This would be an interesting subject for an article describing the TFE731 development. It is rumored the engine was developed from an APU power-plant. I recall when our flight dept operated early conversion applications in which the 731 replaced first-generation turbojets on DH-125 and Lockheed Jet-Star first-gen jets. Jet fuel back then was 50-cents per gallon and the main reason to consider the conversion was not fuel costs but improved range and noise. At that time interest-rates were running in the 8-12% range and so the interest on the $3M conversion cost would pay for all the fuel burned by the older engines. At that time the 71s were also suffering a high rate of fuel pump/control failures, and when the Jet-Star conversion was being considered for the Jet-Star our parent-company CP (who was hoping the company to replace the Jet-Star with a G-II) was overheard to say “I’d rather fly across the Atlantic on one Spey than four 731s.”
The TFE-731 has come a long way since then.