VNAV Making My Aircraft Do 6000 fpm!!!

[Guest Calzoni]

Hey, im a happy PMDG 747-400 user, i

[Guest pierre974]

Do you press the TO/GA switch on take-off ?Do you correctly set the PERF Page - gross weight, fuel weight, cost index ?Do you determine the center of gravity - CG and the trim setting ?What is the speed indicated on the VNAV page - climb / cruise ?I've never undergone a 6000 ft/rpm when using VNAV... Hope to help you with this trouble...

[Guest Robin.B]

I just have to ask how you are fueling your aircraft :)

[Jeff Nielsen 1,385]

I've had the same problem. It occured I think when installing another recent release though. I wonder if there's a shared gauge somewhere.All my FMC entries are correct as far as I know. I think that CG is autmatically updated isn't it on the 47? I might be wrong.I get climbs and decents at the above mentioned rates.

[Guest capntom]

Dead empty with not a lot of fuel, she (IRL) will do 6000-7000 fpm! Sometimes to avoid doing airshow aerobatics when doing short ferry flights it is necessary to speed intervene and use VS as well to set a reasonable number. Another thing, use reduced thrust as able when you are light, TO 2 works well!Tom

[Jeff Nielsen 1,385]

Thanks for the reply.

[Guest D17S]

Take off and climb 'rates of climb' are all about speed on pitch. Just as an experiment, try it manually to get a feel for what the automation is doing. Set your TO page up and get your Vspeeds. (Don't forget to set your stab trim and the other very good advice in the previous posts!). For this take-off, leave Vnav disarmed and you are not going to be using the autopilot. Note your gross weight.Turn on the Flight directors and the autothrottle. Engage the autothrottle (the magic screw) and let it set TO thrust. Roll. Rotate at VR. As you lift off, catch your acceleration with pitch as best you can. If you are light, you will rocket right past V2 (I always do this experiment with a Very heavy airplane . . . just to try to slow things down a little). Bring the nose up to slow the airplane down: Bring the nose down to speed up. Forget about the command bars. Watch the speed tape. Watch its cadence. Do not touch thrust and leave the flaps at 20. Also, watch that you stay in TOGA pitch mode. If you are too aggressive, you will shift to FLCH pitch mode. If this happens, reset and try again. Go a little easier on the pitch adjustments next time. Just climb like this for a while. Get a feel for controlling speed with the pitch of the airplane. Nose down, speed up a little: Nose up, slow down a little. Leave the flaps alone and thrust at TO. This is a test flight. Find V2 with pitch and hold it there. Notice first that the command bars will come to you. You didn't have to chase them at all. They will settle right down on your airplane. The command bars tell you where the autopilot would be flying if it was engaged. Who got there first? If you're good, you can get there first. (Bet you beat it!) Now, note your rate of climb. You are manually doing what TOGA and (pre-acceleration altitude) Vnav does automatically. These modes will do a little adjusting on the actual V2 speed target, but this is exactly what TOGA and Vnav will try to do for you. Again, this is "speed on pitch." You can press buttons or do it your self. Is this the rate you have been seeing with the automation engaged?Here's the sequence including the pitch mode shift to Vnav. The initial pitch mode you will see on the end of the runway is called TOGA. It will give you an initial 8 degrees of pitch for VR (so you won't bang the tail), then 15 degrees for initial climb, then settle into the FMC's V2 from the T/O page for a speed on pitch climb . It can also climb at an adjusted V2 based on . . . too much for now (beginners guide folks, please). Next Vnav will engage at 400 feet if you had it armed for TO, or when you press the Vnav button. Vnav's climb pitch mode is called VNAV SPD. It will fly at V2+10 or the existing airspeed, which ever is greater. At Vnav's acceleration altitude,(LSK1L on the TO page) Vnav's speed target will change to the first speed limit (generally a flap speed limit). At acceleration altitude, the airplane will just pitch down a bit so it can accelerate to the new speed. Another event that Vnav automates is thrust reduction. Power will reduce to climb at the THR Reduction setting from the TO page (LSK 3L?). And away you go.But do it manually to see what's going on. Then you can see the automation more clearly. There's some Very cool systems on this machine. This is just the beginning.

[Jeff Nielsen 1,385]

Sam,Thanks, I think I had all that down. The main thing is there is an abruptness i didn't have before in pitch.I'll do some more checking and try to nail it down.

[Guest D17S]

Yea, I'm getting a real hard pitch over when I initiate a Des Now (within 50 mi of TOD). It drops like a rock for 1000 ft, levels off, then cruises the remaining miles over to "path" . . then starts its normal path descent. (Wierd?) I've always found light TO's are a rocket ride. If I can stop the acceleration by V2+15 without banging the tail, I'm lucky. I peg the VSI a lot on initial climb. Be interesting to see what your seeing though. Keep us informed.