DH Deciphers | What's a tabletop runway? Why is it considered risky?

The tabletop structure of the runway at the Kozhikode airport has been cited as one of the reasons for the fatal crash of the Air India Express plane on the night of August 7. Besides Kozhikode, only two other Indian airports with scheduled flights have such a runway: Mangaluru and Lengpui (Manipur). It's difficult to land aircraft on tabletop runways even during perfect weather conditions. Why are such runways considered risky?

What's a tabletop runway?

A tabletop runway is essentially a runway built on the levelled surface of a hill. The Kozhikode airport runway is located on hilly terrain and is surrounded by deep gorges on both ends. While there is a runway safety area in case an aircraft overshoots (goes beyond) the runway, it's just 240 metres long. An aircraft landing at high speeds can overshoot even this area and finish up in the gorge, leading to a crash. Pilot error or mechanical failures could trigger such accidents. This is what exactly happened in Kozhikode.

Why did Kozhikode opt for a tabletop runway?

The airport was constructed in a hilly region of the neighbouring Malappuram district because the state government faced a severe shortage of space elsewhere. This had also come in the way of extending the runway length. But when safety issues forced wide-bodied international flights to bypass the airport, the runway was upgraded. This was later approved by the Directorate General of Civil Aviation (DGCA). Aviation experts had also drawn attention to the lack of a ground arrestor bed, mandated for tabletop airports. A ground arrestor bed is nothing but a soft bed of engineered materials at the end of the runway to reduce the speed of an aircraft in case it overshoots the runway.

Is weather a factor while landing at tabletop runways?

Since there is a very low margin of error while landing on such runways, pilots have to deal with valley winds rising from both ends of the runway, tailwinds that push the aircraft from behind and headwinds that blow against the aircraft from the front. Visibility of the runway also plays a big role.

What role did weather play in the crash?

According to airport authorities, the pilot had to abort the first landing attempt due to very poor visibility caused by heavy rainfall in the area. In the second attempt, the aircraft wheels skidded and veered off the runway. The pilot could not control it.

How critical is speed control on the runway?

Since the length of a tabletop runway is generally shorter, it is extremely critical that the aircraft brakes the speed quickly. The Kozhikode runway length had been upgraded, but since the flight hard-landed near the middle of the runway, the distance to reduce the speed was far less. The headwinds were low, too.

What's is the optical illusion? How does it affect the pilot?

Once the pilot lands on a tabletop runway, he has to be aware of the optical illusion effect. The runway will appear to him as continuous in front, although it drops off into the valley ahead.