Unable to override FMC V speeds

[Richard McDonald Woods 168]

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?

[scandinavian13 5,192]

 

 

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? 

 

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.

[Richard McDonald Woods 168]

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

[serviceceiling 135]

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.

[Richard McDonald Woods 168]

Thanks Bogdan for confirming that I am not going a little doolally!

 

I shall try to replicate on my next flight.

 

Cheers, R

[scandinavian13 5,192]

 

 

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.

[Mike777 163]

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

[Mike777 163]

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

[pcubine 143]

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?

[scandinavian13 5,192]

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.

[pcubine 143]

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?

[flightwatch 68]

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

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

[scandinavian13 5,192]

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.

[Richard McDonald Woods 168]

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

[scandinavian13 5,192]

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.