Autopilot overshooting and rocking

[Simmer 2]

Hi, I have a very strange problem with the NGX: In VNAV mode the autopilot shows a tendency to overshoot the course. After overshooting, it starts to rock left and right to get back on course. An example where this behavior is really extreme is the PSA 25S transition at EDDF. You can take a look at the charts on Vatsim. I recorded this flight on Youtube (see below). The video starts right in the middle of a left turn over DF607. As you can see, the autopilot is overshooting the new course (248°) to DF608. In fact, it turns until 221° (at 0:55 in the video). Then it starts rocking right and left to get back on course. At 2:10 in the video, the airplane is crossing DF608 and is in a right turn to DF609 und DF611. The autopilot completely messes up this turn: At 03:41 the airplane has finished the 180° turn and is very close to the final leg. ATC will kill me if I fly like that online. The video on Youtube (you should watch with 720p or 1080p and fullscreen in order to be able to read the ND):

Do you have any ideas of what is going wrong here? I haven't seen and topics about this issue here in the forum, so am I the only one with this issue? Something that I am doing wrong with the 737 NGX? Kind Regards,Martin

[lowlevel 0]

I had exactly the same experience couple of weeks ago. On the very same approach. I think you are just too fast for these tight turns. Try reducing the speed of descending at constant speed. You can even try increasing the bank angle limit.Would be interesting to dig further into the specifics of that approach. It feels like a roller-coaster of horror - extremely tight turns, while descending ±2500-3000 fpm. whoa! cheersIgnat Ignatov

[Themis 0]

Martin,The turns are too steep for the speed you have selected. If you select 300 IAS will overshoot more. Lower your speed and the aircraft will perform well.

[Cristi_Neagu 134]

Yeah... that seems to be a speeding issue... Even so... the autopilot on this thing is not exactly a smooth operator. Whenever i engage the autopilot and i'm a bit off the path, the airplane just jerks a bit then banks quite steeply to intercept the track. It is within 45 degrees of bank, but still... just because it can doesn't mean it should.Also, the commanded roll rate seems to me to be a bit to fast. It appears to hold no regard for the actual turn it's supposed to make. No matter it it's a change of heading of 5 degrees or 90 degrees, it will use the same roll rate, and will try to hit 45 degrees for the turn. You can imagine how it would feel to be a passenger on this bird. For every change of heading you'd be thrown on your side and before you even realize what's going on, you're being thrown the other way while the airplane levels. Is this normal? Are we not supposed to leave the banks selector in Auto?

[B747tebs 80]

the autopilot is working as it should. if you drive a car with 250km/h into a corner, do you think curve assistant will help you there (if equipped)? also the bank rate depends on your speed as well.

[Simmer 2]

Hi. Ignat and Themis, you are right. I was too fast for this turn. With my "other" Boeing 767 I am usually already below FL100 at DF608. Thus, the speed restriction of 250 knots jumps in and the turns are fine. I just flew this transition again with the 737 NGX - this time I reduced the speed to 250 knots and the turn was okay. No overshooting. Thanks,Martin

[Paul Deemer 13]

According to the Charts at the link below you should have been much lower at the time and doing about 220 knots on the turns. http://www.fly-sea.com/charts/EDDF.pdf

[Simmer 2]

Hi Paul, the EDDF charts at fly-sea.com are a little bit out of date (Aug 2007). The charts on VATSIM are almost up to date (Mar 2011) and they do not contain any speed restrictions (except 250 knots below FL100) and only one alt. restriction "at or above FL80". But anyway, I was too fast. Martin

According to the Charts at the link below you should have been much lower at the time and doing about 220 knots on the turns. http://www.fly-sea.com/charts/EDDF.pdf

[Paul Deemer 13]

Well I am sure somebody familiar with EDDM will come along eventually and give you the right information. I think 220 knots is reasonable for those sharp loop Turns. Even though the new charts don't show speeds its probably still the same as the old charts.