Thank you for this detailed explanation.

At home I can look into Aircraft Operations Manuals [AOM], in use with KLM until ± 2006. In AOM Volume III, a Buffet Onset Boundary Chart [BOB] and two 1G stall speed charts (Pax/Combi and Freighter) can be found. BOB allows me to check Low Speed Buffet Onset values in terms of Mach-number and IAS: for highest weights from 15.000 ft and above; for weights < 340 metric tons [MT]: 20.000 ft and above. The lowest Mach number on BOB is 0,5 and the highest equals MMO.

In Excel, I created several spreadsheets that quickly calculate some desired value. It is then a matter of experimenting to find coefficients that make my Excel calculations match results, that are read off these AOM graphs; for all weights and altitudes. One of these sheets is calculating low speed buffet. The results, calculated by Excel, are within 1-2 knots (or less than 0,003M) of the values that are read off BOB. For all weights, at 15.000 ft or above.

Stall speed charts allow me to calculate the 1G coefficient of lift (CL), which turns out be ± 0,99-1,01 with Flaps UP. For this I am using TAS, that – once converted into IAS – match the numbers from the 1G stall charts. Pax/Combi numbers are slightly higher than those of the freighter. Adding 5 % to the speeds found, I can then compare numbers for VSS and VLBO.

VSS and VLBO differ quite a bit at 20.000 ft, VSS always being the lower speed. At 270.000 kg (600.000 lbs) I found a difference of 20 kts between the two. At 395.000 kg (870811 lbs) that difference is ± 24 kts. Even if the charted values were for MSL, and those 4-6 kts were added to account for the climb to 20.000 ft, the difference would still be significant. Slightly higher jumps are then calculated for the 1,3G margins (± 27 kts at 395 MT).

Another important difference is the value for CL. At VSS that value would have been ± 0,93 (395 MT). At VLBO, CL is ± 0,75 (still on the linear part of the lift curve): the aircraft isn’t stalled yet, it is just flying at a high Angle of Attack [AoA]. Because of this high AoA, air flowing over the top side of the wing is accelerated to the point where it locally reaches M=1, causing buffet (and perhaps trouble).

Is that the same buffet you were referring to in your last post: “Above 20000 feet, minimum maneuver speed is 1.3g to initial buffet”? Or were you referring to the buffet, that starts as the lift curve slope would begin to flatten out? All of my books address high speed buffet, none of them deals with low speed buffet. Just some internet sources (Skybrary, Flightliteracy…).

If VSS and VLBO indeed differ by so much around 20.000 ft, I would expect to see the red band jump as well. But apparently it doesn’t, as Hardy reported back. Still puzzled by that! Or still missing the point...

Regards,

Simon