EFB vs. FMS VREF

[Dreamflight767]

Hello Folks:

When calculating my VREF speed, the EFB and FMS have displayed two different VREF speeds.  Today, the EFB was showing a VREF of 166 while the FMS was saying 145.

I copied the data from the FMS to the EFB but I couldn't get the two figures to agree.

I know...I must be doing something wrong.

Thanks.

[Stratox]

you can overwrite the values in the FMC if you so choose to. Make sure that the conditions for the calculations are correct. My guess your wind is not entered to either or just one.

[mfrantz]

5 hours ago, Dreamflight767 said:

I copied the data from the FMS to the EFB but I couldn't get the two figures to agree.

There shouldn't be such a difference. Make sure all the data fields on the EFB are the same as in the FMC.

[Ephedrin]

166 is the calculated speed with max landing weight (or at least a calculated higher value.) you need to correct your weight by using the inflight data. I can‘t tell you the exact wording now as I am not at my PC but there is an option in the EFB on the right side when you are on the landing calculation page.

[downscc]

Marc is referring to the fact that there are two landing calculations, Dispatch and Enroute.  It defaults to Dispatch, change it to Enroute to calculate landing when you are doing your predescent flows.

[Dreamflight767]

OK.  I'll make that change.  Thanks.  Just out of curiosity, if I uploaded the FMS data to the EFB, why do I have to change from dispatch to enroute?  Weight and weather are the same so why does the EFB calculate differently. 

[ph-cxz]

Enroute gives you more options when landing. 

[skelsey]

31 minutes ago, Dreamflight767 said:

OK.  I'll make that change.  Thanks.  Just out of curiosity, if I uploaded the FMS data to the EFB, why do I have to change from dispatch to enroute?  Weight and weather are the same so why does the EFB calculate differently. 

Because the regulations are different. 

For planning purposes (ie before dispatch) the aircraft must be able to land within 60% of the available landing distance for a dry runway -- this means you take the actual landing distance at the planned landing weight, multiply this distance by 1.67 and this figure must be less than the available landing distance. If it is not you must reduce the weight until it is. For a wet runway, the factor is 1.92.

Once you are airborne, however, these requirements no longer apply and all that is required is that the commander is satisfied that a safe landing can be made (ie actual unfactored LDR <= LDA).

[Dreamflight767]

I see.  Thank you for the explanation.

[Jlonchambon]

 

I compute my landing weight for arrival i.e. Predicated Remaining fuel at landing and zero fuel weight. Even doing that I still get a high EFB value than FMS. I also remove the 5 knots for comfort.

Vref is supposed to be based  on 1.3 Vso PERIOD,....!

So what's going on here????

 

J. Lonchambon

[jon b]

A quick cross check for vref I use in real world ops is:

flap 30 vref at 240T is 140kts. This speed changes by 1kt per 3T change in landing weight, eg 250T vref 30 is  143kt , 230T its 139 kt