When all else fails, go to the source, in this case, the EAA who did all the tests for the STC's in the first place.
EAA Flight Research Center
Auto Fuel STC
PO Box 3065
Oshkosh, WI 54903-3065
Phone: (920) 426-4843
Fax: (920) 426-4881
Is Vapor Lock a Problem with Unleaded Auto Gasoline?
FIELD INFORMATION
NO.
304
REVISED
5/20/96
Vapor lock is always a problem to consider regardless of what kind of fuel
is being used. Vapor lock has occurred under some conditions while
aircraft were using aviation gasoline.
When using automobile gasoline, vapor lock is an important consideration
because automobile gasoline has been designed to facilitate engines
starting in the winter time and thus has a higher volatility. In the test
work
done by the EAA, this winter fuel of higher volatility was used in the
high
summer ambient temperatures. This combination represents the most
adverse conditions for the formation of vapor lock.
There are other important considerations such as the effect of high
ambient temperatures, very high engine operating temperatures under
conditions of takeoff with high volatility fuel, and the complexity of
the
fuel
system (many bends and fittings). All of these factors and many more
effect the likelihood for vapor lock.
When a fuel is heated, vapor is driven off, which in turn reduces the
volatility of the remaining fuel. As an aircraft sits on the ramp, the
fuel
tanks are heated and vapor is vented out reducing the vapor pressure. In
EAA's test work, it was necessary to refrigerate fuel before loading the
aircraft fuel tank to ensure a high volatility for the test. Then while
attempting to keep the fuel cool and the airplane hot, flight tests
demonstrated satisfactory performance of the fuel system during the
critical takeoff period. Tests were also satisfactorily completed using
the
same high volatility fuel heated to 110 degrees Fahrenheit.
Another area of concern is high altitude vapor lock. All EAA flight test
programs have included an evaluation of the adverse combination of
volatility and temperatures, plus demonstrated climbs to the service
ceiling
of the aircraft, (which in the case of the Cessna 182 was 21,034 feet
density altitude), and to also include some periods of cruise at altitudes
above 7000 feet. Each airplane that we have flown in our 500-hour flight
test program successfully completed these tests.
ASTM specification defines gasoline RVP (Reid Vapor Pressure) for
classes of A through E in areas of the country for various seasons. Any
class gasoline, A through E, is approved for any season anywhere in the
United States.
304-1
Revised 5/20/96
Automobile gasoline volatility has been generally higher than aviation
gasoline volatility. If critical operating conditions, as mentioned,
reach
extremes, vapor lock can occur earlier with automobile gasoline than with
aviation gasoline. Operation conditions that encourage the formation of
vapor in aviation or automobile gasoline are those which raise the
under-cowl temperatures to extremes and provide a source for the transfer
of excessive heat into the fuel lines. After any prolonged period of heat
soak (e.g., hot day ground idling or engine restart a short time after a
long
period of engine operation), perform full power check before taking off.
Ensure recommended fuel pressure is indicated on aircraft so equipped.
Follow this precaution also with aviation gasoline.
At the present time, in most US metropolitan regions, the EPA limits the
volatility of automobile gasoline to about the same as 100LL aviation
gasoline. In California as of June 1, 1996, regular automobile gasoline,
with
the exception of gasoline that has alcohol, is for all practical purposes
identical to 80 Grade aviation gasoline. Future automobile gasoline
changes to meet EPA requirements find both aviation gasoline and
automobile gasoline approaching identical characteristics with the
exception of meeting the 100 octane rating. Oxygenates required in these
urban areas are primarily Methyl Tertiary Butyl Ether (MTBE) and ethanol
alcohol. Alcohol additives, other than some de-icing fluids, are not
approved by the FAA.
Is it all right to mix aviation
gasoline and automobile
gasoline?
FIELD INFORMATION
NO.
310
REVISED
5/20/96
Aviation gasoline and unleaded automotive gasoline may be mixed in any
proportion. Any mixture containing unleaded automotive gasoline in the
aircraft must be handled in accordance with the placards or precautions
established for unleaded automotive gasoline.
Can an Airport Stop me from Fueling My Own Aircraft?
An airport cannot stop you from fueling your aircraft and continue to be
eligible for Federal aid. It may however, require that you conform to its
safety regulations.
In accordance with guidelines per FAA AC 150/5190-2A, section 4.D,
"Any unreasonable restriction imposed on the owners and operators of
aircraft regarding the servicing of their own aircraft and equipment may
be
considered as a violation of agency policy. The owner of an aircraft
should
be permitted to fuel, wash, repair, paint, and otherwise take care of his
or
her own aircraft, provided there is no attempt to perform such services
for
others. Restrictions, which have the effect of diverting activity of this
type
to a commercial enterprise, amount to an exclusion right contrary to law.
Local airport regulations, however, may and should impose restrictions on
these activities necessary for safety, preservation of airport facilities
and
protection of public interest. These might cover, for example,
restrictions
on the handling practices for aviation fuel and other flammable products,
such as aircraft paint and thinners, requirements to keep fire lanes
open,
weight limitations on vehicles, and aircraft to protect pavement from over
stress, etc."
Can alchohol, methanol and ethanol be used?
Do not use fuel which contains methanol or ethanol. They are not
compatible with materials in your fuel system and will cause malfunction
of the fuel delivery system. There is a simple test to determine
significant
alcohol content in fuel. EAA Field Information No. 306 provides details
on
how to do it yourself. The FAA has tested Methyl Tertiary Butyl Ether
(MTBE) and found no harmful effects in aircraft engines and typical fuel
systems at various concentrations up to 100 percent MTBE. (Reference:
Report DOT/FAA/CT88/05 "Alternate fuels for General Aviation Aircraft
with
Spark Ignition Engines."). As of late 1992, FAA clarified the policy
allowing
the use of MTBE for STC-approved installations. As of December 1995,
the FAA has also tested and approved Ethyl Tertiary Butyl Ether (ETBE).
See EAA Field Information No. 305.
310-1
Revised 5/20/96
Why is the exhaust stack of my engine black with soot after
using auto gasoline?
A black soot is the natural by-product of burning unleaded gasoline and
is
to be expected. This is not an indication of a rich mixture, nor an
indication of internal build-up or potential for spark plug fouling. The
black
soot is graphite-type deposit. The gray deposit we usually see when using
100LL aviation gasoline is a lead-type deposit.
Fueling Safety
REVISED
5/20/96
A question frequently asked is, "Is it all right to fuel my airplane using
five
gallon cans?" The answer is "yes," but you must know it is risky business
to handle gasoline in this manner. There is an added risk of contamination
when putting gasoline into your aircraft. Fuel contamination with aviation
gasoline or automobile gasoline, or any fuel is a significant safety
problem
in aircraft. Using five-gallon cans exposes you to the possibility of
contaminating with rust or perhaps solder droplets from brand new cans
and, in particular, from water formed by condensation or water otherwise
entering the fuel system. Our recommendation, when this method of
fueling must be done, is to take extreme care in making sure no water or
other contaminant is put in your airplane fuel tank. Farm stores sell
large
funnels with flanges that will screw on your gasoline tank inlet to
support
the funnel and at the same time provide a 100-mesh stainless steel
screen. This should help ensure the absence of water in the fuel tank.
301-3
Revised 5/20/96
The problem still remains, as with all aircraft and fuel systems, of
possible
condensation forming enough water in the fuel tank to cause a problem.
The recommended practice has been to leave the fuel tanks full whenever
possible in the small aircraft we fly. We highly recommend reading FAA
Advisory Circular AC 20-43 regarding the contamination of aviation fuel.
Caution: Do not use fuel which contains alcohol. It is not compatible with
all materials in your fuel system and will cause malfunction of the fuel
delivery system, reduce your aircraft range and aggravate the
contamination problems and vapor lock potential.
Octane Number
FIELD INFORMATION
NO.
303
REVISED
5/20/96
The number which is posted on the automobile service station pump is not
a true octane number. It is what is called an "antiknock index" number
(AKI). This number is the average of two octane numbers arrived at by two
different kinds of tests. One is called ASTM Research Method and is often
abbreviated R or RON. The other is the ASTM Motor Method, M or MON.
The antiknock index number on the pump is then this average, or R + M
divided by 2 = AKI. A rule of thumb is that the Motor Method octane
number (MON) is approximately five points less than the AKI. The
significance of the MON is that this is identical to the octane number for
aviation gasoline.
Specification D-4814 (previously D-439) for automobile gasoline requires a
minimum of 82 MON when the posted number is 87 AKI or more. When
the EAA requested approval from the FAA, the request was for an AKI
number of 87 to insure a safety margin of 2 octane numbers over the
approved rating for aviation gasoline for these 80 octane engines.
Detonation
Detonation will not be a problem when using any grade of automobile
gasoline with an aircraft engine approved for use of 80 octane fuel.
The Department of Energy's (DOE) semi-annual report giving data on
gasoline's selected at random throughout the United States shows that for
more than 20 years, the lowest octane number measured for automobile
gasoline in the US has been more than 80 octane by the Motor Method,
which is the same as the aviation method. So, for 80 octane aircraft
engines, the octane rating of even the lowest octane automobile gasoline
is more than adequate.
The AKI is only 85 in some states. Is that a legal fuel?
No. The octane number requirement for any engine is reduced with
reduced ambient temperatures and increased altitude. Therefore, mountain
states are permitted to market fuel at lower octane numbers than others.
In terms of the STC, the approved fuel must have an 87 antiknock index
rating. This may mean that sometimes in mountainous states premium
fuel should be used. In this case, the minimum octane number, according
to DOE surveys will be at least 84 MON (equivalent to aviation gasoline
octane number). Using regular automotive gasoline with an 85 AKI could
possibly result in a minimum octane number of 79.1 MON. This difference
is probably not significant, but in order to maintain a larger
conservative
margin, EAA requested approval for automobile gasoline with a minimum
87 AKI to insure 82 MON.
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