Airbus vs Boeing (speeds)

[Check Airman]

Airbus guy here, looking to understand a bit more about American airplanes.

Looking at the 777 documentation, Boeing has a minimum manoeuvring speed and a flap manoeuvring speed. It says the flap speed is the cue for flap extension, while the minimum speed is the 1.3g buffet limit.

What exactly is the difference then? I assume the flap speed is equivalent to the characteristic speeds on an Airbus. Why then, are we required to extend flaps when slowing below the flap speed, when there still remains a healthy margin to the actual minimum manoeuvring speed?

On an A320, you can safely fly below this flap manoeuvring speed. What’s different on a Boeing?

Obviously, we’re going to follow SOP, but the academic curiosity remains.

[rudestuff]

Hopefully the philosophy is the same: on a 767 in the old days they had flap maneuver speeds based on weight, basically Vref30 +20, +40, +60 etc. They were blanket speeds designed to cover all eventualities and still provide the 1.3g buffer. (Flap 15 and flap 20 both used the same speed for example).

More modern versions use speed tape display which has an amber band which is a much more dynamic display: the top of which represents the minimum maneuver speed which still provides full protection at 40° bank. It also moves up and down noticeably with speed brake.

According to the FCTM the Flap maneuver speeds are recommended, and the bug should not be set below minimum maneuver speed, so technically you can fly below the Flap maneuver speed as long as you are above the minimum maneuver speed (amber band) - and cognisant that it is dynamic. Very few do.

[Vessbot]

As an innocent bystander trying to interpret things, it sounds like the "flap maneuvering speed" might be a compromise between minimum speed at the current flap setting, and max structural speed at the next?

[Check Airman]

Hopefully the philosophy is the same: on a 767 in the old days they had flap maneuver speeds based on weight, basically Vref30 +20, +40, +60 etc. They were blanket speeds designed to cover all eventualities and still provide the 1.3g buffer. (Flap 15 and flap 20 both used the same speed for example).

More modern versions use speed tape display which has an amber band which is a much more dynamic display: the top of which represents the minimum maneuver speed which still provides full protection at 40° bank. It also moves up and down noticeably with speed brake.

According to the FCTM the Flap maneuver speeds are recommended, and the bug should not be set below minimum maneuver speed, so technically you can fly below the Flap maneuver speed as long as you are above the minimum maneuver speed (amber band) - and cognisant that it is dynamic. Very few do.

Thanks for that. Referring to the FCTM, on page 5.7, it seems pretty clear that we need to add flaps before slowing below the flap speed. I’m just trying to figure out why this is necessary if there’s still a good margin to the 1.3G buffet at the minimum manoeuvring speed.

[Check Airman]

As an innocent bystander trying to interpret things, it sounds like the "flap maneuvering speed" might be a compromise between minimum speed at the current flap setting, and max structural speed at the next?

You may well have a point, but it then leads me to ask why’s the minimum speed at the current setting so high? Shouldn’t it be at the 1.3G buffet minimum speed above the red bricks?

[rudestuff]

The minimum maneuver speed (amber band) is dynamic, effected amongst other things by speedbrake and anti Ice, giving reduced maneuver margins. The Flap maneuver speeds are set high enough that this shouldn't be an issue. It also means that a few knots of speed loss here and there won't hurt - if you look at the 'maneuver margins to stick shaker' zig zag chart you'll see that there are sizable margins.

Flap maneuver speeds give nice easy 20kt increments that just makes it simpler to fly. Having said that, if you've got a minimum maneuver speed displayed and you understand what it means then you get a little bit of flexibility when speed controlled on final etc.