EFB vs FMC vs EICAS differences

[Wothan 180]

Using the performance calculator in the EFB I get some differences between the readouts on the EFB, FMC and EICAS

Please look here:

https://www.dropbox.com/s/n5m6mchv0242rx1/2018-9-26_19-41-44-429.jpg

V-Speeds are equal, but N1 limits are different - 97.8 on the EFB and 98.7 on the EICAS.

 

Not sure if thats correct - but the V-speeds don´t change on the FMC when the Assumed Temperature value is changed - I tried various numbers between 25 and 99°C. Only the N1 limit changes.

Maybe performance calculations in the FMC isn´t fully featured yet - like in the EFB ?

 

FinnJ

Edited September 26, 2018 by Wothan

[downscc 1,327]

I'm not sure if V speeds should change with AMT.... why would they as long as there's enough stopway up to V1.

I suspect there is a reason between FMS and EFB as far as TO N1 goes, not sure it is an error so let's see what PMDG says when you submit this on a trouble ticket.

[Wothan 180]

16 minutes ago, downscc said:

I'm not sure if V speeds should change with AMT.... why would they as long as there's enough stopway up to V1.

I suspect there is a reason between FMS and EFB as far as TO N1 goes, not sure it is an error so let's see what PMDG says when you submit this on a trouble ticket.

Not sure either - but on the 777ER changing AMT temperature also changes the V-speeds and not only the N1 limits.

But using some logic...if PMDG isn´t ready to do proper calculations of performance data in the EFB, regarding Assumed temperatur thrust reduction, D-TO1 and D-TO2 - how should they then could do it in the FMC !

 

FinnJ

[downscc 1,327]

12 minutes ago, Wothan said:

Not sure either - but on the 777ER changing AMT temperature also changes the V-speeds and not only the N1 limits.

But using some logic...if PMDG isn´t ready to do proper calculations of performance data in the EFB, regarding Assumed temperatur thrust reduction, D-TO1 and D-TO2 - how should they then could do it in the FMC !

 

FinnJ

I think you are mixing concepts here.  First, the use of derates in a twin is significantly different from a quad jet.  There are rare cases where one would want to derate a quad.  Second, it's not if PMDG wants to do calcs or not that is the issue.  The issue is that the data to allow the ATM performance determinations is expensive.  Finally, there is a lot more to the takeoff calculation that accelerate and stop distances (this is V1) in that one has to ensure safe climb out at V2 with various engine out scenarios and obstacle clearances... the FMS does not have the takeoff data, the EFB would were the data affordable.  Every runway used by the EFB must have data included obstacle heights and distances.  They are not there yet.

[Wothan 180]

Why does the V-speeds then change in the 747-400, but not in the 747-8, when I change the Assumed temperature or select one of the De-rates ?

 

FinnJ

Edited September 26, 2018 by Wothan

[Copper. 78]

A split of approx 1pcnt between opt and fmc n1 ref is normal.

On real airplanes you can get a fmc n1 limit split between respective engines by usually less than 1pcnt which aren’t simulated as far as I can tell.

Generally, alteration of thr limit page data will result in t/off speeds deletion requiring reverifying and selection of vspeeds - a safety feature.

im surprised the 748 doesn’t do this because the required data is in fcom and qrh which doesn’t factor required takeoff segment performance and is based on field length, environmental conditions and airplane configuration which is how the 744 and others do it.

Edited September 27, 2018 by Copper.

[mfrantz 45]

Finn,

The N1 difference is because of the Air Conditioning. In the EFB you have set it to ON. However you have your packs off and the FMC detects that and calculates N1 with packs OFF.

[Wothan 180]

18 minutes ago, mfrantz said:

Finn,

The N1 difference is because of the Air Conditioning. In the EFB you have set it to ON. However you have your packs off and the FMC detects that and calculates N1 with packs OFF.

Ah - great find.

 

I also experimentet with V-speeds and found them to change accordingly to RWY length.

My first tests used Copenhagen Kastrup (EKCH), but it seems that the runway length (RWY22R) are too long to affect V-speed due to Flex temp settings.

Using the shorter runway at Billund (EKBI) started to show the effect of Flex temps on the V-speeds.

 

My question now is - if You can do performance calculations in the FMC's for the 747-8, what's then the problem doing so on the EFB's  for ATM and TO1 and TO2 ?

 

FinnJ

[mfrantz 45]

Finn,

Regarding FMC ATM calculations, of course they are fully performed as in all PMDG products.

The differences in V-speeds with assumed temperatures depend on various factors. Running your example I do see a 1KT difference in V2 with a sel temp of 55 deg C :

https://1drv.ms/u/s!At14-KyBvp1Uj6QIFa4ifWnC__YFyA

https://1drv.ms/u/s!At14-KyBvp1Uj6QJZ_0asjyX67JeaQ

If you select flaps 10 instead of 20, there are also differences in V1, VR and V2:

https://1drv.ms/u/s!At14-KyBvp1Uj6QKxeH_9lDwrtBTww

https://1drv.ms/u/s!At14-KyBvp1Uj6QHZhWmzVpxSesv7g

 

Edited September 27, 2018 by mfrantz

[mfrantz 45]

6 minutes ago, Wothan said:

My question now is - if You can do performance calculations in the FMC's for the 747-8, what's then the problem doing so on the EFB's  for ATM and TO1 and TO2 ? 

These are different calculations. The FMC does not know or care about the runway length. It just calculates N1 and V-speeds for a given assumed temperature. Now if these thrust and speeds are enough for the aircraft to be able to safely takeoff before the runway end or abort at V1 and safely stop before the runway end is a different calculation. This is what the EFB does. And in this case additional data are considered as detailed runway surface conditions, obstacle clearances etc.

It is not a problem to do the ATM/derate calculations in the EFB. It is just a very complex task if we want to do it accurately and we decided that we don't want to hold the 747-8 release just for that. This missing part will eventually be provided.

[mfrantz 45]

18 minutes ago, Wothan said:

Using the shorter runway at Billund (EKBI) started to show the effect of Flex temps on the V-speeds. 

You are talking about FMC calculations I assume.

The FMC calculations do not consider runway length. Any differences you might see choosing a different airport/runway are only a result of different airport elevation.

[Wothan 180]

Yeah - I see now.

FMC calculations do not tell if they are valid for a safe take off at the present runway. Those additional checks are (or supposed to) be done in the EFB.

But for V1 I think runway length must be considered, since it determines whether the aircrat, at it´s present GW, can stop safely before reaching the end of the runway. So actual runway length minus expected runway length obtained at V1 (dependent on engine thrust setting), should be equal or greater than the distance needed to deccelerate to a full stop at present GW. For this to be known, runway length and condition (slope, dry/wet, friction) must be known, as well as some other affecting paramters.

Right ?

 

FinnJ

[mfrantz 45]

No, this is not the case. You first calculate V1 based on all other parameters (MCG - Minimum Control Speed is important here)  and then you use the EFB or other software to check against available runway length. If the runway is not long enough you can't reduce V1 - you just are not allowed to takeoff.

For more information check the 747-8 FCOM pages PI.18.1-PI-18.2 for a description and PI.10.2 onward for tables.

[Mroberts95 136]

22 minutes ago, mfrantz said:

No, this is not the case. You first calculate V1 based on all other parameters (MCG - Minimum Control Speed is important here)  and then you use the EFB or other software to check against available runway length. If the runway is not long enough you can't reduce V1 - you just are not allowed to takeoff.

For more information check the 747-8 FCOM pages PI.18.1-PI-18.2 for a description and PI.10.2 onward for tables.

Hey I’m not at home but would the tables allow you to do a manual calculation of the assumed temperature or do we as customers not have access to those data tables?

 

[Wothan 180]

I did read the FCOM pages, but still...

Quote from a Type rating instructor:

"V1 speed is the tool used by pilots of transport category jet aircraft to ensure adequate stopping distance remains if the pilot elects to stop the takeoff due to an engine failure, fire or other event which may limit the ability to safely fly. The key component of V1 is the balanced field length which is the distance to accelerate to V1 and then stop if necessary using max braking, engines at idle and spoilers deployed. If the Balanced Field Length computed is equal to runway available then the Takeoff Distance is said to be Field Length Limited. In some cases the computed balanced field length may exceed runway available and in such cases either the weight must be reduced, a different flap setting used or another longer runway used for takeoff.

To determine V1 the pressure altitude, temperature, runway length, runway slope, wind, condition of runway (dry, wet or icy) must be known and the aircraft takeoff weight and flap setting for takeoff. Above V1 the pilot must continue the takeoff or risk going off the end of runway attempting to stop."

So for proper V1 calculations, runway length and conditions must be known.

All these parameters are available in the FMC, either by pilot input or from the navdatabase (Runway length, filed elevation).

 

FinnJ

Edited September 27, 2018 by Wothan