The Cost Index (CI) function of the Flight Management Computer (FMC) is an important and often misunderstood feature of a modern airliner. Apart from real-world cost savings in fuel, differing CI values alter airspeeds used during the climb, cruise and descent phase of a flight. Certainly, the CI value is not a pressing issue for a virtual pilot flying a simulator, but to an operating airline in a fuel-expensive environment, differing CI values can equate to thousands of dollars saved.
LEFT: Screengrab from CDU screen showing the Cost Index page in 'PERF INIT'.
Simply explained, the CI alters the airspeed used for economy (ECON) climb, cruise and descent; it is the ratio of the time-related operating costs of the aircraft verses the cost of fuel. If the CI is 0 the FMC calculates the airspeed for the maximum range and minimum trip fuel (lower airspeed). Conversely, if the CI is set to the highest number, the FMC will calculate higher airspeeds (Vmo/Mmo) and disregard any cost savings.
In practice, neither of the extreme CI values is used; instead, many operators use values based on their specific cost structure, modified if necessary to the requirements of individual flight routes. Therefore, the CI values will typically vary between airline operators, airframes, and individual routes.
LEFT: CDU showing Cost Index. A CI of 11 will generate significant savings as opposed to a value of 300. FMC is produced by Flight Deck Solutions (FDS) click to enlarge. A review of the FDS CDU can be read here.
Access to the CI is on page 1 of 2 in the ‘ACT PERF INIT’ page of the Control Display Unit (CDU) of the Flight Management Computer (FMC). It is on the left hand side lower screen and displayed ‘COST INDEX’. The range of the CI is 0-200 units in the Boeing 737 Classics and 0-500 units in the Next Generation (NG) airframes.
Fuel Verses Time and Money
There is a definite benefit to an airline’s fuel cost when the CI is used correctly. Bill Roberson in his excellent article ‘Fuel Conservation Strategies: Cost Index Explained’ states the difference between a CI value of 45 verses a CI value of 12 for a B737-700 can be in the order of $1790 - $1971 USD depending upon the price of fuel; the time gained by selecting the higher CI value (CI-12) is in the area of +3 minutes. Although these time savings appear minimal, bear in mind that airlines are charged by the minute that they remain at the gate.
Granted fuel savings are important, but so is an airline’s ability to consistently deliver on time, its passengers and cargo. It is a fine line between cost savings and time management, and often the CI will be changed before a flight to cater towards unscheduled delays, a change in routing, short or long haul flights, cost of fuel, aircraft weight, or favourable in-flight weather conditions (i.e. tailwind).
A higher CI value may be used by airlines that are more interested in expediency than fuel cost savings; the extra revenue and savings generated by an airline that consistently meets its schedule with less time spent at the gate may be equal to, or greater than any potential fuel savings. Boeing realizes that as fuel costs increase, airlines are reticent to only expend what is absolutely necessary; therefore, Boeing works with its clients (airlines) to determine, based upon their operating style, the most appropriate CI value to use.
Changing CI on The Fly'
Although not standard practice, the CI value can be changed during the flight. Any change in the CI will reflect on climb, descent and cruise speeds, which will be updated in the CDU and can be monitored via the 'progress' page of the CDU.
Figure 1 below compares the cost index values against climb, cruise, descent and recommended altitudes for the Boeing 757 air frame. Although these figures do not relate to the Boeing 737-800 NG, they do provide an insight into the difference in calculated CI values for climb, cruise, descent and recommended altitude.
Is the Cost Index Modelled in all Avionics Suites
The CI is modelled by the avionics suite, and whether it is functional depends on the suite used. ProSim737 and Sim Avionics have the CI modelled and functional, as does Project Magenta (PM), Precision Manuals Development Group (PMDG) and I-Fly.
Airline Cost Index Values
As stated above, the inputted CI value is variable and is rarely used at either of the extreme ranges. The following airline list of B737-800 carriers is incomplete, but provides guidance to CI values typically used. Note that the CI is variable and the values below may alter dependent upon airlines operations. A more detailed list can be found on the AVSIM website (Thanks Dirk (ProSim737 forum) for the link).
- Air Baltic CI – 28
- Air Berlin CI – 30
- Air France CI – 35
- Air Malta CI – 25
- Air New Zealand CI – 45
- Austrian CI – 35
- Fly GlobesSpan CI – 13-14
- Fly Niki CI – 35
- Hamburg International CI – 30
- KLM CI – 15/30
- Nord Star CI – 30
- Norwegian CI – 15
- QANTAS CI – 40
- Ryanair CI – 30
- SAS CI – 45-50
- South African CI – 50
- South West CI – 36
- Thomson Airways CI – 9
- Ukraine International Airlines CI – 28
- WestJet CI – 20-25
The CI is an important feature of the avionics suite that should not be dismissed. Whilst real-world fuel savings are not important during simulator flying, the altered airspeeds that a different CI value generates can have consequences for the distance able to be flown, climb, descent and cruise speeds.
CDU – Control Display Unit
CI – Cost Index
FMC – Flight Management Computer
Mmo – Maximum operating speed
Vmo – Maximum operating limit speed