Aberdeen departure weather

[roughneck]

Hi guys, would someone be able to help me with the following?

I'm an offshore worker and always had a big interest in the weather conditions that affects the North Sea (see one of my previous posts), but on a particular cloudy/foggy day what are the limiting weather factors in order to depart Aberdeen airport to any offshore rig?

Before I'm being scheduled to fly on any trip I always have a look on my IPhone App 'AeroWeather' to see what the general conditions at/around Aberdeen airport are.
Also: after having talked to a few pilots about this (and it's still not all very clear to me) what kind of decisions do you guys make if the cloud levels are for example Broken at 600' or Broken at 400' or Broken at 200' or even when it's Scattered at 200' or 300' or Mist/fog etc... (does scattered cloud and mist/fog have the same limits as Broken cloud levels at Aberdeen airport/other airports around Aberdeen )?

Hope my question is not too demanding, but after getting a bit more insight/understanding of weather and decisions that you pilots make it makes us passengers more relaxed, well it makes me more relaxed for sure).

Ta
Roughneck

[iamthetroll]

Hi roughneck,

First off my answers relate to the use of Pumas and make reference to SOPs which are not necessarily identical between the three operators but probably similar.
The decision to fly will be based upon (as far as weather is concerned) conditions at departure aerodrome, destination, and en-route. When we begin planning a flight we always start with the weather and will look through the METARs and TAFs for any aerodromes which could be of use or are relevant in building a bigger picture of what's happening around us. We apply limits to find out to what extent an airfield/rig is "usable". For airfields we look to see if the cloud and visibility are greater than the approach minima for that airfield (this varies between approaches and from place to place ie Aberdeen has different approach minima for ILS or VOR/DME both of which differ to Sumburgh's ILS and VOR/DME approach minima).
If the conditions forecast are less than the approach minima we can technically still fly to this location however we MUST hold TWO alternates with fuel to reach the furthest. If the weather is above minima we can fly to this location and hold one alternate. Then we if we add 200' to the cloud minima, and 400m to the visibility minima of an airfield and find that the wether conditions are forecast to be better than these new minima we can nominate that airfield as an alternate (by applying stricter minima we give ourselves a better shot at getting in). As we fly IFR we must always carry fuel for an onshore alternate.
So by this stage we can classify the different airfields as "destination+alternate", "destination only", and "below minima". One final classification exists for Aberdeen and Sumburgh which is a special one that enable us to travel there without an alternate. This is called coastal and requires the forecast cloud to be 600' or greater, and vis 4000m or greater by day; 1000' and 5000m by night. This can be especially helpful in offering greater payloads as we technically don't have to carry an alternate for an airfield meeting the coastal minima. However this exception only applies to Aberdeen and Sumburgh.
As I'm sure you're aware the TAF will be given as a forecast over a period of 6/12/24hrs. Which can be divided up into TEMPOs (temporary changes) and these TEMPOs come with forecast timeframes, to which we add two hours (one at the beginning, one at the end) so a TEMPO between 1200 and 1400 we would treat it as being between 1100 and 1500. When there is FOG or freezing rain we stay away from it. Flight in freezing rain is prohibited in the flight manual (for Pumas). So extending the TEMPOs and increasing the required minima make it harder for an airfield's weather to meet our requirements but make is safer for everyone as it increases our chances of getting in.

As for departure minima... Well these can technically be extremely low. To the extent you'll find that typically if a flight is delayed or cancelled then it would probably have more to do with destination weather, or a lack of alternates. The low vis procedure for the puma can be used down to 150m vis and cloud at (ahem) 50'. Don't know anyone who's used it in anger.

Onto offshore weather: basically cloud must be 50' above deck height (to an absolute minimum of 200' by day or 300' by night), and vis greater than 3/4mile. I'll leave it there.

En-route weather is not often such a nuisance as the nature of it is that we can fly around it (usually). The most infamous issue is with freezing level (altitude at which the air temperature becomes negative). The Pumas require a band of positive air 500' above the surface offshore or above MEA onshore. This is for the purposes of shedding excessive ice build up. Unlike the S92 the Puma's icing clearance is a "limited icing clearance", specifically as the permission to fly in icing conditions it restricted by a number of factors and the one relevant to your question is the requirement for 500' positive air. So if the temp at the surface is 0 or less, we can't fly in moisture (cloud) because we'd have no way of shedding ice.

I appreciate it's a long a gruelling post, but hope it goes some way toward helping answer your questions.

Cheers.

[HeliComparator]

Wow Trolley, that was a comprehensive answer and I think you get 10 out of 10 for ops man knowledge! Not much to add except this:

For an onshore approach down the ILS, we are likely to be able to land even with weather below minimum in terms of cloud base. So at Aberdeen where the decision height is 200' (ie need to have visual reference by 200' above the runway) we would still be likely to get in even if the cloud was Overcast at zero feet, because at 200' there is only say 400' slant range of cloud to be able to see through, in order to get the high intensity runway lights visual. The visibility inside the cloud will be reported and provided it is not too thick cloud we should be OK. Of course if it too thick, not only would we not be able to get visual, we would not even be allowed to make the approach since there is an "approach ban" with runway visual range (RVR) below 500 metres. 500 metres sounds quite a lot but it is the distance at which you can see the bright lights. If you land with 500m RVR you can barely see the edge of the runway from the middle - it is thick fog!

By contrast offshore the decision point when we need to be visual is 0.75 miles from the rig. If we are not below cloud by this point (minimum height as mentioned by Troll) we are unlikely to get in since the lights are 0.75 miles away through cloud (or fog). In other words we require vastly much better visibility at the decision point offshore, than onshore and so it is normally offshore weather that prevents us getting the job done.

HC

[iamthetroll]

Thank HC, I suspect I might have a bit too much time on my hands... I'm not sure if the ops manual drags on as much as my post. :p

[coalface]

Thanks Troll for explaining it so well. I've never understood our weather minima and have been afraid to ask any of my other pilot colleagues in case I showed my ignorance. I have always just followed the guy in front. Not much good when you are the only 7am departure on a weekend morning. In that event I always ask my co-pilot of he/she wants to operate P1 u/s so he/she has to make all the decisions and I just agree :O

[Fareastdriver]

If you can find your aircraft you can fly it.

[Ant T]

Coalface .............:D:D:D

[cyclic]

CF

The best post in months!

[coalface]

Now I have got the weather minima sorted ( I have printed out Trolls post and stuck it inside my baseball cap) can someone tell me what all this new fangled helideck movement limits are all about? It used to be that as long as the r/o didn't say a number bigger than 3 degrees or 5 metres then everything was fine. Now we have heave rate, inclination, periodicity (which I always thought was something to do with menstruation) and recently someone told me that the limits change depending on whether the helideck is painted green or red. I have seen green ones, grey ones, dirty ones but never a red one. I am verily confused. :confused:

[OvertHawk]

Coalface - Periodicity is indeed a new limit, and is related to what you suspected. They had to introduce it with the rising number of female pilots - depending on where she is in her cycle, you'd better let her have the landing or she'll rip your head off! ;)