We're starting a brand-new chapter together — Chapter 17, Class A — En Route — Page 443, Lesson 546

We're starting a brand-new chapter together — Chapter 17, Class A — En Route. This is where we take everything we've built about mass and balance and performance, and we apply it to the actual cruise phase of a flight. Let me give you the roadmap of what this chapter covers, because it's a big one. We begin with the En Route Phase itself, then we get into the Climb Profile and Climb Schedule — that's how we get the aircraft up to cruise altitude efficiently. From there we move to Cruise Speeds, and then a really important operational concept called the Cost Index, which ties speed choice directly to economics. After that we look at Cruise Altitudes, then the Aerodynamic Ceiling and Manoeuvre Ceiling — these are the limits of how high and how fast we can fly. Then comes Buffet Onset, which is about the aerodynamic warning we get before the wing stalls. We cover the Normal Descent, then the emergency case of Depressurization, and then Engine Failure and Drift Down — that's the critical scenario where we've lost an engine and the aircraft can no longer maintain altitude. We finish with Obstacle Clearance Requirements, the Range Limit Following Engine Failure, and finally ETOPS — Extended-range Twin-engine Operations, which governs how far a twin can fly from a diversion airfield. So the whole chapter is really about the performance envelope of the aircraft during the en route phase, and how we manage it — both in normal operations and in the degraded cases like engine failure or loss of pressurization. Let me start with the first topic: the En Route Phase. This is simply the portion of the flight between the end of the climb and the start of the descent — the cruise segment. But the chapter isn't just about flying straight and level; it's about managing the aircraft's performance within its limits throughout that phase. Then we get to the Climb Profile and Climb Schedule. The climb profile is the path the aircraft follows upward — the relationship between altitude and distance. The climb schedule is the specific plan of speeds and power settings we use to fly that profile. The key idea here is that we don't just climb at one speed; we follow a schedule that's optimized — typically a faster indicated airspeed during the climb, which trades a bit of climb rate for better forward progress and engine cooling. There's a figure showing a typical climb schedule using a faster indicated airspeed — that's . The point is that the schedule is a deliberate choice, not an accident. Now, Cruise Speeds. Once we're at cruise altitude, we have several speed options. The chapter introduces the Long Range Cruise speed — that's the LRC — which is the speed that gives us the best fuel mileage over a long distance, essentially the most economical cruise speed for range. There's also a graph showing the relation between the Long Range Cruise speed and the Cost Index — that's . The Cost Index is a number that the operator sets, and it tells the flight management computer how to trade off fuel cost against time cost. A low Cost Index means we prioritize fuel economy — we fly slower, closer to LRC. A high Cost Index means we prioritize time — we fly faster, burning more fuel but getting there sooner. So the Cost Index directly determines our chosen cruise speed, and the graph shows that relationship. Then we have Cruise Altitudes. This is about choosing how high to fly. The choice isn't arbitrary — it's driven by the aircraft's weight, the temperature, and the performance limits. Higher is usually more fuel-efficient because the air is thinner and drag is lower, but we're limited by the aircraft's ability to climb and maintain that altitude, and by the ceilings we're about to discuss. That brings us to the Aerodynamic Ceiling and the Manoeuvre Ceiling. The aerodynamic ceiling is the maximum altitude at which the aircraft can still maintain a given rate of climb — it's the point where the excess thrust just barely allows us to climb. The manoeuvre ceiling is the altitude at which the aircraft can still perform a specified manoeuvre, like a banked turn, without stalling. These are two different limits, and the chapter treats them separately because they constrain us in different ways. Then Buffet Onset. Buffet is the aerodynamic vibration you feel when the airflow over the wing starts to separate — it's the precursor to a stall. The buffet onset is the point at which that vibration begins, and it defines a boundary on the flight envelope: at high altitude and high speed, or at high load factor, we can hit buffet onset even though we're not near the stall in the traditional sense. The chapter will show us how to stay clear of it. After that, the Normal Descent — that's the planned, controlled descent from cruise to the approach phase, managing speed and altitude in a stable way. Then Depressurization — this is the emergency where the cabin loses pressure at altitude. The immediate response is an emergency descent to a lower altitude where the air is breathable, and the chapter covers the performance implications of that rapid descent. Then the big one: Engine Failure and Drift Down. When an engine fails at cruise, the aircraft can no longer maintain its altitude. It will drift down — that's the term — to a lower altitude where the remaining engine can just barely sustain level flight. The drift down altitude depends on the aircraft's weight and the outside temperature. This is a critical performance calculation because it determines whether we can clear obstacles and how far we can fly. That leads to Obstacle Clearance Requirements — the rules that ensure our drift-down path keeps us safely above terrain and obstacles. Then Range Limit Following Engine Failure — how far the aircraft can fly on the remaining engine, which is a key factor in route planning. And finally ETOPS — Extended-range Twin-engine Operations. This is the regulatory framework that allows twin-engine aircraft to fly routes where a diversion airfield is more than a certain distance away, based on the aircraft's demonstrated reliability and the operator's procedures. So that's the whole chapter laid out. We're going to work through each of these in turn, and they build on each other — the climb schedule feeds into the cruise speed choice, the ceilings and buffet onset define the envelope, and the engine failure cases define the emergency limits. Let's start with the climb profile and schedule in detail.