Simviation Forums

Hi all
I've run into this problem on odd occasions.   I'll be flying along in the default 737-400 when I find that it won't descend easily.

I really have to push the stick all the way forward to get it to descend.  As soon as I let go of the stick (ie in neutral position), the aircraft starts climbing at a high rate.  AT other times (different flights), the aircraft does the opposite (ie it's a battle trying to get it to ascend!)

I don't think it's the AP setting as it occurs even when AP is switched off.   Flaps are at 0.   engine anywhere between 50% and 100%

Speed somewhere between Stall and Overspeed!

Any ideas anyone

kind regards

Satz

Have u tweaked the trim?

Ben

Hi Ben,

I tried adjusting the trim but it didn't seem to make much difference.  Anyway, I thought the trim would do  just subtle trimming...

What I was getting was a major problem which I don't think that trim would fix (though I'm happy to be corrected)

cheers

If you're pushing the stick forward to descend, you're not trimmed properly.

Set your power to idle.  Trim the aircraft for level flight.  Then use your throttle to climb/descend.

Hi,

Like any kind of objects in the nature airplanes "want" to maintain their current kinetic condition (I hope I am using the correct term english is not my native language). Remember what happens to you when you are in a car and the driver pushes the brakes.

When you push the stick in your virtual 737 all the way forward you descend with an extreme nose down angle. This increases your airplane's airspeed dramatically and as a consequence it (the airplane) wants to climb up again in order to reduce speed and this eventually happens when you release the pressure.

In order to descend with a 737 you must maintain a slight nose down pitch (about 2-3 degrees) and ideally maintain clear configuration (no flaps no spoilers). Also keep in mind a 737 is not a Cessna which means do not expect to descend immediately. You should plan about 3 miles for each thousand of feet you want to descend. For example if you want to descend from 20000ft to 12000ft that is a descend of 8000ft and you should plan 8*3 = 24 miles.
Alternatively if you want to work with Flight Levels remove the zero at the end of the FL and multiply by 3. That is if you want to descend for landing from FL120 you have 12*3=36 miles.

Another rule of thumb to find the best rate of descend is to divide your Ground Speed (GS) by 2 and add a zero at the end. That is if your GS is 210 KIAS your rate of descend should be 280/2=140 making 1400 FPMs

The above numbers are not precise under all conditions (in fact it all differs regarding your altitude, the amount of your fuel thus your weight, the temperature and the wind) but it will work in FS in general.

I have just one question is your autopilot altitude hold off
Try pressing F1 which is ground idle if you find that the airplane is still trying to climb either your auto pilot altitude hold is set or you have a major trim problem

Hey there!

I would say that the problem you face is a combination of all the factors already expressed here. You should watch, IMHO, the airspeed, your flap settings and your trim. And don

many thanks all.   Will try to apply this knowledge when next I fly on FS2002.

Another question....  when using Autopilot, what's the difference between using HDG and NAV?

And must NAV be used with the GPS/NAV toggle in the NAV position?

thanks again

cheers

Satz

HI!
If you use heading,your autopilot follow the heading that you have selected on heading panel.
For Nav mode you shoul know something about instrumental navigation.....The NAV mode just let your airplane intercept and follow or just follow the RADIAL (course) that you've selected on COURSE panel.But the course works with the frequency you've selected on radio NAV1 !!
I try to explain better......The course means your relative position respect to the radio aid (VOR) you have selected on your NAV 1.
If you are flying on radial 180

yeah apply as much nose down trim as possible.  ;D
Hey what happened to the good old idle thrust command when you want the aircraft to descend?  ::)

HDG means aircraft will maintain the selected heading (wind drifting not corrected)
NAV can either mean VOR/LOC course tracking or GPS tracking (depending on NAV/GPS switch position). Wind corrections are applied here since the aircraft "knows" where you want to go.

Hi Satz ,

I was flying in multiplayer the other day and my friend and I were both in ultralights ( supposedly no autopilots ) .  His would go into dives , etc. and he only got the problem corrected by going back to his usual 172 and then back to the ultralight  .  I am not  sure what changes he made , but my thought was that it had to be the autopilot in memory from his previous flight ??  

Your problem sounds more like an autopilot setting rather than trim to me .  Z on the keyboard is supposed to disengage the AP .  I always try that if my plane is acting crazy .  I would be curious to know how you solved the problem .

As to getting a large jet down , other than starting a descent far enough out , I have found that the spoilers do a fantastic job.  I never flew jets in the real world so I am not sure if one is supposed to use those in that manner or not and at what speed .  I am one of those who likes to fly rather than read !!  You can also slow to gear speed and lower the gear which adds drag and aids in losing altitude .   In my experience in small aircraft , there was a max. speed at which you should lower the gear and a higher allowed speed after the gear was down  .  The reason for the difference in speed  was due to the fact that the actuation of the gear doors into the slipstream could damage them at too high a speed .

Jamie