# Why are we still putting lead in our fuel?

It is also a fact that there are aircraft piston engines which do run perfectly fine on unleaded fuel, why is it that hardly any airport offers it? Even in cost-conscious and eco-friendly Europe, most airports offer only 100LL and Jet fuel.

• wiki (last sentence of the paragraph) says it's because of pricing – ratchet freak Mar 3 '14 at 13:46
• @ratchetfreak; it does indeed say that, however, unleaded high octane fuel is still cheaper. I think they're looking for an alternative to Tetraethyllead which is economically feasible, that is, for engines which require leaded fuel, it doesn't talk about why said engines require (and continue to require) leaded fuel. – falstro Mar 3 '14 at 13:54
• @ratchetfreak The Katana I fly is certified for, and runs perfectly well on Mogas (it's a Rotax). The question is still, why are lead-fuel-engines still being produced today, more than 40 years after the phase out of lead-fuel-engines began, and more than a decade after the use has been banned in all other circumstances. It can't be that they're old. Take the Continental IO-550 for example, it was first run in '83, it has a FADEC for crying out loud, and still munches lead. – falstro Mar 3 '14 at 14:16
• just as a comment because of lack of time: engine certification is just so expensive that no producer can be bothered while their standard 30-year old products are still selling. Rotax is different because they open up a new market (Ultralight and VLA). – yankeekilo Mar 3 '14 at 14:44
• Some aircraft engines can approved for running 91 or better ETHANOL-FREE autogas through a Supplemental Type Certificate (STC). Each engine and airframe have to have been approved through the STC process and must have the certificate or STC for the individual aircraft to use what is coined "mogas." There is a high % of the GA fleet that can be STC'd for "mogas" but a lot of misinformation has caused the usage to be scanty as a whole. – user4032 Oct 23 '14 at 12:14

There is nothing "special" about an aircraft engine that requires lead in the fuel -- Engines don't much care about the lead in tetraethyl lead, nor do they much care for it (it gets everywhere, fouling sparkplugs and contaminating the oil). What they care about is the octane (anti-knock) rating of the fuel.

Many "low performance" aircraft engines run just fine on unleaded fuels with a lower octane rating (among them the Lycoming O-320 and O-360 family that power a good chunk of the GA fleet), but high-performance aircraft engines (e.g. the IO-520 you'll find in Beech Bonanzas) require a 100 octane fuel. The approved specification for 100 octane aviation gasoline (ASTM D910) lists tetraethyl lead as the octane booster of choice.

It's also worth noting that some of the octane boosting techniques used in automotive gasoline are not acceptable for aircraft engines (the most common in the US being the addition of ethanol, which has two undesirable effects: reducing the energy content of the fuel, and damaging aviation fuel system seals and other components).

So why do we still make 100LL, and why don't we offer the other unleaded options at every airport?

Aviation gasoline is a minuscule slice of the gasoline market, so it doesn't make sense to have 5 tanks with 5 different grades of fuel at every airport:
All aircraft engines will run on 100LL, so 100LL (leaded) aviation gasoline is still produced because it is a "single fuel" solution to piston aviation's needs.

If leaded avgas were to disappear tomorrow the engines that require it would be left without an approved fuel, which would result in those aircraft being grounded until such time as an alternative fuel could be developed or the engine manufacturers develop a procedure for derating the engines (operating them at reduced power). For obvious reasons neither of these options is attractive, particularly to folks who own higher-power engines which were presumably purchased for the performance...

Changing the avgas specification is quite a bit of work - it requires ensuring that the new fuel is a "drop-in replacement" for 100LL -- one which can be mixed with 100LL in any proportion, and will work correctly in any engine designed for 100LL fuel.
The FAA is working with engine manufacturers and major fuel producers on a program which will result in such a specification (a Google search for unleaded avgas transition also produces useful results), and hopefully a universal unleaded aviation fuel specification and one or more products will come out of those efforts.

• Although a question remains: why do most aero piston engines seem so outdated compared to modern automotive counterparts? – yankeekilo Mar 3 '14 at 18:26
• Good answer, but lead also acts as a lubricant and some engines depend on that, so your first paragraph isn't really 100% accurate. – Lnafziger Mar 3 '14 at 18:27
• @yankeekilo That has a lot to do with the process of getting anything through FAA certification -- modernizing an engine would require a mountain of paperwork, and specific approvals (probably in the form of STCs) to install it on certificated airframes as a replacement for the "old" designs. The existing "vintage" engines work, so we keep producing them (just like the existing aircraft designs work -- today's newest Piper Archers are largely identical to the first Cherokees produced...) – voretaq7 Mar 3 '14 at 18:33
• There is also tendency to use diesel engines burning Jet-A/Jet-A1 for the lower power engines and turboprops (burning also Jet-A/Jet-A1) for the higher powered ones and get rid of avgas altogether (because it's already more expensive and less available, at least outside US). – Jan Hudec Mar 3 '14 at 18:59
• @roe - lol, ever been to a car repair shop? Cars all over the world are definitely falling apart... Okay, okay, I'm being silly :). But, keep in mind, aircraft engines do have to be far more reliable than an automobile engine. If an automobile engine quits out on you, you roll to a stop and call a tow truck. In a plane if your the engine cuts out, or even loses power, you may well die. So the engine has to be much more precise. Hence the FAA is picky about engines, hence it's taking a lot of time to get regular 100 out the door. – Jay Carr Mar 4 '14 at 18:39

Lead in fuel is primarily an anti-knock agent, raising the octane rating and permitting higher compression ratios in the engine which in turn provides more power and more efficient use of fuel. Many GA aircraft engines are built specifically to take advantage of the anti-knock, higher octane fuel. Although there is a lot of work underway to develop a non-leaded substitute fuel that will allow the same performance, we are not there yet. There is no alternative fuel certified for these engines, and burning unleaded fuel at high power settings will damage these engines severely and quickly. So we still need the lead in these cases.

With luck and technology, this will change in a few years. If not, diesel engines are a likely replacement for the 100LL burners. But that is a pricey change for most GA planes.

One factor may well be that engines that run on leaded fuel are more fuel efficient than those that don't.
IOW you get more horsepower and/or better range from leaded fuel than from an identical amount of unleaded fuel when burnt in an engine with similar level of technology.
So an aircraft burning unleaded fuel would have worse performance and/or worse range. And of course especially the performance would cause trouble as it means longer takeoff run, lower service ceiling, worse climb performance, and potentially leaving you with an aircraft that can't even get off the ground.
And given the far longer lifespan of aviation engines as compared to car engines, there are far more aircraft around that need leaded fuel, the transition period would be far longer as well. And a car where you put a bottle of lead replacement compound in the fueltank, if it drives a bit slower it's not as bad as an aircraft where you're doing the same and it's not just slower, but can't climb to the same altitude and needs more runway...

• There have been examples of very efficient, Mogas-compatible flight engines, like the Porsche PFM 3200, so the efficiency angle may not be always relevant. Modern car engines are much more efficient than their lead-guzzling counterparts from just 15 years ago... – yankeekilo Mar 3 '14 at 16:13
• @yankeekilo yes, but a car engine with similar fuel consumption to one using leaded gasoline created at the same time would not be. That's the whole point. There are so many aircraft out there using engines that require leaded fuel, and not just museum pieces but in operational use, that would require new engines to be designed for them, it's just not feasible. – jwenting Mar 4 '14 at 7:41

My understanding is that it is simply a case of certification. There is no replacement certified fuel that will take the place of 100LL.