New Delhi, Dec. 4 -- With hundreds of flight cancellations and delays affecting thousands of fliers since Tuesday, IndiGo-India's largest budget carrier by market capitalization-is grappling with one of its most severe operational disruptions, prompting a probe by the Directorate General of Civil Aviation (DGCA).

Crew shortages due to new duty-time rules, technical breakdowns at key airports, and heavy winter congestion have precipitated the crisis, according to the airline.

Mint explains why IndiGo has been hit the hardest.

India's second and final phase of Flight Duty Time Limitations (FDTL), designed to reduce pilot fatigue and improve safety, took effect on 1 November, requiring overhauls to duty schedules, night-landing plans, and...