I am shocked to read this. Why are they still using leaded fuel? There are so many well known research that links lead in fuel to unstable mental health and crime in population.
That's the excuse. Basically the real reason is that they only care about safety of the planes, not of the people handling the fuel (e.g. pilots, ground staff), or breathing the toxic fumes (everybody on or near an airport). So, they only care about safety when it is a very narrowly scoped notion of that concept.
Bureaucracies are weird like that. This is fundamentally not about people's safety but about covering their own safety (i.e. ass coverage). The problem is not something bad might happen but that they'd be held accountable for it.
Never mind that something bad has been known to happen for the last half century or so that they are not being held accountable for. People actually get sick and die because of leaded fuel but it's not their problem. And never mind that the bad thing that might happen is basically some ancient engines not running that well with unleaded fuel. That's why certification processes exist for engines. You can test this and decide to not certify certain engines for unleaded fuel. Ensuring people fly around with certified engines definitely is their problem. Any modern engine is basically certified for unleaded fuel already.
I mean, planes falling out of the sky is a fairly significant safety issue. "Safety of the plane" is misleading if there's people inside the planes lol.
And to be clear, "knocking" means engine detonation, which very quickly leads to catastrophic engine failure. Which is something that you don't want while in flight.
Until now, the only alternative was to fly a turbine-powered aircraft that can run on Jet-A. The problem with those is that turbines are far too expensive, and terribly inefficient at low altitude - both of which have kept them out of reach for most of general aviation.
Admittedly Turboprops can be somewhat more efficent at lower altitudes than turbojet and turbofan engines. But still not terribly efficient at say sightseeing or cropdusting altitudes, and they are still a lot more expensive than avgas engines (and that is not even accounting for the fact that the aircraft designed to use them are bigger and more expensive than many avgas based prop planes).