FLCH mode: pitch changes a lot

[Guest]

Hi,I know that this has already been mentioned, but I still have the problem:When in FLCH mode, the pitch angle of the 757 is not constant (within certain limits). Instead, the pitch goes up and down like a sine curve. As a result, the V/S changes from 2000 to 4000 (example) and back again. Did I make a mistake or is this a problem that needs to be fixed?I haven't entered a flight plan, just wanted to test the FLCH capabilities.greetingsMalte DietschEDDH

[Guest amerton]

Hi,Well no, you did not make a mistake ;-)The speed on pitch modes (VNAV SPD / FL CH) can exhibit this behaviour on some systems, and it seems the slower the system, the more oscillations. It is very very difficult to tune this mode for a wide range of setups, so we had to compromise.To reduce this effect, I suggest to relax a bit the speed target.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest]

Hi,ok then I should use V/S mode instead.Are you planning to fix this problem in the near future? Just asking, because VNAV and FLCH are very important to me.Nevertheless, the 757 is a great product!regardsMalte

[Guest amerton]

Well, as I said it is very difficult to have valid tunings for all systems, unfortunately.I'm not sure I understand why you'd use vs instead. The idea behind FL CH / VNAV is to have engines at set limit thrust and control speed via pitch. Although there might be oscillations the speed control is OK. Having VS set and the AT in SPD mode would not be quite identical in functionality. athonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest]

Anthony,I get that, too, both on climb and descent. I'm not running a slow machine (P4/1.6GHz with GeForce 2 64Mb and SB PCI 512). The policy of the VA I fly for is to set CLB to 300/0.78, which is a bit relaxed from the default 315/0.81 CLB setting. It isn't BAD, but it does happen, and I truly feel for my poor passengers!Bottom line is...I concur, it's a problem.Kurt Kalbfleisch

[Guest amerton]

Oh yeah, no doubt it is a problem :-)We have to model those sick bagsanthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest]

that's right, but think of the passengers...2000fpm,4000fpm,6000fpm,4000fpm,2000fpm! I want a steady climb rate. FLCH sets a certain pitch angle in order to maintain a given speed. When FLCH has found out the correct pitch, the vertical speed will be quite steady (until you change the speed on the MCP).I just don't want to use it because of the oscillations. Just think of the poor passengers! :-)And - it's not realistic.If there were just small oscillations, it would be ok. But in my case, the v/s went from 2000 fpm and 6000 fpm and back again and that is not acceptable for the passengers. In 767 PIC, it is also possible to have a steady FLCH/VNAV climb/descent, so why isn't it possible in the 757? ;-)regardsMalte