Author Topic: Cat approaches  (Read 1351 times)

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Offline Adrian Chitan

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Cat approaches
« on: June 28, 2016, 10:09:26 AM »
Hi guys,

yet another post about the intricacies of civil aviation flying :D.

So, we're in a B777 flying over the Atlantic towards Chicago. As we all know Chicago weather can be a killer so arriving in early morning in Chicago we can find a very thick fog over the airport, not allowing for landing and we have to be directed to a 40-nm alternate that'll ruin the day. But wait a minute, if the company was thoughtful enough (and when you fly to Chicago, you always are! :)) ), the B777 could land with no problems in Chicago.

The thing needed is a certification called Cat II or one of the various Cat III standards. And for you fighter plane out there, I am not talking about the small button changing CAT I to CAT III in an F-16. That is for raising the Flight Control Computer flight limits. It is about a certain certification that lets the pilot use the autopilot in certain conditions.

DH and RVR

The limits of Cat certifications, which have nothing to do with the actual certification process (that has to do with the equipment and the crew the plane is carrying), are given in Decision Height and Runway Visual Range.

Decision Height

Decision height is the wheel height above the runway elevation by which a go-around must be initiated unless adequate visual reference has been established and the aircraft position and approach path have been assessed as satisfactory to continue the approach and landing in safety.

In JAA (Europe), DH can only be assessed using the radio altimeter. In FAA (US), the DH can be assessed by the use of radio altimeter, inner-markers (see the runway above a certain marker by following the published vertical path) or barometric altimeter. So in Europe, when preparing the descent, you should always rely on the radio altimeter for DH. In the US, you can use any combination of Barometric altimeter, Radar altimeter or charts. The implication is that, in Europe, you cannot do Cat approaches with a failed radio altimeter!

Runway Visual Range

Runway Visual Range (RVR) is the range over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings or the lights delineating the runway or identifying its centreline.

RVR tells the pilot how far can he/she see while on the runway. This is measured by specialized equipment (transmissometers) placed at the threshold of runways.


Cat    DH [ft]    RVR [ft]
I        >200      >1800
II       <200      >1200
IIIa    <100      >700
IIIb    <50        <700
         or none   >150
IIIc    none       none
*a note on JAA vs FAA: there is no IIIc for JAA

RVR translation

So what does the RVR limit means for the pilot in the air? As RVR is described as a visibility distance when on the ground, it seems like it doesn't. So how can a pilot say "LAND!" or "CONTINUE!" when at Decision Height without any visual reference? Well, there's a solution.

First, the minimum RVR is translated into rows of lights to see on the runway (in low visibility conditions, as in Cat approaches, the runway lights are ON). For example, for a Cat II and IIIa the pilot looks for 3 rows of lights on the runway at DH.

But what if the pilot doesn't get RVR information but only visibility info from the METAR? How is he/she to do Cat II and III approaches? Well, that's a simple problem of trigonometry.

So we have to transform RVR into Slant Visual Range which is basically the visibility because we are computing this for very low flying (at DH), just as visibility is also computed.

In the image above, the RVR required is 90 m long. The Down Vision Angle is how much the plane lets you look downward. For example, a A320 lets the pilot see 20o bellow the dash. The formula to get the visibility (or SVR) required for a certain RVR is:

SVR = Sqrt{[v+h cotan(w)]2+h2},
where v is the RVR required, h is the DH that you're computing for and w is the cockpit cut-off angle given by the above mentioned down-vision angle minus the pitch angle.

So if we have 1200 ft RVR requirement at 200 ft DH (Cat II) in an A320 the minimum visibility requirement published by the airport for a Cat II approach and landing would be...1979 ft. So if you're a pilot happily certified to Cat II and find the destination METAR containing 1200 (not in the time segment), then you have to hold until the visibility raises to 2000 or ask the airport what their RVR reading is every 5 minutes until they say 1200 ft.

Hope this helps,


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