Hundreds of Air India and IndiGo flights were delayed on Saturday with India joining a global effort to upgrade the software on Airbus A320 family aircraft and fix a safety risk related to flight controls.

Not all the aircraft from the A320 family need the upgrade. Sources said Air India, IndiGo and Air India Express had completed the exercise in 270 of the 338 aircraft they had identified for the upgrade.

Four Air India Express flights were cancelled. The airlines said that slight delays might continue till the software upgrade was completed in all the vulnerable flights. Sources said the process was likely to be over by Sunday evening.

Airbus and the European Union Aviation Safety Agency (EASA) had issued advisories for a mandatory software realignment worldwide on some 6,000 aircraft belonging to the A320 family.

Airbus said an analysis of these aircraft had revealed that intense solar radiation might corrupt data critical to the functioning of the flight controls.

This prompted India’s aviation regulator, the directorate-general of civil aviation (DGCA), to direct airlines to address the issue.

Government sources said that IndiGo, which operates one of the largest fleets of A320 family aircraft in the world, had completed the software upgrade in 184 of the 200 planes it had identified for the purpose.

They said 69 of the 113 aircraft from the A320 family with Air India, and 17 of the 25 with Air India Express, had completedthe task.

The remaining aircraft are undergoing the process at maintenance centres in Delhi, Bengaluru, Mumbai, Chennai, Hyderabad, Ahmedabad and Calcutta.

Airbus A320 family aircraft — which include the A318, A319, A320 and A321 — are among the best-selling single-aisle planes worldwide. These narrow-body aircraft, where the standard seating is three-three across, are designed for short to medium-haul routes.

IndiGo, which operates around 2,300 flights daily connecting many domestic and international destinations, is heavily dependent on its A320 variants.

“Working closely with both organisations (EASA and Airbus), a total of 200 of our aircraft were identified for these checks,” an IndiGospokesperson said.

Air India said its engineers were working round the clock to complete the task at theearliest.

The analysis was carried out after a JetBlue-operated A320 aircraft travelling between Cancun in Mexico and Newark in the US recently experienced a sharp loss in altitude, causing injuries to many passengers.

A preliminary technical assessment by Airbus identified the malfunctioning of the affected elevator aileron computer (ELAC) as a possible contributingfactor.

The ELAC is commonly used for flight control. If not corrected, the snag could in the worst-case scenario lead to a sharp, un-commanded loss of altitude where the forces exerted on the aircraft body could exceed what it’s structurally capable ofwithstanding.

To address this, Airbus instructed airlines to install a serviceable ELAC provided by it.