Cat I. II. III Operations

 

t=General Operating Minima Let’s look at the common operating minima accepted for low visibility operation. Be aware that your applicable minima are found in your operations manual.

 

t=Here you can see the obstacle assessment surfaces in the initial, final and missed approach sectors. Note how this corridor gets narrower as you approach the runway. We can say that the closer you get, the more precise you have to  be. Therefore it is dangerous to deviate from the desired track as the minimum obstacle clearance reduces from the centerline. To be safe, use the published speed, track and altitudes and you will maintain a safe obstacle clearance.

 

t=Let’s summarize the minimum DH (decision height) for each approach category. The minimum DH for CAT I operation is 200 ft. For CAT II it is 100ft. CAT III minima depend upon the sophistication of the autopilot system. CAT III A has a DH of below 100 ft. CAT III B below 50ft or no DH. CAT III C operation requests no DH or RVR. In practice some

kind of RVR minimum is required to facilitate taxi and help from ground vehicles if needed.

 

t=The micro wave landing system, MLS, was designed to provide precision navigation guidance for alignment and descent of aircraft on approach to a runway. In the future it will eventually replace the ILS as the standard landing system. The MLS is segmented capable of providing precision three dimensional navigation guidance with the possibility  perform curved and approaches, selectable glide path angles and to establish boundaries to ensure to clearance from obstruction in the terminal area. The approach azimuth coverage of the MLS is at least 40 degrees to either side of the runway laterally and in elevation up to an angle of 15 degrees and at least 20 000 feet. The minimum range of the approach azimuth is at least 20 nautical miles. The back azimuth coverage is also at least 40 degrees to either side of the runway laterally and in elevation up to an angle of 15 degrees, but the minimum range coverage is only 7 nautical whereas the actual coverage is normally the same. Future goals are a coverage of 360 degrees laterally and 30miles degrees in elevation.

 

t=The micro wave landing may bethe divided into five functions: The approach azimuth, the system back azimuth, approach elevation, range information and data communication. The azimuth and elevation stations transmit angle and data on one of 200 channels within the frequency range of 5031 to 5091 Mhz. MLS identification is a four letter designation starting with the letter M and it is transmitted by the approch and back   azimuth transmitter in international morse code 6 times per minute. The approach azimuth station is normally located about 1000 feet beyond the stop end of the runway and the elevation station is normally located 400 feet from the side of the runway between the runway threshold and touchdown zone. The back azimuth transmitter is normally located about 1000 feet in front of the approch end of the runway. The MLS precision distance measuring equipment works the same as the DME described in 062 01 04 00 and the MLS DME channel is paired with the azimuth and elevation channel.

Range information is provided with an accuracy of about 100 feet.

 

t=Other very important facilities at the airport are markings, signs and lights. As these are often the only visual cues you will have during low visibility, it is vital to know them. They will help you to find your way and allow you to

check your position. Let’s have a look at the approach Lighting System. The basic category I system may comprise a distance coded centerline or a barrette center line system. The centerline leads to the runway centerline and the crossbars provide roll guidance and distance information.

 

t=However, in low visibility conditions you will only see lights that are near to, or on the runway. When you look at the approach light systems for CAT II approaches you will see that the inner 300m are supplemented by red side  barrettes. These red barrettes give additional lateral and roll guidance and alert the pilot that he has not yet crossed the threshold. Here you can see the last 300 m of the Cat. II approach lighting system

 

t=As we know, obstructions like large trucks, cars, aircraft or other mobile objects can disturb and influence the  beams of any navigational transmitting installation on ground, such as the localizer and the glidepath beams. In order  order   to prevent these beams from being disturbed during low visibility operations certain areas have to be defined. There are Critical areas and Sensitive areas. Their size depends upon antenna type and type of operation (CAT I / CAT II or CAT III)

 

t Fog is the most typical problem and forms under certain conditions regarding temperature, humidity and wind. Small changes in any of these parameters results in variations of extent, density and depth of the fog. Even in „stable  conditions” the consistency of the fog varies with the vertical distance from the ground. Its density normally decreases with decreasing altitude but there may also be rapid horizontal variations. Changes in fog density are more  significant at higher decision heights than at lower ones. As a result, fog at CAT III DH tends to be more stable and less dense than at Decision Heights of e.g. 200ft.