SINGAPORE - All 12 flights operated by Firefly between Singapore and Malaysia on Wednesday (Sept 18) were cancelled due to poor visibility caused by the haze.

In response to queries from The Straits Times, a Firefly spokesman noted that Firefly will continue to monitor the situation for the 12 flights scheduled between Seletar Airport and Subang Airport on Thursday.

She said: "We are looking at the reports carefully for now so as to manage possible situations affecting the airspace."

Six flights had been scheduled from Seletar Airport to Subang Airport between 8.40am and 7pm on Wednesday; another six had also been scheduled from Subang Airport to Seletar Airport from the first flight at 8.10am to the last set for departure at 6.30pm.

All were cancelled "due to the haze and the resultant bad visibility", Firefly said.

The airline said it will provide more details on the number of people affected, as well as the actions Firefly took to assist those passengers, when more information is available.

This comes even as the haze worsened in Singapore on Wednesday. Air quality was hovering in the moderate and unhealthy ranges on Tuesday, but deteriorated into the unhealthy range the next day.

Seletar Airport does not have the ground-based Instrument Landing System (ILS) procedures, which the vast majority of commercial aircraft currently use to provide pilots with guidance for landing.

Malaysia had objected to the introduction of ILS at Seletar Airport, saying it would restrict developments in Pasir Gudang town, which is near Seletar Airport.

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Instead, Malaysian Transport Minister Anthony Loke said on April 21, at a joint press conference with Singapore Transport Minister Khaw Boon Wan, that Global Positioning System-based instrument approach procedures would be implemented at Seletar to enhance flight safety.

He said then: "The GPS approach is something we have agreed (on)... We are looking for Firefly to work towards that in a timely manner."

With either ILS or GPS procedures, pilots execute a series of predetermined manoeuvres under the guidance of instrument systems as the aircraft approaches an airport.

The difference between the two systems is whether the signals are received from a satellite or ground-based station.

To use satellite-based GPS signals, aircraft operators would typically require some time to outfit the aircraft with the necessary equipment.

Pilots would also have to undergo training to develop the capability and obtain the operational approvals from their regulators.

In an earlier April press conference, Mr Loke did not say why Malaysia is ready to accept GPS at Seletar Airport but not ILS.