Class A - En Route — Page 457, Lesson 560

Let’s pick this up right where the buffet onset chart left off, because that chart is the key to the first two numbers we’re about to read. We’re looking at a specific aeroplane at a weight of 150 tons. The first thing we want from the chart is the aerodynamic ceiling at that weight. Here’s the procedure, step by step. You start at the 1g point on the chart, and you follow the vertical dashed red line straight upwards until you hit the 150 tons plot. Once you’re on that plot, you move horizontally to the left, and along the way you pass through a CG correction — that’s the centre of gravity correction, which adjusts the result for where the aeroplane’s weight is distributed. You keep going left until you reach M 0.8, which is Mach 0.8, the cruise Mach number. At that point you read across to the interpolated altitude curve, and the chart gives you an aerodynamic ceiling of FL390 — that’s flight level 390, or 39,000 feet. Now the second thing the chart gives us is the load factor and bank angle at which buffet occurs. Buffet is the aerodynamic buffet — the shaking you feel as the airflow starts to separate over the wing. Using the same data, from M 0.8 you follow the dashed blue line, and it gives you a 54° bank angle, which corresponds to a load factor of 1.7g. So at this weight and Mach number, if you bank to 54 degrees, you’re pulling 1.7g, and that’s the point where buffet onset begins. That’s your manoeuvre margin — the bank angle and g-load you can pull before the wing starts to buffet. Now let’s move on to the normal descent. When the aeroplane gets close to the destination airfield, it reaches a point that marks the beginning of the descent. That point is called the top of descent. To initiate a descent, first the thrust must be reduced, and then the nose is lowered so that the weight acts forwards to balance the drag. That balance of forces — weight acting forward against the drag — is what ensures a constant speed can be maintained during the descent. The descent profile is almost the reverse of the climb profile. Let me give you the climb first so you can see the mirror image. A typical 737 climbs initially at 250 knots, then at 10,000 feet the speed changes to 280 knots, and then at the crossover altitude a speed of Mach 0.74 is maintained. The descent is flown in reverse: initially at Mach 0.74, then at the crossover altitude the speed is kept constant at 280 knots, but when 10,000 feet is reached, no more than 250 knots must be flown. So the speed schedule is exactly the climb schedule, just backwards. Now, if at any point air traffic control asks the pilots to expedite the descent — to get down faster — the only action the pilots take is to deploy the speed brakes. That increases the drag. And because the drag has increased, it must be balanced by more weight apparent thrust, so the nose is lowered. Lowering the nose increases both the angle of descent and the rate of descent, exactly as ATC instructed. Now we come to a different kind of descent — the descent that follows either depressurization or engine failure. In flight, an engine failure or a pressurization failure forces a premature descent, and this makes the performance very constraining, especially over mountainous areas. Let’s take the depressurization case first. When we suffer a pressurization failure, the procedure is a little different from the engine failure case. At high altitudes, the oxygen pressure in the cabin is insufficient to support life, so oxygen is provided for both crew and passengers through oxygen masks. However, the amount of oxygen carried is limited. Therefore, the aeroplane must descend as rapidly as possible to 10,000 feet, where there is sufficient oxygen pressure, before the oxygen supply runs out. So the constraint here is time — you have a finite oxygen supply, and you must get down to 10,000 feet before it’s exhausted. That’s the depressurization descent. Next we’ll look at the engine failure case and the drift down procedure, which is a different beast entirely.