Saturday, November 8, 2014

CAT III LANDING ON THE MIRAGE 2000 IN 1993



    CAT III LANDING ON THE MIRAGE 2000 IN 1993. Have you read Part 1, Charles Svoboda's story? Read that first, please.

          Over 90% of today's tourists travel by air. Its so much faster that the time saved more than compensates for the extra cost. In truth, even that statement is not really true in today's modern conditions. The advent of no-frills low-fare airlines have made the cost of flying cheaper than by rail, road or sea.

          That said, travelling by air has its own limitations. These are mostly weather related and, in the odd case, aircraft availability. Sometimes, it is a combination of the two, when your specific aircraft gets held up ay another airport because it has a snag that needs attention, or the prevailing weather does not permit flight.

          Automation will soon make flying possible in what we pilots call zero/zero conditions. An official definition of zero/zero exists: "atmospheric conditions that reduce cloud ceiling and visibility to zero." Current Instrument Landing Systems-ILS- have become so advanced that today's airliners require a Runway visual range (RVR) of 46 metres. The ILS transmits two beams, the Localiser (LOC) along the runway centre line and the Glidepath, along the aircraft's descent path. Both are displayed on one instrument, and the pilot has only to keep them centred to come down safely.

          Runway visual range (RVR), in aviation terms, is the distance over which a pilot of an aircraft on the centreline of the runway can see the runway surface markings delineating the runway or identifying its centre line. RVR is normally expressed in metres. In the US, which has to be different, it is expressed in feet. RVR is used as one of the main criteria for minima on instrument approaches.

Category I:

Decision Height: 60 m or 200 feet. If at this height, the pilot does not sight the runway, he must abort his approach and either divert or try again.

Runway visual range (RVR): 550 m or 1,800 ft.

Visibility: 800 m or 2,600 ft.  

Category II:

Decision Height: 30 m or 100 feet.

RVR: 1,200 feet (370 m).


Category III is subdivided into three sections:
    • Category III A – A precision instrument approach and landing with:
      • a) a decision height lower than 100 feet (30 m) above touchdown zone elevation, or no decision height (alert height); and
      • b) a runway visual range not less than 200 meters (660 ft).
    • Category III B – A precision instrument approach and landing with:
      • a) a decision height lower than 50 feet (15 m) above touchdown zone elevation, or no decision height (alert height); and
      • b) a runway visual range less than 200 meters (660 ft) but not less than 75 meters (246 ft). Autopilot is used until taxi-speed. In the United States, FAA criteria for CAT III B runway visual range allows readings as low as 150 ft (46 m).
    • Category III C – A precision instrument approach and landing with no decision height and no runway visual range limitations. This category is not yet in operation anywhere in the world, as it requires guidance to taxi in zero visibility as well. "Category III C" is not mentioned in EU-OPS. Category III B is currently the best available system. 
    •  
           The main reason for the delay in using Cat IIIC ILS is its prohibitive cost. It might be required for only 6-8 days in a year! So why spend so much?   
Autolanding                 

             The Airbus 380 and Boeing 777 have zero/zero capability as well as an autolanding system. An auto landing process is achieved by an autopilot together with the ILS. As the name suggests, the ILS directs where the plane goes and the autopilot ensures that it does so. The auto landing procedure is executed automatically but the Captain may still have to intervene to check that the speed is as desired when the flaps are selected from 0 degrees to landing position.  

          At 50 feet, the autopilot flares the airplane, a term to describe how it would raise the nose slightly to prepare for a soft landing. The computer would call out aurally the heights every 10 feet and then at around 25 feet, the throttles are closed. At this point, the airplane should sit onto the runway gently and roll along the centreline until it comes to a complete stop by the auto brakes with the pilot aiding it further with reverse thrust. If the Captain is unable to see the taxiway because the visibility has further reduced, he may request a ‘Follow Me’ vehicle to guide the pilot to its parking bay. 

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