LVL CHG engaging A/P on T/O

[Guest TonyDAVIES]

Hi GuysSorry if this is a dumb question but I am new to the 737NG.How do I prevent the LVL CHG mode from automatically engaging when I switch the autopilot on after takeoff.? It has only recently started occuring on T/O since I started using LVL CHG instead of V/S for small altitude changes during the approach phase.What do I have to do to prevent this?Many ThanksTony

[Guest boxjockey99]

TonyThis shouldn't happen assuming you're using TOGA mode for take off. Corect procedure would be:cleared for take off stabilise the engines at about 40%N1 then hit the TOGA button. The A/THR will motor the engines to TOGA power and you'll get N1 and TOGA annunciated on the top of the PFD. The Flight directors will give TOGA Cues until you reach the ALT in the ALT window where it will enter ALT ACQ or you engage a pitch mode (N1, SPEED, LVL CHG, V/S or VNAV). Our normal procedure for a POL departure from Leeds (EGNM) runway 32 is: N1 TOGA mode to 1500ft AAL then N1 to maintain V2+20 till 3000ft AAL where we engage VNAV / LVL CHG and retract the flaps on scheduleThe only thing that might cause this is the underspeed protection. If you have a situation where the aircraft is for some reason not maintaining the commanded speed (excluding use of VNAV which has a slightly different protection law) the aircraft will drop out of the mode you are in back to LVL CHG which is the safest. By the safest I mean that it will pitch the aircraft for the commanded speed so if you are slow with a high nose attitude it will lower the nose allowing the speed to increase then it will ptch up again to continue the climb. Basically here I need to know what your procedure is for take off and so then see where you are going wrong. I suspect it is the underspeed protection or not using TOGA mode but untill I see a selection of screenies or description of what you are doing I cnnot say much more!All the best and I hope this helps a little!Kris

[slip-stream 5]

Kris, did you complete an intergrated course? Or modular? If intergrated where did you complete your training? Cheers.

[Guest boxjockey99]

I was one of the first to be put through JAR ATPL modular when they came on stream. I did my ground school at the now defunct PPSC and then flight training at Professional Air Training both in Bournemouth UK. Hope that helpsKris

[Guest TonyDAVIES]

Hi KrisThanks for your comments and reply to my post.>cleared for take off stabilise the engines at about 40%N1 then hit >the TOGA button. The A/THR will motor the engines to TOGA power and >you'll get N1 and TOGA annunciated on the top of the PFD. yep everything you mention above is correct with me.>The Flight directors will give TOGA Cues until you reach the ALT in >the ALT window where it will enter ALT ACQ or you engage a pitch >mode (N1, SPEED, LVL CHG, V/S or VNAV). Do I engage a pitch mode before the a/p is engaged?.My takeoff procedure is as above then at 1500ft I lower the nose slightly to start cleaning up the aircraft engage A/P then LVL CHG immediately engages and pitches the aircraft nose up sharply. As I suppose the altitude has not yet been aquired, set at 4000ft for first SID altitude restriction. I am obviously doing something wrong although as i mentioned earlier, only since using the LVL CHG for approach does this now occur, not before.Why do I do this? Well I beleive I have seen this type of departure regularly at Gatwick and Heathrow where the aircraft will climb to about 1500ft then lower the climb rate clean up,accelerate and gradually climb to meet the first altitude restriction on the SID. 26L/LAM4M at EGKK for example. This is a huge learning curve for me but I believe I am getting there, well I havnt crashed yet.I have printed the POL SID chart for EGNM and will practice a few POL1W takeoffs as you described. One last question. What Initial clearance altitude does ATC assign to you? and what altitude do you initially enter in the window? Oops 2 questions.Thanks againBest RegardsTony

[Guest boxjockey99]

TonyI had an e-mail from Randy Smith one of the B737NG Beta testers on this. Apparently the problem is that the logic of the 737NG autoflight system means that when you engage the A/P in CMD mode it automatically defaults to LVL CHG unless another mode has been armed previously namely VNAV or V/S. This is not the case in the classic that I fly so far as I am aware anyway although I will check. What may have changed is simply that you are now NOT arming the V/S mode prior to entering into A/P CMD thus the system is now doing what it is meant to and defaulting to its safest mode. Lets take the POL 1W from R/W 32 at Leeds, here's what I would do on the real thing!1) stabilise and then hit TOGA, verify you have N 1 TOGA annunciation 2) Passing 400ft engage LNAV (or HDG SEL if you are a sadist)3) lift off at Vr and climb away in TOGA mode till passing 2200ft (above sea level - this is the equivalent of 1500ft above airfield level as leeds is 670ft up). At 2200ft I would press N1 and follow the flight director bars. basically the speed window opens at the speed you are at when you push N1 and the system pitches to keep the speed4) Passing 3700ft AMSL (3000ft above the aifield) press VNAV and allow the aircraft to accellerate retracting the flaps on the schedule. Once in VNAV this is where I would tend to put the a/p in although you can put it in at any time after 1000ft most put it in either after pressing N1 or after pressing VNAV. I personally like to fly the aircraft so try to keep my hands on it as long as I can!In answer to your questions your initial cleared level is 4000ft on the QNH not a flight level and this is very close to the 3700ft accelleration alt we just discussed so I would expect the jet to go into ALT ACQ before you get to the 3700ft. If it does just hit VNAV early thus preventing the thrust from retarding and thus allowing you to accellerate. If 4000ft is your stop height then that is what you put in the box! Hope that helpsKris

[Guest TonyDAVIES]

Hi KrisThanks for the talkthru on the POL 1W. I have just completed about 20 takeoffs and did everything as you described.Re 1) Yep no problems Re 2) LNAV engaged at 400agl. I totally agree with the sadist comment.Re 3) Yep no problemsRe 4) Once in VNAV this is where I put the a/p in also. Having flown the Capt Sim 727 previously I like to fly the aircraft on departure, often as high as transition level before the so called a/p in engaged.On a couple of occasions tonight I have shot the altitude restriction but by only 200ft. As you say the two altitudes are very close. Also on a couple of times the LVL CHG and SPEED modes engaged before pressing N1 at 2200ft. But overall a great successOk back to reading the manuals before someone tells me to RTFM.Thanks again Kris for your time and help I have found many of your posts in this forum invaluable.CheersTony"Every day I learn a little more but forget much more than I have learnt that day"