We abbreviate the Mach number for the low‑speed buffet as MMIN — Page 452, Lesson 557

Let’s start with the big picture, because this is one of the most important ideas in high‑altitude jet operations. For a given weight and configuration, the aeroplane will always stall at the same indicated airspeed. That’s a fixed number on your ASI regardless of altitude. But the Mach number at which that stall occurs — the low‑speed buffet — increases as you climb. So the same indicated airspeed corresponds to a higher Mach number the higher you go. We abbreviate the Mach number for the low‑speed buffet as MMIN. That’s your first term: MMIN, the Mach number at which the low‑speed buffet or stall occurs. Now, a similar buffet can happen at high speed. At very high speeds, close to the speed of sound, the compressibility of the air ahead of the aeroplane leads to the formation of shock waves, or high‑pressure waves. Those shock waves disturb the airflow over the wing, causing it to separate and create turbulent eddies. Just like the low‑speed buffet, those eddies buffet the elevator. That phenomenon is the high‑speed buffet. If you fly faster than this speed, and your wings aren’t designed to overcome those effects, you can get a high‑speed shock stall. The Mach number for the high‑speed buffet decreases with increasing altitude. We abbreviate it as MMAX, and on the graph it’s the backward‑sloping red line to the right. So now you have two limits: MMIN on the left, MMAX on the right. Below MMIN you stall at low speed; above MMAX you buffet at high speed. The range between them is the buffet margin — the speed range within which the aeroplane can fly. Here’s the critical relationship: the buffet margin decreases as altitude increases. The two lines converge. There is an altitude where, under 1g conditions, the low‑speed and high‑speed buffets are equal — the margin is zero. It is impossible to fly higher than that altitude. At that point, flying slower or faster than the single speed shown will stall the aeroplane. Even manoeuvring will initiate a stall, because manoeuvring increases the effective weight and raises the stall speed. That altitude is called the aerodynamic ceiling, or coffin corner. Now, to keep aeroplanes from operating too close to that dangerous point, an operational limit is set below it. Notice that a 1.3g manoeuvre moves the buffet speed lines to the faded red position on the graph. With that manoeuvre load, the Mach numbers for the low‑speed and high‑speed buffets become coincident at a lower altitude. That altitude is the 1.3g buffet limit altitude, or manoeuvre ceiling, and it’s usually about 4000 to 6000 feet below the aerodynamic ceiling. Finally, to calculate the buffet boundary more accurately, the pilot uses the buffet onset chart in the aircraft flight manual. That chart gives you the precise high‑ and low‑speed buffet limits for your actual conditions, rather than the simplified graph. So, to tie it together: MMIN rises with altitude, MMAX falls with altitude, the buffet margin shrinks, and the aerodynamic ceiling — coffin corner — is where they meet. The manoeuvre ceiling sits 4000 to 6000 feet below that, and the buffet onset chart is your precise tool for finding the real boundary.