Vref 30 on the 767-300ER

Date:         06 Sep 97 02:55:09 
From:         ranga@batelco.com.bh (Chris Ranganathan)
Organization: Batelco
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Could someone shed some light on the following?

On the B767-300 ER, Vref 30 increases almost linearly with weight at
1.23 * Vs1g for weights of up to approximately 152 tons.  At weights
above this, it increases to a factor greater than 1.23 * Vs1g,
resulting in a Vref 30 of 1.30* Vs1g at 181 tons.

This apparent "artificial" increase in Vref 30, results in Vref 25
being greater than Vref 30 at weights greater than approximately 160
tons; (i.e.), a situation where the smaller landing flap setting (Flap
25) has a lower landing speed.

As one can imagine, there are several "theories" currently in vogue
regarding this apparent anomaly: the two most prominent being:

1.	At weights greater than approx. 152 tons, Vref 30 has been
"artificially" increased to above the minimum of 1.23 Vsig, to meet
the climb gradient required in the "land climb" or "balked landing"
scenario.  My problem with this theory, is that as the magnitude of
increase above the minimum speed, is 5 kts. or less. I am not sure
this small increase in speed can cause a significant climb gradient
improvement ; please correct me if I am wrong.

2.	As all manoeuvring speeds are based on additives to Vref 30,
this "artificial" increase is to ensure full manoeuvre capability at
heavier weights, for some particular flap setting: (Flaps UP / Flaps
1)?.

Vref 30 - Landing reference speed with flaps set to posn. 30.
Vref 25 - Landing reference speed with flaps set to posn. 25.
Vs1g    - Stalling speed for the particular configuration at 1g.

--
Chris Ranganathan
ranga@batelco.com.bh