Class A - En Route — Page 457, Lesson 560

I want to walk you through the Class A en-route performance work, and we’re starting with the aerodynamic ceiling and buffet onset, then moving into the descent profiles. Let’s begin with the aerodynamic ceiling. This is the maximum altitude at which the aeroplane can maintain straight and level flight at a given weight — and here we’re told to determine it at 150 tons. The method is a graphical one on the buffet onset chart. You start by following the vertical dashed red line upwards from 1g — that’s the load factor of 1, which is straight and level flight. You continue that line up until it meets the 150 tons plot. Then you move horizontally to the left to M 0.8 — that’s Mach 0.8, 80% of the speed of sound — and you do this via the CG correction, the centre of gravity correction. The interpolated altitude curve then gives you an aerodynamic ceiling of FL390 — flight level 390, which is 39,000 feet on the standard pressure setting. Now, the same chart also gives you the load factor and bank angle at which buffet occurs. Buffet is the aerodynamic buffet — the airflow separating and causing the airframe to shake — and it marks the limit of the manoeuvre envelope. Using the data supplied, from M 0.8 you follow the dashed blue line, and that gives you either 54° of bank angle or 1.7g — a load factor of 1.7 times gravity. So the chart ties together weight, Mach number, load factor, bank angle, and altitude all in one picture. That’s Figure 17.9, the example of a buffet onset chart. Now let’s move to the normal descent. When the aeroplane gets close to the destination airfield, it reaches a point that marks the beginning of the descent — this is called the top of descent. To initiate a descent, firstly the thrust must be reduced, and then the nose is lowered to get weight to act forwards to balance the drag. That balance of forces — weight acting forward against the drag — 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 for a typical 737 first, so you can see the mirror image. The climb is initially flown at 250 knots, then at 10,000 feet this changes to 280 knots, and then at the crossover altitude, Mach 0.74 is maintained. The crossover altitude is the altitude where the indicated airspeed and the Mach number give the same equivalent airspeed — that’s where you switch from flying a constant IAS to a constant Mach. Now the descent is flown 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 you see the reverse: Mach 0.74 down to the crossover, then 280 knots, then capped at 250 knots below 10,000 feet — that’s the airspeed restriction below 10,000 feet. Figure 17.10 shows a typical descent profile for a medium range jet, and it shows the characteristics of the descent — what happens to the gradient and rate of descent throughout the profile. The gradient is the slope of the descent path, and the rate of descent is the vertical speed in feet per minute. Now, if at any point air traffic control asks the pilots to expedite the descent — to get down faster — the only action by the pilots would be to deploy the speed brakes. This increases the drag, which must be balanced by more weight apparent thrust — that’s the component of weight acting along the flight path. Therefore the nose is lowered, which increases both the angle and rate of descent, as per the instruction of air traffic control. The next descent to consider is the descent characteristic following either depressurization or engine failure. In flight, engine or pressurization failures force a premature descent, and therefore the performance becomes very constraining over mountainous areas — because you need terrain clearance while you’re coming down. Let’s look at depressurization 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 will be insufficient to support life, so oxygen will be 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 key constraint here is time: you have a limited oxygen supply, and you must get down to 10,000 feet before it’s exhausted. That’s the driving performance requirement for the depressurization descent.