Menu Sign In Contact FAQ
Banner
Welcome to our forums

Mixing UL91 / 91UL and 100LL, and need for additive LW-16702, or Aeroshell 15W/50

From —> here, got me thinking…

Aircraft has Lycoming IO-540-T4B5 and has always run on Avgas 100LL. My understanding is that Lycoming SB 1070S approves us of UL91 and 91/96 for this engine model, so I need to undertake a paper exercise (Update FM, stickers, log book signoff etc..) to formally start using UL91.

Now suppose I flyin with 100LL in the tanks, arrive at an airfield where the only option is to top up with UL91, and the two fuel types are mixed in the tanks. What performance or operational issues can I expect ?

Would it be prudent to try this at a home airfield rather than testing in a far away touring location ?

United Kingdom

What operational changes do you have in mind? Using UL91 in a IO540C4D5D, I could not observe any significant change in any of the parameters.
Mixing it with 100LL – practically inevitable most of the time did not change that.

...
EDM_, Germany

flyingelephant wrote:

Now suppose I flyin with 100LL in the tanks, arrive at an airfield where the only option is to top up with UL91, and the two fuel types are mixed in the tanks. What performance or operational issues can I expect ?

Short answer: None.

Unless you have a FADEC with knock sensor, there is no performance difference between 100LL and UL91 as the engine can’t make use of the potential performance advantage of higher octane fuel. Both fuel types are AVGAS so they can be mixed in any proportion.

Note that to run the engine on unleaded fuel, you need to use an oil additive (or an oil quality which has the additive from the start, which all major brands make).

Last Edited by Airborne_Again at 09 Jun 11:36
ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

Note that to run the engine on unleaded fuel, you need to use an oil additive (or an oil quality which has the additive from the start, which all major brands make).

Could you elaborate on that? Thanks!

EGTR

As Airborne Again says you can expect absolutely no performance difference using 100LL in an aircraft certified for 91UL, I have been mixing the two fuels for a few years and have had no problems.

My preference is for using the 91UL as it results in cleaner oil and will not leave lead deposits that clog the oil control piston rings or centrifuge out blocking oilways. However I often find 91UL is not available .

The only issue with running 91UL is you have to put the Lycoming additive in the oil or run on an oil that has an anti-wear package that contains the additive. ( Aeroshell W15/W50 or Aeroshell 80 plus or 100 plus ).

There is widespread confusion as to what Fuel can be used and this is made clear in Service Letters issued by the engine manufacturer. As most flight manuals had been written before 91UL came on the market the flight manual won’t include 91UL in the approved fuels list so a flight manual amendment is needed and the placards on the fuel tanks need changing to include 91UL as an approved fuel .
This can be done under CS-STAN referencing the engine manufacturers data.

That argument is obsolete, was a factor with all cast iron heads. With aero engines you get valve seat inserts so no problem here. Vic
vic
EDME

A_and_C wrote:

The only issue with running 91UL is you have to put the Lycoming additive in the oil or run on an oil that has an anti-wear package that contains the additive. ( Aeroshell W15/W50 or Aeroshell 80 plus or 100 plus ).

We use oil from TOTAL which also has such additives in its XPD oil grades.

ESKC (Uppsala/Sundbro), Sweden

My assumption would be non as it works (tested) also with 100LL and Mogas. I would expect UL91 to produce even less (than the experienced none) issues than Mogas.

EDDV, EDVM, Germany

That sounds positive… now where do I find UL91 stickers ? Can only find 100LL stickers online.

United Kingdom

Given that nobody here reported any change, mixing them should be completely safe, in any engine certified for 91UL.

Administrator
Shoreham EGKA, United Kingdom
30 Posts
Sign in to add your message

Back to Top