# FMC calculated takeoff speeds

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Dear All

When a headwind is entered on TAKEOFF REF 2/2 page on line 1 left (LSK1L), the FMC decreases the V1 speed, why?

A runway headwind component affect the acceleration, therefore the FMC should show a higher V1.

Michael

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A headwind shortens the stopping distance in case of a rejected takeoff. Therefore, V1 is lower when there's a headwind. At least, that's how I always understood it. Others will correct me if I'm wrong.

Edit:

Some info from the FAA:

The effect of the wind speed and direction on takeoff distance is very straightforward. At any given airspeed, a 10-knot headwind component lowers the ground speed by 10 knots. Since V1, rotation, and liftoff speeds are at lower ground speeds, the required takeoff distance is reduced. The opposite occurs if the wind has a 10-knot tailwind component, producing a 10-knot increase in the ground speed. The required runway length is increased, especially the distance required to stop the airplane from V1.

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Hi Donor

First of all thanks for your feedback.

But,  you got it wrong.

Slope and Wind:

- A runway upslope or headwind component affect the acceleration - a higher V1 is needed.

- A runway downslope or tailwind component affect the deceleration - a lower V1 is needed.

The value that V1 needs to be corrected for, is available in the FCOM/PI SLOPE AND WIND V1 ADJUSTMENTS table.

Example:

Aircraft type   = B737-800W/CFM56-7B26

Condition       = Wet Runway

Weight           = 80 (1000 KG)

Wind              = 20 kts headwind component

Required V1 adjustement = V1 + 2 kts

Michael

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But, you got it wrong.

If you knew the answer, why did you ask the question?

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First of all thanks for your feedback.

But, you got it wrong.

Could be  wrong  here  some  one  can  correct me, your info  maybe  good  in rw   rwys  but   don't thing  fsx   doesn't  simulate  slope  runways

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If you knew the answer, why did you ask the question?

I think the point being made was that the FMC's behaviour appears to be incorrect.

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Hi Pete

For me, flying from A to B without knowledge about the real things does not make sense.

Either I do it right or not at all.

Hi Skelsey

You are right, I wanted to complain about the error.

I thought the PMDG team is interested.

Maybe they can correct it with the next update.

Michael

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I thought the PMDG team is interested.

Maybe they can correct it with the next update.

Submit a ticket at the link in my signature with the correct behavior, and references (FCOM, or some other official document) and we'll look into it.

EDIT: Full names in the forum, please - first and last.

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Hi Pete

For me, flying from A to B without knowledge about the real things does not make sense.

Either I do it right or not at all.

No problems   can  confirm  your  findings,  just did  a test   v1  went  from 148   to 143    after  I entered   head  wind  as  you have  described in your  original post

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The speeds the FMC gives you are for balanced field length and may well need adjusting for the conditions. They are a starting point, the FMC does not contain all the flight manual data.

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Don't use the FMC Takeoff Ref page 2. Only enter the EO ACC ALT and your ICAO proc A or B (thrust reduction alt and acceleration altitude) and then use TOPCAT for your V SPEEDS. FMC speeds may be used if they differ from the calculated TOPCAT speed by no more than 1 knot.

Happy Contrails

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Hi Kevin

The speeds the FMC gives you are for balanced field length and may well need adjusting for the conditions. They are a starting point, the FMC does not contain all the flight manual data.

We speak about the PMDG 737, I am right?

In the real airplane V1 needs only correction when:

- Equipment is inoperative.

- Runway is contaminated.

- Unbalancing (Clearway > Stopway, Stopway > Clearway).

DRY RUNWAY:

• With actual ZFW and PLANNED TOF entered on PERF REF-page of the FMC, TAKEOFF REF-page 1/2 shows correct VR and V2 speeds.

• With FMC/TAKEOFF REF-page 2/2 filled out correctly with actual wind and runway slope, TAKEOFF REF-page 1/2 shows balanced V1 corrected for VMCG.

WET RUNWAY:

• With actual ZFW and PLANNED TOF entered on PERF REF-page of the FMC, TAKEOFF REF-page 1/2 shows correct VR and V2 speeds.

• If FMC/TAKEOFF REF-page 2/2 is filled out correctly with actual wind and runway slope, and RWY COND “WET” is highlighted, TAKEOFF REF-page 1/2 shows balanced

V1WET corrected for VMCG.

****************************

Hi Driver

Don't use the FMC Takeoff Ref page 2. Only enter the EO ACC ALT and your ICAO proc A or B (thrust reduction alt and acceleration altitude) and then use TOPCAT for your V SPEEDS. FMC speeds may be used if they differ from the calculated TOPCAT speed by no more than 1 knot.

Firstly I have not available the TOPCAT software, and it also not required to find out the correct takeoff speeds.

Inputing values in such a tool and using the output makes life easier, thats right.

However, without any background knowledge (knowing what is standing behind the automatic calculation), ... ,for me this makes no sense.

When the procedure is known, determining the values by use of the tables is easy and takes only a few minutes :smile:

****************************

PMDG team

I would recommend to inhibit the wind correction feature (in case you do not want to correct the error). It do not make sense and is also not serious to simulate the reverse logic.

Michael

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PMDG team

I would recommend to inhibit the wind correction feature (in case you do not want to correct the error). It do not make sense and is also not serious to simulate the reverse logic.

Michael

Michael,

Full names in the forum, please - first and last.

If you would like anyone to look at this, please submit a ticket at the link in my signature, citing specific sources (FCOM, FCTM, Boeing official documentation, and so on).

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Firstly I have not available the TOPCAT software, and it also not required to find out the correct takeoff speeds.

Inputing values in such a tool and using the output makes life easier, thats right.

However, without any background knowledge (knowing what is standing behind the automatic calculation), ... ,for me this makes no sense.

When the procedure is known, determining the values by use of the tables is easy and takes only a few minutes

Looks like you have the 737 performance reference handbook great little thing.

I totally understand why you do it from the books its good to know yes, but topcat is to easy and even in the real world its done by computers now. But good on you, i'll have to try it sometime!!!

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Michael,

Full names in the forum, please - first and last.

If you would like anyone to look at this, please submit a ticket at the link in my signature, citing specific sources (FCOM, FCTM, Boeing official documentation, and so on).

It looks like Michael is right. The tables in FCOM vol 1 shows that V1 adjustment is positive for a headwind, negative for a tailwind. The adjustment is only 1-2 knots either way. The FMC calculates a much greater V1 wind adjustment for a given wind speed and in the wrong direction.

The same problem is apparent in the 777 FMC.

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Dear All

Thanks for your interest, and feedbacks.

Hi Vernon

Yes you are right, firstly with your assumption PRH :-), and secondly with the computers.

Hi Kyle

I do not understand why, however, it`s clear now :-)  -> Full name Michael Domenego

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