Guwahati, Aug. 3: Unexpected air turbulence led an Indigo flight approaching Guwahati to ascend 250-300 feet and then descend sharply to maintain the mandatory distance from another aircraft last evening, leaving some passengers giddy and two crew members in the need for first aid.

In a statement today, Indigo said its Mumbai-Guwahati flight (6E-813) had to ascend 250-300 feet because of clear-air turbulence (CAT) minutes before landing at Lokapriya Gopinath Bordoloi International Airport at Borjhar here.

As the ascent reduced the aircraft's mandatory vertical separation of 1000 feet from another aircraft (Indigo Guwahati-Chennai flight 6E-136) to 700 feet, the pilot of the Mumbai-Guwahati flight, following the prescribed protocol, immediately applied the resolution advisory (RA) triggered by the TCAS (Traffic Collision Avoidance System) and descended.

The sudden descent left four passengers giddy while two cabin crew members were hurt and had to be administered first aid at the airport, the release added.

The Guwahati-Mumbai flight with its 112 passengers and an infant landed around 8.30pm. Indigo officials said the incident took place between 8pm and 8.15pm.

The flight was scheduled to take off at 4.20pm from Mumbai but was delayed till 5.07pm because of bad weather, one of them told The Telegraph this afternoon.

The Guwahati-Chennai flight had 91 passengers and an infant on board.

The Indigo release said the plane experienced turbulence because of prevailing monsoon activity.

"During such weather situations, there are two types of activities - cumulo-nimbus and clear-air turbulence. While the former can be detected by the weather radar, enabling the flight crew to take deviations to avoid any turbulence, the second is experienced occasionally and cannot be detected by the weather radar. Flight 6E-813 experienced a CAT situation which could not be detected by the weather radar. This caused the aircraft to climb by about 250-300 feet," the airline said.

Wing Cdr (retd) Atul Rajkhowa said cumulo-nimbus clouds are best avoided and are an aviation hazard. They contain hailstorm and high velocity winds and static current and can cause serious loss of height and disorientation for an aircraft. "Clear-air turbulence, on the other hand, comprises air currents of high velocity and can change the movement and direction of an aircraft in seconds," he added.

TCAS is an onboard equipment that advises pilots about traffic in the proximity of the aircraft and also instructs them about "avoidance action" to maintain the desired separation between the aircraft, the Indigo statement said.

Later this evening, reacting to media reports of aversion of mid-air collision, Indigo issued another release saying: "Initial analysis indicates that there was no threat of collision - both the aircraft were laterally and vertically well separated. Due to unexpected turbulence, the vertical separation reduced to 700 feet and flight crew immediately began to address the altitude deviation at the very moment TCAS-RA advisory was generated. As per the standard operating procedure, the flight crew immediately took corrective action in interest of safety."

Assamese actor Akashdeep Deka, who was on the Mumbai-Guwahati flight, told The Telegraph today that he never had such a "horrible experience" although he was a frequent flyer.

"There was chaos among the passengers for about 30 minutes as the plane suddenly started descending. It came down so fast that my head banged into the overhead luggage compartment. I lost my mobile phone in the chaos. We could not hear what the crew was saying as everybody was screaming and praying. After the plane movement became normal, it felt like we had got back our lives. I kept my eyes shut for about another 30 minutes after the incident, trying to calm down. I can still hear my heartbeat, even after more than 20 hours," he said.

Deka is the husband of BJP legislator and popular Assamese actress Angoorlata Deka.

Following the incident, the pilot alerted Air Traffic Control about the situation on board and requested a doctor and ambulance at Guwahati airport. The flight made a normal landing. The doctor's report says those hurt did not suffer any major injuries, the Indigo statement added.