Twice a week I flew a Britannia from Kathmandu via Calcutta to Hong Kong during Jan/Feb/March 1973. Crossing Vietnam eastbound in the evening, between FL200 and 280, we often saw B52's trailing above us and on several trips we saw bombs exploding on the northern edge of the airway. Quite a bit of tension on the flight deck until we coasted out and headed for Hong Kong for a few well earned pints of San Miguel!