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Aviation: Highways in the sky
Most of us travel by air so often that we have stopped marvelling at the technologies behind air travel. We may know a thing or two about jet engines or aerodynamics, but most of us know precious little about how aircraft travel from one destination to another without colliding or crashing.
It’s simply amazing how aeroplanes locate that long thin strip of land from hundreds, even thousands, of metres above or how an aircraft makes it to the runway in one go without deviating once from that straight line. Well, it’s state-of-the-art navigation which makes all this possible.
VFR and IFR: The basics of navigation begin with “VFR” – for Visual Flight Rules – and “IFR” – which stands for Instrument Flight Rules. VFR, as the term implies, involves flying the plane on visual rules, based on landmarks and navigation maps and without relying on Air Traffic Control guidance or instrument navigation aids. You still need takeoff/landing clearances and air space clearances, though.
Commercial aeroplanes carrying passengers do not fly VFR as VFR flights are limited to weather clearances that roughly translate into three miles’ visibility with at least 500 feet clearance from the clouds above you. VFR flights cannot be done in Class A airspace (above 18,000 feet). To fly an IFR flight, a flight plan is filed with the ATC, which then guides you along.
Flight altitudes: There are some basic standards vis-a-vis altitudes when flying. Above 3,000 feet, when flying East (0 to 179 degrees on the compass) the airplane has to fly at odd altitudes or odd plus 500 feet (37,000 feet, 35,000 ft and 35,500). When flying in a westerly direction, an aircraft has to fly at even altitudes or even plus 500 feet (26,000, 36,000 and 36,500).
Altitudes are measured using altimeters that are set according to barometric pressure. Above 18,000 feet, altitude is referred to as Flight Level whereas an altitude of 36,000 feet would be called Flight Level 360. Altimeters at 18,000 feet are set at a standard pressure of 29.92 inch Hg.
Runway directions: Runways are numbered according to the direction they are in. If, on approaching one end of the runway, the magnetic direction is 253 degrees, the runway would be called 25. Divide the direction by 10 and round off. That’s why 187 degrees would mean runway 19.
A TCAS system guards against mid-air collisions. It should be credited with averting a collision between a PIA and an Airblue aircraft not long ago
A single strip of runway would, therefore, have different numbers for each of its ends. If one end is 25, the other end would be 7, since the other end is 180 degrees opposite. The R (right) and L (left) symbols stand for the relative positions of the runway.
If there are two parallel runway strips in the same direction, say 253 degrees, both of them would be called 25. On approach, if the strip you are landing on, lies at the right of the other strip, it will be called 25R and the other 25L.
Aircraft navigation has evolved quite a lot from ground-based light-emitting beacon towers to Global Positioning System. Airborne navigation has evolved to the extent that pilots can now fly their planes in zero visibility. Here we will discuss three main navigation instruments/methods that help pilots find their way around the globe.
NDB or Non-Directional Beacons are ground-based facilities that outwardly radiate radio waves. An onboard antenna and related equipment, namely ADF (Automatic Direction Finder) translates this signal into the movement of a needle to show the direction in which the beacon is. The ADF needle points towards the direction from which the signal is coming and hence the location of the beacon.
When flying towards a beacon, the flag in the ADF meter is pointed upwards and when flying away from such a beacon, the flag is pointed downwards. To fly towards a beacon the pilot has to turn in the direction such that the needle points straight ahead. Pilots can navigate from one NDB to another for point-to-point navigation.
VOR navigation: VOR stands for Very high frequency Omni-bearing Range. VORs were introduced in the fifties and are still the primary navigation tools. VORs too are ground-based radio frequency emitters that transmit two signals. One of these is constant in all directions while the other is rotated about the station.
The onboard VOR equipment interprets this in the form of radials coming from all three sixty degrees. To perceive a VOR, imagine the base station as a circle and the radials as beams going out in every degree from the circle. Hence, there would be 360 beams in all directions.
Some VOR stations have a DME (Distance Measuring Equipment) broadcast too. Based on the DME information, the onboard display shows the distance from the VOR station the aircraft has. The VOR shows the radial as a bar on the display. When in range, the VOR bar gets visible on the display.
To understand in layman’s terms what VOR navigation is all about and how it works, let us get our imaginations running. Suppose the arrow shown in the figure is an airplane flying at 12,000 feet. The mission of the airplane is to reach the black dot shown where an audience is gathered in an air show.
To get to the park the pilot is instructed to maintain 12,000 feet and intercept the 090 degree radial of VOR station A. The pilot sets his equipment to 090 after tuning in to station A. The VOR currently shows the bar to be tilted at an angle from a centre-aligned position.
As the airplane reaches the point where it intersects the 090 radial of VOR A, it turns in to intercept the radial (by turning towards A) and gets aligned with this path. As the airplane begins flying towards A, the ATC instructs the pilot to tune in to VOR B, 045 degree radial and intercept it and fly away from the VOR station. As the pilot approaches the radial, his VOR equipment shows the needle centring in as the pilot turns right to intercept the radial.
With the needle aligned and the pilot flying in the direction of the radial, the ATC informs that at exactly 24NM (nautical miles) from VOR B, he would pass over the park. The pilot watches his DME display and around 24 NM he sees the park right in front of him!
Instruments landing
Just as aeroplanes are able to make their way from one point to another, they can locate and home in to a runway with so much precision that without even looking at the airfield outside, the navigation aids can lead pilots to a perfect landing.
Once the plane is approaching its destination, the ATC instructs it to descend to lower altitudes. As the airplane gets closer to the airport using some of the navigation aids described earlier and guidance from the ATC, the pilot sets his NAV instrument to the ILS (Instrument Landing System) frequency of the runway he intends to land on.
On the ILS display, the pilot sets the bearing to the direction of the runway he is landing on. The runway appears as a bar in the ILS display. If the runway is equipped with DME, the display also shows the distance in nautical miles between the aircraft and the runway. An indicator on the side of the ILS display shows some dots with an arrow pointing at one of them.
The centre dot represents the perfect height that the aeroplane should reach on a decent path towards the runway, called the glide slope. If the arrow is at the dots above the centre dot, it shows that the glide slope or the decent path to the runway is above the current height. Alternatively, if the pointer is below the centre dot, it means that your approach path is below your current altitude. The aircraft is on a perfect approach path towards the runway when both the runway centre line and the glide slope are aligned on the ILS.
By intercepting the ILS radial, the aircraft can intercept and land on runways without physically seeing where they are. However, depending on the category of the runway (CAT I, II and III) the minimum visibilities and visual ranges are defined for the usage and reliability of the instrument landing systems.
TCAS: A Traffic Alert and Collision Avoidance System is a vital instrument on board which prevents mid-air collisions. It works via the information sent through transponders that send information to ground-based radars about the location, altitude and speed of the plane. This transponder information is shared between aeroplanes in TCAS and the aircraft in the vicinity can been seen as symbols on the display, along with their altitude information, direction, distance, and ascending or descending trend.
TCAS works by calculating the time to reach the Closest Point of Approach with the intruder, by dividing the range by the closure rate. This time value is the main parameter for issuing alerts. TCAS can issue two types of alerts:
— Traffic Alerts to assist the pilot in visually searching for the intruder aircraft and prepare the pilot for a potential Resolution Advisory.
— Resolution Advisories (RA) are alerts that recommend manoeuvres that will either increase or maintain the existing vertical separation from an intruder aircraft. When the intruder aircraft is also fitted with TCAS II, both systems ensure that complementary resolution senses are selected — that is if one aircraft is advised to ascend, the other be advised to descend.
TCAS can operate in traffic densities of up to 0.3 aircraft per square nautical mile (nmi). This translates to 24 aircraft within a 5nmi radius. However, TCAS has its limitations too: it can detect aircraft with active transponders only.
If intruding aircraft are equipped with TCAS, then the RAs would be complementary, otherwise TCAS would issue RAs based on the behaviour of the incoming aircraft, which implies that any sudden manoeuvre by the intruder would result in changed instructions in the TCAS, and in worst cases, in a collision.
Additionally, TCAS advisories do not take into account terrain or any other limitations. Hence TCAS may advise the pilot to descend sharply whereas descending could actually bring the aircraft to a crash. In such cases pilots have to exercise their superior judgment.
Nevertheless, TCAS greatly rules out mid-air collisions and it should be credited with preventing the collision between the PIA and Airblue aircraft recently.
The writer umer.asif@gmail.com is an engineer
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