Sunday, February 08, 2015

Controlled flight into terrain -By A W K Senaratne

By A W K Senaratne

Aviation is the safest mode of transport at present, when compared with others port such as road, rail and sea. Safety in transport is measured by comparing the number of casualties (deaths) with the number of passengers multiplied by the number of kilometers they travelled. Consequently, in aviation, the number of casualties is the lowest when considered in terms of passenger- kilometers. However, when an aircraft accident occur the probability of survival is least in air transport mainly because of the speed they travel.

Any flight can be divided in to five phases namely takeoff, climb, cruise, descend and landing. Out of these five phases landing phase is the most critical and sixty percent of aircraft accidents occur during this part of the flight. The aircraft accident that occurred last month at Athurugiriya close to Ratmalana airport involving a Sri Lanka Air Force aircraft AN 32 is one of those accidents that took place during the landing phase of the flight.

When analyzing aircraft accidents during the landing phase, eighty percent of these accidents can be grouped into a category called "Controlled Flight In-to Terrain" (CFIT). It simply means that there is nothing wrong with the aircraft and the pilot is in control of it, yet the aircraft hit high ground or land short of the runway. The accident at Athurugiriya falls into this category.

In early nineteen seventies, International Civil Aviation Organization (ICAO) accepted an avionics (aviation electronic) system designed to prevent these so called CFIT accidents suitable to be installed on aircraft. This new avionics system was called Ground Proximity Warning System (GPWS). It gives oral and visual warnings in seven modes to the pilots, if the aircraft gets too close to the ground in an unusual or unintended manner during landing or take off phases of the flight. For example during the landing phase, if the aircraft is descending at the rate of five feet per second but the aircraft radio/ radar altitude decreases by ten feet per second, then the GPWS will shout to the pilot "PULL UP - PULL UP TERRAIN - TERRAIN", indicating that the aircraft is getting closer to ground in an unusual manner (ground is rising or aircraft is heading towards a hill). A similar warning will come up if soon after takeoff the radio/ radar altitude of the aircraft decreases rather than increasing at the rate of climb of the aircraft, indicating that the ground is getting closer unusually. ICAO made it compulsory to have this system fitted on all commercial aircraft carrying more than nineteen passengers.

This very useful and important avionics system has paid dividend by reducing the percentage of CFIT accidents which was at eighty percent of all landing phase accidents to that below forty percent at present. With the advancement of the present day modern computer systems with large memory capacities, a more improved version of GPWS was introduced, and is called Enhanced Ground Proximity Warning System (EGPWS). Aircraft equipped with EGPWS and having "Glass Cockpits" (aircraft with modern TV type instrument display) can give a graphic three dimensional display of the terrain in front of the aircraft on its way to land at the runway. This display will show all tall buildings, high tension electricity lines raised ground contour etc. To provide this view the EGPWS computer memory has to be loaded with the terrain data of each airport where the aircraft is intended to be used. This data is obtained from GPS(Global Positioning System) terrain data. Also the computer terrain data has to be updated regularly as required by the ICAO. This type of display will enhance the confidence of the pilots and safety of the flight to a greater extent during the landing phase of the flight. Since the introduction of EGPWS only a few aircraft installed with this system has met with CFIT accidents.

The ill fated aircraft AN 32 operated by Sri Lanka Air Force (SLAF) under Helitours commercial operation was doing a VFR (Visual Flight Rules) landing at Ratmalana airport. There is no radio landing aids available at Ratmalana airport other than a basic NDB (Non Directional Beacon) which is a navigational aid rather than a landing aid. It is highly dangerous and unwise to do a visual landing without any radio landing aid with the level of visibility that was prevailing due to mist around 6 am at Ratmalana airport on that fateful day. This type of weather occurs only on few days per year at Ratmalana or Katunayake airports. To do a safe landing in poor visibility there should be ground based radio aids.In such situations IFR (Instrument Flight Rules) landing has to be carried out.To do an IFR precision landing the airport should have the main radio aid used for landing in bad or normal weather, called Instrument Landing System (ILS). This system consists of two radio beams radiated from the runway towards the aircraft. One beam guides the pilot/ autopilot in the vertical plane to approach and land at a three degree angle with the runway surface. The other beam guides the pilot/ autopilot in the lateral plane (horizontal plane) to come along the extended centre line of the runway with the magnetic heading of the runway. With this radio aid the pilot/ autopilot can carry out a safe landing looking at the instrument panel alone, and without seeing the runway at all. This type of landing is called auto-landing.

In Sri Lanka only Katunayake and Mattala airports are equipped with this ILS radio landing aid. Even these two installations fall in to the category one and no auto landing is allowed. To carry out auto landing the ground based ILS system should be in category three. Since we have over ninety five percent of clear visibility throughout the year auto landing facility will not be essential. However, a less expensive non precision landing aid that guides the pilot only in lateral plane would be useful for Ratmalana airport which could have prevented this type of accidents. Most common less expensive non precision radio landing aid is the VOR/DME (Very high frequency Omni Range/ Distance Measuring Equipment). These two units are co-located and the first one gives magnetic heading to fly along the extended centre line of the runway to carry out a safe landing. The second unit gives the distance to reach the runway threshold (landing end of runway). If the NDB that is available at Ratmalana airport is located along the extended centre line of the runway, it could be used as a navigational aid as well as a non-precision landing aid even though its accuracy is much less than that of VOR/DME and it does not provide the distance to touch down point. At present the NDB at Ratmalana airport is installed not in line with the extended centre line of the runway (for reasons unknown) thus preventing its use as a radio landing aid (non precision) in addition to a navigational aid.

Factors that could cause CFIT accidents can be identified as wind shear, micro burst, poor visibility, miss-orientation, pilot error, etc. Wind shear/ micro burst is caused by turbulent weather conditions where thick columns of air moves vertically up or down at places. If this happens across the path of the aircraft on its way to land it can create a disastrous situation for the aircraft. Detection of wind shear condition is not easy and needs expensive radar equipment. Under poor visibility condition, even though there were no landing aids at Ratmalana airport, the Air Traffic Control (ATC) radar could have been used to vector (direct) the aircraft on to the runway by the air traffic controller on duty had the pilot made such a request. With poor visibility in a visual landing condition the pilot tend to look out to locate the runway, and in the process, they lose concentration on aircraft altitude and ends up in disaster, when suddenly a build up or hill appears in front of the flight path. This could be a likely cause of the accident at Ratmalana involving SLAF AN – 32 aircraft. However, the real cause/ causes can be identified only after a detailed investigation including the analysis of the flight data recorder and cockpit voice recorder (black boxes).

(The writer is a Commonwealth Expert on Aviation and was in charge of Airworthiness and Accident investigation Division at the Department of Civil Aviation of Sri Lanka. He has investigated over twenty aircraft accidents including four Sri Lanka Air Force aircraft accidents. He can be reached at

Story and photos:

No comments:

Post a Comment