Low-cost airline SpiceJet Ltd staff illegally allowed children, who should have been given separate seats, to travel on the laps of adults to accommodate more passengers on a Delhi-Mumbai flight earlier this month, according to a complaint being investigated by the regulator.

The complaint was made in a letter sent to the Directorate General of Civil Aviation (DGCA), aviation minister Praful Patel and other ministry and regulatory officials.

Safety check: Passengers queue at a SpiceJet counter at the Indira Gandhi International Airport in New Delhi. The carrier had deployed a 189-seat aircraft instead of a scheduled one with 212 seats on 2 June. Madhu Kapparath / Mint

“DGCA is investigating the matter," a civil aviation ministry spokeswoman said in reply to questions sent on Monday.

SpiceJet chief executive Sanjay Aggarwal’s office said he was out of the country and would only be able to comment after he returns on 24 June. Emails and calls to Aggarwal remained unanswered. The airline is in the process of being taken over by media baron Kalanithi Maran, who runs Sun TV Network Ltd.

The 2 June flight, SG-103, had been delayed by four hours after planes had to be switched. SpiceJet deployed a 189-seat Boeing Co. 737-800 aircraft instead of one with 212 seats as scheduled.

“There was a mother and her maid sitting behind me with two daughters. They already had four seats, but were asked to move to two seats," said Shuchita Srivastava, who was travelling on the same flight. The complaint letter was written by her husband Sandeep Srivastava.

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About 15 passengers were asked to get off the plane as there weren’t enough seats, provoking them to protest. The airline then asked that the children sit on the laps of adults they were travelling with, though the rules prescribe that those above the age of 2 have to be accommodated in seats.

“There was a big fight," recalls Shuchita Srivastava. “Passengers were not getting off the plane, they would not let the door close. The passengers insisted they had valid boarding cards and it was the airline’s problem to give them seats. One lady entered the cockpit with her child and said, ‘I am going to sit here if you don’t take me to Mumbai’."

SpiceJet’s internal staff duty roster for 2 June, reviewed by Mint, states SG-103 was scheduled to be operated by a Boeing 737-900, 212-seater aircraft by Avinash Chikte and Abhishek Sinha. It was to depart at 12.15pm and land in Mumbai at 2.20pm.

In a 3 June letter sent to Srivastava, SpiceJet said it had taken note of the incident.

“We would like to inform you that we are investigating the matter and shall revert to you with an update at the earliest," Megha Chadha of the airline’s customer service department, wrote in the email.

Srivastava said he has not heard anything further since then.

The incident took place around two weeks after an Air India Express plane crashed while landing at the Mangalore airport, killing 158 people.

“This is not practised in the UK and is indeed forbidden under ICAO/Iata/CAA regulations," said Saj Ahmad, a London-based aerospace analyst, referring to the International Civil Aviation Organization, International Air Transport Association and the Civil Aviation Authority of the UK.

“If, God forbid, there was an accident of any kind, fatality or otherwise, insurance companies would almost certainly refuse to cover airlines that engage in this practice—seats are there for a good reason and must be used, irrespective of yield or revenue ‘damage’ due to a less-than-full-fare paying passenger using the seat(s)," he added. Wearing seat belts is critical to protecting passengers during air turbulence or sudden loss of altitude, which can cause injury.

“It is a very serious matter. In the past also, another erstwhile low-cost airline was found doing the same thing," said a DGCA official, who did not want to be named. Most such cases get brushed under the carpet, he added.