how do pilots

[jrpilot]

Hello,

How do pilots select a good criuse speed and a good criuse height because ussually going west a pilot is lower and slower than a pilot going east is higher and faster is there any way I can find out by math or any sort of how to get these measurements.

Thank you

[Nexus]

Well, the optimal cruise speed (and flight level) is calculated by the FMC or resp. aircrafts flight management system. Factors to take into account here is gross weight, zero fuel weight, cost index etc. With this information, the computer can give you best (which for an airline translates into MOST ECONOMICAL) cruise level and cruise speed. The Aircraft Performance page looks like this on Boeing FMC's


Regarding the east and west thingie you mentioned, it is a wee bit more complex than most people think.
Above FL245 (where most jets fly) you obey under the semicircular rule =
Tracking 000 - 179

[OTTOL]

Geez!  Am I glad I don't fly one of those NEW jets with FMS, or else I might end up getting confused!!! Step aside Nexus before you hurt yourself!  ::)  
It's actually very simple. I'm going to give you a real world example. I planned a quick(from a planning standpoint)trip from Atlanta to Seattle using the current weather conditions. The great circle(GPS direct)true course for this trip is approximately 300degrees. The winds and temperatures over LWS(Lewiston, ID)are shown below. I use this weather reporting point because it has the most extreme conditions for the route.
LWS--->  Winds and Temperatures aloft:

FL 180: 270degrees,54kts,-18degrees
FL 240: 270degrees,71kts,-29degrees
FL 300: 260degrees,92kts,-45
FL 340: 250deg,111kts,-52
FL 390: 250deg,112kts,-57
With these numbers, I can now make a quick decision as to which altitude I'm using. Normally, I would use 350 or 390, but obviously I wouldn't want to fly into a 112!! knot headwind. At 240 the wind is over 40 knots less!! Why not go all the way down to 180? or lower? ! ?   A jet needs to stay at a relatively high altitude for fuel economy. So NOW it's a question of weather the goundspeed EARNED is worth the extra FUEL BURNED.
  Now it's time to go back to ATL. Guess what altitude I want. 230-250-270-290-330-370-410-450 are all proper altitudes for an Eastbound flight(360deg-179deg). I'm using 450! If I can climb that high(key sentence). This is where Nexus' FMS is nice to have! Aircraft weight, and OAT(Outside Air Temp)are NOW crucial planning factors. ISA(standard temperature) at 390 is about -63degrees. If you look back at the 390 numbers, you'll see that it was ONLY -56, or ISA + 7degrees. In a BIG, fully loaded aircraft, like Nexus' 737, you can't climb to 450, or probably even 410 under these conditions. This should explain why you will see airliners "down low" during the winter.  ;)

As far as SPEED, the basic "rule of thumb" is: high mach with a headwind(fly closer to the A/C's MMO),and pull the power back with a tailwind(Usually closer to .74-.77 Mi on the big planes). These are realworld figures, and IMO don't apply to flying F$. I have tried to "pull-back the power" and save fuel, and I just don't think that the programming is that detailed, re: fuel burn dynamics.