F/D and VNAV question

[Guest jlm]

Dear allI have a few issues I would like to clarify concerning use of both VNAV in general and then also the Flight Director.Taking the 737 800/900 as an example:VNAVAfter takeoff, when should VNAV be activated?I understood at or after the thrust reduction altitude, but not before, since the speeds VNAV applies for a SID will be the climb speed (for example 240 kts) raher than any constraint speed. Is this correct.So, if I push VNAV (after LNAV) at say 400ft AGL my speeds will rocket up to 240kts (for example) rather than maintain V2+10 ?So, I fly the initial climb at V2+10 using pitch, then at thrust reduction alt (say 1500ft) I can activate VNAV.Concerning F/DI have found that switching F/Ds on just before take-off roll doesn't guide me to the pich to maintain V2+10, in fact the F/D bars follow my pitch input, ragter than the F/Ds providing me with a pitch to follow. Am I doing omething wrong here, my F/D bars are completely latched onto my horizontal reference bar.When should F/D be switched on - I understand it is not a guide for rotation, only for pitch command for V2+10 and thereafter, so should I only activate after say 400ft AGL?Advise appreciated to set me straightthanksjlm

[Airbrakes 0]

Engage VNAV at the acceleration height (1000FT AGL). When VNAV is engaged, the TMC selects CLB thrust so you lose TO power.F/D's should be turned on during preflight and left on. They don't provide rollout guidance as a automatic takeoff was not included in the list of the AFDS capabilities. On the takeoff roll, increase pitch about 1.5 - 3 sec per 2.5 degrees. Look through the F/D's and pitch for V2+10. The F/D's and AFDS pitch for V2+10 and view what's set in the MCP speed window as V2.

[michal 44]

When should F/D be switched on - I understand it is not a guide for rotation, only for pitch command for V2+10 and thereafter, so should I only activate after say 400ft AGL?

F/D should be switched on the ground - it should definitely give you pitch commands through rotation, etc. Tim Metzinger's tutorial guides you through this process step by step.

[Guest jlm]

thankyoujlm

[basa 0]

When should F/D be switched on - I understand it is not a guide for rotation, only for pitch command for V2+10 and thereafter, so should I only activate after say 400ft AGL?

Once you get the "pre-flight complete" message in the FMC, the F/D bars have got all the information they need for the flight (including takeoff), that is the perect moment to turn on the F/D.I've found that in the very initial climb, the F/D bars are a bit unstable, I prefer to "Pitch for Speed" watching the speed-trend green arrow.

[ExNusquam 11]

In the Real plane, I believe the F/D bars are unstable until about 1000', as they are chasing the airspeed set in the MCP, which should be V2.

[JHepburn 0]

Engage VNAV at the acceleration height (1000FT AGL). When VNAV is engaged, the TMC selects CLB thrust so you lose TO power.

Kevin,I _believe_ that this is incorrect for the 737 (although it is correct for 767). It has been a while since I've flown the 737, but I believe arming VNAV below accel altitude (set in FMC) maintains takeoff thrust untl that altitude. If I remember correctly, engaging VNAV at 400AGL (for example) will maintain the greater of v2+20 (target airspeed for 1st climb segment) or current airspeed. At acceleration altitude, VNAV will drive the speed bug to the altitude constraint on the climb page (usually 250kts below 10,000k).I know in the Level-D 767, if you engage VNAV while still in a takeoff profile...meaning nose up to maintain v2+20 (or so), then you'll get a rude awakening since it automatically sets climb thrust regardless of altitude.Hopefully this helps and isn't too confusing. I'll play around with it tonight to confirm as well...

[JHepburn 0]

In the Real plane, I believe the F/D bars are unstable until about 1000', as they are chasing the airspeed set in the MCP, which should be V2.

Per Mr. Bill Bulfer (excited to have a chance to pull these books out again tonight!)... --Takeoff Mode"AFDS commands 10 degrees nose down, wings level though 60kts; 15deg nose up after 60 kts; 84kts autothrottles go to throttle hold - THR HOLD annunciates; 15 degrees noes up after lift-off until a sufficient climb rate is acquired, then pitch is commanded to maintain MCP + 20 kts"

[Airbrakes 0]

Kevin,I _believe_ that this is incorrect for the 737 (although it is correct for 767). It has been a while since I've flown the 737, but I believe arming VNAV below accel altitude (set in FMC) maintains takeoff thrust untl that altitude. If I remember correctly, engaging VNAV at 400AGL (for example) will maintain the greater of v2+20 (target airspeed for 1st climb segment) or current airspeed. At acceleration altitude, VNAV will drive the speed bug to the altitude constraint on the climb page (usually 250kts below 10,000k).I know in the Level-D 767, if you engage VNAV while still in a takeoff profile...meaning nose up to maintain v2+20 (or so), then you'll get a rude awakening since it automatically sets climb thrust regardless of altitude.Hopefully this helps and isn't too confusing. I'll play around with it tonight to confirm as well...

I don't have FS9 so it's been a while since I flew the 737, but I thought that feature was on the 744 only. You can select when CLB thrust will be activated by the TMC by either altitude or a certain flap setting. I don't remember if this was featured on the 737.

[craig_read 43]

Hi,I am not sure what you are meaning by "engage VNAV" so let me use the 747-400 as an example.The VNAV and LNAV are illuminated at the gate with engines off, it is NOT engaged here. It's certainly done before the aircraft takes off, VNAV and LNAV should be ARMED (not ENGAGED) modes on the FMA (indicated in white).On takeoff your FMA will show nothing / TO/GA / TO/GA... and LNAV and VNAV below armed in white. This means that once the height restrictions for these modes are passed, they will become that active pitch and roll modes and replace the green TO/GA pitch and roll modes. On take off you will spool engines to 70%, stabilise and engage the autothrottle with the TO/GA switches (assuming your autothrottle is armed and correctly configured).Your FMA modes will be ... THR REF / TO/GA / TO/GA.... the engines will spool up to take off power and should reach this stable state around 80 knots.. once they reach this state the FMA will show.. HOLD / TO/GA / TO/GA .... still LNAV and VNAV are white armed pitch and roll modes.... HOLD means the autothrottle has met the thrust target, you can adjust the thrust with the levers manually at this point. The autothrottle IS NOT.. trying to maintain a set speed, this is a common mistake!V1.... VR... nose up... 3 degrees pitch a second.. to 10 degrees.... V2... watch the radio altimeter confirm wheels off the ground and climb out to about 50 feet.. at this point your climb is shallow and the speed will bust your takeoff ref quite easily.. remember the autothrottle is NOT maintaining a speed, it's holding the thrust, otherwise it would throttle back to your V2 speed which is BAD BAD BAD at this cruical stage...Now your job is to turn that power from accelleration to climbout... so pitch up till she stops speeding up... and hold that.. ok so we have a safe climb now.. TO/GA and TO/GA are still our pitch and roll modes.. as we pass 400 feet AGL.. the autopilot thinks.. "aahh... VNAV is valid now.. I can make that active" and it will replace the pitch mode TO/GA with VNAV... same happens for LNAV at it's active altitude..You do not "turn these on" (VNAV / LNAV that is) yourself.. The autopilot has them as armed modes and when they are valid makes them active automatically. What you "turn on" is the autopilot... by invoking the VNAV and LNAV buttons you are setting the pitch and roll modes.. just as you could invoke HDG SEL, V/S, ALT HOLD etc etc.. The autopilot will use the active modes within the FMA to determine how to fly.. And remember.. nothing in your FMA for throttle pitch or roll, then no autopilot modes and it's not doing anything.Accelleration height is the height at which the aircraft will stop the initial climbout phase of the flight, and begin the process of reverting to the climb phase. At this height VNAV will be active, LNAV will be active modes. What acceleration height does (and someone correct me if I am incorrect) is change the operation of the throttle system slightly, NOW, it's got a speed target! But it's stepped in conjunction with flap retraction. The aicraft pitches down.. the speed begins to climb.. you approach your flaps 5 setting now... set flaps 5... watch your EICAS now.. as you acquire flaps 5 the thrust mode will change from TO to CLB (or CLB1 or CLB2 depending on your thrust limits).. You will hear the engines throttle back a little to the climb thrust setting. You can adjust these heights if you wish within the FMC at pre-flight. But the A/C objective now is to get to a clean configuration and safe speed for that configuration, once that is acheived the power will go back into the climb again.Does that help you? CheersCraig

[Guest jlm]

thats clear, very easy to push the buttons wtih these systems but not have a clue what's going on in the backround.many thanksjohn