in FACT is,
No poblems.
The command speed bug is set to the target speed you will be flying on the approach for both manual flying & with the autopilot engaged. So you set the speed you want to maintain - in your 1st example, 138kts.
Bled off to what speed?
In this case 130kts (Vref), as your headwind correction was 8kts.
would it be correct if you have steady strongth head wind (no gust at all) just to set Vref+5 on the bug than fly manually with the Vref+ head wind correction, and what is the different between command speed and approach speed....??
No, the bug is set to the speed you wish to fly. You shouldn't be flying a speed above the speed you have set on the bug.
Approach speed is the speed you have decided is appropriate to fly the aircraft at, for the given conditions on the day. Basicaly, it is an adjustment for wind to the Vref calculated for the current weight of the aircraft.
Command speed is the speed you have set on the bug. On approach it is the same speed as the approach speed. With the autothrottle engaged & the appropriate mode selected, this is the speed that the autothrottle will try to maintain during the approach. We set it for manual approaches as well in order to keep manual & automatic approaches as similar as possible & also to have the target speed that we are attempting to fly shown on the ASI.
what should I set on the MCP, if for example I have 10 kts head wind correction and 10 kts gust correction, and fly mannually, so I have to set on the MCP Vref+20 kts (in this example 150 kts) then approaching touch down to bled off the steady wind correction to 140 kts at 50 ft AGL, looking at the figur on speed tape, while command speed bug give better indication
if you set to 140 kts to maintain until touch down
You are correct in having 150kts set on the bug. I can see what you mean by having 140kts set, however this means that you will be trying to fly the entire approach at a speed higher than the bug & will have no immediate reference when you check the ASI of where you are in relation to your approach speed. Dropping below the bug speed approaching the flare is of less consequence, as you spend more time looking outside & you are reducing speed anyway. Vref is still bugged as well, so you have an indication of minimum speed. As mentioned previously, the ideal touchdown speed is Vref to Vref -5, so you have a little lattitude in the final few feet to touchdown.
You have to find the balance between reducing the speed too soon & carrying too much speed into the flare, resulting in floating & perhaps going off the end on a short runway. Most people seem to be more afraid of getting too slow & end up carrying too much speed too far into the flare. We have seen the possible consequences of that (along with unstable approaches) in a number of accidents over the years.
Hope this helps.