A330 to buffet or not to buffet?

[9 points]

Does the A330 experience high speed buffet?

Reference 1: The A330 FCOM Unreliable speed procedure preamble says:
"Buffet .... is a symptom of real overspeed."

Reference 2: The BEA AF447 Final report page 198 says:
"On the Airbus A330, the buffet phenomenon is only encountered on the approach to stall."

SO WHICH IS IT?

[ahramin]

Overspeed buffet is different from stall buffet in small but distinguishable ways. And overspeed buffet will be heard before it is felt. I've heard 330s as slow as .84 starting to give you high speed signs.

Stall buffet on the other hand is felt though the whole airframe and is much more sudden.

[HazelNuts39]

9 points,

Both quotes are taken out of context and therefore incomplete.

Re reference 1: The procedure says: Depending on the failure, the OVERSPEED warning may be false or justified. Buffet, associated with the OVERSPEED VFE warning, is a symptom of a real overspeed condition.

Re reference 2: BEA should have said that this applies at 1g within the normal envelope. High speed buffet is encountered beyond Mmo at 1g, or at more than approx. 1.5 g below Mmo.

The characteristics of 'low speed' buffet at low altitude are different from those at high altitude. The 'buffet onset boundary' shown in the FCOM or QRH is associated with local supersonic flow phenomena and I would expect the buffet to be similar at the 'low' and 'high' speed limits.

[9 points]

HN39

Clearly you have a better understanding of this than I.

Regarding reference 1 Thanks for pointing out the VFE aspect. My mistake.

Regarding the BEA report. If I understand you correctly you are saying that the quote in the report is taken out of context and that a better reference is the FCOM limitations buffet onset graph which shows examples of "M" buffet at .82 and 1.75g AND "Low speed" buffet at .58 and 1g.

So clearly the A330 does experience high speed buffet.

Correct?

[HazelNuts39]

9 points,

Yes, that is correct. The final report gives some background to this in paragraph 1.18.3.2 on page 151.

[9 points]

Thank you Sir.

[HazelNuts39]

So clearly the A330 does experience high speed buffet.

The A330 only experiences high speed buffet below Mmo at substantial 'g'. Pulling more 'g' increases the buffet, and less 'g' reduces buffet or gets the airplane out of it. So for recovery from buffet is doesn't really matter whether it's low or high speed buffet: stop pulling, pitch down, not up, gets the airplane out of buffet. Correcting the airspeed comes next - same as for stall recovery.

Buffet is a function of angle of attack and Mach number. Angle of attack is managed with the pitch control, airspeed is managed with thrust and drag and flight path angle.

[Louietheflyer]

I was going to advise you to have a little bit of lots of things and don't overload your plate...But then I saw you were using the other meaning of Buffet.

[White Knight]

Originally Posted by hazelnuts39

The A330 only experiences high speed buffet below Mmo at substantial 'g'.

Have experienced the hi-speed buffet a few times on 330 in cruise during wind-shift/ mountain-wave activity that causes speed rise. All around 1g. and below Mmo.

The aeroplane 'buzzes' for want of a better description! That's hi-speed rumble for sure!

[HazelNuts39]

White Knight,

Well, whatever it was, it doesn't sound like hi-speed buffet to me. More like small-scale turbulence in a shear layer.

Is there a report on the incident? Weight, altitude, Mach? Overspeed warning? Law change? A/P disconnect?

[hikoushi]

Definitely get the ".84 Rumble" in the 330-200. Some of them at least. If anyone knows, where does the first shockwave form on the 330 at the critical Mach number? Is it over the nose or the wing?

[Gysbreght]

hikoushi,

if you have a look at the buffet onset boundary in the QRH or FCOM, you'll see that the critical Mach number varies quite a bit, depending on angle of attack. At low Mach I would expect shockwaves to occur first on the wing, at high Mach I don't know. You'll have to wait for our resident aerodynamicist to chime in.

[320busboy]

Its not an A330, but on an 320 test flight at the over speed prior to spoiler deployment test I noticed a "sonic wave" unless anyone has a better explanation midspan on the wing (OB of the pylon). It was a blurry line running chordwise distorting your view looking through it. It moved inboard and outboard by a few feet as the wing loading changed. On a 727 I saw a similar thing at overspeed test but it was a v shape originating at the leading edge on the very wing tip as it was vibrating madly. Mach .917 if I remember correctly. Im not an aerodynamic person, so if someone knows exactly what it is, please let me know.

[320busboy]

On the subject of buffeting.. not high speed tho..
One thing for the 330 drivers, If you are on descent and use speedbrake 1/4-1/2 and notice any above normal buffeting you have a good excuse to have a chat with the guys/gals down the back of the plane. Ask them was there any buffeting.. Usual response is from the front.. a little (same as what ou felt).. from the mid.. nope all good... from the rear.. we were wondering when you were going to put the seatbelt sign on (difficulty in standing up). Ask your friendly engineer to check elevator actuators (all 4) and hinge bearing freeplay...

[Owain Glyndwr]

If anyone knows, where does the first shockwave form on the 330 at the critical Mach number? Is it over the nose or the wing?


Difficult to tell unless one has access to the actual data. The velocity over the forward fuselage is mainly dependent on skin curvature with a bit of AOA influence. Supersonic flow usually starts on the roof just behind the glazing panels - deceleration back to subsonic through a shock wave a bit aft of that.


Well placed that any boundary layer noise generated by shock wave instability is transmitted to the crew!


Guessing a bit, but it is not impossible that the "rumble" is that sort of noise rather than full-blown wing buffet.