Originally Posted by
Escape Path
Twin Otter figures (at max weight): V2: 80kias. Vx flap 10 (T/O config) is 75kias. ...on actual engine failure on takeoff we use V2 and stick the ASI needle at 80. ...the POI doesn't like it when we go below 80kias on engine failure trainings...
...we should accelerate to V2+5 (85kias) before flap retraction. The SOP also states that our climb speed on normal circumstances is Vy flap 0 which is 100kias (is there a Vy flap 10?).
By the way, which is the equivalent of, or the closest speed to, best L/D ratio on aircraft like the Twin Otter?
Hello Escape Path:
Although your description suggests you are generally doing things right (right = in accordance with the approved AFM) with your Twin Otter, I am a bit perplexed by some of the figures you have cited above.
For a Series 300 or 400 Twin Otter at MTOW, V2 (Take-off Safety Speed) is 80 KIAS, as you have indicated. During normal two-engine operations, this is the target speed to maintain between 35 feet AGL and 400 feet AGL. Flap retraction is typically commenced at 400 feet AGL. Should an engine fail after V1, the same 80 KIAS is maintained for the entire duration of the climb following take-off, because 80 KIAS is the best rate of climb speed for the Twin Otter wing in the 10° flap configuration, and 10° flap is the only approved take-off configuration for the Twin Otter landplane.
Twin Otter take-off performance calculations are particularly simple because the take-off configuration (flaps 10°) is the same as the configuration for Vyse (also flaps 10°), and the V2 at MTOW (80 KIAS) is identical to the Vyse at MTOW (80 KIAS). Thus, when you make a take-off, you know that your aircraft configuration and speed target will be flaps 10° and 80 KIAS, no matter whether both engine work perfectly or one engine fails after V1.
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. The single-engine take-off climb gradient (again MTOW, ISA) is 4.2%. The only published Vx for the DHC-6 is in the flaps 0° configuration, and at MTOW, that is 87 KIAS.
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.
Originally Posted by
Escape Path
...(is there a Vy flap 10?)
Certainly. At MTOW, it is 80 KIAS. This is one of the key reasons why 80 KIAS is the V2... because, as mentioned earlier, it is also the Vyse. If something goes wrong on take-off, you don't need to change your flap configuration or your airspeed.
Consider, for a moment, that 'Vy at flap 10°' is 80 KIAS whether you have both engines operating, one engine operating, or even if you are being towed on the end of a rope like a glider. The wing has no idea how many engines are operating, and the speed at which the wing (in a specified configuration) achieves best rate of climb is not going to change no matter how many or how few engines are operating.
Originally Posted by
Escape Path
...By the way, which is the equivalent of, or the closest speed to, best L/D ratio on aircraft like the Twin Otter?
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.
Michael