buffet margins

[Mango]

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

[Mad (Flt) Scientist]

From the Airbus publication "Getting to Grips with Aircraft Performance", page 145:

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.

[zonnair]

Airbus excepts the cruise altitude (ALT CRZ) that provides a margin of .2. Higher it will not except.

[latetonite]

.3 is a luxury. In the old non-FMC planes you (and the flight engineer) decide for yourself. .2 and .25 were common.

[Mango]

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

[TruthHurts]

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