 

Not that it matters a whole lot, but I'm intrigued to know whether I'm just doing something wrong.

 

Paul Scholey

[QF 737 2]

I can't answer that as i'm not a technician. Hopefully Andrea will be along soon to answer your question.

 

I have a question though. Why aren't you arming LNAV during the Before Start Procedure (FCOM1 NP21.29)? LNAV is typically armed during this procedure, in real life as well as in in the simulator.

 

Michael McKenna.

[Paul_Yorks 97]

I'm not arming it because I can't arm it if my route deviates from runway heading by over 5deg. I am currently flying lots of legs according to plans I download from my VA, and nearly all of them end up with me in this situation.

 

Now I know I could add a waypoint a mile or 3 from the end of the runway, and that would fix the issue, but if I choose not to, then this is where I get to - like I said, it's not a deal breaker, since there are at least 2 fixes, but I'd still be interested to see if there's something I'm missing.

[pete_auau 1,154]

Think you can arm it above 400 feet i think some one can correct me and vnav after 1500 feet

[NM5K 1]

I consider the arm/engage alitude as 400 feet. That's the minimum legal alltitude in the US anyway,

as far as I know. You can watch your modes during the ascent and know exactly when you can engage.

I run quite a few short flights that won't let me arm LNAV on the ground.

Both VNAV and LNAV can be armed after 400 feet, and you will see the modes light up in

green boxes on the PFD. Say I'm taking off with only VNAV armed.. After 400 feet, I will

engage the autopilot, and then note that VNAV is armed, then I will arm LNAV. Make sure no

strong yoke pressure when engaging autopilot. Shouldn't be much if you trimmed correctly before

takeoff. Too much pressure, and the AP won't engage. And off I go into the wild, usually blue, yonder... :/

You do not have to wait until 1500 AGL to arm VNAV. Both will arm after 400 AGL.

[AirbusCG 52]

If I'm taking off without LNAV engaged (say because my first WPT is more than 5deg off rway heading), at what height can I engage LNAV?

 

FCOM2 says 50', but I'm finding that its a good bit higher than that. The issue came up when I realised that choosing LNAV in FS2crew in NADP1 wasn't activating LNAV - but I find that the same issue arises when I manually mouse-click LNAV - it's not activating till a good bit higher.

 

Not that it matters a whole lot, but I'm intrigued to know whether I'm just doing something wrong.

 

Paul Scholey

 

Hi Paul,

why don't you use SIDs? Even when flying for your VA they should give you flightplans with valid SIDs. If you choose to use those you should always have your first waypoint in the correct heading. Otherwise you can still use HDG Select to get the aircraft to a proper interceptheading and then switch over to VNAV.

 

By the way @ Peter Kelberg: LNAV can be engaged at 50ft and VNAV at 400ft. But you're right, you're not allowed to engage the autopilot below 400ft anyways.

[Paul_Yorks 97]

Thanks for the replies folks, very helpful.

 

I'll look at SIDs in more detail - the plans I've downloaded don't as a rule have headings that are LNAV compliant.

 

Emi - I am convinced you cannot engage LNAV at 50', though that's what FCOM2 says. I have repeatably found that the NGX ignores the LNAV button press at that level, indeed up to several hundred feet - hence my OP.

[Tabs 2,790]

It's 400 feet if you don't have LNAV pre-armed on the ground. If you have it pre-armed it engages at 50 feet. There are some rare SID cases where you can't arm it on the ground because of the 5 degree limitation - this is accurate to to the real airplane and you just have to handfly it or if you have the option for HDG SEL TO/GA instead of wings level you can use that. I'd think there's very few SIDs that absolutely require you to make a turn before 400 feet though, and if there are they probably are coded wrong in the navdata or something - there should be a psuedo waypoint at the altitude where the turn happens - then your first waypoint is on the runway heading and you don't have the 5 degree issue and can arm LNAV on the ground.

[Paul_Yorks 97]

Thanks Ryan, that's great.

 

Paul Scholey

[pete_auau 1,154]

By the way @ Peter Kelberg: LNAV can be engaged at 50ft and VNAV at 400ft. But you're right, you're not allowed to engage the autopilot below 400ft anyways

Hi read ryan's post