737NG Vnav acceleration altitude
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737NG Vnav acceleration altitude
Just a quick question regarding the Vnav logic regarding the accel altitude after takeoff in the NG. Does the NG fmc + vnav command a pitch down for acceleration when Vnav is selected? I ask this because in the procedure guidelines I've seen the pilot is told to pitch down to 10 degrees and check accelerating before asking for VNav.
In the 744 and 767 there is an entry in the takeoff page of the fmc for acceleration altitude - Vnav will command a pitch change at this point for acceleration and flap retraction. The NG fmc does not have a entry for accel altitude - are different procedures needed (manual pitch down, disregarding flight director)?
Odd question I know, but someone may be able to help me. Cheers.
In the 744 and 767 there is an entry in the takeoff page of the fmc for acceleration altitude - Vnav will command a pitch change at this point for acceleration and flap retraction. The NG fmc does not have a entry for accel altitude - are different procedures needed (manual pitch down, disregarding flight director)?
Odd question I know, but someone may be able to help me. Cheers.
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"Vnav logic regarding the accel altitude after takeoff in the NG"
No such thing in our 800's with FMC U10.6. There is no entry prompt like that on the Take-Off Ref or CLB pages, so after departure if you hit/call for VNAV the A/P or F/D will command an acceleration to 250 kts (if you have a speed restriction at 3L on the CLB page, something like 250/10000' that is...)
Our company procedures are to call for/engage VNAV when Flaps are UP, reason being is that VNAV will accelerate to 250 kts regardless of Flap position. I believe that new FMC updates are already out that allow VNAV to be engaged before T/O on the ground, but I haven't seen 'em yet.
(you can mess around with the old FMC's to be able to engage VNAV right after departure, but it's a drag...I wouldn't mess with it)
Big Boeing s are more sophisticated....
No such thing in our 800's with FMC U10.6. There is no entry prompt like that on the Take-Off Ref or CLB pages, so after departure if you hit/call for VNAV the A/P or F/D will command an acceleration to 250 kts (if you have a speed restriction at 3L on the CLB page, something like 250/10000' that is...)
Our company procedures are to call for/engage VNAV when Flaps are UP, reason being is that VNAV will accelerate to 250 kts regardless of Flap position. I believe that new FMC updates are already out that allow VNAV to be engaged before T/O on the ground, but I haven't seen 'em yet.
(you can mess around with the old FMC's to be able to engage VNAV right after departure, but it's a drag...I wouldn't mess with it)
Big Boeing s are more sophisticated....
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Are you sure?
If the auto throttle is in, and you pitch down, then surely the speed will increase, so thrust will decrease to compensate.
I suspect that there is either an interpretation issue with the procedures, or christmas cheer.
Might I suggest the following is true.
At acceleration, call for either-
N1-bug up, or VNAV.
N1-bug up will ensure you are within the flap limits as you are commanding clean speed.
VNAV is also safe because the flap limits are 250kts. But the structure is little changed from earlier aircraft, and so it is worth considering 230kts because of leading edge devices. So flaps-up, lights out should have occurred before 230kts. A light 737 could bust this very quickly.
VNAV will accelerate to the first hard speed constraint. in ICAO, climb is 250kts below 10000' (occasionally 12000' in spain). If there is another speed constraint in the sid, say 230kt maximum in the first turn, then this is the first commanded speed in VNAV.
In either of the above cases, when selecting the next pitch mode, check accelerating, before commanding a flap retraction. (is this where the comment comes from, in your original post?)
If the auto throttle is in, and you pitch down, then surely the speed will increase, so thrust will decrease to compensate.
I suspect that there is either an interpretation issue with the procedures, or christmas cheer.
Might I suggest the following is true.
At acceleration, call for either-
N1-bug up, or VNAV.
N1-bug up will ensure you are within the flap limits as you are commanding clean speed.
VNAV is also safe because the flap limits are 250kts. But the structure is little changed from earlier aircraft, and so it is worth considering 230kts because of leading edge devices. So flaps-up, lights out should have occurred before 230kts. A light 737 could bust this very quickly.
VNAV will accelerate to the first hard speed constraint. in ICAO, climb is 250kts below 10000' (occasionally 12000' in spain). If there is another speed constraint in the sid, say 230kt maximum in the first turn, then this is the first commanded speed in VNAV.
In either of the above cases, when selecting the next pitch mode, check accelerating, before commanding a flap retraction. (is this where the comment comes from, in your original post?)
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On our -600s, there is in fact an acceleration altitude entry. (I don't know the version number)
So at acceleration altitude, it commands the next speed. I don't recall for sure the selections required, as it is a while since I flew it. But it makes no difference to what is happening, except that the acceleration command speed
is automatic.
So at acceleration altitude, it commands the next speed. I don't recall for sure the selections required, as it is a while since I flew it. But it makes no difference to what is happening, except that the acceleration command speed
is automatic.
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People who have better access and understanding of the manuals than I do, have told me, that you will loose certain layers of protection should you select V NAV before flaps are fully retracted on at least our NG’s ( 800).
I am not sure if this has to with:
Climb profile during flap retraction
Speed protections
Company variations (ie, your company can specify for V NAV, min climb gradients for various faces of flight- this might be a problem during flap retraction?)
Maybe SMYD
Or what ever.
One thing he told me was: that while he was visiting Mr Boeing (½a year ago) he enquired about it, and was told that this was very far from being a simple subject, at least with the current FMC updates: 10,6 and 10,7 that we fly around with. So in the company I work for only TOGA and LVL change is allowed up to flaps fully retracted.
Not sure if this helps
