mtsu_av8er
Well-Known Member
With regards to a Flight Management System - what is a "cruise index", and how does how does it affect efficiency?
Cruise Index . . .
- Thread starter mtsu_av8er
- Start date
Ooh, ooh! I know this one!
On the -88/90 it's a cost index.
We've got values like OT1 which is 168 and values like ECN (Economy) which is 38.
The higher the cost (cruise?) index, the higher the cruise speed and it tries to plan climbs and descents to best improve your total TAS at the cost of fuel.
The lower settings will climb a little faster, cruise a little slower and descend nearer to CLmax (or something like that).
A lot of guys will spend most of the trip at ECN (38) cost indexes, but on the last leg where either of the pilots commute, that value can get as high as 300 which is beyond OT1.
Clear as a heavy lead curtain?
On the -88/90 it's a cost index.
We've got values like OT1 which is 168 and values like ECN (Economy) which is 38.
The higher the cost (cruise?) index, the higher the cruise speed and it tries to plan climbs and descents to best improve your total TAS at the cost of fuel.
The lower settings will climb a little faster, cruise a little slower and descend nearer to CLmax (or something like that).
A lot of guys will spend most of the trip at ECN (38) cost indexes, but on the last leg where either of the pilots commute, that value can get as high as 300 which is beyond OT1.
Clear as a heavy lead curtain?
mtsu_av8er
Well-Known Member
Ah-ha!!! It makes perfect sense!
No, really - it does!
So, it's not something that's preset in the FMS, but instead can be completely manipulated by the crew?
What role does dispatch play in this? Is reverse planning used? That is to say, is the cruise index a product of the flight planning process, or is the cruise index used initially to determine the altitude/fuel/times?
Am I making any sense whatsoever?
No, really - it does!
So, it's not something that's preset in the FMS, but instead can be completely manipulated by the crew?
What role does dispatch play in this? Is reverse planning used? That is to say, is the cruise index a product of the flight planning process, or is the cruise index used initially to determine the altitude/fuel/times?
Am I making any sense whatsoever?
For the most part, it stays at the ECN cost index. When you're running behind, they'll give you the "OT1" (on time - 1) cost index on the flight plan so you can up the climb/cruise/descent profiles.
However, there's a caveat.
At the big airlines, if you arrive too early, there's always a chance that you'll arrive at the airport and your arrival gate is still occupied, so dispatch will also print another cost index to insert in order to slow down the cruise and descent to minimize the time you spend on the tarmac waiting for gate space.
For the most part, dispatch at my airline will plan everything based off of the ECON setting. However, sometimes in the remarks section of our flight plan, it'll be denoted that it was planned on "OT1" because of enroute winds or whatever.
However, there's a caveat.
At the big airlines, if you arrive too early, there's always a chance that you'll arrive at the airport and your arrival gate is still occupied, so dispatch will also print another cost index to insert in order to slow down the cruise and descent to minimize the time you spend on the tarmac waiting for gate space.
For the most part, dispatch at my airline will plan everything based off of the ECON setting. However, sometimes in the remarks section of our flight plan, it'll be denoted that it was planned on "OT1" because of enroute winds or whatever.
mtsu_av8er
Well-Known Member
OK, I think I've got it now. I've run across the term a few times here and there, and it was beginning to kinda drive me wild!!!:banghead:
Let's put it this way. Telling the gate agents that you're ready to board ASAP, pestering ATC for shortcuts, telling the girls 'short taxi!!!' (thus starting the safety tape mega early) and running the cost index up to 300 saved about 30 minutes on a flight blocked at 1:58.
mtsu_av8er
Well-Known Member
That much of a difference, huh?
So, what did pilots do back in the old days?
In the 206, my Cruise Index is running 25 degrees hotter than normal on the EGT's, keeping an extra 1/2" MP and praying that it's smooth enough to go deep into the yellow on the Airspeed!:nana2:
So, what did pilots do back in the old days?
In the 206, my Cruise Index is running 25 degrees hotter than normal on the EGT's, keeping an extra 1/2" MP and praying that it's smooth enough to go deep into the yellow on the Airspeed!:nana2:
You can't make up that much time in the air beyond flying a little faster, direct routing and straight-in visual approaches.
But between flying .75 and .81 is about 40 kts I think.
All of that times savings is eaten up when the inefficiencies of ground control and not having the ramp ready (sorry Tim!) when you arrive.
But between flying .75 and .81 is about 40 kts I think.
All of that times savings is eaten up when the inefficiencies of ground control and not having the ramp ready (sorry Tim!) when you arrive.
BobDDuck
Island Bus Driver
For those of us flying shorter legs (.5 to 1.8) going faster in flight doesn't really help on the times too much. We use 290/M.74 for everything and pushing up to .77 only will save a minute or so at a large cost of gas. Dispatch gives us the figures to use, as there is no place in our FMS to input numbers for that. The only thing we can control is altitudes and then look at the gas/time usage, but as far as I know that doesn't take into account real time wind data.
typhoonpilot
Well-Known Member
mtsu_av8er said:
Cost index is derived from a formula based on the cost of operating the aircraft per hour in relation to the cost of fuel. The higher the cost of fuel, the lower the cost index. Since the massive run-up in jet fuel prices over the last two years we've seen our cost indexes come way down. We used to cruise around .84 mach, now it is closer to .825 with very slow descent speeds to boot.
In the 777 a cost index of 0 would be the most economical fuel burn. I can't remember the max value off the top of my head because we never use it, but it could be 999 which would be a very high speed climb, cruise, and descent.
TP