Performance Tool: V1 check against V-MCG ...?

SimBrief Dispatch
1

Today I planned a short flight for a B747-400 (Aerowinx PSX) from EHAM RWY 24 to ELLX, TOW was calculated to be 200 MT. Simbrief’s performance tool calculates V-speeds - using Flaps 10, D-TO2, assumed temperature 54 degrees Celsius - as:

  • V1: 111
  • VR: 116
  • V2: 136

The following checks, however, are not performed by this performance tool:
Check V1 against VMCG (Minimum Controllable Groundspeed); If V1 after applying corrections is less than VMCG, increase V1 to VMCG;
Check VR against VR MIN; If VR after applying corrections is less than VR MIN: Increase VR to VR MIN, and Increase V2 to “V2 at VR MIN”.
Corrections factors to V1 are given for slope and wind, correction factors to VR are given for Pressure Altitude and assumed temperature.

For this given flight, VMCG would have been ± 123 kts, VR MIN ± 128 kts, V2 MIN ± 148 kts.

Kind regards,
Simon

2

Hi Simon,

VMCG should already be considered. Are you sure the min speeds you listed are for TO2? They look more like the min speeds for “TO” to me. :thinking:

One of the side-effects of using TO2 (fixed derate) is that since the engines are limited to lower thrust, VMCG is lower due to less asymmetric thrust in case of engine failure. If I force the thrust setting to “TO”, SimBrief returns speeds that are closer to your quoted figures.

Can you double check if the minimum speeds you posted are indeed for TO or TO2?

Cheers,

3

Hi Derek,

SimBrief provides both the D-TO2 and Assumed Temperature. However, only one can be used at a time. Using the assumed temperature, the Thrust Limit page on the CDU will show a D-TO N1 %. Selecting the TO-2 option, the inserted assumed temperature gets deleted and (again) a D-TO N1 % will be shown for this option, which is very close to the assumed temperature value. Hence, either method can be used.

I can look into (obsolete) KLM Aircraft Operations Manuals [AOM], with Flaps 20 data only. I can also look into KLM’s FCOM, which only contains data for self dispatch Flaps 10 takeoff performance at Maximum Takeoff thrust - and intended only as last resort data (when the normal performance tool [LINTOP] is not available).

KLM does not allow fixed derate takeoffs so I have no data available for D-TO1 or D-TO2.

Using KLM’s AOM and a Flaps 20 Takeoff Speeds Chart, values are given for V1 DRY, V1 WET, VR, V2 for Takeoff Weights between 450 MT and 200 MT. Corrections to V1 and VR have to be applied, based on the calculated assumed temperature and Pressure Altitude. On top of that, corrections to V1 for wind and slope have to be applied. If the resulting value for V1 is less than VMCG, determined at the actual OAT, V1 has to be increased to VMCG. Similarly, corrections to VR are based on the assumed temperature.

For the Pax/Combi version of the 747-400 at EHAM, Flaps 20 and 22 degrees OAT, VMCG equals 125 KIAS. VR and V2 MIN are 125 and 145 KIAS. These values only decrease as Pressure Altitude increases.

Although the FCOM Flaps 10 data - both for the Pax/Combi and Freighter versions - is for maximum takeoff thrust and actual OAT, looking at an actual temperature of 50 degrees VMCG = 123 and VR MIN = 128 kts (Freighter), or 116 and 116 KIAS (Pax/Combi).

If you’d like to discuss these data tables in more detail, I would prefer to do this outside of this forum.

Kind regards,
Simon

4

Hi Simon,

I wonder if the combined TO2 and assumed temperature entry is an operator-specific option? The PMDG 747 for example lets you enter both (it doesn’t revert to TO if you enter an assumed temperature). And after a brief Google search I have found forum posts from real 747 pilots that mention their company uses combined entry.

In any case, it sounds like if you are simulating KLM you should simply force SimBrief to use “TO” for all takeoffs (you can do this through the “Thrust Setting” option). I think this should solve both the combined entry issue, as well as result in your expected VMCG values.

Best regards,

5

Hi,

Most certainly the option to use the assumed temperature or derated takeoff is airline specific. And as said above, either method - for this specific Cargolux 747ERF flight - produced more or less the same calculated N1 percentage (in my Aerowinx PSX simulator).

The topic, however, is about V1 being below V-MCG. For this flight with a TOW of slightly less than 200 MT (197.4), V1 is calculated to be 114 KIAS at either D-TO2 or at 54 degrees assumed temperature. The lowest value for V-MCG that I see in the manuals for the actual OAT is ± 125 KIAS. VR MIN and V2 at VR MIN are ± 125 and ± 145 KIAS respectively.

For what it is worth…: entering Simbrief’s values into the Aerowinx CDU returns values for V1 MIN, VR MIN and V2 MIN that are more or less in line with the values as seen in the manuals.

Kind regards,
Simon

6

Hi Simon,

As I said in a previous post, VMCG is lower at TO2 than it is at TO, due to the reduced thrust.

You mentioned that you only have charts for TO, and it sounds like you are assuming VMCG for TO2 would be the same as those for TO, but this is incorrect.

Critically, this VMCG reduction only applies to the fixed derates (TO1/TO2), VMCG is not reduced when using the assumed temperature method. There are a few threads online that discuss why and how this is, for example here.

For practical examples, consider the following SimBrief calculations for KJFK at 200 MT:

Thrust Assumed Temperature Speeds Link
TO No 123 / 127 / 148 Link
TO Yes - 54 123 / 127 / 148 Link
TO2 No 114 / 117 / 136 Link
TO2 Yes - 54 114 / 117 / 136 Link

You can see that the assumed temperature thrust reduction makes no difference to VMCG, however using TO2 reduces it quite a lot. This is accurate as far as I know.

There might be some small variances between your manuals (123 vs your quoted 125 for example), due to the different thrust ratings of the GE, PW, or RR engines. But it should generally be within a few knots as long as you are using the TO rating.

Best regards,

7

Presumably you would select the derate first and assumed temperature second, when you want to use both as provided by SimBrief.

If your airline doesn’t do derated takeoffs then you need to manually select a TO rating in SimBrief’s performance calculator.

Regards,

Tim

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Hi Derek,

Thank you for the link to the online threads. That helped me out understanding the difference between TO and D-TO1/2 with respect to V-MCG. Thank you as well for your patience in answering my questions.

With kind regards,
Simon

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