IndiGo, Air India and Air India Express on Saturday started carrying out software upgrade of A320 family planes to address a potential flight control issue, and requisite modifications have been done for 80 per cent of the affected 338 aircraft.
The exercise, conducted across major aviation hubs, also caused delays and a handful of cancellations.
According to the Directorate General of Civil Aviation, IndiGo and Air India did not cancel any flights, while Air India Express cancelled four flights due to the software modification process.
Flight delays ranged between 60 and 90 minutes at various airports, sources said.
Airbus on Friday warned that intense solar radiation might corrupt data critical to flight controls in a significant number of A320 family aircraft.
The manufacturer said the software changes required to fix the issue would lead to operational disruptions.
The A320 family includes A319s, A320 ceos and neos, and A321 ceos and neos, with more than 8,100 such planes in service globally.
Nearly 6,000 of these worldwide require software upgrades and some may also need hardware realignment.
DGCA data showed that of the 338 A320 family aircraft operated by Indian carriers that require upgrades, 270 had been completed by 5.30 pm on 29 November.
IndiGo has 200 affected planes and has completed upgrades on 184 of them. Air India has 113 affected aircraft and has upgraded 69. Air India Express has 25 impacted planes and has completed work on 17.
Officials said Air India has completed upgrades on 80 per cent of its affected planes, while Air India Express has taken the requisite actions for 22 of its 25 impacted aircraft.
The upgrades are being conducted at airline bases in Delhi, Bengaluru, Mumbai, Chennai, Hyderabad, Ahmedabad and Kolkata, and all affected aircraft are to be updated by 5.29 am on 30 November.
DGCA on Saturday issued an Airworthiness Directive instructing airlines to immediately carry out the software upgrades.
The directive followed an alert issued by Airbus to operators worldwide and an Emergency Airworthiness Directive from the European Aviation Safety Authority.
EASA noted that Airbus had asked airline operators to install a serviceable Elevator Aileron Computer in the impacted aircraft. The ELAC system governs flight controls.
IndiGo, which operates more than 2,300 flights daily, said no flights had been cancelled due to the checks and that only a few may experience minimal delays.
Air India said there had been no cancellations due to the task and that there was no major impact on schedule integrity, although some flights could be slightly delayed or rescheduled.
IndiGo later said upgrades had been completed on 160 of its 200 impacted planes by noon on Saturday and inspections on remaining aircraft were progressing well.
Air India said it had completed the reset on more than 40 per cent of its affected aircraft and expected to cover the entire fleet within the timeline set by EASA.
Former pilot Ehsan Khalid told PTI Videos that ELACs are the brain and nervous system of the aircraft, and the software problem with one of the ELACs is significant.
When the pilot moves the control forward, it will cause the aircraft to pitch down and when it is moved backward, then the plane pitches down. This particular movement is done by pilot control and if that happens on its own, then there is a problem, he said.
On October 30, he said a JetBlue aircraft experienced an uncommanded pitch down for seven seconds, which caused the aircraft to lose 100 feet and injured more than 15 people.
"At 35,000 feet, neither a loss of 100 feet nor seven seconds is critical. But if you remember the Ahmedabad crash, it happened during take-off, and seven seconds would have been enough to cause a catastrophic end. So humanity has been lucky this time.
"The aircraft manufacturer has openly acknowledged a software problem and has said it will fix it," Khalid said.
Airbus said its analysis of the recent event revealed that intense solar radiation may corrupt data critical to flight controls and that a significant number of A320 aircraft may be affected.
The manufacturer acknowledged that recommended actions would result in operational disruptions for airlines and passengers.