Help - Boeing OPT Vref question

[Capt R Swipe]

Hi,

Been scratching my head for a while on this one. It's about the VREF ADD box on the Landing Enroute page of Boeings OPT. I have flown for 2 747-8 operators, one has 0 as default and the other has +5. Here's the case for both:

Case for default 0 additive - OPT data give you LDR based on touching down at Vref, The OPT performance data will have taken into account the deceleration from approach speed to touchdown speed during the flare. This is not a guess but clearly indicated by the fact that when you select AUTOLAND with a VREF additive of 0, the system adds 5kts (because it knows speed won't be reduced to VREF during the flare). The VREF ADD option is for wind gust corrections and control surface/configuration malfunctions.

Case for default +5 additive - The speed that is displayed in the results window is the speed at which the threshold is crossed.The threshold speed should be Vref plus half the steady headwind component plus the gust intensity, not to exceed Vref + 20. The total addition to Vref is entered in the VREF ADD input window. In absence of wind, the threshold is crossed with Vref + 5. For this reason, Vref ADD is set to 5 by default.

Any boffins out there who can tell me who is correct?

[Skyjob]

The default additive is set by your operator.
By default OPT uses +5 as Vfly equals to a minimum of Vref+5 on approach.
Vthreshold also requires to be not less then Vfly on approach, thus Vref+5 as the flare and thus some speed bleeding off during the flare has yet to be commenced.
For Landing Distance Required (LDR) calculations, which is what you may want to determine when checking a Landing Enroute capability, you require landing distance which is based on Vtouchdown, thus no +5 additive is required for this anymore. However a correction as per your FCTM must be included if an additive for current ambient conditions requires you to do so, such as gust correction.

[Capt R Swipe]

Thanks Skyjob, This was really keeping me awake at night (I know, I to to get out more!) I don't suppose you have a reference to LDR being based on Vtouchdown? Would help my discussions with the tech department.

Cheers.

[Skyjob]

The reference is actually in many manuals, but not directly described.

Think in the quote in the 737 FCTM reads: "airplane attitude at a normal touchdown speed (VREF to VREF - 5 knots)"

In all Performance Inflight charts for LDR it refers to 305m (1000') air distance in accordance with FAA rules, so JAA operators or airfields need to be adjusted according to JAA markers set at 400m, add an additional 95m in other words unless TDZ is promulgated non-standard JAA markings (for example LDY).

Furthermore the charts add a Vref additive which is applied to find the LDR.

[Capt R Swipe]

Thanks,

Just to be clear then. Both operators in still wind conditions fly the approach at VREF+5 (as do all 747 operators as far as I know). Is it necessary to enter 5 knots in the VREF ADD box in the OPT to determine the LDR?