Calcutta, Sept. 18: Two special planes sent to check communication and navigation systems at Calcutta airport returned to Delhi with the job unfinished, allegedly because the air traffic control didn’t cooperate.

“Whenever they sought information from the ATC, they were told the officials were busy handling other flights. The aircraft returned after flying for about two-and-a-half hours,” an official of the Airports Authority of India (AAI), which sent the planes on Monday, said from Delhi.

The airport authorities had been told about the drill “well in advance”, the official said.

The planes — called calibration aircraft in aviation parlance — would have checked the newly installed Category II instrument landing system.

The checks are usually carried out when traffic is light. “Since night traffic is very heavy at Calcutta airport, with planes bound for Southeast Asia taking off or flying over the city, afternoon was thought to be the best time. The ATC had agreed to the plan,” the official said.

But when the two Donniers took to the skies around 1.30pm on Monday, they were allegedly not given information by the ATC. The planes later landed in Calcutta and returned to Delhi today after an overnight stopover.

AAI officials in Calcutta denied reports about the lack of cooperation. “There was no lack of cooperation on part of the Calcutta ATC. The flying safety hours of the aircraft were over and so they had to return,” said AAI executive director (eastern region) S. P. S. Bakshi. He added that the drill would resume on Thursday.

However, ATC sources said communication units were under-staffed and officials overworked. “The officials had to look after the operation system for other flights,” said a source at the ATC.