PMDG T7 FMC and PFPX Step Climb Discrepancies

[Remys 4]

hi,

 

upon setting all the correct parameters in the PFPX I am gettting for instance sometimes step climbs in the FP and others not.

 

what is happenind ithat after setting the FMC of the T7 with all the parameteres received from PFPX it is calculating step climbs (even if in the original flight plan generated there are none). and sometimes other values for step climbs

 

 

[hopskip 253]

Winds calculated too? And inserted in the FMS?

 

PFPX calculates how much weight the 777 will have at any given moment along the flightplan. It uses winds data to predict this.

 

The FMS also does this. It uses the winds data you provide it either via the uplink or typing in manually from the Legs page to do this. If no data is inserted, it will assume that the wind in the whole world is 000° at 0kts, and the temperature is ISA+0°C

 

While ISA+0 is possible, 000°0Kts across a whole flightplan at FL3xx isn't.

 

Nevertheless, without any data to the contrary, that's what the FMS will assume.

 

The Opt/max altitude on the VNAV page of the FMS should be the most accurate, though that only shows data for "Right now, in the conditions that are on the other side of the window" (assuming all temp/irs/air data sensors are working right).

[Remys 4]

Winds calculated too? And inserted in the FMS?

 

PFPX calculates how much weight the 777 will have at any given moment along the flightplan. It uses winds data to predict this.

 

The FMS also does this. It uses the winds data you provide it either via the uplink or typing in manually from the Legs page to do this. If no data is inserted, it will assume that the wind in the whole world is 000° at 0kts, and the temperature is ISA+0°C

 

While ISA+0 is possible, 000°0Kts across a whole flightplan at FL3xx isn't.

 

Nevertheless, without any data to the contrary, that's what the FMS will assume.

 

The Opt/max altitude on the VNAV page of the FMS should be the most accurate, though that only shows data for "Right now, in the conditions that are on the other side of the window" (assuming all temp/irs/air data sensors are working right).

so pfpx should fix this as i am entering the exact winds fuel gw ..... and generating accurate weather files

[JC75 22]

"Fix it" may be a strong phrase to make. Number of factors to be considered in pfpx vs fmc. Key one is the weather being injected into fsx vs the weather you using to generate a plan in pfpx. Also take into account that the pfpx weather regardless of source will be based on forecast winds rather than actual winds which hours later you might experience. Also things like extra fuel burn you maybe have on stand running apu, longer taxi or holding on ground etc will all change things.

 

Key is the weather you use to build pfpx plan, is what needs to be in fsx.

But you also need to remember that pfpx is creating a "plan" based on forecast. And just like if you ask me should I wear a tshirt tomorrow, I may say yes thinking it be sunny, but when it comes it rains. Regardless of if you enter winds into fmc, an hour into your flight, the fmc may say climb even when pfpx didn't predict then, because your fmc is now reporting that based on weight of aircraft now, and that may be different from what was planned, due to extra fuel burn, from differing winds, or even effects of how aircraft been flown etc.

[Tabs 2,790]

PFPX is not using our 100% exact performance library/model guys. That's really not possible unless Christian were to do something like take the 777 and fly every possible permutation of weights, temperatures etc to log the data. We are not going to be giving anyone our FMC code for making third party applications - that's completely out of the question.

 

There's probably always going to be small discrepancies and there often are in real life dispatch too. Often companies will not do step climbs even if the FMC is recommending it anyway too - for instance in the tutorial flight, the VRMM-OMDB route does generate a step climb to FL400 depending on the conditions but if you look at it it's only something like 20 or 30 miles at FL400 before you have to descend. That just increases the pilot workload for very very minimal benefit and the pilots who helped me with the tutorial said they never actually do step climbs like that in those "borderline" cases. You just fly the whole thing at FL380.

[scandinavian13 5,192]

...what Ryan said.

 

In addition to that, I'm not sure how PFPX handles this precisely, but non-radar segments (like the NATs or PACOTS) are constant level / constant speed segments.  Generate a plan that crosses the NATs and take a look at the fix right before NAT entry and right after.  Note that you'll see a speed/altitude assignment right before and right after the NATs.  Why?  PFPX is trying to put you on an optimal altitude for the NAT just prior to entry because you're stuck at that altitude for the crossing.  On the other say, you might see that the altitude remains the same, but the speed changes from a Mach back over to whatever it was prior to the NAT.  It would be at this point that you'd change back over from constant speed to ECON (by actually writing ECON in the scratchpad and upselecting it to the exit waypoint).

 

So, other than what Ryan mentioned, you're likely seeing a discrepancy between the planned ideal (the FMC's calculation) and the ideal case with known ATC restrictions/SOPs/LOAs/etc (PFPX's calculation).

[phil747fan 14]

i may add that is true that the datas used are not coming from PMDG FMS for sure but this is the same datas used by real ops to generate their own flightplans so yes we didnt dig into the PMDG FMS but im pretty sure there is no need to do that if you have access to some other datas.

 

all the best.

 

Phil

[captain420 3,099]

So I guess step climbs are not necessary then when flight planning and we should set the step climb in PFPX to 'none'? and that we should fly the entire route at one flight level?

[pcubine 143]

See Post #81 of this thread -http://www.avsim.com/topic/475382-fuel-at-destination-prediction/page-6.

[downscc 1,327]

Yeah, I pretty much ignore the altitudes given by PFPX for both the NGX and the 777. It's close enough for planning but not for execution.