NavList:

Here is a quote from Britain's Imperial Air Routes 1918 to 1939 (Robin Higham 1960). The Company is Imperial Airways.

"From the pilot's point of view the Christmas of 1926 was rather dismal, for the Company was making them comply with  the new Air Ministry dictum that aircraft carrying ten or more passengers for 100 miles or more must carry a Second-class Navigator. All piiots had, therefore, to take the examinations. Though the Company followed maritime practice and made pilots buy their own sextants, it did encourage them to take their First-class Navigator's licences even before the Air Ministry made it mandatory."

My question is when did bubble sextants become available? Would the Imperial Airways pilots have purchased marine sextants?

From the same chapter

"In 1926 thinks looked brighter all around. The pilots sported a new uniform which had less gold braid than the old Instone [a] one, though gold rank badges were added in 1931".

[a]   Sanuel Instone and his brothers ran a profitable collierly and shipping business.

David C