Article written by D. Nepal
A new term has come into a light, after Air India Express Flight 1344 skidded off the runway of Kozhikode’s Calicut International Airport, on Aug 7. The term, ‘table-top runway’ has made the headlines as the Boeing 737 touched down on the rain-slicked runway with 11 knots tailwind and raced down until it fell off the hillside, killing 18 people on board.
The ‘table-top’ runway was mentioned in the final investigation report of the 2010 Mangalore crash. The same type of aircraft of the same company had overshot the runway and tumbled into the gorge in Mangalore. In the report, Airport Authority of India was recommended to bring the downward slopes on the same level as the runway, on all table-top airports in India.
As the name suggests, the ‘table-top’ runways are located at either or both opposite ends on the edge of a hill or plateau with steep drops. Such runways raise the risk of deaths or accidents if pilots force their approach under or over.
Both crews died during Goma Air Flight 409 in 2017; operating cargo flights to Lukla dropped too low below the threshold altitude, at which pilots increased the angle of attack at the low speed that caused the aerodynamic stop. The plane had an impact under the runway level of 400 ft.
During the Mangalore crash, the plane was too high on the approach to Mangalore, nonetheless, the captain decided to land, and the Boeing 737 touched down 5200 feet from the start of the runway, resulting in runway overshoot into the gorge and bursting into the flames.
These table-top runways are mostly found in the mountainous region, as the topography poses constraints for constructing a flat, larger airfield. Nepal has several airports that have table-top runways, especially in the mountainous region. Along with the infamous Lukla Airport, Talcha Airport in Rara, Juphal Airport in Dolpo, Manmaya Rai Airport in Diktel, Okhaldhunga’s Rumjhatar Airport and Tumlingtar Airport in Sankhuwasabha are some examples of the airports having table-top runway that offer frequent scheduled passenger flights.
Kathmandu Airport also lies on top of a plateau; i.e. on a ‘table-top’, and the threshold of runway 20 leads to a slope down into the bank of Bagmati river. This drop-off terrain poses risk in the airport, especially in the case of runway overrun. Kathmandu is frequented by heavy jets like B777 and A330 and considering the large momentum of these aircraft, the downward slope in case of overrun can worsen the outcome.
According to ICAO, most of the accidents occur during landing and take-off phases, with a large number of runway excursions and aircraft overrunning into the runway safety area (RSA).
In April 2018, Batik Air Malaysia rejected high-speed takeoff resulting in a runway overrun in Kathmandu. The aircraft came to stop about 250 feet past the end of runway 20. So, there is always a potential chance of a runway overrun.
TIA has RSAs each 300 m in length, at both ends of the runway after the runway extension project was completed in June. The safety areas are aimed to prevent severe consequences during the case of a runway overrun. However, the Federal Aviation Administration (FAA) suggests that it is not practicable to achieve the full standard RSA if there are obstacles such as bodies of water, highways, railroads, and populated areas or severe drop-off terrain.
Most airports are equipped with an Engineered Material Arresting System (EMAS) to improve runway safety areas. EMAS arrester bed can stop an aircraft at approximately 128 km/h. The Mangalore accident report also suggests that “considering the large number of runway excursions leading to hull loss accidents, ideally an arresting system like the Engineering Material Arresting System (EMAS) should be installed on the runway overshoot areas, especially for table-top airports”