Cost index

[FoxTROT]

Hi

I often hear the term Cost Index but I must admit I don't fully understand the term. How is the Cost Index for a particular leg determined and what factors are taken into account?

[beckers]

In simple terms it is:

Time(Maintenance) Cost
______________________ = Cost Index
Fuel Cost

So the higher the times costs, for example you are leasing the aircraft per hour at a high rate or maintenance is expensive, then you get a high cost index number. If fuel becomes expensive you get a low cost index.

Typical cost indexes for a 737 is 0-30
For a 747 is about 600.

The computer(FMC) then applies this cost index to find the most cost effective speeds to fly for that leg. The higher the cost index the faster it will fly. The lower the cost index the closer you fly to best endurance speeds. The cost index flight will also take winds into account. For instance fly faster into a headwind to save time and slower with a tailwind to save fuel.

Whereas flying on a long range cruise setting is the speed to fly long range in zero wind conditions. LRC only takes lift/drag for current weight and pressure altitude into account.

A good airline will adjust the cost index for each route as the costs vary per airport fuel, ticket pricing, maintenance.

So in simple terms us pilots can fly faster than cost index and save time. Us pilots can fly slower and save fuel. But the cost index will to both a bit both in the correct ratio to save the airline money.

[FoxTROT]

Thanks, appreciate it! Good explaination. Makes Cents (for the airline! )

[Jayson v Schalkwyk]

Beckers,

Do the crew then get a different CI for each sector when you doing a 4 leg day?

[sky waiter]

Yes we do get different cost indexes every leg however it takes a matter of seconds to program it into the FMC, the FMC needs a new cost index after every sector when we re-align the IRS.

We use a system called avplan which gives us fuel required flight time etc etc and takes into account weather which is updated 6 hourly.

A typical cost index for us though is around 16 however on some of the durban-jnb legs it goes up to around 28 (headwind component as per beckers expalantion)

[Robbie]

Attached a document by Boeing - "Cost Index Explained"

[Robbie]

..... and a document by Airbus.

[HAAN]

Before the price of oil went through the roof the typical CID (Cost Index) for a B737NG was about 27-30 now you looking at a CID-19.
This means the cruise speed goes down from M0.79 to about M0.76-77. So airlines do fly slower now to save fuel.

[FoxTROT]

@ Robbie

Really informative documents! Thanks very much for posting!