Landing at Luton -- or not...

[Self Loading Freight]

Had a non-optimal experience last night on Ryanair from Dublin (if you've got nothing better to do, you can tune into the Passengers and SLF forum for the full 4AM rant...). Towards the end of it, we had an aborted approach to Luton and ended up on the ground at Stansted... Very on the ground: no shilly-shallying, but the full denture-rattler. Excellent.

Which makes me think: I've got a vague idea about all this CAT II/III business, but obviously not a very good one. Could someone let me know why we could land at Stansted and not Luton (lots of fog at both), why Ryanair moved us to a different 737 that was 'better equipped for the conditions at Luton' (you mean they're not all OK for fog?), why we hung around for 45 minutes outside Luton waiting for a momentary change in conditions and... why too much fog stops play anyway?

I think the thing that puzzles me most is the hanging around above Luton. I presume that we were waiting to actually see the runway, but in that case Captain Speaking (*) must have seen it to commence the aborted approach... what happens subsequently to make it no longer safe to continue?

Thanks for any advice. Any definitive replies will make their way into the SLF FAQ, to inform future generations as yet unborn.

R

(* actually one Captain Reddy, I think. Looked like Captain Birdseye, and could do the authentic Voice of God on the PA. I'm surprised the weather *dared* be so uppity).

[Jetdriver]

SLF,

The short answer is, It's not the quantity of the fog, but the quality !

Generally speaking for a Cat 1 ILS approach, there is a visibility requirement of around 550 metres ( this figure may be subject to some adjustment for ground lighting etc). Provided this visibility is reported then the Crew may commence an approach or perhaps more accurately may continue the approach beyond the final approach fix or equivalent position. Of course upon reaching the Decision Altitude usually around 200'agl they must have the required visual elements to complete a landing ( manual or automatic). If they do no, then they must go around and decide on the next course of action.

Cat 2 and Cat 3 enable a suitably equipped aircraft at a suitably equipped airport to operate in reduced visibilty with regard to the above requirements. Cat2 and Cat3 usually require ( not always) that the aircraft is carrying out an automatic approach and landing. Cat 2 reduces the visibilty requirement for passing the final approach fix to around 350 metres( measured) with a reduced Decision height of around 130 ft. Cat 3 (there are 3 sub categories) further reduce the visibility requirement to around 75 metres ( theoretically zero) with a decision height of around 14 ft.

It has to be said that whatever the reported visibility,it does not mean that upon reaching the decision height you will necessarily be able to achieve the required visual cues to complete the landing. In that case around you go (even though the aircraft may actually touch down in the process)!

Even on an aircraft equipped for Cat 3B conditions (14ft DHt and 75 metres measured vis), if the airport has degraded facilities and there is a long list of what is required, a Cat 3b approach may be ruled out. Likewise any degradation of aircraft equipment would also have the same effect. On top of that there are certification requirements for the crew and currency considerations.

I cannot comment on your particular flight, but as you can see the requirements are fairly complex. If for example the aircraft is certified for Cat2 then 300 metres would not be acceptable for one airport but 350 metres at another would be ok. Both is pretty thick fog to me!

I have tried to keep this simple. Hope it makes sense.

[Self Loading Freight]

JetDriver--

Thanks for that. Presumably the 'required visual elements' are the minima which get mentioned in hushed tones from time to time? And I do get a bit confused by the idea that by the time you've decided to do a go-around, you've practically landed... what's the disaster you're saving yourself from? Hitting something that might be hiding in the mists along the runway?

And has anyone got a pointer to somewhere online where the various categories are listed or discussed? It was clear from listening to the conversation of the pax around me that the whole business of flying safely through muck isn't understood -- a lot of people were genuinely scared by the end of it all.

R

[Jetdriver]

SLF,

The minima is a statutory requirement. For example ( Cat 3b approach) the measured and reported visibility must be 75 metres or more to commence the final segment of the approach ( say the last 4 miles) the Decision height ( also part of the minima) is the height by which either the required visual reference is aquired or a go around MUST be initiated. In the case of the example I have given that visual reference may be as little as "One centreline runway light". The higher the minima the more lights that must be visible.

The simple reason for this is that the Pilot must be able to manually complete the landing in the event of failure of the automatic systems. In the case of a manual approach the Pilot must be able to correct drift etc and maintain enough visual guidance to complete the landing.

The formulae for calculating the various minima is fairly complex in that takes many factors into account. "Minima" provides a very simple definition for the Pilot to make a safe decision in this regard.

[Self Loading Freight]

Thanks very much. Just what was required.

R

[ETOPS]

SLF

One further point - some of Ryanair's older B737-200 aircraft are ex Lufthansa and are fitted with large hush-kits at the rear of each engine. When BA looked at this mod some years ago a drawback was that it downgraded the autoland capability from CAT 111A (50'/200m) to CAT11 (or even CAT1). This was because only 25 degrees land flap could be used. What sort of 737 did you start on? An old one I guess, hence the change at STN...

[Vmike]

Try not to land at Luton. I got diverted there from STN last week and it took us seven hours to get out of the bloody place! Total DUMP!

[Self Loading Freight]

ETOPS--

That's the odd thing. The 737 we moved to had the hush kit fitted (never got close enough to the original one to see whether it did or not). (It was also definitely ex-Lufthansa, since I was instructed "Kein Rauchen" by the little lights above my head.)

Are there more alternate airports available if you're quieter?

R