Register Forms FAQ Wikiposts Calendar Search Today's Posts Mark Forums Read

 Tech Log The very best in practical technical discussion on the web

 15th Nov 2010, 03:23 #1 (permalink) Thread Starter   Join Date: Jun 1999 Location: over there Posts: 103 buffet margins Can anybody give me the answer to the following; 1)What is the minimum acceptable margin against the stall buffet at cruise? My answer is .3g but not sure. 2)What is the normal margin against buffet at optimum cruise? My answer would be 1.3g but then again not 100% sure Thanks Mango
15th Nov 2010, 03:56   #2 (permalink)

Join Date: Sep 2002
Location: La Belle Province
Posts: 2,029
From the Airbus publication "Getting to Grips with Aircraft Performance", page 145:

Quote:
 In order to maintain a minimum margin against buffeting and ensure good aircraft maneuverability, it is necessary to determine an acceptable load factor limit below which buffeting shall never occur. This load factor limit is generally fixed to 1.3. This value is an operating limitation, but not a regulatory one. The corresponding altitude is called the “1.3g buffet limited altitude” or “buffet ceiling”.
So 1.3 is a "normal minimum" but I'm sure someone has a counterexample out there somewhere.

The example that follows indicates a case where the selected cruise Mach could be in a range where the buffet margin is 1.3 or greater. There will be cases where the optimum lies in the middle of that range, and so the margin would be higher than 1.3.

 15th Nov 2010, 19:13 #3 (permalink) Join Date: Jul 2007 Location: The Netherlands Age: 33 Posts: 81 Airbus excepts the cruise altitude (ALT CRZ) that provides a margin of .2. Higher it will not except.
 15th Nov 2010, 20:00 #4 (permalink) Join Date: May 2001 Location: A few degrees South Posts: 806 .3 is a luxury. In the old non-FMC planes you (and the flight engineer) decide for yourself. .2 and .25 were common.
16th Nov 2010, 10:01   #5 (permalink)

Join Date: Jun 1999
Location: over there
Posts: 103
Quote:
 Airbus excepts the cruise altitude (ALT CRZ) that provides a margin of .2. Higher it will not except.
The .2g buffet is a airbus FMGC limitation but as "Mad (Flt) Scientist" stated the 1.3g is an operating limitation. So I'm still not sure what the answer is.

If anybody could clarify this for me that would be great.

mango

 18th Nov 2010, 07:48 #6 (permalink) Join Date: Oct 2010 Location: Below Radar Age: 47 Posts: 16 G margins and their representation on the PFD I am not aware that 0.3 is a limitation, but think of it like this: 0.2g you need to bank the aircraft at least 33 degrees to get the low of high speed buffets, and as you know that the airbus A320 family VLS represent 0.2g. 0.3 you need at least 39.7 almost 40 degrees of bank and the airbus long range VLS is representing 0.3g. VLS is corrected for Mach effect to maintain either 0.2g or 0.3g buffet margin (aircraft specific), I think it happens at or above 25000ft but not positive about the altitude. From an operation point of view you might want to have 0.3g margin in turbulence conditions. TruthHurts