Class A - En Route — Page 443, Lesson 547

Let’s start with the en route phase itself. I want you to picture the whole flight as a series of regulated segments. The en route phase begins at 1,500 feet above the departure aerodrome, and it ends once the aeroplane has reached 1,500 feet above the intended destination aerodrome. So it’s not the whole climb and not the whole descent — it’s the middle portion, bracketed by those two 1,500-foot reference points. Now, the key regulatory idea here: for Class A aeroplanes, 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. So the whole en route performance story is built around the assumption that an engine may quit, and the aeroplane must still comply with the rules. Let me lay out the structure of what we’re covering. First, the climb to the en route altitude. Then the en route altitudes themselves and how they’re calculated, plus the various flight speeds. Finally, the descent — both the normal descent and the descent forced by either engine failure or depressurization. Let’s go into the climb profile, or climb schedule. After a normal take-off, climbing to the en route altitude is straightforward. Once the aeroplane configuration is clean — meaning the flaps and landing gear are retracted — a set climb profile or climb schedule is flown. Here’s the core idea. 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 as you go higher. 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 the climb profile in summary: initially climb at a constant indicated airspeed, then at the crossover altitude, climb at a constant Mach number. But there’s a regulatory limit on that initial indicated airspeed. ICAO limits the maximum indicated airspeed to 250 knots below 10,000 feet. So for the majority of the 737 family, the climb profile works like this: 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 increases, and when the Mach number reaches 0.74, the aeroplane maintains a climb speed of 0.74 until the en route cruise altitude. Now, using Figure 17.1, you’ll notice the crossover altitude is at about 25,700 feet. That’s the point where the climb transitions from constant indicated airspeed to constant Mach number. Let me make sure you’ve got the key numbers and terms locked in. MMO is the maximum operating Mach number, and for the 737 family it’s 0.82. The crossover altitude is where you switch from constant IAS to constant Mach — about 25,700 feet for this example. The climb schedule itself: 250 knots IAS up to 10,000 feet, then accelerate to 280 knots, climb at 280 knots until Mach 0.74, then hold 0.74 to cruise altitude. And remember, the 250-knot limit below 10,000 feet is an ICAO rule, not just a 737 preference. That’s the climb profile. Next we’ll move into the en route altitudes and how they’re calculated, and then the various flight speeds.