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Journalism of Courage
With the national capital enveloped in a blanket of noxious smog, flight operations at the Indira Gandhi International Airport in Delhi have been impacted.
At least 15 flights were diverted — 13 to Jaipur, one to Dehradun, and one to Lucknow — and many more delayed on Monday (November 18) due to poor visibility levels. According to sources at the airport, these diversions were made due to some pilots not being trained in CAT III operations.
Put simply, CAT III refers to an Instrument Landing System (ILS) which allows planes to land in extremely poor visibility conditions, like what is currently being seen in Delhi.
Landing is considered to be the most difficult aspect of flying an aircraft — and the most dangerous. According to research by Boeing, the final descent and landing takes about 4% of the average flight time. But a whopping 49% of fatal accidents occur in this tiny window.
Poor visibility is one of many factors that make landing aircraft difficult. In such conditions, pilots have little to no visual cues they can rely upon while landing an aircraft, making it very hard to accurately judge the glide path to the ground and the alignment of the aircraft with the runway. Mistimed or misaligned touchdowns significantly increase the risk of accidents.
This is why modern aircraft and runways are equipped with various kinds of instruments which allow aircraft to land even in poor or no visibility. The ILS is a specific ground-based radio navigation system that provides pilots accurate information about their aircraft’s glide path and alignment with the runway.
The ILS provides the pilot with two key pieces of information that she needs while landing an aircraft.
First, it provides azimuth guidance. Azimuth refers to the horizontal angle between a reference direction (in this case the runway) and a line to a point of interest (the aircraft). This information is provided with the help of the ILS localiser (LOC), whose antennae are usually located on the far end of the runway.
These radio antennae transmit two narrow intersecting beams, one from slightly to the right of the runway’s centreline and the other to the left. Where they intersect provides the centreline of the runway. This information comes to the pilot flying the aircraft in the form of the vehicle’s displacement from the runway centreline, which then allows her to course-correct horizontally.
Second, the ILS provides the correct vertical descent profile for the aircraft. In other words, it tells the pilot whether the aircraft is too high or too low at any given distance from the runway. This is done with the help of a glidescope (GS).
The GS antennae work in the same manner as the LOC antennae, they are just turned on one side. Located generally somewhere along the middle of the runway, they transmit two narrow intersecting beams, one slightly below the required vertical profile and the other slightly above it which, with the point of these beams intersection providing the optimum descent position of an aircraft.
ILS approaches are categorised based on two parameters: decision height (DH) and runway visual range (RVR). (Refer to Table below).
DH refers to the lowest altitude during descent till which pilots can rely solely on the ILS. At this altitude if the RVR — the distance over which a pilot can see the runway’s lights or markings — is below a certain threshold, then the landing has to be aborted.
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Table: Categories of ILS approaches |
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Category |
DH |
RVR |
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CAT I |
Not lower than 60 m |
Not less than 550 m |
|
CAT II |
Lower than 60 m but not lower than 30 m |
Not less than 350 m |
|
CAT III A |
Lower than 30 m, or no decision height |
Not less than 200 m |
|
CAT III B |
Lower than 15 m, or no decision height |
Not less than 50 m |
|
CAT III C |
No DH |
No RVR |
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Source: Airports Authority of India |
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Whether an aircraft is capable of making a certain category of ILS-based landing is a product of the equipment it has aboard, the equipment on the ground, and a pilot’s training.
Currently, six Indian airports — Delhi, Lucknow, Jaipur, Amritsar, Bengaluru and Kolkata — are equipped to handle CAT IIIB operations. India does not have a single airport capable of CAT IIIC operations. Airports such as New York’s John F Kennedy Airport and the London Heathrow Airport support CAT IIIC landings.
