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

Let’s pick this up right where the buffet boundary work begins. I want to walk you through how a pilot actually uses the buffet onset chart from the aircraft flight manual — the Airbus example is Figure 17.9 — to find two specific altitudes: the Manoeuvre Ceiling at 1.3g and the Aerodynamic Ceiling at 1.0g. First, let’s get the terms straight. The Manoeuvre Ceiling is the altitude at which you can sustain a load factor increment of 0.3g above 1g — that’s 1g plus 0.3g equals 1.3g — without the wing starting to buffet. The Aerodynamic Ceiling is the 1.0g ceiling, the altitude where you can just maintain straight and level flight at 1g before buffet onset. The chart lets you read both off for a given mass and centre of gravity. Now let’s follow the worked example for the 1.3g altitude. The data supplied: aircraft mass is 110 tons, CG is at 30%, and we’re looking at Mach 0.8. Here’s the path on the chart. You start at the 1.3g point on the vertical axis and follow the vertical solid red line upwards until it meets the 110 tons line. Then you go horizontally across to the 30% CG vertical line. From there you run parallel to the CG reference line — that’s the sloped line that accounts for how CG shifts the buffet boundary — and again horizontally until you hit the M 0.8 vertical line. Now you must ‘parallel’ the altitude curve, meaning you follow the shape of the neighbouring altitude contours, and read off the flight level. That gives you FL405. So the 1.3g altitude is 40,500 feet. Here’s the operational meaning. If you fly this aircraft — at this mass and this CG — above FL405, then a gust, or a bank angle of less than 40°, could cause the wing to buffet. And note the caution in the text: 40° of bank at high altitude is excessive. A normal operational maximum at high altitude would be 10° to 15°. So the manoeuvre ceiling isn’t just a number — it’s telling you how much manoeuvre margin you have before buffet, and at high altitude that margin is deliberately kept small. Now the second part: the buffet restricted speed limits. Same data — 110 tons, 30% CG — but now we’re looking at the speed envelope at a given altitude, FL350. Follow the vertical dashed red line upwards from 1g to the 110 tons line, then horizontally to the 30% CG vertical line, then parallel to the CG reference line. Now observe the FL350 curve. Here’s the key point: the curve does not reach the horizontal dashed red line at the high speed end, because M 0.84 — that’s MMO, the maximum operating speed limit — is the cap. At the low speed end of the dashed red line, the FL350 curve is intersected at M 0.555. So under these stated conditions, the low speed buffet restriction is Mach 0.555, and there is no high speed buffet restriction — because MMO, the maximum operating Mach number, may not be exceeded under any circumstances. That’s the contrast to hold onto: low speed is limited by buffet onset, high speed is limited by the structural/operating Mach limit, not by buffet. Let me tie it together. The buffet onset chart gives you two things from one set of inputs — mass, CG, and Mach or altitude. The 1.3g altitude tells you your manoeuvre ceiling, where a small gust or a modest bank will trigger buffet. The buffet restricted speed limits tell you, at a fixed altitude, the low speed Mach below which buffet starts, and remind you that the high speed side is governed by MMO. Both are read by the same tracing technique: vertical to the mass line, horizontal to the CG line, parallel to the CG reference line, then along the altitude or speed curves.