Hi Michael. Always great to hear from you
I am perplexed by your mention of "75 KIAS as Vx flap 10°". I cannot find this specification anywhere in the AFM or the approved AFM supplements. In light of the quite remarkable climb gradient a DHC-6 achieves with both engines operating (approximately 17% at ISA), it seems pointless to even think about a Vx for the take-off configuration.
In the very short time I've been in contact with the aircraft and those who fly it at my company I've never seen or heard of someone using Vx Flap 10. I've told you previously that we operate with a SOP very similar to the one used by the defunct ACES Colombia (which I've heard was the largest DHC-6 operator at a given point somewhere in the late 80's) so I think you should be familiar with the name and the reputation the airline had. So I am a bit perplexed as well, that there's no reference of that speed other than our company's AFM. I'll scan and send you the page on the AFM that contains the speeds.
Here's what our AFM says about the use of Vx flap 10:
If a Vx-10° flaps climb is necessary after takeoff, corresponding speed should only be set at or above 500ft AGL; accelerate to V2+5 before flap retraction. It is also possible to climb at Vx-0° flaps at pilot's discretion.
With respect to flap retraction on the Twin Otter, the AFM states that flap retraction should not be commenced prior to 400 feet AGL (minimum), and that the aircraft should be accelerated from 80 KIAS (V2, also equal to Vy with flaps 10°, also equal to Vyse with flaps 10°) to 100 KIAS (Vy with flaps up and two engines operating) as flaps are retracted.
Company SOP states flap retraction should be done at or above 1000ft. I think such a high figure is used due to the high elevation/terrain we operate in/around.
Glide speed at MTOW (both engines out, both propellers feathered, flaps up) is 100 KIAS for best range and 77 KIAS for best endurance. These figures are published in the AFM for the Series 400 Twin Otter, and will be published in the AFM for the Series 300 Twin Otter when Revision 53 is promulgated.
Thank you very much.
Best regards