Guwahati, Aug. 3: An IndiGo flight caught in air turbulence was forced to ascend 300ft and descend sharply last evening to maintain the mandatory distance from another aircraft, leaving some fliers dizzy and two crew members in need of first aid.

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

The ascent reduced the aircraft's mandatory vertical separation of 1000ft from IndiGo's Guwahati-Chennai flight 6E-136 to 700ft. Following prescribed protocol, the pilot immediately applied the resolution advisory (RA) triggered by the traffic collision avoidance system (TCAS) and descended.

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

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

The Guwahati-Mumbai flight with 112 passengers and an infant landed around 8.30pm. 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, an official said.

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 were an aviation hazard. Containing hailstorms, high velocity winds and static current, they 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 said.

Late 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."

Assamese actor Akashdeep Deka, who was on the Mumbai-Guwahati flight, said he had never had such a "horrible experience" although he was a frequent flier.

"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," he said.

"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 another 30 minutes after the incident, trying to calm down. I can still hear my heartbeat, even after 20 hours."

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

According to the IndiGo statement, a doctor's report said those hurt did not suffer any big injuries.