B738 flaps 40 through 30 command speeds
 

Hello folks!

Question for those boeing drivers around:

We are performing an approach with flaps 40, we are heavy weight, so we decide to go with Flaps 1-5-15-30(!)-40.

Now lets assume that for some reason we have to stay on 30 for a while. What command speed will you set? Approach one (Vref40+additive), which may be less than Vref30, or such for flaps 30 (Vref30)?

FCTM is quite unstrict with this tellng us that we should go "30 or 40" after 15, and set "Vref30 or Vref40 +additives". So it can be read both as "set any flaps and final speed" and as "set any flaps and RESPECTIVE speed".

Where is the truth?

Why not use F25 as an interim stage and then F40?

if you have selected F30 then you set F30 green bug. However, the trap is Vref 40 + wind additives could be higher than F30. So Vapp would be set. It is more comfortable and better for the trim changes to sequence F15 - F25 - F40. It's less strenuous if going F15 - Vref30, but if landing F40 it can be a trim strain going F15 - F40 in one step.

Going with flaps 25 after 15 renders the issue void and nil. Also can reduce the chance of forgetting to set the final flaps.

In practical terms Vref 40 is usually only about 7 KTS less than Vref 30 and given that Vref for current flap setting is also V2 for the go-around there is sufficient margin to continue with Vref 40 speed but with flap 30 selected under normal circumstances, usually we set 5 its on top in the MCP, but also consider if we do a dual channel approach the Vref is commanded as per the FMS speed, so in a nutshell there may be only 2 Kts difference. Obviously if we decide to land Flap 30 then the designated Vref should be set. As commented, selecting F25 can drag the speed down. The other consideration is if you are near the speed limits and the wind shifts you could end up auto-retracting from 40 to 30 in any event.