767 threshold speeds

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Any bright sparks out there that can tell me why the threshold speed on the 767 300 is lower at max gross weights using flap 25 as opposed to flap 30.
Many thanks.

[None]

My company recently sent this same question to Boeing for a response.

Their reply was that it is due to the deck angle as it pertains to the two flap positions.

The question came up because company changed the policy concerning overweight landing flap settings (emergency return after takeoff scenario). Flaps 25 is now used at high gross weights, and the slower Vref was noticed. By using Flaps 25 and Vref 25, in addition to the slightly slower Vref, you decrease the possibility of flap blowup, therefore lessening last minute changes in stability due to flaps blowing back down to the set position.

[Expert]

All minimum manoeuvring speeds on the B767 are based on a reference speed, this speed being VREF30. For example, minimum clean speed is VREF30+80, minimum speed at Flaps 1 is VREF30+60, at Flaps 5 VREF30+40, etc. To protect the manoeuvre margin at high weights in the clean configuration, the VREF30 reference speed has had to be increased artificially to allow the VREF30+80 flown to be sufficient/correct. As the VREF25 is only used as an actual approach & landing speed as opposed to a reference speed for other that flap 25 configurations, it is the correct speed for the flap 25 configuration, not an artificial one.

At high weights this equates to a lower speed than the equivalent flap 30 (VREF30) speed. An additional benefit of this anomaly is that in the case of a high AUW overweight landing it is more beneficial to use flaps 25 for the landing as this setting will produce a lower approach speed and will also ensure that you do not get the flap load relief system activating at an inappropriate stage on the approach, resulting in flap blow back (flap 25 limit speed being higher than flap 30).

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Excellent replies,thank you.

[None]

Expert,

That makes a lot more sense than what we were told. Thanks.