Class A - En Route — Page 443, Lesson 549

Let’s pick this up right where the climb profile left off, because the crossover altitude is the key idea that ties into the speeds we’re about to look at. I want to walk you through what happens when you climb using a faster indicated airspeed. In the standard climb schedule, you might hold 250 knots indicated airspeed up to 10,000 feet, then accelerate to 280 knots and climb at that speed, and once the Mach number reaches a certain value, you switch to climbing at a constant Mach number. That point where you switch from climbing at a constant indicated airspeed to climbing at a constant Mach number is called the crossover altitude. Now, in this example, we increase the second part of the indicated airspeed climb from 280 knots up to 325 knots. So the aeroplane still starts climbing at 250 knots, but once it reaches 10,000 feet, it accelerates to 325 knots and climbs at that speed. Then, once the Mach number has increased to 0.74, the aeroplane climbs at that constant Mach number of 0.74. Here’s the crucial relationship: notice that with this faster indicated airspeed, the crossover altitude is now lower — it’s 18,000 feet instead of a higher value. So the rule to remember is: if you increase the indicated airspeed in the climb profile, the crossover altitude decreases to a lower altitude. That’s the direct cause-and-effect you need to hold onto. Now, before we move on to the en route altitude itself, I want to briefly detail a few important speeds, most of which you’ve already covered in earlier work. These speeds are commonly expressed as Mach numbers. The first is the maximum operating speed. When it’s expressed in terms of indicated airspeed, we call it VMO; when it’s expressed as a Mach number, we call it MMO. For the majority of the 737 family, VMO is 340 knots and MMO is 0.82. Flying beyond these speeds in a commercial operational context is not permitted — it may cause either structural damage or a high-speed stall. So these are hard limits you must respect. The next two speeds are used in reference to describing the range of the aeroplane, and they were mentioned in the en route chapter of the general performance principles section. These are the maximum range cruise speed, abbreviated MRC, and the long range cruise speed, abbreviated LRC. When these speeds are referenced in terms of a Mach number, the abbreviation changes: MMR stands for Mach number for maximum range, and MLRC stands for Mach number for long range cruise, respectively. So to summarise the naming convention: VMO and MMO are the same limit expressed two ways — one in knots indicated airspeed, one in Mach number. And MRC and LRC are the range-related speeds, which become MMR and MLRC when you’re talking about them as Mach numbers. Keep those abbreviations straight, because they’ll come up again when we discuss en route altitude selection.