Thank you for your answers.
FAR25.1001 part (a) and (b) is quoted below, and as far as I can see, is it only approach/go around climb gradients all engines (25.119) and one engine out (25.121(d) ) that dictates whether or not a fuel dump system is required to have on a given aircraft model.
Many aircraft have larger differences between MTOW and MLW w/o fuel dump system intalled, so it seems to concentrate whether or not you can do a go-around at MTOM, ie. you have been attempting to land at a weight over MTOW, in an emergency I suppose.
Anyone care to correct if this is wrong?
Thanks.
FAR 25.1001 a+b (same as JAR 25.1001)
(a) A fuel jettisoning system must be installed on each airplane unless it is shown that the airplane meets the climb requirements of §§25.119 and 25.121(d) at maximum takeoff weight, less the actual or computed weight of fuel necessary for a 15-minute flight comprised of a takeoff, go-around, and landing at the airport of departure with the airplane configuration, speed, power, and thrust the same as that used in meeting the applicable takeoff, approach, and landing climb performance requirements of this part.
(b) If a fuel jettisoning system is required it must be capable of jettisoning enough fuel within 15 minutes, starting with the weight given in paragraph (a) of this section, to enable the airplane to meet the climb requirements of §§25.119 and 25.121(d), assuming that the fuel is jettisoned under the conditions, except weight, found least favorable during the flight tests prescribed in paragraph (c) of this section.