NEW DELHI, June 20: A Delhi-Jabalpur Spicejet flight returned to Delhi airport on Sunday evening shortly after takeoff as the plane failed to maintain the requisite cabin pressure.

As the plane was climbing after takeoff from Delhi, the crew found that the cabin air pressure differential was not building up in sync with the rise in altitude. So it levelled off at 6,000 ft (most of the planes maintain cabin air pressure equivalent to the atmospheric pressure available at an altitude of 6,000 to 8,000 feet as they climb to and beyond 30,000 ft - a little lower than the ground level pressure). The pilot decided to return to Delhi, where the plane landed without any incident.

(The aircraft cabin, or the section the passengers travel, is pressurized to maintain the air pressure as it attains altitude so that the flyers can breathe without difficulty and the oxygen supply is also assured. Because higher the altitude, like on the mountains, the atmospheric air pressure goes down.

(Pressurization systems keep the cabin pressure between 11 and 12 psi (pound per square inch) at cruise altitude. The atmospheric pressure at 30,000 ft altitude is 4.4 psi. At sea level (or ground) it is 14.7 psi. Passenger planes typically fly between 31,000 and 38,000 feet).

This was the second such incident the airline faced in a space of six hours during the day. A Delhi-bound aircraft had made an emergency landing in Patna right after take-off around noon Sunday as its left engine caught fire reportedly after a bird hit. The engine was shut and the plane made an emergency landing with the right engine. There were no injuries.

"On 19 June, SpiceJet Q400 aircraft was operating SG-2962 (Delhi- Jabalpur). During the initial climb, the crew observed cabin pressure differential was not building up along with rising in cabin altitude. The aircraft was levelled off at 6000 ft. Pressurization was not regained," the airline said.