Cost Index Zero Climb and Descent Data

[Mariano]

Hardy,

Would you be able/willing to share how/from where (tables?) PSX calculates CI0 climb and descent fuel, time and distance for B1F engines?

Thank you,

Mariano

[Hardy Heinlin]

Mariano,

that's a long story. First there is an algorithm (based on observations in the BA sims, not a table) that sets a command IAS/Mach referring to a given cost index, weight, SAT, altitude. Then there are tables for fuel flow referring to the selected THR REF or IDLE thrust. Then there are algorithms to mix RTE DATA and FORECAST wind and OAT values with the current IRS wind and ADC SAT. At that point it can predict the TAS and the groundspeed. With that it calculates the turn radius at waypoints where the course is to be changed, and so it can compute a more precise distance to go. Finally it calculates the ETA. -- Mmm ... I'm not willing to post the entire internal code of this :-)


Regards,

|-|ardy

[Mariano]

Thanks, Hardy.

Best regards,

Mariano

[Hardy Heinlin]

By the way ...

... to mix RTE DATA and FORECAST wind and OAT values with the current IRS wind and ADC SAT. At that point it can predict the TAS and the groundspeed.

It's actually plural. Speeds, not speed. Speeds for each leg, and even within each leg there are multiple speeds as each leg is split in multiple segments. The size of a segment varies, but is never shorter than 2 nm. Shorter segments are typically used in climb and descent legs as in that flight phase the wind & OAT data quickly changes with altitude. In cruise on long legs the segments can be longer. Imagine a 600 nm leg from airport A to airport B -- just a single waypoint on the LEGS page -- wasn't internally split in multiple segments: There would be just one groundspeed computation at the takeoff location and another one at the landing location; the FMC would predict the whole flight is to be flown at 160 kt or so.

In other words, a route with 100 waypoints may internally consist of thousands of waypoints, i.e. thousands of different speed segments, i.e. thousands of different leg time segments, i.e. thousands of different fuel subtraction segments ...


Regards,

|-|ardy

[Mariano]

Thanks, Hardy.

Seems that this subject is way above my pay grade ;-) Gladly, I found a Cost Index Zero descent (approximation) table, just missing a climb one.

On another matter.

I have noticed for a while that the PW4060s and PW4062s on our 767s start much slower than they do in PSX (PW4056s and PW4062s - APU supplying duct to an average of 30 PSI). It is a slower, gentler and cooler start compared to GEs. (I meant to record one today but forgot). Can any 747 guy confirm that this is also the case on the 744?

I assume that for the same pneumatic pressure, the start spool up rate should be similar regardless of aircraft type on which the engine is mounted, considering all other variables to be the same. (I could very well be dead wrong).

Best regards,

Mariano

[Hardy Heinlin]

The PW start model in PSX agrees exactly with a video of a 744 PW start which I used for my reference to tune my algorithms. I don't think that every PW engine of any age, under any conditions, installed on any aircraft type, behaves exactly the same. So it's certainly correct to say that some engines may behave differently. But it's also correct to say that PSX agrees with one of those real-world examples. The one doesn't exclude the other.


Cheers,

|-|ardy

[Mariano]

Excellent, thanks.

Best regards,

Mariano.