Class A - En Route — Page 443, Lesson 549

We’re in the middle of the climb discussion, so let’s pick up with the crossover altitude and what happens when you change the climb profile. In the previous example, we had a climb where the aeroplane held 250 knots indicated airspeed up to a point, then accelerated to 280 knots, and eventually transitioned to a constant Mach number. Now I want you to see what happens if we increase that second indicated airspeed segment from 280 knots up to 325 knots. So the profile becomes: the aeroplane still climbs at 250 knots initially, 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, it settles into a climb at Mach 0.74. Now, the key observation here is the crossover altitude — the altitude where the indicated airspeed climb and the Mach climb intersect. With this faster profile, the crossover altitude is now lower, at 18,000 feet. So the rule you need to remember is: if you increase the indicated airspeed in the climb profile, the crossover altitude decreases to a lower altitude. That’s a direct cause-and-effect relationship. 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. It’s called VMO when you’re referring to it in terms of indicated airspeed, and MMO when you’re referring to it in terms of Mach number. 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. Now, here’s a subtlety in the nomenclature: when these speeds are referenced in terms of a Mach number, the abbreviation changes. MRC becomes MMR, which stands for Mach number for maximum range, and LRC becomes MLRC, which stands for Mach number for long range cruise. So you have to be careful — the same concept, but the abbreviation shifts depending on whether you’re talking about the speed in knots or the Mach number. Let me make sure you’ve got the distinction clear: VMO and MMO are the same limit expressed two ways — one in indicated airspeed, one in Mach. MRC and LRC are the range-optimising speeds, and their Mach-number counterparts are MMR and MLRC. The crossover altitude drops when you climb faster in indicated airspeed — that’s the key operational takeaway from this section.