FMC Fix Speed Restrictions overridden by Flap Schedule Speeds?

[Guest meyomyx]

Hi all,When flying a STAR to an Approach (or actually on departure too) with speed constraints, I note that all is well as we approach the first fix with a speed constraint but then ....... it would seems as though the fix speed constraints are overridden by flap schedule speeds or sub 10k speeds so -we have a 285kt descent to a decelleration point before a fix with a 175 kt speed restriction. The FMC reflects this. As soon as I select flaps, the limited speed increases to 211. The aircraft will then merrily only fly to the flap schedule speeds rather than fix speeds.Similarly on departure, ascent above 10000 seems to remove any speed restrictions at subsequent fixes (this is a high airport!) despite them showing in the legs page?Can anyone help?RegardsNick Kidd** hasn't bothered to do a fancy signature yet :) **

[Guest amerton]

Nick,On your first point : if you look at your des page you'll probably see 211/FLAPS. Speeds below that are not safe for your current flaps setting, so it effectively overrides your 175 restriction. If you extend your flaps further, you'll eventually be able to meet your 175 restriction.Second point : ? I have not observed this. Maybe a picture posted could help ?anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest meyomyx]

Hi TonySo in descent, the speed restriction is a minimum speed? Surely not? Not safe to fly below 211 kts without flaps extended etc?I need to go to work and think about that further ;)Nick

[Guest amerton]

Nick,Yes, that's the case. Flaps allow the aircraft to fly at "low" speeds.The FMC found out from its tables it was not safe to fly at your current weight below 211 in a clean configuration, so as long as you stay in this configuration, it will impose a (safe) minimum of 211.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com

[Guest meyomyx]

Yes yes yes Anthony I'm very comfortable with the concept of flaps, slats, Kruger flaps, fences, turbulators etc ........ it was the concept of an upper speed limitation (as implied in the FMC manual) becoming a mimimum speed limitation.I was unaware that this was the case!If the aircraft is commanded to accellerate (a go around for example), does the limitation speed switch to an upper limit for flap extension? Or ..... are the speeds assumed to be the same? In this way is the limiting speed 211 - even though that may not be the structural limiting speed for that flap setting?Will Speed Intervention still be governed by these mimimum speed restrictions?Interesting.Thank you for your interest :)Nick

[Guest amerton]

Sure, there are upper limitations, or Vmos, depending upon flap configs.anthonyAnthony MertonPrecision Manuals Developmenthttp://www.precisionmanuals.com