VNAV at Takeoff

[Jean-philippe]

Hi everybody,

I'm runing a complete set of PM software with a FS9 B747-400. My setup uses the PMDG airfile with Posky Model/Textures. It gives me a very accurate flight model without the problems of compatibility between PM and PMDG.

My question is about the takeoff. I make it with TO/GA mode, LNAV and VNAV armed. The first unrealistic thing happens when the VNAV mode engages at 480' AGL. The thrust jumps from TO (or D-TO) to CLB. It happens too soon. In the reality it happens at about 1500' AGL. I don't find where to tune this parameter within the CDU or the .ini files, even via the ACF files. Does somebody have an idea about this issue ?

The second thing is about the speed management during the first segment of the climb. After the rotation, the target speed must be V2+10, that's what I set in the MCP IAS window before takeoff. But when the VNAV mode engages at 480' AGL, the target speed increases and the MCP IAS window extinguishes. It's a too low altitude for the acceleration, in the reality this acceleration height is at about 1500', until 3000' during noise abatment procedures. Again, in the reality, this "acceleration height" is available for tuning in the CDU TAKEOFF REF page. This is not implemented in the PM CDU and I don't find how to set it up. Any help will be appreciated.

Thank you and have nice flights

Jean-philippe

[Sean Nixon]

Hi Jean-philippe, welcome to the forum.

I'm not 100% up to date with flying the AFDS at the moment (and I fly 737's), but are you not engaging VNAV too soon? Shouldn't it be engaged once the aircraft is cleaned up (flaps), typically around 3000', like you said?

Regards

Sean

[Sean Nixon]

From the 737NG FCTM:-

6332

[maussuam]

Hey Jean-Philippe,

I'm not shure if I remember right, but wasn't it the second perf init page where you can edit the thrust reduction alt ?

Regards
Bastian

[Jean-philippe]

Some companies takeoff with VNAV armed, sone others engage it later. In all situatons the VNAV mode is built to manage the speed and thrust correctly, and PM doesn't. I'll check this second page on the PERF INIT.

[Jean-philippe]

No thrust reduction field on the second page of PERF INIT. There is a T/R ALT field on LSK 5L of the THRUST LIM page. I tried to set it to 1500 but the autothrottle still engages CLB at 480' AGL. There is a long time I don't wait anymore for PM for support. I hope the comunity we are will help.

[maussuam]

The Boeing 737-CL Operations Manual says "Above 3000 feet AGL, engage VNAV or select normal climb speed and verify announciation."
I remember a pilot statement, that it is his company procedure to set VNAV when reaching clean configuration and not arming it on GND.

[Jean-philippe]

I have to appologize. I checked the 747-400 crew trianng manual and it says the same thing. So the pitch mode for takeoff is TO/GA until clean configuration and VNAV engagement, is that correct ?

[Sean Nixon]

In the 737NG, the autothrottle switches from TO/GA to ARM at around 800'. You can then press N1 to initiate thrust reduction and enter CLB mode, otherwise leave it and it'll do it automatically once the TR ALT set in the FMC is reached (normal method).

Edit: once the aircraft is cleaned up, VNAV is normally selected, perhaps around 3000'.

Sean.

[Jean-philippe]

Understood. But back to my question, where can I set the TR ALT in the FMS ? I don't see any field in the PM FMS
and I have no answer from PM.

[maussuam]

Hey Jean-Philippe,

I checked the 747-400 AOM from an european airline, they usually take off with LNAV and VNAV armed, so there must me a solution your problem.
I found a very good manual of the PDMG 747-400 FMS here: click me (http://www.ifpz.be/tutorials/Working%20with%20the%20Pmdg%20B747%20FMC.pdf)
According to that (Page 40), you can set Thrust Reduction Height and Acceleration Height independent from each other in the Takeoff Ref page (which in reallity is also made for some Noise Abatement Procedures).
You set the ACC height together with flap setting in the flaps field (LS1L). If it is set to 1500, the commanded speed should remain at V2+10 until reaching the height. I hope this is working. ;)

Regards
Bastian

[Sean Nixon]

Try page 2 of the TAKE OFF page.

[Jean-philippe]

The PM CDU has these features displayed in the THRUST and TAKEOFF pages but they are not functional. I tried to set 10/1500 in the LSK 1L of the TAKEOFF REF page to set acceleration haight at 1500' but the VNAV continues to set his own speed targets below 1500'. About the thrust reduction altitude there is also an field in the LSK 3L of the TAKEOFF REF page but it is not functional at all and the setting remains to FLAPS 5 CLB (witch is not followed by the autothrottle).

Again PM software limitations ?

[Jean-philippe]

Try page 2 of the TAKE OFF page.

I don't have any page 2 on the TAKEOFF REF page. I use 747-400 CDU build 492.