AOPA Pilot Magazine - July 2022 - 85

RUDDER & WRENCH
The FAA explains, " These aviation
fuel operating limitations may be listed in
the product's Type Certificate Data Sheet
(TCDS), installation manual, service
instructions, or as limitations associated
with a Supplemental Type Certificate
(STC). " Lycoming has been active in
revising its fuel service instruction SI1070
to reflect approved fuels.
So, where does that leave us? Start
by checking the type certificate data
sheet (available on the FAA's website,
or your type club's website) for fuel
requirements. Make sure to consider
any STCed changes that may have been
imposed by modifications.
For instance, most higher-compression
piston mods revise the
minimum grade from 80/87 or 91/96 to
100. Similarly, turbocharger retrofits typically
require the higher-octane fuel. If
the TCDS or STC calls for 80/87 or 91/96,
then according to the FAA SAIB, you can
Just fuel and go, safely. Paperwork may
be required.
If you want to use mogas, then determine
if EAA or Petersen Aviation have
STCs for both the engine and airframe
of interest. Note that mogas STCs can
require modifications because properties
other than octane are affected: Some lowwing
airframes require boost pumps, or
fuel line re-routing, for example, because
of mogas' higher vapor pressure.
We've asked the FAA to clarify
use avgas rated at 91 or 94 octane. (This
does not extend to mogas, whose octane
ratings are inflated relative to avgas by
less severe octane test procedures.)
If the TCDS or STC requires 100,
100/130, or 100LL, then 100 unleaded
fuels should all work. Everything the
developers are saying now points to
those fuels being completely fungible
(intermixable) with 100/130 or 100LL.
its guidance on UL94. If Lycoming
or Continental service information
list your engine as being suitable for
UL94, then you're likely good to go.
Technically, you also need guidance
from the airframe manufacturer, since
the airframe TCDS also prescribes a
fuel. But since the only salient properties
that are different on the currently
proposed unleaded avgas is octane,
there's no general airframe implication.
What about the valves?
" My dad swears that some of his high-performance Ford
engines (Lotus Cortina and others) had valve seat recession
when lead was removed from mogas back in the 1980s. Did
leaded fuels aid the valves somehow to prevent valve seat
recession, and if so, how? "
The popular press conventional wisdom circa 1985,
when the lead phaseout began in earnest, was that the
lead provided a sacrificial surface on the valve seat to
prevent micro-welding of the valve to the seat, which was
experienced as valve seat erosion (aka recession). What's
now believed to have happened is an economic effect on
gasoline blending that impacted engines.
With the phasedown of lead, mogas octane became much
more difficult and expensive to achieve. The oil industry
responded to that by blending to octane number much more
carefully. This led to a real reduction in fleet mogas octane,
although the pump octanes stayed the same.
In 1983, 87 grade was really about 89 because octane
was cheap; by 1990, 87 grade became really 87, plus or
minus a bit. By 1995, regular unleaded AKI was still 87 on
the pump. But, because of harsh economics, the industry
was holding delivered octane to that number within one-half
an octane number.
This caused difficulties with EPA enforcement of octane
labeling, as octane test engine repeatability varies by about
that much. So the industry proposed and the EPA accepted
statistical control of octane: If a refiner could show that
statistically its octane met spec, it wasn't necessary for
every gallon to test to that octane. The net effect of this
is that while the advertised octane on the pump stayed
the same, the delivered octane, on average, declined. And
lower octane fuels led to light to moderate detonation under
conditions that slightly higher-octane fuels would not allow.
The conventional wisdom now is that valve issues didn't
arise from the lack of lead, per se, but rather from the actual
decline in mogas octane as delivered to the tank-even
though the spec number never changed. The " giveaway "
of octane that the industry was accustomed to went away
because of the much greater cost of delivering octane
without lead.
This was documented in papers published by the Society
of Automotive Engineers (SAE). In addition, in the early
days of unleaded avgas investigation, the FAA carefully
formulated leaded and unleaded avgas of matching octane,
and operated a twin with each engine dedicated to either
leaded or unleaded fuel. Teardown of the engines after
several hundred hours showed no difference in wear.
There's no doubt some people saw differences in engine
performance and wear with the phaseout of lead in mogas
in the 1980s and early 1990s, but we now have evidence
that suggests those differences came from differences in
octane, rather than from the absence of lead. -PM
AOPA PILOT / July 2022
85
AOPA Pilot Magazine - July 2022

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