CAT I RVR minima...why?

[Gonzo]

Now, I don't mean why are there CAT I minima, but why was 600m chosen as the figure?

I've tried to find out but it seems lost in the mists of time...

[Skyjob]

Your statement of 600m is only true depending on approach light system in use and decision height for the approach you are referring to.

Appendix 1 (new) to JAR-OPS 1.430 defines the minima and can be used to derive the adjustments to the minima in case of equipment downgrades such as a FALS being reduced to IALS.

Minima are set by the authorities in accordance with JAR-OPS or a variation thereupon in some States.

Generally speaking: the better the Approach Lighting equipment installed (FALS, IALS, BALS or NALS) in combination with a lower published DH, sometimes defined for aircraft category type and occasionally defined with a higher then standard Missed Approach Climb Gradient (defined in 10' increments) will give lower RVR minimums.

[Jumbo Driver]

Surely the 600m limit pre-dated JAR-OPS by about 30 years ... ?

Or maybe even more ...

[bucket_and_spade]

KISS.

Initially I'm sure it was based on how far ahead you needed to see in order to pick up the runway or lights when popping out of cloud at 200' AGL.

So the answer is probably...geometry!

[renard]

Most of the airports I go to have 550m RVR for CAT I ILS's.

As the answers above mention, the lighting facilities and decision height determine the minimum RVR.

The table in our Ops manual has four lighting capabilities, Full,
Intermediate, Basic and Nil.

[Gonzo]

Thanks all, I'm aware that EVS, and LTS CAT I etc etc can mean CAT I approaches can be conducted in less than 600m RVR, and all the intricacies regarding failures of lighting etc, but I am trying to find out the reason why the figure of 600m was chosen.

There is a move within ICAO to harmonise 'LVP' provisions across the US and Europe, and across the world (There are 65 CAT III capable airports in Europe, 64 in the USA, and only 15 in the rest of the world - that surprised me when I found that out!), and as part of the discussion yesterday we were trying to find out why the figure of 600m was chosen, and is it relevant/suitable today?

All very interesting.

[BUGS/BEARINGS/BOXES]

Possibly an arbitrary figure deduced from those boffins in Farnborough and BEA during the Trident days of auto land testing. IIRC the CAA never had a speed limit for RETS until airlines pushed them for a figure.

[Denti]

Dunno where you get the 600m from. It has been 550m for a very long time now without any need for LTS or EVS.

And I guess it has been a number chosen that allows to see the minimum visual segment when looking out at minimum. Of course it is quite possible as well that it is just an arbitrarily chosen number.

[Skyjob]

The trick is in geometry as stated:

At 200' AGL on a typical 3 degree path distance to travel is 3821', which converts into 1165 meters. As lights are usually easier to determine than their surroundings a factor of 2 is applied to take into account their effect, reducing therefore the RVR minimum to 582m.

As angle may not be exactly 3 degrees and MAHt may not exactly be 200' AGL, changes to this figure are made using simple trigonometry for creating usually higher RVR values when higher decision heights are used.

[Gonzo]

Well, yes, 550m, but at 550m ATC protects to CAT III, so ATC in the UK assume you're doing a CAT III at that RVR.

[Skyjob]

but at 550m ATC protects to CAT III, so ATC in the UK assume you're doing a CAT III at that RVR.

Not necessarily so, as you stated there are few airports which have CATIII capability, mostly have only CATI, sometimes an intermediate CATII.

But the minima is based on any aircraft being capable of making an approach without the use of augmented automatics being available such as required for an autoland to be carried out.

Thus these RVR minima are based on arbitrary trigonometry so if our wonderful CATIIIB aircraft has for instance an engine failure or hydraulic system failure rendering it incapable of its CATIIIB capability, at least we can determine if we can make an approach using the published minima for RVR using a CATI approach, like any other (non-)commercial plane could do if unable to fly better then CATI, and have a chance of seeing some approach lights to determine the possibility of identifying the correct runway to land on.

[rudderrudderrat]

Hi Gonzo,
After the 1965 Vanguard crash at LHR
ASN Aircraft accident Vickers 951 Vanguard G-APEE London-Heathrow Airport (LHR) (http://aviation-safety.net/database/record.php?id=19651027-0)
The AAIB findings said PROBABLE CAUSE: "The cause of the accident was attributed to pilot error due to the following combination of events: 1) low visibility; 2) tiredness; 3) anxiety; 4) disorientation; 5) lack of experience of overshooting in fog; 6) over-reliance on pressure instruments; 7) position error in pressure instruments; 8) lacunae in training; 9) unsatisfactory overshoot procedure; 10) indifferent flap selector mechanism design; 11) wrong flap selection"

The 1st item of probable cause was attempting to land with visibilities of only around 350m to 500m.
The Board recommended " The system whereby no positive approval of an operator's weather minima is required to be given by the Ministry is unsatisfactory. It gives power whilst withholding responsibility. Positive approval or disapproval should be required."

If the DH is 200 feet and the glide slope is 3 degs, then the distance from flight deck window to touch down is 3,821' or 1164m. It was decided to make the limiting RVR half that distance (582m) rounded up to 600m, later rounded down to 550m

[zerozero]

Hmm.

Word of the Day. Thanks.

Lacunae | Define Lacunae at Dictionary.com (http://dictionary.reference.com/browse/lacunae+?s=t)

:8

Anyway, what is this "overshoot" procedure?

[underfire]

RVR is determined at each location based on Allards law...

http://i41.tinypic.com/2m2v129.jpg

[Gonzo]

Rudderrudderrat,

Perfect, thank you very much, just what I was looking for.:ok:

[safetypee]

Gonzo, there is some history from BLEU before the 1965 accident – see ref.
The originating Cat 1 limits were 200ft and 800m; much of this considered the then accuracy of the ILS and/or flight system, where the final approach position often required a lateral manoeuvre back to the centreline. Thus the visibility requirement had to be sufficient to both identify the aircraft’s position and to enable a correcting manoeuvre; these tasks were supported by the Calvert lighting pattern.

Over time, the ILS accuracy improved such that the need for lateral manoeuvring was reduced, and the accuracy of approach delivery further aided by FD and Autopilot use. Then with a process of demonstration by aircraft type (usually the manufacturer) or by operator application, lower visibility limits could be approved by individual authorities (use of UK CAA ‘Fog Model’); the 1965 accident noting ‘the lowest limit’.
Formal recognition of these aspects cumulated in JAR-OPS 1 (E), which with due harmonisation process settled on the lowest values used by all European authorities (previous ref was ECAC Doc 18). This harmonisation actually increased the risks in some circumstances (manual, no FD, poor beam quality), but this was either accepted or more likely overlooked due to the high landing success rate for Cat 1.

Subsequent work by BLEU (RAE Flight Systems Bedford) identified some of the underlying parameters governing the minimum visual segment and decision making, and some authorities have further lowered the limits for special circumstances or equipment use (FD / Autos to 80%DH, aircraft type). Much of this is now being accepted as the norm, which further erodes the approach safety margin vs min RVR.

http://aerosociety.com/Assets/Docs/Publications/The%20Journal%20of%20Aeronautical%20History/2011-01Allweathe%20landing_Charnley.pdf

[de facto]

At 200ft DA without a vis lower than 550 rvr you wont see the approach lights..:E
If you look at your DA and MDA in feet and multiply by 3 it will give you a good idea of what visibility ud need for the approach in order to see a standard approach light system.
Ie an MDA of 400 ft would require at least 1200 m vis,otherwise forget about it:E