optimum altitude selection...
 

Hi all,

on the B 737 was once a recommendation (flight crew training manual) that selecting the next higher flight level, wich was stated in the flight plan, should be made, when optimum plus 1000' showed in the FMC. (e.g. current FL 360 ....Flight plan final FL 380 and FMC CRZ page showed OPT FL 370...when needed to climb to FL380 than its FL 370 plus 1000')

Is that still valid for the NG?

thanks

Basic rule of thumb for all a/c - when you are 1000' below OPT it's time to climb 1000' above OPT ALT.

former 737-800 nose gunner.

If you're flying the -800 with winglets you might see that the split between OPT and MAX is only about 800 ft. (possibly depending on CG location, haven't checked).
So if you are cruising at 360, and OPT indicated as 370, your max level could be 378 so in fact you're not able to take the next level yet.

Usually the inefficiency in being too low is greater than the inefficiency in being too high, so the most efficient way is to step climb to the next level (2000' higher) when the Optimum altitude is 500' above the aircraft (so that after the climb, the optimum altitude is 1500' below the aircraft, and moving towards it).

Our FMC's (wingletted data) had a split of 1700(+/-) between OPT and MAX ALT.

Non-wingletted split was 2500'.

It's the same for A320/330/340. You climb to Opt plus 1000' when in RVSM airspace as long as it not above Max. If you only climb when to say FL360 when Opt is FL360 then you're net Alt will always be below Opt. Airbus has a nice picture to demonstrate this but I have no idea how to cut it out of the FCTM and paste it here.

Checkie,

I must disagree. Whenever you are off optimum you lose, a fact of life.

When below optimum you can claw back a bit of the fuel burn by slowing down but when above no such remedy exists so it's all lose. Hence it works to climb later than FMS shows. This is regularly demonstrated on our B744 fleet.

Hi mustafagander,

I agree with your analysis and so do Airbus:

http://www.blackholes.org.uk/pp/step climb.jpg

The diagram above shows three step climb strategies with respect to OPT and REC MAX FL.
Strategy 1 provides the best trip cost, followed by 2 then 3.

And as the diagram 1 illustrates it is best to climb above Opt when Opt if halfway between where you are and where you want to be.

Eg: You are at FL350
FMS Opt is FL360
You should therefore climb to FL370

When Opt then reaches FL380 you will climb to FL390.