Vref as defined is not limited to 15 deg bank + 15 deg overshoot. If it was then all the flap manoeuvre speeds based on it would be equally limited.

 

The regulations say that Vref must be a minimum of 1.23 x Vs1g. This, by definition, is less than the 1.3g margin required to provide 40 degrees of bank.

 

However, there is nothing saying that Vref cannot be higher than 1.23 x Vs1g (great example on the 767: if you plot Vref30 against weight on a graph it starts off at 1.23 x Vs1g and increases almost linearly up to about 158T. Above this weight, Vref30 suddenly increases more rapidly, ending up somewhere near 1.3 x Vs1g at MTOW. Boeing found that the standard flap schedule didn't provide adequate manoeuvre margin throughout the range of configurations at higher weights, so they artificially increased Vref30 in order to preserve the flap manoeuvre speed schedule).

 

So it's not possible to make sweeping statements about how much manoeuvrability one has at Vref: you may or may not have a full 40 degrees depending on the type, how the manufacturer has designed the speed schedule and even current weight. Whilst the manoeuvre margin decreases (linearly?) to zero at the red bricks, it's not out of the question for an airline to make the sort of statement Vernon quotes from his FCOM: i.e. if you don't have the full 40 degrees, you may only use 15, even if the actual margin allows for 37 degrees.

 

That said, as already mentioned it's fairly academic as you're unlikely to be carrying out that sort of manoeuvring at Vref.

The regulations say that Vref must be a minimum of 1.23 x Vs1g. This, by definition, is less than the 1.3g margin required to provide 40 degrees of bank.

 

 

 

However, there is nothing saying that Vref cannot be higher than 1.23 x Vs1g (great example on the 767: if you plot Vref30 against weight on a graph it starts off at 1.23 x Vs1g and increases almost linearly up to about 158T. Above this weight, Vref30 suddenly increases more rapidly, ending up somewhere near 1.3 x Vs1g at MTOW. Boeing found that the standard flap schedule didn't provide adequate manoeuvre margin throughout the range of configurations at higher weights, so they artificially increased Vref30 in order to preserve the flap manoeuvre speed schedule).

 

According to the manuals the 737 has a manoeuvre margin of 40 deg (25+15) at Vref so I expect Boeing are using more than the minimum definition of 1.23g. I thought they used 1.3g. That's the figure I've always heard quoted.

 

 

it's not out of the question for an airline to make the sort of statement Vernon quotes from his FCOM: i.e. if you don't have the full 40 degrees, you may only use 15, even if the actual margin allows for 37 degrees.

 

I agree, which is why I suggested he might be misinterpreting what his FCOM said as a general statement of what 737 Vref was. My 37 deg calculation was based on a misreading of your earlier post. I used 1.26g by mistake for some reason. 1.23g would be 35.6 deg bank.

 

 

That said, as already mentioned it's fairly academic as you're unlikely to be carrying out that sort of manoeuvring at Vref.

 

That depends who's flying. :wink:

 

 

According to the manuals the 737 has a manoeuvre margin of 40 deg (25+15) at Vref so I expect Boeing are using more than the minimum definition of 1.23g. I thought they used 1.3g. That's the figure I've always heard quoted.

 

As far as I know and understand it, the certification used to require 1.3Vs in the landing configuration. The problem with this was that this was that the stall speed in the landing configuration could be demonstrated at slightly less than 1g (within reason: you couldn't do a 0.5g bunt and use that, but you could certainly 'massage' the numbers) which would result in a lower value of Vs (and thus a lower Vref).

 

In order to close this 'loophole' and make things more consistent, it was decided that aircraft should be certified under 1g conditions -- hence the use of Vs1g, the unaccelerated stall speed in the landing configuration. It was determined/accepted that this new method of certification would result in speeds roughly 6% higher than under the old regime: hence 1.3 x 0.94 = 1.23 to give the same margin under the new rules.

As far as I know and understand it, the certification used to require 1.3Vs in the landing configuration. The problem with this was that this was that the stall speed in the landing configuration could be demonstrated at slightly less than 1g (within reason: you couldn't do a 0.5g bunt and use that, but you could certainly 'massage' the numbers) which would result in a lower value of Vs (and thus a lower Vref).

 

In order to close this 'loophole' and make things more consistent, it was decided that aircraft should be certified under 1g conditions -- hence the use of Vs1g, the unaccelerated stall speed in the landing configuration. It was determined/accepted that this new method of certification would result in speeds roughly 6% higher than under the old regime: hence 1.3 x 0.94 = 1.23 to give the same margin under the new rules.

I don't doubt that, but 1.23g represents the minimum Vref allowable. The actual Vref can be higher than that, and seems to be in this case according to the FCTM and FCOM information.

Absolutely -- like I say, 1.23 is a minimum and may be exceeded, especially as Boeing base the flap schedule on Vref and need sufficient margin to manoeuvre at Vref+20/40/60/80 etc throughout the weight range (as in the 767 example).

 

The point really is that Vref itself doesn't automatically guarantee you full manoeuvrability in all types: if Boeing say it does in the NG, of course, that's fine!

I've read in the FCTM that the amber band is the minimum maneuvre speed and vref is above this by some kts? So will that not make vref have full maneuvre capability?

I guess I tend to approach things with too much of the bigger picture in mind. Just to satisfy my own curiosity...why does all of this theoretical discussion of maneuver margin matter?

 

You're only supposed to be a VREF (with no wind addition) when over the threshold (+/-), which means you're going to be at about...what...30 feet?

 

...and we're debating the finer degree extremes of maneuverability?

I've read in the FCTM that the amber band is the minimum maneuvre speed and vref is above this by some kts? So will that not make vref have full maneuvre capability?

In a word, yes.

I should say at the top of the amber band

 

FCTM

 

Flaps Down Amber Band

For all flaps-down operations (any time the flaps are not full-up) the minimum maneuver speed is the slowest speed that provides full maneuver capability, 1.3g or 40° of bank (25° of bank and 15° overshoot) to stick shaker. The top of the amber band does not vary with g load.

As airspeed is decreased below the top of the amber band, maneuver capability decreases. In 1g flight, the speed in the middle of the amber band provides adequate maneuver capability or 30° of bank (15° of bank and 15° overshoot). The speed at the bottom of the amber band (top of the red and black tape) corresponds to stick shaker activation for the current g load. If the g load is increased during maneuvering, the stick shaker activation speed increases also.

 

 

So anything below the top of the amber band you loose your full maneuvre capability