What is the origin of 1000ft ceiling when using RRSM

[ramzez]

Does anyone have info on where the 1000ft ceiling limit comes from when using reduced runway separation minima?

I'm looking for some consistency and wondering why it is not for example 1500ft which would be consistent with when SVFR clearances need to be given.

[Equivocal]

I can't tell you for sure but I had some involvement in the UK with development of previous and similar rules.

When writing rules one tries to draw up a comprehensive set of rules which cover all possible situations (including those which 'only an idiot would do', because there are plenty of idiots around), but does not unnecessarily constrain either operations or application of the procedures. Cloudbase of 1000ft will allow a controller to see the positions of aircraft between which reduced separation is being applied, in reality, it's the visibility (if only eyes are used) which will likely limit the controller's ability to see the aircraft involved. Using a cloudbase limit of 1500ft would mean that the controller cannot use reduced separations as often as is possible using the 1000ft standard. If the change is made only for consistency with some other procedure you will lose flexibility and, potentially, capacity for no benefit.

[jmmoric]

.

I agree.

It comes straight out of DOC4444

7.11.6 Reduced runway separation minima shall be subject to the following conditions:
a) wake turbulence separation minima shall be applied;
b) visibility shall be at least 5 km and ceiling shall not be lower than 300 m (1 000 ft);

It will ensure the succeeding aircraft is able to see the preceding aircraft, and also the controller to see the aircraft involved. <- this is my take on it.

Once the ceiling and visibility get that low, you're also close to the limit of being able to use the separation anyway, since you still need to apply minimal separation between then when they dissapear from view from the tower.