737-800 VNAV arm before takeoff.
 

For what technical reason(s) if any, would you not arm VNAV before takeoff on the 737-800, if you have U10.8A installed?

Is there hardware issues? Or does it have something to do with having a mix of CL/NG aircraft in the fleet and pilots who fly both?

I've been searching for an answer for weeks; no dice. Pprune was my last resort.

Noise abatement procedures are one. Windshear, or expected windshear, is another.

Thanks for the response - with respect to noise abatement - Is it a case of accepted best practice, company ops or do the manuals preclude arming VNAV for takeoff (assuming the appropriate SB's/U10.8A have been incorporated).

I'm currently reading dated study guides; can't find a thing... bloody annoying!

Noise abatement?
I don't think so.
Takeoff Ref page 2. You could set values for
Thrust reduction altitude
Acceleration altitude
EO accel alt

VNAV armed before TO will take into account all these values and manage thrust and speed accordingly.

And windshear recovery mode will be shown with FD on/armed regardless of VNAV armed before take off.

The only technical reason i have to not use that feature is whenever i want to do a FD off departure, which admittetly is quite often. Other than that it works very well.

I don't fly the -800 but my first answer would be if you don't intend on following the vertical profile of the departure.

I don't think there is any technical reason. There is a software issue: on FMC 10.7 VNAV cannot be engaged, on FMC 10.8A VNAV can be engaged. We have both softwares on our fleet. Aircraft equipped with FMC 10.7 procedures prohibit arming VNAV on the ground, aircraft equipped with FMC 10.8A procedures stipulate to arm VNAV during the "Before Start Procedure".
Hope this helps,

Wasn't there a notice to users or some such that there's a bug with VNAV regarding windshear? I think the jet would exit TO/GA and you'd have to hit it again while in the recovery.

NADP-1 is easier to accomplish in VNAV than is NADP-2 (at least with my then airline's profile).

We have both versions on our fleet i.e v10.7 and V10.8. VNAV can be engaged on V10.7. I've done it a few times. This is the first I'm hearing of it being prohibited. Guess I'll have to take that up with the technical pilot.

The reason Boeing recommends against it on 10.7 is there's no flap limit speed protection on that version.
For reasons unknown to me we don't arm VNAV on the ground, we do have fleet wide 10.8A.

From Boeing FCTM;

On airplanes with FMC U10.7 and earlier, after flaps and slats retraction is complete, select VNAV or set the desired climb speed in the MCP speed window. Before selecting VNAV, flaps should be retracted because VNAV does not provide overspeed protection for the leading edge devices.

With FMC U10.8 and later, VNAV can be engaged anytime because VNAV provides overspeed protection for the leading edge devices.

I'm aware that 737-800, vnav shouldn't be armed for takeoff with anything less than U10.8A... but thanks nonetheless.

The best explanation I can come up for myself is due to the fact our tech crew operate both the 400/800.
I think the practice of not arming vnav for takeoff, in an aircraft that has the capability for it to be armed, stems from the 400 (or classics in general).
I think the procedures have been designed to be uniformed; that is, an appropriately endorsed pilot can jump from the 400 to the 800 <and not arm vnav for takeoff>.

I'm just having trouble finding the link, and confirming my theory.

one reason not to arm Vnav on the ground, could be it the airport has special convoluted noise abatement procedures, ie aircraft is light, and you dont want to chase 230kts but instead stay at up speed (and you dont want to use SPD int)...

or more likely because you want to delay part of the flap retraction and keep lower groundspeed, to cater for adverse strong tail/X wind turing a tight turn on departure... (thinking about those places with rocks hidden i the clouds)

Thanks P-P. No doubt the principles of airmanship apply in this case. Also, I'm aware some ports have specific procedures for departures which may discourage/prevent arming vnav for takeoff.

Perhaps I should be asking if 737-400's have VNAV armed for takeoff (an educated guess would be, no). I know the -300 manual doesn't call for vnav until 3000 or above.

Now approaching the veeery annoying stage. I can't find any technical reason for not arming vnav on the 737-800 :ugh:

EagleFlyer,
We have a Boeing Bulletin that states:
"Boeing has received reports of inconsistencies associated with the arming of
VNAV prior to takeoff on 737NG airplanes equipped with FMC Update U10.7.
These inconsistencies can occur even on those airplanes with the configuration
specified in Service Bulletin (...) installed, i.e., FMC Update U10.7,
CDS Blockpoint 06 and Collins Autopilot P4.0.
Additionally, VNAV may incorrectly engage rather than arm if an FCC version
earlier than Collins FCC P4.0 or Honeywell -710 is installed. For FCC P4.0 or
Honeywell -710 or later installations, VNAV may engage rather than arm under
certain conditions when the departure is not aligned within 5 degrees of the
runway. With VNAV engaged on the ground, when TOGA is pressed VNAV will
disengage and the MCP IAS/MACH display will open and display 120 knots
(Collins FCC) or 100 knots (Honeywell FCC).
This anomaly exists only with FMC Update U10.7. It will be corrected in a future
software upgrade of the FMC and/or FCC."
Hence the fact that, arming VNAV on ground is prohibited for aircraft equipped with FMC 10.7.
MooseKnuckle,
Mixed Fleet Flying restrictions could be a reason as you state it.

My company does not arm VNAV for NADP 2 departures (up speed till 3000 feet). We do change parameters on takeoff page 2/2 for NADP 1 procedures, allowing for VNAV use.

There is a Boeing bulletin that talks about VNAV and windshear, which recommends VNAV not to be used with 10.8A software.

I see. So (maybe) I'm wrong. Can you advise why?

VH ABC, Boeing doesn't recommend NOT to arm VNAV. The objective of the bulletin is: " This bulletin informs 737NG flight crews of the need to revise the windshear escape maneuver if VNAV has been armed on the ground for takeoff (FMC Update 10.8 and 10.8A)."
Now if you operate mixed FMC's (10.7 and 10.8A), there could be a technical reason NOT to arm VNAV on the ground: You don't want your crews to arm VNAV on 10.7 and you don't want them to follow an AFDS windshear maneuver with VNAV engaged on 10.8A. To escape the consequences of these errors, fleet managers could prohibit the use of VNAV on the ground until softwares issues are solved (not the case in my Company).

Arming VNAV on the ground is not permitted with FMC Update 10.7 or earlier.

With U10.8 or 10.8A installed, if unstable weather conditions are present prior to takeoff, consider not arming VNAV for takeoff.

Direct quote from the Boeing bulletin under OPERATING INSTRUCTIONS.

Mooseknuckle, you originally asked the question as to a technical reason why you would NOT arm VNAV for takeoff. Isn't Boeing giving you one in this bulletin?

Thanks a million mate!!! Case closed at last.