Landing of an Aero
plane under Zero Visibility Condition
- Manikandan J, Trichy, INDIA I was sitting inside a plane and was about to land at the airport early morning. Flight attendant announced that the plane is about to land in few minutes, but I could not see anything out of the window inspite of the plane being just 500-800 meters above the ground. It was dense fog outside and I was thinking how pilot is able to see and land perfectly onto the runway under this zero visibility conditions? I wish to share my research in this area with many readers having the same question in mind during flight under foggy environment. Fog is a layer of mist that reduces the distance of human eye visibility. But there are radio waves invisible to human eyes that penetrate this fog and do the work of human eye. There are systems on the aeroplane that view the outside environment under foggy conditions using radio waves and give all the information to pilot on cockpit displays and as sound on the headphone. Now we will see how does these systems make pilot to land the aeroplane safely under zero visibility condition? First the pilot should know the height at which he is flying and the height of descent during landing. A system called Radio Altimeter is used to measure the height above ground and this works on radio frequency. The concept behind its working can be explained with an example. Consider the example of a jet-train traveling at the speed of 500kmph from station A to station B and returning to station A without halting anywhere. Assume that the train completed the one-round trip in 2 hours. Using simple mathematics, we can conclude that the distance between station A and station B is 500kms. The same concept is used in radio altimeter, but vertically down with train analogous to radio wave. The time of transmission and reception of radio waves is recorded in the system to compute the height. The radio wave emission and reception by radio altimeter is analogous to a child throwing a ball down to ground and catching it back in hand. After height, next requirement is the direction towards which the plane should land. The system VHF omnidirectional range finder on the aircraft is basically a type of "radio-compass" unit, broadcasting a directional signal that provides coded information that gives the compass angle of the signal between the aircraft and the airport. The functionality of this range finder is similar to magnetic compass, but magnetic compass gives direction using earth magnetic field and the range finder uses coded radio frequency signals transmitted from ground station. The radio compass measurement is available on the cockpit display to pilot as an arrow mark shown in figure 1. Figure 1 shows that aircraft has drifted to the off the desired course. Now that the height above ground and direction of landing approach is known, next issue is – when and what should be the angle at which the plane should land so that the plane has sufficient runway left till it comes to a halt. There is a system called
glidescope on the side of runway that transmits signals in air at an angle of 3° uphill and once the aeroplane enters the glidescope region of signal transmission, the aircraft receives vertical guidance for landing. Refer figure 2 to understand the glidescope details that is visible to the pilot. Consider yourself as the pilotwho has to control the aircraft at the glidescope angle to land the plane on runway perfectly so as to avoid shortage of runway after landing. The horizontal line displays the angle of aircraft and the horizontal dots give the glidescope angle required for landing. So the aim for landing should be to make the horizontal line superimpose on the horizontal dots as shown in the second display. This can be done by simple maneuvers of the aircraft. Standard glide-slope path is 3° downhill to the approach-end of the runway. Now you shall follow glidescope faithfully and the altitude will be precisely correct when the plane reaches the touchdown zone of the runway. The glide slope is normally usable to a distance of 10nautical miles. During landing, one important parameter to be noted is pitch angle which is the angle of aircraft. Aircraft during level flight will have pitch angle as 0O and during landing the safe value of pitch angle depends on length and design of individual aircraft. Pitch angle during take off and landing is around 10O. This parameter is important to avoid tail hitting the runway first before the wheels and the pitch angle of aircraft for landing should always be verified during landing from the cockpit display unit. All of you would have seen landing of aeroplanes and would have noticed that all aircrafts land in nose-up position so that the rear wheels touch the runway first and then the front nose wheel touches the runway. Now that the plane is gliding down at the recommended angle and following glidescope too, but is it gliding towards the center of the runway? Localizer antennas located at the end of the runway transmit signals to aircraft giving information about the deviation of aircraft from runway centerline to the pilot on cockpit display. Refer figure 2 again to understand the localizer details. Again consider that you are the pilot and your task is to make the plane land at the center of the runway. The vertical line displays the direction of aircraft Figure 2 shows the cockpit display available to pilots using glidescope and localizer approaching the runway. This is similar to range finder but provides direction only with respect to runway landing using localizer signals. As mentioned earlier for glidescope, here the vertical line shall superimpose over the vertical dots for the aeroplane to land on the centerline of runway. The localizer signal is normally usable 18 NM from the field. Now the plane is gliding down at the recommended angle and also towards the centerline of runway. Can aeroplane detect any landmarks before runway in zero visibility condition to reconfirm that it is heading towards correct path? Yes, there are outer markers, middle markers and inner markers available on ground at a distance of 4-7 nautical miles, 3500feet and 100feet respectively from the runway. Markers send signals vertically up towards sky in an elliptical form with different tones, so that aeroplane can detect as to which marker is below the plane right now. Also an indicator bulb is there on cockpit that would glow when the plane is above the concerned marker - blue colour for outer marker, amber for middle and white for inner. The markers on ground can be considered analogous to children throwing stones vertically up. It can be assumed that there are one, two and three children at outer, middle and inner markers respectively. Above outer marker, the pilot may hear a single “thud”, above middle he may receive two “thuds” and three “thuds” above inner marker. Figure 3 shows pictorial representation of the signals and aeroplane path to be followed Figure 3 gives a pictorial representation of all the signals mentioned above and the concept of landing in zero-visibility. The glidescope signals intersect the middle marker at about 200 feet, outer marker at about 1,400 feet above the runway elevation. The center line of glidescope and localizer is the recommended line of path of aircraft for a perfect landing.