Singh, who turned 81 today, did not cut a cake on board the special Air India aircraft, he took to travel to Washington from Frankfurt where he had stopped overnight on his way to US.
It was a sombre mood in the plane in the wake of terror attacks that claimed nine lives -- a Lt Col, two sepoys, four policemen, including ASI, and two civilians.
On previous occasions, during visits to US the Prime Minister had cut cake though reluctantly because he says he is not given to birthday celebrations.
Three terrorists who attacked the Army camp in Samba were killed by security forces.