Nine flights for departure and 38 for clearance delivery at the Ninoy Aquino International Airport were affected on Sunday morning as the Civil Aviation Authority of the Philippines (CAAP) said it replaced a blower for a second uninterruptible power supply to power the communications, navigation and surveillance/air traffic management (CNS/ATM) system.

In a statement, CAAP said it implemented the maintenance activity from 4:20 am to 5:20 am.

''Operations at the ATMC (Air Traffic Management Center) returned to normal at 6:09 with the NOTAM (notice to airmen) subsequently being cancelled,'' said CAAP.

CAAP spokesperson Eric Apolonio told dzBB that two UPS are now operating as a result of the maintenance activity.

CAAP said the notice to airmen was issued on January 21 at 8:53 pm to concerned airlines.

The maintenance activity took place three weeks after glitch in the CNS/ATM caused hundreds of flights to be delayed and thousands of passengers stranded at the Ninoy Aquino International Airport (NAIA).

Senator Francis Tolentino, who was going to Iloilo, told dzBB that the pilot mentioned an ''unannounced air traffic control maintenance.''

Tolentino said there were 11 planes whose take off had been delayed. ''We were number 11 for takeoff,'' he said. DMS