At least seven IndiGo pilots were allegedly found using offensive language over salary issues on a frequency used for emergency communications, sources said on Thursday.

On April 9, these pilots were allegedly found venting their ire over low salaries by using offensive language on 121.5 MHz frequency, which is used for emergency communications only for the aircraft in distress.

The Directorate General of Civil Aviation (DGCA) has launched a probe in the matter. IndiGo is yet to issue a statement on the development, the sources said.

Frequency 121.5 MHz, used for emergency communications, has to be compulsorily monitored by the air traffic controllers who are in the vicinity of the aircraft.

For air-to-air communication among the pilots of different aircraft, 123.45 MHz frequency is used, which is not monitored by the air traffic controllers.

Days before the incident, IndiGo had suspended a few pilots who were planning to hold a strike on April 5 against the pay cuts affected during the COVID-19 pandemic.

During the peak of the pandemic, the airline had cut the salaries of its pilots by as much as 30 per cent.

On April 1, IndiGo announced its decision to increase the salaries of the pilots by 8 per cent, saying another 6.5-per cent hike will be implemented in November in case there are no disruptions.