Incorrect calculation of approach speed correction in the EFB landing distance calculator

[donstim]

Aircraft Version

Development

Build info

Current

Describe the bug

The EFT landing distance calculator currently subtracts the no wind approach speed from the actual approach speed (including any additives), then multiplies that by the appropriate speed correction factor to determine the additional stopping distance for a higher approach speed. This is incorrect. A distance correction for speed should only be made if the approach speed is higher than the normal VLS + 1/3 of the steady headwind. For example, if VLS is 125 KCAS and the landing is being made in a 15 knot headwind, Vapp will be computed by the FMCG to be 125 + 15/3 = 125 + 5 = 130 KCAS. There would be no landing distance added for this Vapp. Currently, the landing performance calculator in the EFB will apply a distance correction for a 5 knot speed difference between Vapp and VLS.

Here is the description from the QRH for how the landing distance increase for speed additives (denoted as SPD) should be made:

Some important things to note:

1. APPR COR has one important difference from the approach speed additive applied in the FMGC. In the FMGC, a minimum 5 knot increment is always applied. For the landing distance calculator, a minimum of 5 knots is only applied if the landing will be made with autothrust on. There needs to be an option in the EFB to identify whether autothrust will be used for the landing. (Note: Our documentation should explain that a minimum of 5 knots will be added to VLS by the FMGC. This can either be manually removed from the FMGC Vapp if autothrust will not be used, or, if the pilot still desires to use the 5 knots speed additive, it should either be added as a pilot increment, or autothrust on should be selected in the EFB landing distance options to apply it automatically.
2. The ice accretion additives are not included in the the current EFB landing distance calculation. They should be added.
3. There should be an entry for an extra approach speed additive optionally added by the pilot. Any pilot extra approach speed additive would always result in applying a distance increment for the speed additive.
4. Summary - If, and only if, the approach speed is higher than VLS + 1/3 of the headwind component (due to ice accretion, autothrust usage, or a pilot speed additive), then the landing distance should increase according to the appropriate speed additive for this additional speed. It is now being added for any Vapp higher than VLS, which is incorrect. (The reason that there is no landing distance for a normal headwind speed additive is that the distance associated with this additional speed is more than canceled out by the reduction in landing distance due to the headwind, which is not otherwise accounted for.)

As an additional enhancement, I think we should consider displaying both the unfactored and factored landing distances on the runway pictogram. Currently, only the factored distance is displayed. (By factored, I mean the distance including the 15% safety margin applied. The calculator multiplies the distance calculated from the various parameters by 1.15 to include this safety margin.) This will give the pilot a more complete picture of both the expected landing distance and the size of the safety margin that has been applied. The FlySmart EFB of at least one of our IRL NEO pilots displays both the factored and unfactored distances. An abbreviation like "FAC" or whatever fits best might be used to denote the factored distance. Of course, this will need to be explained in our documentation.

Expected behavior

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Steps to reproduce

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