Hi All,
Just a query re: Wake Turbulence and stability- if anyone can share their thoughts or knowledge to clear my mind would be greatly appreciated.

Firstly, in the CASA exam they might ask you for distance OR time amounts for separation, is there a basic speed that we should be applying between the two, 2nm/min?



Last but not least, another concept i'm having trouble with and understanding is what exact term should be used to describe a the characteristic of an a/c that is straight and level, experiences a disturbance from the left and then returns to its' original position.
What is it called if it adopts the new direction? Having strong/weak...

Is that lateral static stability? Directional stability? The tendency to return to the original position? To sideslip? I'm at a loss.
Thanks in advance

Hi Ben,

Take a look at the values listed in the CPL AERO book on page 5.10 in the book. Those are the figures you need to remember. As far as I am aware there in no speed which applies but rather it depends on the wake turbulence category of the aircraft and the positioning e.g. light landing after landing heavy, light intersection takeoff after departing heavy etc

As for your stability question, here's a quick rule of thumb:

Static Stability (refers to an aircraft in a wind tunnel)
"Is there a tendency for the aircraft to return to a state of rest after a disturbance?"
Yes, and it wants to return to the original position = Statically Stable
Yes, but it will just stay at the point you displaced it = Statically Neutral
No, there is no tendency to return to a state of rest = Statically Unstable

Dynamic Stability (considers the flight path of the aircraft)
"Does the aircraft actually reach some state of rest after a disturbance?"
Yes = Dynamically stable
No = Dynamically unstable.

An aircraft can be any combination of these except statically unstable and dynamically stable. That's a contradictioin in terms.

If an aircraft has strong dynamic stability it will tend to settle very quickly. Weak dynamic stability means it will settle but it may oscillate for a while first or only slowly return to rest. If it is dynamically unstable it just never gets back to a settled position again.

Aircraft tend to be strongly directionally stable and have weak lateral stability. Longitudinal stability (pitch) is somewhere in between. So in your example, if you are talking directional stability and you have a displacement of the nose to the left and the aircraft returns to the original direction then it is statically stable (it wants to return to the original orientation) and it is also dynamically stable since it actually returns to the original heading.

If it is displaced left and then settles heading in the new direction, the aircraft has neutral static stability and is dynamically stable since it steadies with the new heading.

If the nose was displaced left and the aircraft yawed left and then back to the right and then left again in a wobble of unstable yawimg motion then it is probably statically stable (it wants to come back to some rest position) but it is dynamically unstable (it never actually achieves it).

Cheers,

Rich