[GrahamB]

The obvious downside being that 100 octane fuel is required. Hopefully the 100UL will be available and approved by the time 100LL is phased out, assuming I'm still flying by then.

The 100LL requirement is entirely due to that being the only suitable fuel commonly available at the time Bill Scott got the STC approved. 80/87 (or whatever it was) was permitted for the unmodified engine. Had there been a 91/96, unleaded or otherwise, around in the US at the time I’m sure he would have got it approved for that.

The HC STC raises the compression ratio from 7:1 to 8.5:1. The model it effectively turns it into is approved for 91/96UL, so there is no technical reason why our modified engines couldn’t run (and wouldn’t thrive) on 91/96UL, it’s pure paperwork that stops it being permitted. Anecdotally (ahem) they run really sweetly on it. There’s little incentive for Bill to go to the expense for the relatively few engines running where 91/96UL is available, but I understand he will be doing it for 100UL once that’s available.

[TrickyWoo]

As ever lots of great and detailed info and what I get from it is up to a 1/3 (!) of fuel could be being wasted? Like driving in 2nd on the M1? Why are we not taught this? Can't be thought too complicated shurely?

[Flyin'Dutch']

As ever lots of great and detailed info and what I get from it is up to a 1/3 (!) of fuel could be being wasted? Like driving in 2nd on the M1? Why are we not taught this? Can't be thought too complicated shurely?

Folklore and mis-information.

[TopCat]

what I get from it is up to a 1/3 (!) of fuel could be being wasted? ... Why are we not taught this? Can't be thought too complicated shurely?

Beats me. I was taught it.

And in ordinary light singles, it's very easy. Pull knob out until it just runs rough, push knob in until smooth again. Readjust after any significant altitude change.

Back to rich as part of pre-landing actions/checks in case you need full power for a go-around.

There is literally no more to it than that.

It's not like schools charge for amount of fuel used, so there's no incentive to game the system by over-leaning.

[Milty]

I had a chat with my instructor on leaning and whether it would be taught. He’s happy to teach me, but it’s not part of the ‘syllabus’ it seems.

I recall that the main concern from a ‘renter’, especially if dry renting is over-leaning causing over heating and engine damage. Taking an aircraft out of rental/training service or requiring early replacement is more of a concern than saving a few litres of fuel that you can charge a student for anyway.

I get the impression that leaning is something that you can learn once you have learned to fly and got your PPL.

Not saying that I particularly agree or condone this thought process, just relaying the info I have.

I’m encouraged though that if I ask my instructor something, he normally agrees to teach it. I guess it may depend on if you are hell bent on being a minimum hours student or happy to spend a couple of hours more to finesse your training. I suspect most schools would be happy to take up a recently qualified PPL to show some ‘best practice’ techniques (maybe I’m deluded).

[JAFO]

@Milty - I don't think it's about rushing to do it in minimum hours. I did my PPL in 38 hours back in 1986 when such things were possible. I learnt to lean, spent time on instruments and even did some spinning.

Now I fly an aeroplane with no gyro instruments and no mixture control so don't get to practise what I learned but there was plenty of time for it.

[GrahamB]

… to spend a couple of hours more to finesse your training.

It takes about two minutes, not hours, to demonstrate correct leaning technique. Call it five if you wanted to throw in some background theory on the ground.

I just can’t understand why flying schools don’t insist on leaning being taught and used. It would save them tons of money in fuel and maintenance.

[T6Harvard]

I was shown leaning very early on in training. As every airborne check includes Mixture I see no reason why we don't do it more as Students.

[Milty]

I guess like school teachers, they’re all different.

@GrahamB apologies, I was pretty sure that leaning won’t take that long but was being a bit more general and thinking about other skills like grass landings if only done tarmac for example.

[GrahamB]

The other thing about leaning which is often overlooked is that the closer you can run the engine to the stoichiometric mixture, the less CO there will be in the exhaust.

If you do get a crack in the exhaust, you are less likely to poison the occupants the leaner the mixture is.

[TopCat]

The other thing about leaning which is often overlooked is that the closer you can run the engine to the stoichiometric mixture, the less CO there will be in the exhaust.

If you do get a crack in the exhaust, you are less likely to poison the occupants the leaner the mixture is.

Excellent point. That had genuinely never occurred to me before.

[Paul_Sengupta]

it’s not part of the ‘syllabus’ it seems.

It's part of the basic operation of the aeroplane. It's like learning to drive in an old car - starting it first thing in the morning with the choke on and then leaving it on all day. You just wouldn't do it.

It isn't going to damage the engine if it's under 65% power (Lycoming say 75%). It's only leaning when increasing power, especially in a full power climb where airspeed for cylinder cooling is low, where you'd really need to be careful.

@GrahamB apologies, I was pretty sure that leaning won’t take that long but was being a bit more general and thinking about other skills like grass landings if only done tarmac for example.

It doesn't take much longer to land on grass rather than tarmac either!

Though training for short and soft field is part of the syllabus, useful if you want to use grass all year round.

[Milty]

To be clear for all, I’m in no way defending the fact that it’s not taught - just confirming that in my case it hasn’t been so far. I’ve just shared what my instructor told me.

This is one thing that I like about this forum is that it provokes thought that otherwise in my training, I may not have considered yet. Thanks to all.

I do like the analogy of driving an old car with the choke out all the time. That said, my old mini did seem to run better if the choke wasn’t all the way in. A wooden clothes peg clipped around the shaft was the perfect setting.