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Comment by p1mrx | Hacker News Reader

> some local governments are taking action to ban leaded avgas from being used at airports in their jurisdictions.

What happens if a 100LL aircraft lands at such an airport? Do they have to disassemble/tow it, or is there some sort of financial penalty?

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From what I can google it looks like they ban the sale of leaded avgas at those airports. If you run out of gas at one of those airports I imagine you're purchasing it elsewhere and one way or another arranging for it to be delivered to you at the airport at your expense + hassle.

can't it just run unleaded, albeit with slightly less power?

I assume based on this article that running unleaded would have violated FAA regulations.

I honestly think there should be civil consequences for the FAA slow-rolling the sunsetting of leaded avgas. Red tape is causing very real harm to vast numbers of people.

As with many problems with hard-to-see externalities, I think the FAA can't win here. If a plane falls out of the sky because of a fast-tracked approval, or people go out of business because of a fast-tracked ban, then the FAA is the bad guy. The number of people with the ability to assess the impact of atmospheric lead is much smaller.

In general: no. In aviation you don't do "this should probably work so let's do it". You follow approved guidelines and procedures and only approved guidelines and procedures. When there are a hundred separate things that can go wrong and kill you, you don't stray outside the lines.

And the issue isn't only slightly less power.

I know nothing about this and was surprised to learn leaded fuel is still a thing in any gas. Found this: https://www.faa.gov/about/initiatives/avgas/

Avgas is the only remaining lead-containing transportation fuel. Lead in avgas prevents damaging engine knock, or detonation, that can result in a sudden engine failure.

If engine knock can knock you out of the sky, it's probably not something you want to just take a chance with.

Internal risk (&/or cost of a better engine) vs externality of lead slowly poisoning the population.

The risk of getting hit by a plane whose engine has failed is probably the bigger externality.

To somewhat expand on the other answers: The only reason there are cars that "require 93 octane but can run 87 with lower power" is that they have knock sensors and ECUs that can limit ignition timing to avoid knock. This is a sophisticated system and nothing like it is available in any aviation engine, being designs from the 50s (except maybe some new experimental engines.)

The effect of running too low octane fuel in an aviation engine is not "slighly less power", but likely detonation and catastrophic engine failure.

And that's why it's a violation of FAA regs to run aircraft on non-approved fuels.

Just because something is old doesn't mean it should get a free pass on continuing to use leaded fuel though... there must be other reasons why leaded is still used besides simple knock sensors and ignition timing.

Exhaust valve seats. If you don't use appropriately hardened seats, you need lead to help cushion the wear between the exhaust valves and their seats in the head.

If your perfectly reasonable next question is, why aren't standard aircraft engines just built with the appropriate seats, well, then... The answer is low market volume coupled with high regulatory hurdles. There aren't any villains here, just written-in-blood safety regs coupled with a small enough market that there is a lot less money available to fight inertia.

Exactly. Flying for sport is expensive as it is today because safety has to be a priority (as it should). Add a requirement for new engines or major overhauls on old planes and the cost will spiral out of control. Besides, leaded fuel was a problem when literally every single car on the street was burning it. The total volume of fuel consumed by aviation piston engines is a joke compared to that.

In the case of Lycomings at least these valve guides have been hardened on cylinders produced since 1998, without having to certify a new type. On the Lycoming engines approved for 94UL I'm not aware of a restriction that these new guides must be installed for unleaded use.

> just written-in-blood safety regs

To be clear, this is literally true. Those safety lessons were paid for in blood.

Also, aviation engines are in general pushed far closer to their operational limits than a regular car engine. The latter might see full throttle and need to generate its rated power for a few percent of its life. Failures are not too dangerous. The former needs to generate rated power for a far larger fraction, and is also held to much higher reliability standards. Hence the requirement for high octane fuel and massive displacements relative to power output.

The difference in operational regime is true, although it's my impression that automobile engines are pushed through some very severe tests as part of their development. For example, this movie, https://youtu.be/GNPB3RtHN2M, claims that KIA tests their engines for 300 straight hours at full throttle.

As for reliability, I'm not convinced. In practical terms, I think automobile engines are far more reliable than aircraft engines. When was the last time your automobile engine broke an exhaust valve, cracked a cylinder head, or had a magneto come apart?

When was the last time your automobile engine broke an exhaust valve, cracked a cylinder head, or had a magneto come apart?

Exactly. An automobile engine is subjected to far less stresses in normal operation. 300h at full load is considered an extreme torture test for one, while a typical aircraft engine is meant to run for over 1000h between overhaul ( https://en.wikipedia.org/wiki/Time_between_overhauls ) and spends a significant amount of that time at full load --- that's normal operating conditions.

No. The compression ratio is fixed, as is the type of valve seat, and both need specific octane ratings or anti-wear additive, respectively.

The Pilot Operating Handbook (POH) contains numerous tables that pilots use to calculate what will be safe operating conditions for an aircraft. These include how much runway will be required at specific temperatures/elevations to clear obstacles. Those numbers were based on a 100 octane fuel so if the fuel is changed, those values must be determined once again and I believe that is largely done with flight testing. To do that on the entire fleet would be prohibitive. Without these small planes the misery you experience flying commercial would increased greatly due to the lack of pilots. It's pretty bad as it is.

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