In the PMDG 747-400X the FMC-calculated V speeds are not usually accurate. Software such as TOPCAT is therefore required to calculate accurate V speeds. These speeds may be entered without the FMC subsequently overriding them.

 

The B777 manuals from SmartCockpit state:

 

After an engine is started, the FMC recalculates the takeoff speeds. Any

combination of gross weight, OAT, or pressure altitude resulting in a

takeoff speed change of two or more knots from the previously calculated

speeds, causes the FMC to recalculate takeoff speeds.

 

Does this mean that the FMC-calculated V speeds override any pilot or TOPCAT-calculated V speeds? If so, are the B777 calculated V speeds dependable and can thus be relied upon?

Does this mean that the FMC-calculated V speeds override any pilot or TOPCAT-calculated V speeds? If so, are the B777 calculated V speeds dependable and can thus be relied upon?

 

As far as I know, the V-speeds are calculated via the EFB, not the FMC. From the manual you quoted, the FMC will recalculate the speeds, not override the speeds as you suggested.

 

I have always found the FMC speeds calculated by the NGX work well, but I don't have Topcat, so I can't really comment on that aspect. Plus, if you have two different sets of v-speeds, how do you know which one is correct? It's like two watches showing different times.

 

However, I am no expert in this subject, maybe a real-world 777 "driver" could answer this one fully.

In the PMDG 747-400X the FMC-calculated V speeds are not usually accurate.

Really? Similar to the previous user, I am wondering how you concluded or determined this. The only observation I have made of takeoff speed calculation involving the PMDG B747 FMC is that VR and V2 seem to be very close together (at least in real-world videos they do not seem so close).

 

Also, despite trimming the aircraft according to the FMC-calculated value and trim readout on the 2-D panel, I often have to use almost full elevator deflection to achieve the 2.5 degrees/s pitch increase rate while using derated takeoff thrust, but this probably only seems odd because the Saitek yoke feels cheap when used to such extremes.

 

Otherwise, I have never called into question the accuracy of PMDG FMC-calculated takeoff speeds, as they seem to adjust realistically when I change essential parameters such as aircraft weight and takeoff thrust. Nevertheless, I am interested in hearing what you have to say.

As far as I know, the V-speeds are calculated via the EFB, not the FMC. From the manual you quoted, the FMC will recalculate the speeds, not override the speeds as you suggested.

The EFB is a relatively recent addition to the 777. I'm fairly sure the FMC calculates V speeds. As you say they will be recalculated, they should not override a pilot input V speed, though sometimes the FMC clears all V speeds.

I think there's some confusion as to the role of the calculators here:

 

Much like TOPCAT (or any takeoff performance calculator), the EFB's calculator is going to give you numbers. I'm not sure how airlines use these numbers, or which ones override what in which cases, but they should be similar to the ones the FMC will spit out at you. In any case, the real ones (meaning real external application calculators) are likely driven by the same calculations that are present in the FMC (as is the case with the fuel planning software airlines use). The FMC, however, is getting its information from the plane itself, as well. The EFB may or may not.

 

So, when you calc out your speeds, you should see similar results from external program to FMC. In my case, I stick to the FMC for V2 and VR, but will go with the lower of the V1 speeds between any external calc and the FMC.

 

Additionally, as far as that excerpt goes, based on how I'm reading it, I see it like this:

The internal magic runs a check of all calculated speeds once the engines have started. If those calculations result in a speed change of two or more knots, it will override that speed (and likely display differently - smaller, bluer, brighter, something - until confirmed or overridden again).

Hi Kyle,

Thanks for your detailed answer. It does, though, just leave me wondering why will might need an external tool, like TOPCAT.

Cheers, R

t does, though, just leave me wondering why will might need an external tool, like TOPCAT.

 

FAR 91.103 (specifically B 2) :wink:

 

Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This information must include—

a) For a flight under IFR or a flight not in the vicinity of an airport, weather reports and forecasts, fuel requirements, alternatives available if the planned flight cannot be completed, and any known traffic delays of which the pilot in command has been advised by ATC;

b) For any flight, runway lengths at airports of intended use, and the following takeoff and landing distance information:

1) For civil aircraft for which an approved Airplane or Rotorcraft Flight Manual containing takeoff and landing distance data is required, the takeoff and landing distance data contained therein; and

2) For civil aircraft other than those specified in paragraph b)1) of this section, other reliable information appropriate to the aircraft, relating to aircraft performance under expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature.

Emphases mine

 

Also, it makes determining what derate/assumed temp is acceptable a lot easier.

 

EDITED: kept adding stupid smileys in the FAR reference...beat it into submission.

Hi Kyle,

Thanks for your detailed answer. It does, though, just leave me wondering why will might need an external tool, like TOPCAT.

Cheers, R

 

The FMC won't calculate optimum flap setting, or an appropriate assumed temp. You're just guessing otherwise, so topcat is still very useful. Good for working out landing speeds and distances too.

 

That said, I wouldn't be surprised if PMDG include the EFB performance calculator anyway, in which case no need for topcat (at least for the 777).

TOPCAT is useless if they don't make a 777 profile.

Our 777's doesnt calculate any speeds by itself (not even sure if it does any crosschecks except to compare entered v-speeds to make sure they are not less than Vmin for RR or Vmin-5 for the GE). All performance is done by each pilot individually via the OPT module of the EFB and then compared before being entered into the fmc during the preflight (after final loadsheet received).

After reviewing some of my JustPlanes 777 videos, I have found that ACA use the EFB, in a similar fashion to that described above. However from a book I read a while ago, I seem to remember that BA use the datalink to recieve performance data from dispatch. I'm not sure if this is normal procedure for any aircraft without an EFB, but I think it is.

 

Hope this clears this up a bit.

Performance data via ACARS is probably not in the PMDG plan, so it will normally be TOPCAT for me, if the TOPCAT program then supports the B777.

 

BA do not use assumed temps or derates and all takeoffs are with flaps 20, I believe.

BA do not use assumed temps or derates and all takeoffs are with flaps 20, I believe.

 

Based on what? Are you trying to say that all BAW 777 take off's are at maximum take off power?

No, not at all. The calculations are then for normal takeoff power to reach Vr and V2 appropriate to the runway length and meteorological conditions.

 

I should add that my experience is entirely for the B744.

I am confused.... your two posts contradict themselves. If T/O power is calculated in regards to certain runway/wx conditions, how is it not de-rated?