Now, let’s define the terms precisely — Page 452, Lesson 557

Let’s start with the big picture. For a given weight and configuration, an aeroplane will always stall at the same indicated airspeed — that’s a fixed number on the airspeed indicator. But the Mach number at which that stall, or the low speed buffet, occurs actually increases with altitude. That’s the first key relationship in Figure 17.8. Now, let’s define the terms precisely. The Mach number for the low speed buffet is abbreviated MMIN. So as you climb, the Mach number at which the low speed buffet begins gets higher. The graph shows this as a line that slopes upward to the right. Now, there’s a similar phenomenon 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. These shock waves disturb the airflow over the wing, causing it to separate and create turbulent eddies. Just like the low speed buffet, these eddies buffet the elevator — that’s the high speed buffet. If you fly faster than this speed, you may get a high speed shock stall in an aeroplane whose wings are not designed to overcome such effects. The Mach number for the high speed buffet is abbreviated MMAX, and it decreases with increasing altitude. On the graph, it’s shown by the backward sloping red line to the right. So now we have two Mach numbers: MMIN below which you can’t fly, and MMAX above which you can’t fly. The speed range between them is called the buffet margin. Here’s the critical point: the margin between the low speed and high speed buffets decreases with increasing altitude. There is an altitude where the low speed and high speed buffets are equal under 1g conditions — and it is impossible to fly higher than that. Flying slower or faster than that speed will stall the aeroplane. In fact, even manoeuvring will initiate a stall, because manoeuvring increases the effective weight and increases the stall speed. This altitude is called the aerodynamic ceiling, or coffin corner. To prevent aeroplanes from operating too close to that altitude, 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. Now the Mach numbers for the low speed and high speed buffets are coincident at a lower altitude. That altitude is called the 1.3g buffet limit altitude, or manoeuvre ceiling, and it’s usually about 4000 to 6000 feet below the aerodynamic ceiling. Finally, to more accurately calculate the high and low speed buffets, or the buffet boundary, a pilot uses the buffet onset chart found within the aircraft flight manual. That’s the practical tool you’d use in the cockpit. So, to tie it together: MMIN rises with altitude, MMAX falls with altitude, the buffet margin shrinks as you climb, and the aerodynamic ceiling is where they meet under 1g. The manoeuvre ceiling, at 1.3g, is lower — typically 4000 to 6000 feet below. That’s the core of Figure 17.8.