New Delhi, July 30: As Defence Minister George Fernandes embarks on MiG-21 flight on Friday, government today said in the Rajya Sabha that the aircraft would be phased out gradually. While not specifying the time-frame for the phase out, the defence minister told the House in a written reply that factors like calendar life and maintainability would be kept in mind before taking this step. Stating the IAF was flying 14 variant of MiG series, the minister said the older aircraft would be phased out first. Bureau Report