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I use a tool called SimBrief to build virtual flight plans for a desktop flight simulator. As I've been getting more familiar with procedures, I've also been exploring more advanced settings in the builder as well as some parts of the OFP output.

One thing that I can't figure out even after consulting the tool's docs is how ascent and descent profiles are represented: for example, for a 737-800 on a medium-haul route, one of the available ascent profiles will be 250/280/78, while one of the decent profiles is 78/280/280.

How should these be read?

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    $\begingroup$ My guess would be 250 to 10,000 ft; 280 from 10k to crossover altitude, then Mach .78 after. Crossover altitude for that scenario would be roughly FL 325. Mach.78 seems a little high for a climb Mach, but I don't fly the 737. Also, 280 in the descent profile below 10k doesn't work, so my guess may be far from right. $\endgroup$ – J Walters Feb 28 '18 at 1:16
  • $\begingroup$ I agree with @JWalters, although I wouldn’t say .78 is at all fast for a modern airliner’s climb profile. That’s about what the 737 NG uses, with some adjustments for head/tail wind (typically maybe +/- 20 kts and .02M, ballpark). Agree that I’d expect the last # in the descent profile to be 250 kts, not 280. $\endgroup$ – Ralph J Feb 28 '18 at 15:23
  • $\begingroup$ @RalphJ That could well be; as I said, I don't fly the 737. The jets that I fly have a typical climb profile of 250 up to crossover then .62. $\endgroup$ – J Walters Feb 28 '18 at 19:13
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For the climb profile, the first two values are indicated airspeed -- below 10,000 and above 10,000 feet MSL, and the final value is a Mach fraction. So 250/280/78 would be a climb of 250 knots below 10,000', 280 knots above 10,000', and transition to a climb of Mach .78 somewhere in the high 20's or low 30's (i.e. at the point where 280 KIAS = M 0.78).

For the descent profile, it is the reverse, initial Mach number from Cruise altitude until reaching the transition to indicated airspeed, then the IAS above 10,000', then the IAS below 10,000'.

The 250/280/78 climb profile is very standard for the 737 NG and Max aircraft; most airlines fly them at something fairly close to those speeds, using a Cost Index which starts there and then the FMC applies an adjustment for headwind or tailwind. In a stiff headwind, the climb speed will increase up to maybe about 310 KIAS (as a ballpark) and Mach .80; with a strong tailwind the speed will go down to maybe 270 and Mach .76 (again, ballpark - exact Cost Index being used affects these adjustments).

The typical descent profile looks similar, although at least in the US we'd never use 280 knots below 10,000' -- it would always be 250 below 10. If the country you were flying in didn't regulate that, you might do 280 down to close to pattern altitude, but I don't know what countries would allow that, and what you'd gain in time saved (not much) you'd pay for in terms of risk if you hit a big enough bird on or near the windscreen.

But that's what those numbers are almost certainly referring to.

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