During takeoff from Madras, the crew reported an unusual noise from the nose landing gear during retraction. The flight continued to Bangalore. Upon approach, the nose gear appeared to be not locked down, and a low pass was performed. Air traffic control confirmed the undercarriage was fully down. Upon landing, the crew maintained the nose up as far as possible. The nose contacted the runway after a ground run of 4000 feet, resulting in a stop 1300 feet further on. A probable cause was non-locking of the nose landing gear in down position, likely due to downward shifting of the nose landing gear actuator attachment point following fatigue and ‘T’ section longitudinal beam under stress corrosion cracking.During takeoff from Madras, the crew reported an unusual noise from the nose landing gear during retraction. The flight continued to Bangalore. Upon approach, the nose gear appeared to be not locked down, and a low pass was performed. Air traffic control confirmed the undercarriage was fully down. Upon landing, the crew maintained the nose up as far as possible. The nose contacted the runway after a ground run of 4000 feet, resulting in a stop 1300 feet further on. A probable cause was non-locking of the nose landing gear in down position, likely due to downward shifting of the nose landing gear actuator attachment point following fatigue and ‘T’ section longitudinal beam under stress corrosion cracking.