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zachlog

STEP SIZE in FMC/PERF INIT Page

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I understand the definition of the is parameter (the size of one step for changing altitudes) but how and when is this used in flight ? I typically perform altitude changes through the MCP/ALT so what role does STEP SIZE play ? Is it useful, should it be used ?The same applies to COST INDEX. Should this be used ? Are there any guidelines anywhere on how values should be set ?

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The step climb is a fundamental technique used to maintain cruise efficiency. The default step size can be changed to suit operational requirements. I usually change it to 2000 from ICAO, but I'm not very well versed on NAT operations. It's very helpful in letting you know when it's beneficial to climb to the next higher cruise altitude, which occurs as you burn fuel and become lighter. I assume in the real world the step climb is not executed until you have clearnace, but it's helpful to have the FMC keeping track of it for you.

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Dan....thanks ?Do you know anything about the significance of COST INDEX and how it should be set (values) ?Zach

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Hi,both entries are NOT required for a normal flight, but it is very helpful. . .As Dan mentionned before I also change to 2000 from ICAO - differencebetween both is the area you fly in. In most areas the RVSM or MNPS rule governs, which means that you will fly from 0 to 179 ODD levels ( 310/330/350 ) and from 180 to 359 at EVEN levels ( 300/320/340 etc ) and the entry into the FMC helps you to define the time for step clb.In the other areas the conventional or ICAO rule takes place which means that the steps are 4000 ft ( from 290 to 330 then 370 for ODD ). Whereby the FMC time for step climb is not very accurate - I use the indication of MAX level minus 20 as my point of climb which is earlier than the FMC time and saves even more fuel !The COST INDEX is related to the fuel consumption and fuel price - I use in the 747 a C/I of 135 which gives me a speed of M .855 and a TAS of 495 kts - you can play around with this function of the FMC and find out for yourself . . .EDIT : Even in flight you can change the C/I and will let you see the change of TAS/Mach nbr and fuel flow, but this is very limited in the engines page - you will notice the FF change only over longer period of time.Karl-Heinz - EDDI/THF " Tempelhof "" The mother of all airports " - Sir Norman Fosterhttp://www.precisionmanuals.com/images/forum/747400.jpg

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