Why is the Go around magenta speed bug based on MAX takeoff MASS and not current?

 

The commanded speed (magenta bug) is determined by actual flap position and based on max take-off mass. The speed window on the MCP is closed therefore the target speeds are controlled by actual flap position. Vref 40 for 74990 is 152 knots,

Because a speed that is based on max weight also works for any weight below that.  It's simply a fixed speed schedule based on flap position.  While they could have programmed the FMC to base the GA speeds on the actual weight there would be nothing to gain from it, so why waste time/money/FMC memory doing so?

But can this not cause flap overspeed if using high thrust go around due to terrain or low level off?

But can this not cause flap overspeed if using high thrust go around due to terrain or low level off?

 

It's not going to command a speed higher than Vfe for the actual flap position.  If you fly the command bars you are not going to overspeed the flaps.  If you only press the TOGA button one time it's not going to give you full thrust, it will give you enough to climb at 1000-2000 feet/min, which keeps the flap retraction to a managable pace.  If you hit TOGA twice then you better be getting the nose up into the flight director because you could overspeed the flaps if you are not agressive enough with the pitch.

So doing a Go-around from manual flight would you set roughly Boeing arm length on the throttles or roughly 80-85 % N1 ?

 

So VFE on a go around is the maximum flap extension speed?

Is VFE different from max extension speed you find on the flap placard speed

VFE is the max flap extension speed, same as on the placard. I said it won't give you a speed higher than VFE, it does not command VFE. If my memory is correct you will end up with a commanded speed around 222 knots once the flaps are up. If you fly the commanded speeds during a go-around you won't exceed the placard speeds.

But in the placard you have MAX 250 for flaps 1 etc?

Why are you so fixated on the placarded max speeds?  They have nothing to do with the commanded GA speed.  During a go-around the command speed bug is driven by a pre-programmed speed schedule base soley on flap position, that's it.

But can this not cause flap overspeed if using high thrust go around due to terrain or low level off?

This is a good reason the get the a/t back on ASAP.

This is a good reason the get the a/t back on ASAP.

One of the perks of being able to leave it engaged for landing.

Question: I disconnect A/T when the decision to land is made. G/A is still a possibility of course but a remote one.

 

Why do you consider getting the A/T back on ? Do you disconnect before ?

One of the perks of being able to leave it engaged for landing.

I was thinking that exact same thing when I was typing it.

 

I hear we're doing a major SOP revision this January - fingers crossed.