Details of their responses could not be immediately known. Officials of the Directorate General of Civil Aviation (DGCA) are examining the replies filed by the three no-frill carriers on the show cause notices, official sources said here.
The DGCA had asked them to explain why they were carrying full-load of passengers and cargo to Patna and Jammu where they should have been flying with 20 per cent less load to ensure safe landing and take-offs on the small runways there.
The airlines were also asked why their operations to these places should not be stopped as they were found to be flouting safety norms and payload restrictions.
The DGCA is shortly expected to strengthen the existing rules for operations to airports which have shorter runways and come out with specific instructions for the airlines to strictly follow, they said.
The aviation regulator had carried out real-time checks on flights of these three airlines to the two cities and found them carrying 170-180 passengers, instead of 150-155 as laid down in the Standard Operating Procedures (SOPs) for airports with short runways.
The officials had also examined the flight manifests of these airlines for the past fortnight and asked them to explain why they were not doing so and assure that they would not fly a fully-occupied aircraft.
They had said that these airlines, in particular, were found to have been violating these SOPs almost on a daily basis to earn more revenue but endangering safety.
The three carriers were also asked to cancel bookings for the next few days if these were more than 150-155 to these two destinations.
There are strict norms restricting payload for smaller runways to ensure safe and smooth landings. Payload means the total weight of passengers and cargo that an aircraft can carry.
While IndiGo and GoAir operate Airbus A-320s, which can carry 180 passengers in a full-economy configuration, SpiceJet flies Boeing 737-800s which have a capacity of 189.
The Patna Airport's runway length is 6,411 feet, Jammu's 6,755 feet, while the shortest runway in Delhi is 9,229 feet where the two types of aircraft can land with full load of passengers.
DGCA officials had yesterday said that barring Air India and Jet Airways, most flights of IndiGo, SpiceJet and GoAir were found operating almost 100 per cent full, instead of 80 per cent of the total capacity.