Vne for the Turbine Duke

[Dingle]

Hi everyone,
The BKSQ manual states Vne for the Turbine Duke is 198kt, about 35 kts less than for the Piston Dukes. Is that correct or am I reading the manual wrong?

On second thought, it may well be correct...

[Black Square]

That's correct. It's a quirk of turbine conversions. The FAA does not permit turboprop aircraft to have the same "smooth air penetration" speed band as reciprocating engine aircraft. You will see on the airspeed indicator that the new VNE is exactly where the old green band (normal operating range) ends in the Piston Duke. Great question, though!

[Dingle]

Oh, that's interesting. What exactly do you mean by the same smooth air penetration band? As far as I understand the yellow arc is pretty much removed.

[Black Square]

That's exactly correct. The yellow band would allow you to fly the aircraft faster in smooth air, but in a turboprop aircraft, there is no such band, meaning that VNO and VNE are the same speed. Unfortunately, I have been unable to locate the logic behind this regulatory decision, but there are several accident reports of structural failure in turboprop conversions flown beyond VNO in turbulent conditions.