When most of Singapore was asleep at 2am yesterday, a pair of F-15SG fighter planes were scrambled from Paya Lebar Airbase, racing to intercept a "threat" flying over the South China Sea.
Back on land, surface-to-air missile systems, manned by Ground Based Air Defence (GBAD) units, were placed on high alert.
The threat - a commercial airliner suspected of being hijacked - may have been a simulated one, but the exercise was a real demonstration of how Singapore would respond to potential air threats, such as a Sept 11-type terror attack.
Called Vigilant Shield, it involved about 400 participants from the Republic of Singapore Air Force (RSAF), the Civil Aviation Authority of Singapore (CAAS), the Changi Airport Group and the Singapore Police Force.
Collectively, they work together under a round-the-clock Air Defence Task Force (ADTF).
ADTF's commander, Brigadier-General Tan Chee Wee, said: "On a daily basis, we do investigate aircraft that may not have a very clear identity or intent, and where the situation requires, we will respond to it." He said some of the incidents investigated could be categorised as potential terror threats, and are taken "very seriously". "And where needed, we will activate our air defence responses to deal with them."
Besides F-15SG fighters, the RSAF also deployed its AH-64D Apache helicopters, along with GBAD missile systems - such as the I-Hawk, Spyder and RBS-70 - for yesterday's exercise.
The RSAF works with the CAAS to clear the affected airspace of civilian air traffic. "This allows the fighters to scramble out in the most expedited departure route to intercept the suspicious aircraft," said an F-15SG pilot, Lieutenant Julian Low, from the 149 Squadron.
Besides using radio communications, the fighters also fly as close as 660ft, or 201m, to the suspicious aircraft, to signal them visually.
Major Zanna Lee, chief planner for Exercise Vigilant Shield, said: "Our primary response is always to make sure they remain compliant and land in Singapore. If not... (it) is to turn them away."
In 2008, a float plane piloted by two Australians was intercepted and escorted by fighters to Changi.
In such situations, the planes are handed over to the police for investigations.
While the ADTF was formalised in 2010, Brig-Gen Tan said work started soon after the Sept 11 strikes to develop a robust system to protect Singapore against such terror attacks.
He said the ADTF also steps up its vigilance during high-key events like the National Day Parade.
•Additional reporting by Fabian Koh