Hi There, Still getting the hang of this aircraft so hoping someone might be able to help out. I've been flying the 737 NGX series for a couple of weeks now and after following the tutorials and much research I feel that I have takeoff procedures mostly down pat. I encountered a strange problem when attempting to takeoff with the 900 from Kennedy airport; for some reason the LNAV did not want to arm, VNAV was fine. I thought nothing off it and proceeded to taxi to the runway and after being cleared for takeoff I still couldn't arm the LNAV, I then decided to start the takeoff roll without it until airborne. After initiating TO/GA at 70% N1 the annunciator changed to N1 and I soon realised I was unable to go faster than about 130 knots preventing me from reaching V1 speed (I then realised the auto-throttle must have put a hold on N1), I panicked and attempted to rotate when I should really have aborted takeoff at this point and crashed a short way past the runway... Is there any reason why the LNAV wouldn't arm when on the ground? Would this have attributed to my failed takeoff after pushing TO/GA? Is there anything that I should check in future to prevent this from happening? Mikey.

I don't think your procedures can be that good if you are trying to engage LNAV or VNAV while still at the gate. Nor are you supposed to proceed if you have a problem that you haven't cleared. And I think you already know that you are not supposed to rotate until you are at rotate speed. Unfortuantly, I don't know what else you have done wrong so I can't really help you fix it. Try following the procedures and checklists in the manuals and let us know if you still have a problem.

Not sure about the speed thing - but the LNAV issue is probably due to the first fix location, it wont allow LNAV if its within certain distance/track - Others will be able to give you a more detailed response to the exact conditions but its also within the FCOM

LNAV arming criteria on the ground: •origin runway in flight plan•active route entered in FMC•track of first leg within 5 degrees of runway heading•LNAV selected prior to TO/GA.•LNAV guidance becomes active at 50 feet AGL [Option - Bank angle limit is 30 degrees above 200 AGL][Option - Honeywell FCC -708 and on]•bank angle is limited to 8 degrees below 200 feet and 30 degrees above 200 feet AGL.

Does your flight match this criteria when you're attempting to arm LNAV?

^From what I recall, LNAV can not be engaged on the ground when the first fix is more than 5 degrees from the runway extended centerline(not entirely sure though..)

Make sure both FD are on for pilot and co pilot, otherwise you can't engage both LNAV and VNAV on the ground.

Make sure both FD are on for pilot and co pilot, otherwise you can't engage both LNAV and VNAV on the ground.

If he got the VNAV, then he got the FDs on.

I don't think your procedures can be that good if you are trying to engage LNAV or VNAV while still at the gate. Nor are you supposed to proceed if you have a problem that you haven't cleared.

You can arm LNAV/VNAV at the gate. That's SOP at some airlines. Proceeding without figuring out the problem, I agree 100%!

I don't think your procedures can be that good if you are trying to engage LNAV or VNAV while still at the gate.

Well I've seen the airlines do it, so it can't be that wrong. EDIT didn't see your reply Matt, nm -

I don't think your procedures can be that good if you are trying to engage LNAV or VNAV while still at the gate.

You can definatly do this on the ground with the NGX. Funny I couldn't do this with the old 737 PMDG NG. Maybe LNAV but not VNAV.

You can definatly do this on the ground with the NGX. Funny I couldn't do this with the old 737 PMDG NG. Maybe LNAV but not VNAV.

Most likely attributed to non U10.8A as FMC version. Not sure when that one came out, but certainly VNAV wouldn't arm on ground with 10.7 or earlier.

Most likely attributed to non U10.8A as FMC version. Not sure when that one came out, but certainly VNAV wouldn't arm on ground with 10.7 or earlier.

I think it's NG vs. Classic. Both fleets of NGs I flew you could arm the LNAV on the ground with either U10.7 or U10.8a, but not on the Classics.

SAS arms LNAV after engine start, where applicable, according to their SOP.

Which sounds like its down to the dep out of JFK, first fix if you loaded data for a dep out of there wouldn't have fix with 5 degrees of runway.

Thanks for all the advice. I tried the takeoff again from JFK and all seemed fine this time around, LNAV engaged and TO/GA behaving properly.

^From what I recall, LNAV can not be engaged on the ground when the first fix is more than 5 degrees from the runway extended centerline(not entirely sure though..)

I think you were correct and this is what may have prevented LNAV from arming as the first fix for the particular SID I chose was definitely outside of the 5 deg.

Which sounds like its down to the dep out of JFK, first fix if you loaded data for a dep out of there wouldn't have fix with 5 degrees of runway.

I chose a different SID this time and it seemed OK. One thing I did notice though is that all departures at JFK from runway 31L/R seem to follow this same pattern.

Thanks for all the advice. I tried the takeoff again from JFK and all seemed fine this time around, LNAV engaged and TO/GA behaving properly. I think you were correct and this is what may have prevented LNAV from arming as the first fix for the particular SID I chose was definitely outside of the 5 deg. I chose a different SID this time and it seemed OK. One thing I did notice though is that all departures at JFK from runway 31L/R seem to follow this same pattern.

Thats probably because the navdata for this SID contains first fix as JFK (it doesnt contain the various departure routings - breazy etc) which would put it outside of the 5degrees.needed - not being an expert on JFK - but maybe a real world guy on here that flies out of there will be able to explain that there are quite a lot of conditions and turns on this dep which are often hand flown - this dep is a good one for using the FIX page to put in all the restrictions and radials and turns.