FJ use of Transponder Mode C
In the 80's and 90's I had the pleasure of operating unpressurised turboprops in Scottish airspace. Many of our routes were on Air traffic advisory routes. Air traffic did their best to provide separation from the (then) plentiful fast jet traffic but this was severely hampered by the fact that the big majority of the contacts had no height information. This very much increased the stress and difficulty, especially when there was weather to avoid as well. As a result ATC sometimes had to endure the buttock-clenching experience of watching two contacts merge. Mode C presumably must have been fitted. Was there some operational reason why it was mostly turned off?