NG FMC cruise winds

[Pin Head]

Hi

Can I ask please does anyone know what is the FMC logic for the Perf INiT wind and the RTE DATA winds on the wind forecast page.

Does the box use one for fuel and time estimate or vice versa. Do people regularly update the PERF INIT wind by every hour say by putting the current wind in. Why do this when you have the OFP wind in which is the basis for the fuel checks.

In my CDU preflight procedure I always put the forecast winds in on the RTE data but saw a colleague every 30 mins update the actual wind in the FMC!

Discuss

:)

[B737900er]

Pinhead are you new on type?
I highly recommend asking your base TRE or line checker these questions, or even your company standards pilot, seeing that it is SOP at your company.

I suggest buying the following books/apps:

Bill Bulfner Cockpit companion
Bill Bulfner FMC Guide
Pat Boones management reference guide.

Merry xmas

[Pin Head]

Thank you for the reply.

[Goldenrivett]

Do people regularly update the PERF INIT wind by every hour say by putting the current wind in.
No. Since the FMC calculates the current wind using present track & ground speed / TAS & Heading vectors, then it only needs the forecast winds further ahead on route for its predictions.

[Pin Head]

So why do these guys update Perf INiT page. They drag the current wind into scratch pad and then on top of last wind

Thanks for help so far.

[wiggy]

So why do these guys update Perf INiT page.

Can't comment specifically about the 737NG but seems a bit strange.

I agree with Goldenrivett, In general and AFAIK "Boeing" FMCs use the sensed wind for close in predictions and then gradually blends in the forecast winds as it number crunches further downroute. How it works It may be all explained in the FCOM (it is in ours).

As for company SOP..as has been said, you'll have to ask a checker...

[RAT 5]

I'd always thought the FMC time/fuel predictions were based on current wind for current sector and the NEXT sector. Then time/fuel to DEST was calculated using FMC entries for the down route sectors. Then, as you crossed into a 'new' sector the whole process started again. This is all OK if the actual wind is similar to the entered forecast. Good luck with that.
It was quite common, more for simplicity, speed & preflight gross error check, to enter the average wind component on Perf Int. This would then give you a rough guesstimate for flight time and ARR Fuel. Many guys left this and allowed the FMC to do its thing following the 'now' sector & 'next' sector calculation as you progressed down route. This didn't work if there was a large change in angle or strength during the flight. I used to do the Pre FLT Perf Int thing, and then at TOC enter RTE Data winds for those portions of the route where there was a steady angle and strength, only changing the entries for significant Gnd-Spd changes. I used to keep a note of the winds & Gnd-Spd's incase there were any fuel/time discrepancies, and also for return expectations. It was all a black art, what went on inside 'the box'.
Adding to the discussion about FMC & winds, consider the effect of all this forecast mumbo jumbo for the descent. Mostly not the most helpful, and if you see the VNAVPTH going haywire just reselect DCT TO and watch the PTH come up with plan B, which often made more sense.
There are forecasts and forecasts, but making predictions is becoming more difficult, especially about the future.

[B737900er]

The PERF TOC wind is used to calculate a better ECON climb speed. During the cruise current wind is used for fuel predictions.

On short flights you can get away with average wind component, but on Oceanic or longer flights putting the forecasting winds in the RTE page will give you a more realistic prediction.

[ImbracableCrunk]

So why do these guys update Perf INiT page. They drag the current wind into scratch pad and then on top of last wind

Thanks for help so far.

Sounds like a tribalism to me. You're not in Korea, per chance, are you?

[Pin Head]

Indonesians.

No comment! Look at all my other questions posted in last week.


But why would they do that? Am I right there in zero benefit from updating wind on Perf INiT as the fuel forecast comes from RTE data?

[B737SFP]

Guess what ?

I've seen that also here in South America.

Please, don't ask me how this kind of thing propagate !

Please, despite of others may say, keep your questions coming. It's always nice to discuss about the airplane we fly, and even nicer to read so many different point of views, from various operators around the globe.

Fly safe.

:ok:

[Skyjob]

Perf init uses entries (cost index, flight level, toc isa, toc winds, calculated weight based on fuel and zfw) for optimising the ias climb to toc as well as determining initial crz Mach based on head/tail-wind component calculated (based on track direction and entered winds of a fixed distance after liftoff) which then equals climb Mach for transition.

Cruise winds can be preprogrammed to enhance econ cost index taking into account forecast winds but executions of any directs will use actual winds and taper towards next forecast wind if entered.

Descend winds are entered to attempt a cda to be flown with idle engines tod to approach. In econ cost index only determined the ias and the Mach is taken from last known cruise Mach.

On an outfit where short turn around times are used, entering perf init will optimise the econ vnav profile for climb, where then subsequently using average cruise level wind will give a close enough estimate for fuel/time to be used during flight. This will allow crew good enough information pre-flight which can be updated using waypoint forecast winds on longer flight eg in cruise or on ground when time permits. Care should be taken in this case that toc isa and winds are used in ALL cases and not average winds in perf init pages.

[Pin Head]

Good stuff guys.

So simply is there any benefit in scratch pading the actual wind into Perf INiT Enroute.

Our flights are 4:30hrs

[B737900er]

Pin Head, in short there is no benefit at all. Having experienced flying over in Indo land, a lot of things are done because the "best pilot in the company" says so.

[FlyingStone]

So simply is there any benefit in scratch pading the actual wind into Perf INiT Enroute.

Our flights are 4:30hrs

Well, compare EFOB at destination before and after the entry and you will most likely see zero difference, so no.

There is benefit of putting TOC wind to perf init page to have optimum economy climb speed and then put the cruise wind into RTE DATA where the wind changes significantly between the waypoints. I believe changing this only affects wind data in RTE DATA where it was not previously modified - I could be wrong though.

[Skyjob]

Let me expand on the cruise portion of flight as there is a misconception that it does not matter if/what is entered there.

The forecast winds are used by vnav cost index to work out head/tailwind component by using track direction and forecast wind angular difference and relate the wind vector component. For headwind the econ Mach is then increased, for tailwind the Mach is decreased.

So yes it would be important to enter them especially on longer flights. As per previous poster, only significant changes need be recorded, and when executing direct clearances using the abeam function will retain the wind information this maintain most accurate cost index flying, as per many company sops. Some operators do not like the retention of abeam points for a variety of reasons, but these operators probably are not used to clearances direct to a waypoint 3 hours away crossing multiple jet-streams. They are not incorrect flying without this feature in eg EU dense airspace, but may be inconvenienced later with i4D introduction when this type of data retention can assist very ably in prediction of data.

[c100driver]

Regarding the selection of abeam waypoints this should not be done if you are operating with ADS-C and/or with CPDLC.

SKYJOB gave a good explanation of how it works and a point that many miss is that the FMC will do a good job if it is allowed to operate without intervention of ATC requirements.

The FMC already uses the current actual wind and blends it out to the forecast wind over a time/distance. Changing at waypoint passage is pointless.

The descent is slightly different in that if you change the route or profile the FMC takes the current wind and blends the wind out at steady rate towards zero at the landing field.

The only reason I can think of to change the INIT winds if you are at a significantly different altitude. Other than that I would suggest it would be a futile exercise at best and if the INIT initial wind was a route average wind then you could be setting yourself up with incorrect fuel predictions at worst.