Class A - En Route — Page 443, Lesson 547

I want to walk you through the en route phase for a Class A aeroplane. This is the phase that sits between take-off and landing, and the regulations treat it very seriously because they assume an engine could fail at any point during it. Let me define the phase boundaries precisely. The en route phase starts at 1500 feet above the departure aerodrome, and it ends once the aeroplane has reached 1500 feet above the intended destination aerodrome. So it's not measured from the runway itself — it's measured from that 1500-foot point on each end. And because this is a Class A aeroplane, the en route regulations account for engine failure. That means manufacturers and operators must ensure that the performance of the aeroplane after an engine fails is still able to meet the regulation requirements. The whole performance case is built around that engine-out scenario. Now, the structure of what I'm going to teach you. First, the climb to the en route altitude. Then the en route altitudes themselves and how they're calculated, along with the various flight speeds. And finally the descent — both the normal descent, and the descent forced by either engine failure or depressurization. Let's start with the climb profile, sometimes called the climb schedule. After a normal take-off, once the aeroplane configuration is clean — meaning the flaps and landing gear are retracted — a set climb profile is flown. Initially, the aeroplane climbs at a constant indicated airspeed. But here's the problem: if you keep climbing at a constant indicated airspeed, the Mach number rises. Beyond a certain altitude, the Mach number gets too high, and serious aerodynamic forces start to affect the aeroplane. In the 737 family, the maximum Mach number — that's MMO — is 0.82. So at some lower altitude, the aeroplane needs to change its climb profile to a constant Mach number climb. The altitude at which this change occurs is called the crossover altitude, or the changeover altitude. So in summary, the climb profile is: constant indicated airspeed initially, then at the crossover altitude, a constant Mach number climb. But there's a regulatory constraint on that initial speed. ICAO limits the maximum indicated airspeed to 250 knots below 10,000 feet. So for the majority of the 737 family, the climb profile is 250 knots indicated airspeed up to 10,000 feet. Then the aeroplane is accelerated to 280 knots, and the climb continues at 280 knots. As the aeroplane climbs, the Mach number will increase, and when the Mach number reaches 0.74, the aeroplane maintains a climb speed of 0.74 until the en route cruise altitude. Now, if you look at Figure 17.1, you'll notice that the crossover altitude is at about 25,700 feet. That's the point where the climb changes from the constant indicated airspeed of 280 knots to the constant Mach number of 0.74. Let me make sure you've got the key numbers straight. MMO is 0.82 — that's the maximum operating Mach number for the 737 family, and you must never exceed it. The climb schedule is 250 knots IAS up to 10,000 feet, then 280 knots IAS. The crossover altitude is about 25,700 feet, and from there you climb at Mach 0.74. And remember, the whole thing is governed by the engine-failure performance requirement — the aeroplane must still meet the regulations even if an engine fails during this phase. Take a moment to look at Figure 17.1 and see how the climb profile transitions at that crossover altitude.