Ask Captain Lim

Dear Captain Lim,

I wonder if you can help me on this one, since there seems to be no consensus amongst pilots (well, this last part is nothing new...).

By definition, the approach ban concept implies the existence of a DA (H) or MDA(H).

Roughly, it states (and it can vary slightly from state to state in Europe) that if the WX conditions fall below the minimums for the approach being flown, before the OM (Outer Marker) or equivalent position, the PIC (pilot-in-charge) can continue the approach down to the OM (or equivalent); if it falls below minimums after having passed the OM (or equivalent) the PIC can continue the approach down to the DA(H) or MDA(H) for the APP.

Having said that, the problem is: If flying a CAT III APP with no DH, where the minimum required RVR is 75m, after passing the OM the tower reports that the RVR decreased to 50m (or any other value below minimums). What is the most correct procedure to apply?

Of course we can always be on the safe side and simply go-around, but what I am trying to find out is if it would be correct to keep flying for a landing with no DH, where there is no requirement for any contact with the Runway lights? Or is it mandatory to, at least, insert any DH, for example, 20 feet, where at least one Runway light will have to visible to the pilot, in order to be within the approach ban concept and therefore legally protected?

Thank you very much for your time.

Sincerely,

Luis Miguel Silveira

Hi Luis,

In order to perform an autoland, three requirements must be satisfied - namely, the plane must be capable of autolanding; the pilots must be trained and certified and the airfield must have the proper facilities. Basically, there are 3 categories for CAT III approaches - CAT IIIA with 50 feet ceiling and 250 meters visibility, CAT IIIB with 20 feet ceiling and 100 meters visibility and CAT IIIC with no ceiling and visibility limitation.

Remember, an airport complying with Cat IIIC approach capability (no decision altitude and zero visibility limitation) is pretty expensive to maintain. As far as I know, CAT IIIC approaches are rarely flown due to the lack of suitable airports that can maintain the necessary equipment to a very high standard.

Even if a CAT IIIC landing is performed, the more pressing problem is not the landing part but the taxiing bit. A Boeing 777 requires at least 75 meters before it is allowed to taxi.

Your question - what is the most correct procedure to apply when the visibility decreases to 50 meters after the plane has passed the approach ban? Normally, if I were performing a CAT IIIA or IIIB approach - I would continue until the decision altitude and then decide if I could see 3 centerline lights/the runway marking for IIIA or one centerline light/the runway marking for CAT IIIB. If not, I would go around.

Aha, in the case of CAT IIIC where there is no decision altitude, how do I decide when the visibility decreases further? Well, I am not absolutely clear on this as I have never done a CAT IIIC approach in my life (anyway, we are not required to train up to this level yet!) But from what I have gathered, an airplane with a fail-operational landing system with automatic ground roll control (or ground roll guidance) may be certificated for operation without a decision height (when the pilot is not required to make a decision). If the ground roll system is fail-operational, no visibility (RVR) limit is necessary. (Fail-operational means that, during the approach, flare and landing can be completed by the remaining part of the automatic system) Based on this, I would continue with the approach if the visibility reduces below 50 meters but going around is equally a safe option!

Happy flying!