Originally Posted by
kcockayne
I wouldn’t make a definitive statement that it was 3000 ft., but when I took up air band listening in the early ‘60s the lowest level allocated on Red 1 south of IBY was regularly FL40.
kcockayne, the lowest even flight level is 40. I can't recall whether the quadrantal system applied within the airway but either was 40 was the lowest westbound flight level.
Transitioning from QNH SPS and a flight level depended on the transition layer. FL30 could be above or below TA. If it was below TA the next available FL would be 35 or higher depending on magnetic course being flown.
Then the allocated FL also had to allow for safety altitude. This would be the MSFL or minimum safe flight level.