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 capable of providing precision three dimensional navigation guidance with the possibility to
perform curved and segmented approaches, selectable glide path angles and to establish boundaries to ensure
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 miles whereas the actual coverage is normally the same. Future goals are a coverage of
360 degrees laterally and 30 degrees in elevation.
t=The micro wave landing system may be divided into five functions:
The approach azimuth, the back azimuth, the 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
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
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.
t=The characteristics of fog change easily. We should look at another special form of fog, called shallow fog.
Shallow fog leads to a fundamentally different visual sequence. As the aircraft is still above the fog top, the limit is
given by a constant angle, thus increasing the SVR with height, or decreasing the SVR when following the glide
slope. When the aircraft enters the fog, the SVR drops to its minimum and starts increasing again until it reaches the
RVR value again when the aircraft is on ground. This unique effect sometimes allows you to see the whole runway
length over the OM (OUTER MARKER) or even further out. You might be expecting good visibility on ground, but
you should expect to fly into a wall of fog when entering the top of the shallow fog. Remember: Be aware of a rapid
deterioration of the visual segment and of the intensity of the approach and runway lights at very low heights.
Therefore whenever „shallow fog” and RVR are reported it is strongly recommended to apply Low Visibility
approach procedures, even if you can see the runway from far away.
t=A permanent change of pitch can be influenced by the aircraft configuration,especially the flap setting. A higher
landing flap position increases your visual segment due to a lower pitch angle and a better downward view. A higher
landing flap setting may implement a second advantage, namely a lower approach speed. A lower approach speed
will give you more time for visual assessment. Please be aware that when flying manually, after changing from
autoflight to manual, you might tend to lower the nose to increase your visual segment. Therefore, rely on your
instruments and crosscheck them with your visual reference. However, the best method to avoid such situations is to
be prepared. So take into account the effect of the pitch on your visual segment.
t=Now we will take a look at the causes of visual illusions, and we will try to find out how we can react to these
phenomena. Certain weather conditions enhance the possibility of disorientation. First of all, we have rain. When
flying in rain, a refraction error may occur. The reduced windshield transparency and the deflection of the light
beams due to water will cause objects to appear lower than they actually are in relation to the pilot. Therefore rain
removal equipment is vital. However, this equipment itself can create problems. The continued movement can have
a hypnotic effect, so be careful.
t=Ground lighting can cause another form of illusion. Bright lights draw the eye’s attention, so that they appear to be
nearer then dim lights.If for any reason, like fog patches, runway contamination or failure in the lighting system, the
lights on one side of the runway are brighter than on the other side, you tend to bank away from the brighter side
towards the weaker side. Visual illusions can also be experienced when your visual segment changes from the bright
approach lights to the weaker runway lights, especially at the end of the touchdown zone. This may give a false
impression about distance or create a feeling of changing visibility.
How do you think that such an effect influences your approach?
If the approach lights are brighter you may have difficulties in distinguishing the runway lights.
t=Low visibility conditions are also often associated with icing. Remember that aircraft anti-icing systems may
require additional fuel.
On ground you are faced with the risk of airframe icing. You have to perform your takeoff within the hold over time.
Have a look in your operations manual to check the appropriate holdover time against the prevailing weather
conditions. What will you do if your anti-ice fluid hold over time has been exceeded and you are still waiting for
take-off clearance? You have no alternative. You must return for another anti-icing.
t=Until now, we have assumed that no equipment failure occurs. How do malfunctions of the airborne or ground
equipment influence our approach procedure? If a serious malfunction occurs during approach you have to evaluate
whether the approach can be continued and a safe landing can be made or if a go-around has to be performed. You
are responsible for making the right decision. The corresponding procedures and effects of failed or downgraded
system can be found in the Operations Manual Parts A and B.
t=As a rule of thumb we can say that before you reach the Outer Marker you may troubleshoot and reconfigure the
aircraft systems to meet the applicable requirements and check the effects of the deficiencies. After the OM
(OUTER MARKER) a go-around is preferable, as troubleshooting diverts your concentrationfrom the task of
landing. Another checkpoint for the influence of failures is the Alert height. Below alert height all failures not
directly affecting a safe landing can be disregarded. We will go into more detail on this when we look at the different