Sign in to follow this  
maukro_1990

Have some problems after take off in the "V/S" mode (LNAV and vNAV are disengage)

Recommended Posts

Hello! 

 

I´ve got a strange problem but i´m sure that the problem is my fault because i do something wrong. Today i wanted to take off without arming VNAV and LNAV. I engaged the V/S Mode and set a VS of about 3000 fpm, and the V2 speed on ground. After that i clicked on HDG select. Now i rolled on the runway and pressed the TOGA button and the take off are working until i´m in the air. And NOW the problem occurs: At about 500 ft over ground i enganged the AP and the selected ap holds the selected vs of 3000 ft for reaching the first initial altitude. And i thought that the AP will maintain my selected speed but it´s not the case. The autopilot does not hold my selected speed, i got a overspeed. My question: What was wrong? I thought that the AP will maintain my speed right after i enganged the Autopilot. 

 

I know: It´s not usual for not using lnav and/or vnav on take off... but i thought that i can set the VS and the speed on take off manually. 

 

I really hope that you understand my question etc. 

 

 

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

You are supposed to take off in take off mode (TOGA) for both pitch and roll.

 

TOGA mode is engaged (green) as soon as you turn on the FD while still on the ground.

The only thing that should be armed for take off is VNAV.

Because Vnav will automatically engage in 400ft AGL and will reduce thrust from TO thrust to CLB thrust and will start acceleration for flap retraction at the FMC programmed altitudes for this!

LNAV can be armed (and 99% of the time it should be armed) to follow an FMC prgrammed SID.

Only if you get an ATC heading for take off would you not arm LNAV for take off.

 

other than that....dont press VS or HDGSEL before being airborne!

 

Take off with TOGA mode and let Vnav do its thing.

From 400ft mose airline SOP allow you to change mode....so if really required, at 400ft AGL you can hit HDG SEL and VS

 

I think in your case the system was just totally confused because you armed LNAV,VNAV and then disarmed it and then engaged HDG SEL and VS before being airborne.

 

When you hit VS at 400ft (which is not normally done) and set +3000ft/min the aircraft will do that and the AT will set thrust to keep the MCP selected target speed.

Ofcourse if you set something crazy like VS +6000ft/min then the nose will go up so high that even with full TO thrust the AT can not maintIn this speed.

 

Just Allow Vnav to do its thing.

Let it reduce to CLB power. (standard value is at 1000ft)

Then see how it increases the target speed for acceleration. (standard acceleration value is also at 1000ft)

Retract flaps as you accelerate.

Then, once clean, you can select VS mode if required.

But you would normally never set VS+3000 ft/min......what we use VS for after take off and clean up, is to reduce VS to +1000 to +1500ft/min to avoid rapidly climbing towards your cleared level off altitude. If you are cleared to climb to 5000ft for example you would reduce to VS +1000 to +1500 ft/min at 3500ft. There is a reason why you are only cleared to climb to 5000ft!......other Traffic!

By reducing VS you avoid getting traffic warnings and avoid other aircraft getting traffic warnings due to you climbing to them like a rocket.

Share this post


Link to post
Share on other sites

 

 


Because Vnav will automatically engage in 400ft AGL and will reduce thrust from TO thrust to CLB thrust and will start acceleration for flap retraction at the FMC programmed altitudes for this!

 

Hi Rob, 

 

I have a doubt there and didn't find a clear answer in the FCOM v2.

Does the autothrottle needs the VNAV to be armed to reduce from takeoff thrust to climb thrust in the T7? 

I didn't check that in the sim as I always arm VNAV on the ground, but on the NGX where I usually takeoff without LNAV/VNAV, the thrust reduction occurs even in TOGA pitch mode.

Share this post


Link to post
Share on other sites

Hi Rob, 

 

I have a doubt there and didn't find a clear answer in the FCOM v2.

Does the autothrottle needs the VNAV to be armed to reduce from takeoff thrust to climb thrust in the T7? 

I didn't check that in the sim as I always arm VNAV on the ground, but on the NGX where I usually takeoff without LNAV/VNAV, the thrust reduction occurs even in TOGA pitch mode.

I think no. If A/T is engaged it reduces the thrust to CLB at thrust reduction height, which is set in TO page 2/2. (The default is 1500 ft AGL, not 400 ft.)

Share this post


Link to post
Share on other sites

Never ever use V/S as pitch mode in initial climb.

 

Your pitch should be determined by speed, not climb rate.

 

Using climb rate to determine pitch means the aircraft could reduce speed to maintain climb rate... never a good thing. Ever.

 

Fly the tutorial.

Share this post


Link to post
Share on other sites

 

 


My question: What was wrong?

 

Always pay attention to your FMA. Stopping to come to grips with what you're telling the AP would help, too. Watch what happens when you hit TOGA. After you're through the THR HOLD mode, you're in N1. This is a pitch for speed throttle mode (essentially), in that it's setting the N1 on the N1 limiter. What you do with all that thrust is up to you, but in most cases, you're using this to get the max amount of climb that you can, for that rated thrust.

 

What was wrong was that the pilot decided to try something non-standard without fully understand the tools he was trying to use in doing so.

Share this post


Link to post
Share on other sites

 

 


At about 500 ft over ground i enganged the AP and the selected ap holds the selected vs of 3000 ft for reaching the first initial altitude. And i thought that the AP will maintain my selected speed but it´s not the case. The autopilot does not hold my selected speed, i got a overspeed. My question: What was wrong? I thought that the AP will maintain my speed right after i enganged the Autopilot.

Put simply, the AP can't maintain the airspeed because you told it to use V/S. In V/S mode airspeed is controlled by autothrottle, but for takeoff this till be set to TOGA thrust. So by engaging V/S with takeoff thrust set nothing is controlling airspeed.

 

You appear to be using techniques learned from the FSX default aircraft, where speed is always controlled by thrust and you set V/S to set climb rate. As others have said, fly the tutotial and learn how the real aircraft is flown.

Share this post


Link to post
Share on other sites

You are supposed to take off in take off mode (TOGA) for both pitch and roll.

 

TOGA mode is engaged (green) as soon as you turn on the FD while still on the ground.

The only thing that should be armed for take off is VNAV.

Because Vnav will automatically engage in 400ft AGL and will reduce thrust from TO thrust to CLB thrust and will start acceleration for flap retraction at the FMC programmed altitudes for this!

LNAV can be armed (and 99% of the time it should be armed) to follow an FMC prgrammed SID.

Only if you get an ATC heading for take off would you not arm LNAV for take off.

 

other than that....dont press VS or HDGSEL before being airborne!

 

Take off with TOGA mode and let Vnav do its thing.

From 400ft mose airline SOP allow you to change mode....so if really required, at 400ft AGL you can hit HDG SEL and VS

 

I think in your case the system was just totally confused because you armed LNAV,VNAV and then disarmed it and then engaged HDG SEL and VS before being airborne.

 

When you hit VS at 400ft (which is not normally done) and set +3000ft/min the aircraft will do that and the AT will set thrust to keep the MCP selected target speed.

Ofcourse if you set something crazy like VS +6000ft/min then the nose will go up so high that even with full TO thrust the AT can not maintIn this speed.

 

Just Allow Vnav to do its thing.

Let it reduce to CLB power. (standard value is at 1000ft)

Then see how it increases the target speed for acceleration. (standard acceleration value is also at 1000ft)

Retract flaps as you accelerate.

Then, once clean, you can select VS mode if required.

But you would normally never set VS+3000 ft/min......what we use VS for after take off and clean up, is to reduce VS to +1000 to +1500ft/min to avoid rapidly climbing towards your cleared level off altitude. If you are cleared to climb to 5000ft for example you would reduce to VS +1000 to +1500 ft/min at 3500ft. There is a reason why you are only cleared to climb to 5000ft!......other Traffic!

By reducing VS you avoid getting traffic warnings and avoid other aircraft getting traffic warnings due to you climbing to them like a rocket.

Wow...thanks for this reply! Yes, the ATC gave me a specific HDG and that was the reason i don´t armed VNAV. I´ll arm VNAV next time and select the needed HDG etc. 

Thank you all for these great replies! 

Share this post


Link to post
Share on other sites

 

 


I´ll arm VNAV next time and select the needed HDG etc.

 

If you have an ATC assigned heading use HDG SEL. VNAV will still work with that.

Share this post


Link to post
Share on other sites

If you have an ATC assigned heading use HDG SEL. VNAV will still work with that.

Thank you for your reply. 

Share this post


Link to post
Share on other sites

When assigned an ATC heading you do still select HDG SEL along with VNAV on the runway correct, as opposed to selecting only VNAV then HDG once airborne?

Share this post


Link to post
Share on other sites

I'm not a real pilot but according to what I understand of the procedures, HDG SEL should be selected only after passing 400ft during the climb, not on ground. The only roll mode that can be armed on ground is LNAV.

 

Anyway, whatever the heading given by the ATC, you wouldn't start turning below 400ft AGL.

 

Don't hesite to correct me if I'm wrong.

Share this post


Link to post
Share on other sites

 

 


Don't hesite to correct me if I'm wrong.

 

This part varies, which is why the option is there in the first place (note the option of TOGA Wings Level v Heading Select). Additionally, safe turn altitude is usually a company SOP thing. When going into and out of Leesburg, I don't turn until I'm about 500 AGL, because in the smaller aircraft I'm usually flying, if anything happens there's still usable runway to set down on. Heading out of ROA last weekend, though, I turned at about 200 AGL because they had us departing on Runway 34, which is aimed at a mountain...

Share this post


Link to post
Share on other sites

 

 


HDG SEL should be selected only after passing 400ft during the climb, not on ground. The only roll mode that can be armed on ground is LNAV.

 

You can arm HDG SEL on the ground, it doesn't become 'active' until you turn on the A/P. For example, I just left KORD on Rnw 28R with departure heading of 010.  During preflght, I set VNAV and HDG SEL to 010 then after takeoff I choose to hand fly and I can follow the FD to make the turn to 010 so it was advantageous in this case to preset the HDG SEL.

Share this post


Link to post
Share on other sites

Just setting the heading bug to an ATC instructed departure heading is one thing.

 

But if you disarm Lnav by pressing on the HDG SEL switch while still on the ground, then the TOGA roll mode (which is like track hold for take off) will also disengage.

 

You then have the FD roll bar deflected left or right the whole time (depending on the set heading) and it is quite annoying to have to fight the urge to follow that FD roll bar.

And you are fighthing it because most airlines will follow Boeing SOP which means you want to fly runway track untill 400ft AGL (which is what the TOGA roll mode gives you) and then, above 400ft, you can start turning by engaging heading select mode.

Share this post


Link to post
Share on other sites

 

 


it is quite annoying to have to fight the urge to follow that FD roll bar

 

Really?  I don't think so but it's an individual preference.  I like knowing I spun the knob in the right direction before the takeoff roll by looking at which direction the steering bar deflects. To each his own.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this