Cloud cover on non-precision approaches

[NorthSouth]

I'm trying to compile some figures on the likely regularity of an operation into an airport with non-precision approaches given the known met data.

I'd welcome anyone's views on what proportion of cloud cover (oktas) at or close to the MDH would give you a reasonable chance of acquiring visual reference and making a successful landing.

For example, if your MDH is 650ft, how would you rate your chances of landing off an approach if the lowest cloud is 3 oktas at 600? How about 4 oktas? or 5?

Logically you'd say 4 oktas would give you around a 50% chance of getting in, but it's obviously more complex than that due to slant ranges etc.

Any views welcome.

NS

[Piltdown Man]

What sort of aircraft? The smaller the aircraft, the more likely you are to get in due lower speeds and the ability to avoid cloud without unstablising the approach. The way an operator's manual is worded will be the next thing that affects whether or not an aircraft will make an approach. Some are precluded from making an approach if a go-around is likely, some are allowed to give it a shot no matter what.

We should also consider the navaid giving the guidance. Is it very close to the runway or is it behind you on the approach? In general, given appropriate viz, say in excess of 2,000m then I'd expect to get in most times given cloudbase at minima. But as soon as you put any cloud below minima then the chances of getting in reduce rapidly, like FEW at 600 say 50% chance down to 20% at BKN.

However, if you are asking these questions because you are an airfield operator who would like to save a a few pennies by not installing an ILS, then truly I would forget it. From what I understand, the commercial people in scheduled airlines do not like going to airfields which don't have the full set of navaids or the weather which means they don't need them (I'm excluding the cheap-cheap operators who would land on a road if they could save a few euros).

[Seat1APlease]

There are many other factors just as important as cloud base if not more so.
Firstly how skilled and experienced are the crew? The secret of a good landing is a well flown approach.
It's no good popping out through the cloud unless the runway is where you expect it to be, or unless you stand a good chance of getting it back in the slot.
If the A/c has an autopilot approved for non-precision approaches and good secondary information such as drift information, gps for distance guidance, etc. then you are going to stand a much better chance than if it is being hand flown using just a wobbly NDB needle and a stopwatch.
What are the runway markings and lighting like? You are much more likely to see a well light runway with clear markings rather than some obscure bit of concrete.
Are the pilots familiar with the field, i.e. is it their home base and they are going to get used to the local landmarks? For example on runway 06 at Inverness which had an NDB approach there was a very distinctive chimney which helped orientate ones-self in poor visibility.
In short I suspect that the cloudbase oktas has a relatively small input into the success of the operation.

[fireflybob]

Not 100% sure about this but I always understood that part of the philosophy beyond the Aerodrome Operating Minima was that if you had the required Vis/RVR you stood a fighting chance of attaining the required visual reference prior to or at MDH etc.

The AOM criteria take account factors such as OCH/MDH/DH, accuracy of approach aid, lighting etc.

Hope this helps

[Dan Winterland]

Technically, the amount of cloud cover is irrelevant - it should be considered as an overcast, ie 8 okta. How much of a hole in the cloud do you expect to be able to see the runway? And where will that hole be when you reach the minima?

However, most approaches give an RVR minima and not a ceiling mnima. This is because bright appraoch lights can penetrate cloud for a certain distance and that distance will depend on how bright the lights are. However, some operators place a cloud limit on approaches. for example, one operator i have wored for stipulated that ceiling was assessed as 5/8ths or more and another placed a limit on F/Os flying the approach of the cloud ceiling 100' above minima.

[tailwheel76]

Not sure about cloud cover but I was told recently that if you mulitply the DA by 5 it gives an approximation of the inflight vis required to see the runway/lights at the DA. Haven't tried it though.

[bookworm]

I'd welcome anyone's views on what proportion of cloud cover (oktas) at or close to the MDH would give you a reasonable chance of acquiring visual reference and making a successful landing.

Three points for your model:

1) There are likely to be substantial non-linearities. The probability of getting in with 2/8 below MDA is likely to be greater than 3/4, because that cloud can be avoided by limited manoeuvring. Conversely, I don't think the chances of getting in with 7/8 of obscuring cloud are as good as 1/8, as the chance of blundering into it at some point and having to go around is much higher.

2) The lights may play a significant part. Without lights, cloud in pretty much fully obscuring. With full approach lights, you may well see the runway environment through 200 ft of cloud below.

3) CANPA vs dive & drive plays a part. The timescale for the instrument visual transition is shorter for CANPA, so I would imagine there's a higher probability of partial cloud obscuring the view at the critical time.

But no rules of thumb, I'm afraid, which is what you're really after.

[NorthSouth]

Thanks everyone, all useful stuff.

On a related topic I found a reference to ICAO defining cloud ceiling as lowest cloud 5/8 or more, however I've trawled Annex 3 and can't find it in there. Maybe it's in a related Doc.

NS