Let’s pick this up right where the buffet boundary leaves off — Page 452, Lesson 559

Let’s pick this up right where the buffet boundary leaves off. I want to walk you through how a pilot actually uses the buffet onset chart from the aircraft flight manual to find two critical altitudes: the Manoeuvre Ceiling at 1.3g and the Aerodynamic Ceiling at 1.0g. First, the chart itself. It’s a graph that plots Mach number against altitude, with lines for different aircraft masses and centres of gravity. You’ll see a vertical solid red line and a vertical dashed red line, plus horizontal dashed red lines. These are your working lines. The example is from Airbus, and it’s built around a specific set of data: a mass of 110 tons, a centre of gravity at 30% MAC, and a cruise Mach number of M 0.8. Let’s start with the 1.3g altitude. The notation “1.3g” means 1g plus 0.3g. So at this altitude, the aircraft can sustain an additional 0.3g increment — say from a gust or a bank — without the wing starting to buffet. To find it, you follow the vertical solid red line upwards from the 1.3g mark until it meets the 110 tons line. Then you go horizontally across to the 30% CG vertical line. From there, you move parallel to the CG reference line — that’s a slanted line on the chart that accounts for how CG position shifts the buffet boundary — and again horizontally until you hit the M 0.8 vertical line. Now you’re at the altitude curve. You parallel that curve — meaning you follow its shape — and read off the flight level. The answer is FL405, so the 1.3g altitude is 40,500 feet. Here’s the operational meaning. If you fly above FL405 at this mass and CG, a gust, or a bank angle of less than 40°, could push the wing into buffet. And note the parenthetical: 40° of bank at high altitude is excessive. A normal operational maximum at high altitude would be 10° to 15°. So the 1.3g ceiling is your manoeuvre protection — it tells you how much load factor you can safely command before buffet onset. Now the buffet restricted speed limits. This is a separate read on the same chart. You follow the vertical dashed red line upwards from the 1g mark to the 110 tons line, then horizontally to the 30% CG vertical line, then parallel to the CG reference line, just like before. But now you observe the FL350 curve — the buffet boundary at flight level 350. At the high speed end, the curve does not reach the horizontal dashed red line. Why? Because M 0.84 — that’s MMO, the maximum operating Mach number — is the hard limit. You may not exceed MMO under any circumstances. So there is no high speed buffet restriction here; MMO is the binding limit instead. At the low speed end of the dashed red line, the FL350 curve is intersected at M 0.555. So under these stated conditions — 110 tons, 30% CG, FL350 — the low speed buffet restriction is M 0.555. That means you must not slow below M 0.555, or the wing will buffet on the low speed side. So the takeaway: the chart gives you two distinct answers. The 1.3g altitude of FL405 is your manoeuvre ceiling — the highest altitude where you can still pull 1.3g without buffet. And at FL350, your low speed buffet limit is M 0.555, while the high speed side is governed entirely by MMO at M 0.84. Both are read by tracing the same path — mass line, CG line, CG reference line — but one uses the solid red line for the 1.3g case, and the other uses the dashed red line for the 1g case.