Enter a CI in FMC. The CLB & CRZ speeds vary depending on gross weight; 1 parameter. Why does the descent speed remain constant for differing weights?
A friend has just been instructed to enter a very low CI which gives 245kts in descent. He wonders if he then speeds up to 250kts passing FL100? (I know this is a max, but everyone does it) Many years ago there was an easyjet thread on here about using very low CI's and it causing very slow descent speeds at high level and ATC asking them to speed up. This could only be done with power, thus negating the low CI to save fuel. i.e. it is a known failing. I wonder why another airline would try something that has already caused problems.
I realise this is 2 threads, but they are related, if only distant cousins.
Skyjob
3rd Nov 2012, 01:02
More in-depth VNAV knowledge is required, a reference to the FMC document is used:
ECON CLB: FMC generates CAS/Mach speed schedule based on a fixed speed throughout the climb, as a function of cost index and initial climb weight. The speeds are calculated using a reference point 180nm downpath of origin, at which point the TOC wind component and predicted OAT are used to make corrections to the fixed speed. CAS is increased up to 40kts with 150kts headwind, decreased up to -20kts with 150kts tailwind, increased up to 4kts with ISA -40C and decreased up to -30kts with ISA +40C [no effect between ISA 0C to +10C]. Econ CLB Mach is set equal to ECON CRZ at the estimated initial cruise conditions. Thus to derive the Econ CLB Mach, estimate TOC Weight, Altitude, Wind and Cost Index are required. PERF INIT provides input area for all calculations.
ECON CRZ: FMC generates a variable Mach based on altitude, cost index and weight. Secondary inputs are derived from wind and ISA deviation. Secondary input effects on speed are most pronounced at lower levels as margins are greater.
ECON DESC: FMC generates a fixed speed descent profile from final ECON CRZ Mach, used for initial descent, to a CAS which provides the lowest total cost over a fixed distance for a given cost index. At CI=0 the CAS is near L/D thus stretching the glide from an earlier TOD, allowing an earlier thrust reduction from CRZ thrust (higher fuel flow) to idle (lower fuel flow), resulting in significant trip fuel savings even though actual descent fuel used is higher due additional time taken to descend.
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ECON descent speeds are thus constant, but consider:
Once on a CAS below a restrictive value, a speed limit is placed upon VNAV not to exceed +10kts on the restriction (even if high on profile). This is the reason why the FMC defaults to 240/FL100 thus not to exceed 250kts in VNAV. Prior to FL100 in this case, VNAV will 'dive off' any excess hight in a trade to regain profile, but below FL100 the restrictive speed is always observed +10kts maximum.
Should VNAV thus command 245kts in ECON, it will plan for 245kts all the way from conversion until a more restrictive speed is encountered, eg 240/FL100 or 220kts at IF, etc.
Hope this helps. ;)
Mikehotel152
4th Nov 2012, 14:08
Nice clear explanation, thanks.