On Dec 25, the Civil Aviation Authority of Malaysia (CAAM) declared a permanent restricted area in the airspace over Pasir Gudang for the purpose of military activities.

The information was disseminated through a Notice to Airmen (Notam) - a notice that is filed to alert the global aviation community of potential hazards along a flight route or at a location that could affect the safety of flight. In its notice, the CAAM said the restriction would take effect from Jan 2.

What it meant was that flights from any country, including Malaysia, would need prior approval from the Royal Malaysian Air Force to operate in that zone between 2,000 and 5,000 feet.

The restricted zone was in the flight path for landing and take-off at Singapore's Seletar Airport, from the north.

It meant planes heading to and from Seletar would have to ensure that they avoid the stipulated area.

This can be done by going into an upward spiral soon after take-off to go over the 5,000 feet threshold and spiralling down when landing in Singapore, experts said.

While there are no commercial airlines that operate scheduled services to Seletar Airport, the restriction had affected private and chartered flights.

While it is not uncommon for countries to declare restricted air zones when they conduct military activities, this is usually done in areas where there are limited commercial flying activities, the experts pointed out.

There are also procedures and guidelines set by the International Civil Aviation Organisation (ICAO) on this. For example, countries should first consult and give advance notice to key stakeholders.