Rides

[Colonel]

No placard, eh?

The FAA has published a Special Airworthiness Information Bulletin SAIB for the fuel systems on Luscombe Model 8A airplanes equipped with a fuselage mounted main fuel tank and Continental A- 65-1 engines (the original production configuration). The SAIB addresses the need for procedures to prevent the possible loss of power on takeoff and climb.

Since certification in 1939, there have been reports of power loss on Luscombe Model 8A airplanes. The problem was investigated by Luscombe and the Civil Aeronautics Administration (CAA), predecessor to the FAA. Flight testing revealed that fuel flow could be interrupted from the fuselage mounted tank as a result of acceleration and higher pitch attitudes commonly encountered in takeoff and climb.

Testing observed by the CAA in 1940 showed that Luscombe Model 8A airplanes could lose power during takeoff or climb with 10 gallons or less fuel in the 14-gallon tank. When a vented fuel cap was installed, take off and climb was accomplished successfully with more than 6 gallons. Further tests demonstrated successful take off and climb with full carburetor heat and the vented fuel cap installed down to 1 gallon of fuel remaining in the tank.

The vented fuel cap (shown below) and use of carburetor heat on takeoff work together to prevent interruption of fuel flow to the engine. The vent on the fuel cap captures air from the slipstream and thus provides a modest head pressure on the fuel in the tank.

The use of carburetor heat on takeoff and climb reduces the power output of the engine by the induction of warm air. As a result, the pitch attitude of the airplane decreases. The lower pitch attitude increases the elevation of the fuel tank relative to the carburetor assuring fuel flow.

The CAA issued Airworthiness Maintenance Bulletin No. 40 February 25, 1941, specifying the installation of a vented fuel cap and a placard revising carburetor heat usage. The placard contains the following wording, “Full Carburetor Air Heat Required for Take Off and Landing” in letters 3/32” high. The placard was fabricated from .032” thick aluminum (pictured here).

Bulletin No. 40 also recommended installation of Luscombe Part No. 486021, Fuel Cap-Pressure Vent. Subsequently, other cap designs were approved by the manufacturer. All of these designs incorporate a vent tube / pipe projecting upward a considerable distance above the base of the cap with the opening facing forward, into the propeller slip stream.

A later design fuel cap utilizes a rigid steel 3/8” diameter tube section extending about 8” above the cap with a 1.25” radius bend forward into the airstream. These caps have a marking at the base with an arrow and black letters “FORWARD.” The information in Bulletin No. 40 was subsequently placed in Aircraft Specification No. A-694 Note 2.

The Luscombe 8A has unique procedures. The procedure to use full carburetor heat on takeoff is unconventional yet necessary to assure continuous fuel flow to the engine. Also, the vented fuel cap must be installed with the vent opening facing forward into the prevailing air stream. It is physically possible to install the fuel cap backwards; this condition will decrease fuel flow from the tank. The cap should have lettering indicating the forward direction. If the “Forward“ lettering is missing or obscured it should be renewed. During pre-flight inspection, while the cap is removed, it is advisable to check the vent function by blowing into the vent tube.

Q29. What's this I hear about using carb heat on takeoff?

A29: Refer to ATC694 and service letter. It is required to reduce the power on an A-65 or -75 when using a fuselage tank. With low fuel, (1/2 tank or less) and a cool day, when the engine is making lots of power and you are in a steep(er) climb angle it is possible to get the engine fuel inlet ABOVE the fuel tank outlet, which causes fuel flow to cease (and the engine quits). Carb heat on was a simple and cheap fix to this as it reduced the power the engine could make and thus reduced the deck angle. It is also why wing tanks are required on higher HP engine conversions, and why wing tanks are strongly recommended for all Luscombes in general.

Systems knowledge. Fuselage tank can bite you on a steep takeoff.

[Slick Goodlin]

You know, I gave the whole airplane and its systems a good think before hopping in but never would have expected a certified airplane to have fuel feed issues related to deck angle in a climb. Good to file that away.

In any case, the one I flew had wing tanks.

[Colonel]

Look at the date. Pre-WWII. 80 years ago (!!)

We're flying museum pieces, man. Watch your @ss.

the one I flew had wing tanks

Bueno. Chuck once told me that good head was important.

I thought I understood him at the time, but ...

An interesting test with a taildragger - either very new or
very old - is to disconnect the fuel line to the carburetor
and measure the time it takes to fill a gallon jug.

From that you can calculate your GPH. It had better be
more than one-tenth of your horsepower.

See, you can have a lot of shit broken on an airplane. I have
flown airplanes with no radio, no flight instruments, no battery,
one magneto, one cylinder not firing, flight controls broken,
structural damage, fabric taped on .... the list goes on and on.

But you better have fuel going into the engine as fast as it
needs it, or you're going to have a short, exciting flight.

[Slick Goodlin]

Chuck once told me that good head was important.

It helps keep one sane.

My ‘should I just go fly that thing’ mental checklist used to be simple and certified=yes, homebuilt=learn more first but I’ve come to realize the dinosaurs I fly have had a lot of time to be modified (through approved STC or just making crap up and jamming it on) in the years since they were made.

In the case of this Luscombe I figured fixed gear, fixed pitch prop, gravity fed fuel from the wings, carbureted, no flaps, minimal electrics and a popular enough plane to show that most people survive them was enough to convince me I could probably operate the thing fine. I could be wrong though, I feel like I’ve been going a bit crazy lately.

[Colonel]

The really cool thing about Aviation, is that there is always something
else to learn. And that something new that you learn today, could keep
you alive tomorrow. Or decades down the road.

Nobody knows everything about Aviation, despite their posturing.

In this case, getting fuel to feed from fuselage tanks is useful when you
are flying single engine across the Gulf of Mexico with ferry fuel:



I don't care what the dick-swinging blowhards on TrashCan say, when that
front seat tank runs empty and the single engine quits over the water, it
gets your attention, regardless of how many tens of thousands of hours
are in your logbooks, or what rank of TC Inspector you are.

In the Weldon boost pump we trust.