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Richard McDonald Woods

Unable to override FMC V speeds

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I have a problem when inputting revised V speeds into the FMC.

 

For my flight VHHH-OMDB in a B77W today, i received computed V speed values of 162, 167 and 172 kts. Using the TOPER calculator to recalculate the V speeds for the first time, I was given 166, 174 and 184 kts.

 

Overriding the V1 speed with 166 was accepted. On entering the 174 for Vr, I received the 'INVALID INPUT' message. Likewise if I tried to override the V2 value.

 

Why is this happening?

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Why is this happening?

 

It will (realistically) only accept values within a certain range of the computed value. I can't remember what the threshold is (I'm on my day job computer, and I can't get to dropbox where I keep my FCOMs). Moreover, I'm currently logged in from a Starschmucks in downtown Philly, so that download would take...oh...who knows...a year?  :lol:

 

If you get a rejected value, then it might be good to double check your PERF INIT entries versus the values that were used in the takeoff calculation.

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Hi Kyle,

 

I have searched the manuals for the 'INVALID ENTRY' message, but can find no more informative explanation than 'range is incorrect'.

 

I shall have to await my next flight to test this again.

 

Cheers, R

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This seems odd. I couldn't find anything in the manuals either (searched for anything "V Speed" and "INVALID INPUT" related). The values you mentioned above are actually within the envelope. 

 

I'd say try to recreate the scenario one more time and check the runway lenght, your payload and other external factors, which might affect the performance. Maybe some of our RW 777 drivers on the forum can help you out.

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I have searched the manuals for the 'INVALID ENTRY' message, but can find no more informative explanation than 'range is incorrect'.

 

 


This seems odd. I couldn't find anything in the manuals either (searched for anything "V Speed" and "INVALID INPUT" related). The values you mentioned above are actually within the envelope. 

 

Searching for the error messages isn't going to get you too much, honestly. Those are pretty generic messages.

 

My source says that you cannot reduce the v-speeds more than 5 lower than the QRH speeds, but I'm looking for a specific Boeing reference for you.

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I have a problem when inputting revised V speeds into the FMC.

 

Hi, Richard,

 

Don't have the NGX in front of me right now, but I think you have to confirm the V speeds.  So I'm wondering if, 1) you are trying to change them after you have confirmed them and 2) maybe after you have confirmed the V speeds, you have to click on delete next to the v speed for each of the three speeds, and then you will be able to modify them (should go from bold to smaller numbers after "delete")?

 

 

 

My source says that you cannot reduce the v-speeds more than 5 lower than the QRH speeds, but I'm looking for a specific Boeing reference for you.

 

His numbers were higher than the FMC-recommended V Speeds.  Hard to believe the FMC would prevent the pilot from entering higher V speeds, at least for Vr and V2 (a higher V1 might lead to a runway overrun, but oddly that's the one the FMC did allow to be changed).

 

Mike

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OK, did some testing.  Wrongly thought this was NGX forum.  For the T7:

 

w/ suggested Vspeeds of 152, 158 and 165, did not allow me to enter speeds more than 5 below the suggested ones.  It did allow V speeds from 5 below the recommended speed to anything -- I tried various entries at least 10 kn faster than the recommended speed up to @ 190, w/ no problem.  It doesn't matter whether the original FMC-recommended V speeds are confirmed or not.

 

The NGX, BTW, does not appear to refuse any speeds whatsoever, including ones more than 5 below the recommended.

 

Mike

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Overriding the V1 speed with 166 was accepted. On entering the 174 for Vr, I received the 'INVALID INPUT' message. Likewise if I tried to override the V2 value.

Why would want to override the V-speeds calculated by the FMC. It TOPER more accurate?

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Why would want to override the V-speeds calculated by the FMC. It TOPER more accurate?

FMC calculates balanced field and not runway specific values.

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FMC calculates balanced field and not runway specific values.

Even though I am using TOPCAT for assumed temperature and de-rate for the THRUST LIMIT page, and flap settings for the TAKEOFF page, I should also use the v-speeds from TOPCAT. Is this correct?

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 Moreover, I'm currently logged in from a Starschmucks in downtown Philly, so that download would take...oh...who knows...a year?  :lol:

Starbucks uses Google internet with a download speed of around 25mbps.  Just some food for thought.

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Starbucks uses Google internet with a download speed of around 25mbps.  Just some food for thought.

 

Yeah, it was a lot better that time than it had been in the past. My upload speed was still pretty bad, but that only affected the day job work.

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Hi Michael,

 

Yes, you definitely ought to use the TOPCAT takeoff speeds, if supported for your aircraft type. Wikipedia discusses the concept here.

 

Unfortunately, I am flying the B777-300ER which is still not supported. Hence my install of TOPER which I am testing for reasonableness.

 

HTH

Cheers, R

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Even though I am using TOPCAT for assumed temperature and de-rate for the THRUST LIMIT page, and flap settings for the TAKEOFF page, I should also use the v-speeds from TOPCAT. Is this correct?

 

Yes and no. With the real plane, using a real EFB that uses Boeing data, I'd use the runway-specific calculations that the EFB would give you.

 

With TOPCAT, I usually use its V1 and just verify that the VR and V2 are similar and use the FMC value for those.

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With TOPCAT, I usually use its V1 and just verify that the VR and V2 are similar and use the FMC value for those.

Thank you

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Gentlemen,

 

I have now tried to alter my V speeds for my flight OMDB-EGLL.

 

FMC calculated 163,174 and 181. TOPER 777 calculated 176,180 and 186 for runway 30R.

 

Without confirming the FMC speeds, I was able to change the values to those by TOPER no problem.

 

Untested still, confirming the speeds and then trying to change them. Another time!

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I got some additional info for those interested.

 

The EFB performance software we use not only calculates the Vspeeds and assumed temp for thrust reduction.

 

It additionally calculates the optimum EO clean up altitude (usually between 1000-1200ft AGL)and presents a text description of the EO SID that needs to be flown if an engine fails during T.O. or shortly thereafter.

(As said in another thread, these EO SIDs are stored in our FMC and pop up automatically in case of Engine fialure).

 

The software is build on an obstacle database so that you are granted obstacle clearance while following the EO SID and while accelaring to clean up (very shallow flight path!)

 

So besides the FMC only being able to calculate balanced take offs, it also can not take the terrain into account!

 

Further more we can input all kinds of runway contamination parameters (braking action/snow depth/standing water depth/etc) and calculate a safe max take off weight.

(I never tried TOPCAT, so I dont know if it allows for that).

 

We can also select MEL items from a dropdown list if they have an impact on performance, select a shortened runway at either end if there if work in progress (KJFK R22R for example is shortened at the moment) and we have a couple of tricks in there that we can use to increase max take off weight (packs off take off for example)

 

 

We used to compare the EFB calculations with the FMC calculations to make sure they were similar. To do so we entered the same parameters for wind and temp in the FMC and EFB. The EFB calculation was mainly for max take off weight calculation plus how much thrust could be reduced, taking the obstacles along the EO SID into account. The EFB speeds were pretty much the same as what the FMC calculated.

 

Since a recent software update that is not required anymore.

There is no need anymore to enter any performance data into the FMC either (wind/temp)....it does not matter what the FMC calculates. There is quite a split between the FMC and EFB calculated Vspeeds sometimes now (more that 5kt) and we overwrite all three of them with the EFB speeds.

To still be able to compare our calculations to something, both pilots have to now calculate the take off performance independantly on his/her EFB.

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I never tried TOPCAT, so I dont know if it allows for that

 

TOPCAT will allow runway shortening and basic dropdown selection of dry/wet etc but we've got the basic constraints of MSFS. The takeoff calculations then identify which limitation is affective such as climb, performance or obstacle. It uses departure data from navdata files from same hobbiest sources as PMDG navdata so simulation is consistent.

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TOPER 777 supports runway shortening,  wet/dry, wind, OAT, flaps, A/C, A/I and QNH but not obstacles, E/O or derates. So it is an improvement on the FMS calculated numbers, but not comprehensive.

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This morning on EGLL-CYYZ I confirmed the FMS-calculated V speeds and was then able to overwrite them with the TOPER-derived values without trouble.

 

So now I am not sure why I originally had the overwrite problem.

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@ Richard,

 

I often get this too,

The best way around it is going to takeoff page 2, and inputting some wind and wet runway for example. And that will align the v speeds to your calculated ones (i use a real world 300ER calculator and the values are sometimes off quite a lot if the runway is wet etc. etc. )

 

Hope that helps :)

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