When your flight will ignore Cost Index...

[Will]

Let's say hypothetically that your dispatch office knows ahead of time that your flight will ignore Cost Index for some arbitrary reason, and proceed instead with a constant speed in cruise, perhaps M0.85, and they set up your OFP flight accordingly.

It's easy enough to tell the aircraft to fly at M0.85 while in cruise, but what would you put for a Cost Index in PERF INIT when you are doing your preflight setup? I assume it would be desirable while on the ground to see the FMC's fuel burn predictions, and that would be difficult to do unless the FMC knows your intended flight profile. So can you tell the FMC on the ground to ignore Cost Index and keep a constant speed? Or would you set a CI that you guess would result in your flight planned speed?

Thanks for your help... I've been flying with CI for so long that I realized I don't know how to preflight the aircraft without it.

[Hardy Heinlin]

The predicted command IAS/Mach on each leg is based on the planned speed mode on each leg. It can be a SEL SPD, LRC, ECON (cost index), E/O, or RTA speed or a mode mix (if the NG FMC is installed) -- it's just what you set on the VNAV pages, especially on the CRZ page, or on the LEGS pages. The CI is just relevant if any leg has the ECON speed mode.

The LEGS pages show you the planned command IAS/Mach on each leg. That's the relevant data.


Regards,

|-|ardy

[Will]

I see. Set the intended airspeed, verify on the LEGS page. CI will only be taken into consideration when the speed mode is ECON. Where the speed has been changed via the FMC, the CI will be ignored. Thanks.

[Hardy Heinlin]

RTA also uses the CI when the RTA target is an "after" or "before" time frame rather than a point in time, and when that time frame can be reached at the current ECON speed. I.e. when the RTA mode is free to vary the Mach number, it will use the ECON speed (but "RTA" remains displayed).

[DougSnow]

Let's say hypothetically that your dispatch office knows ahead of time that your flight will ignore Cost Index for some arbitrary reason, and proceed instead with a constant speed in cruise, perhaps M0.85, and they set up your OFP flight accordingly.

Thanks for your help... I've been flying with CI for so long that I realized I don't know how to preflight the aircraft without it.

I'll do that crossing the NAT, as in the new NAT DOC 007 Constant Mach is now mando on the NAT again. So I'll plan a CI to the Ocean, Const Mach Across, then back to CI on the far side.  It's not arbitrary, its due to airspace requirements. Except when planning a flight with a specific speed restricting MEL/CDL, we always plan CI. If you as a crewmember know that CI will not work for today's flight (say widespread Mod Turb), it is your responsibility to call the dispatcher (under a 121 joint control mindset) and we can discuss it.

----------------------------------------------------------------------
N573A                          350   073 .83  251/039  M55  90.6  62.3
            N5224.0   0103 ... 055   056 480     P038  M01
NEEKO      W05550.0   0313 ... 579  2364 518        1  372 ..... .....
----------------------------------------------------------------------
NOTE: CRUISE SPEED CHANGED TO M.83 AT 5450N
----------------------------------------------------------------------
DCT                            290   083 .83  300/038  M39  84.1  68.8
54N050W     N5400.0   0027 ... 025   063 494     P027  P03
5450N      W05000.0   0340 ... 231  2133 521        0  375 ..... .....
----------------------------------------------------------------------
blah blah
----------------------------------------------------------------------
DCT                            290   096 .83  234/013  M34  43.0 109.9
            N5500.0   0004 ... 010   090 500     P012  P08
NETKI      W01400.0   0603 ...  34   883 512        0  415 ..... .....
----------------------------------------------------------------------
NOTE: CRUISE SPEED CHANGED TO CI 150 AT DEXEN
----------------------------------------------------------------------

I know some of our captains (usually a check airman demonstrating to new pilots, and when we're on a random out bothering no one) who will, when the air is calm on the NAT, will fly a CI anyway. They'll find that CI that equates out to M0.83 and fly CI as opposed to M0.83.  The other oceans it's always CI.

Don't sign for a plan that you know will not fly. That is very poor airmanship.  In 121, the dispatcher is also a part of the crew.