AOPA Turbine Pilot Magazine - April 2019 - T-6

TURBINE PILOT
Vent check
valves

SYSTEM RUNDOWN

Outboard
fuel cells
Inboard fuel
cells
Center wing
fuel cell

Inboard fuel
cells

Outboard
fuel cells

Vent check
valves

Boost
pumps

Vent

Flapper
valve

Vent
Fuselage
fuel cell
Fuel sump

Flapper
valve

Twin Commander
690 fuel system
Where gravity does the work

WHO SAYS TURBINE-POWERED AIRCRAFT

are always saddled with complex systems? Sure, many airplanes' electrical
systems can be baffling, but that's the
price of redundancy. And in the October
2018 Turbine Edition of AOPA Pilot we
discussed the King Air B200's fuel system,
which some may consider a busy tangle of
tanks, pumps, and switches. But the Twin
Commander 690, 690A, and 690B fuel systems are models of simplicity.
Built from 1972 to 1979, these Twin
Commanders' fuel systems take advantage
of the airplane's high-wing design-like the
Aero Commander piston twins that preceded them. An interconnected network of
10 rubber-bladder fuel cells in each wing
gravity-feeds fuel to two central tanks. One
of these is in the center section of the wing,
and it gravity-feeds to a central fuselage
tank, where two boost pumps send fuel on
to the engines, where engine-driven fuel

pumps deliver it to the combustors. If one
of those boost pumps fails, fuel will still
be sent to the engines, although there's a
25,000-foot maximum operating altitude
limitation when using Jet-A. (It's 15,000
feet when using Jet-B, and 10,000 feet if
you're forced to use avgas).
With the fuselage tank as the single point of fuel collection and delivery,
there's no need for fuel selectors-or multiple fuel gauges. That means there's a
single fuel gauge that shows the airplane's
total fuel load. This makes fuel management a breeze.
Sure, the 690s have check valves to prevent fuel from flowing the wrong way, fuel
filters, sump drains, fuel vents, and shutoff
valves-the same components you'd see in
any airplane. And like any airplane there
are also some gotchas. When gassing up,
fill the center tank first, then alternate filling the wing tanks so that the fuel load is
BY THOMAS A. HORNE
I L LU S T R AT I O N B Y K E V I N H A N D

T-6 | AOPA PILOT April 2019

balanced. If you top off one wing while the
other is low on fuel, a big gotcha can kick
in: the airplane can tip over on the heavy
wing. Also, because of fuel sloshing in the
fuselage tank, it's strongly advised to get
the airplane on the ground should the low
fuel warning light come on. It's set to light
up when fuel levels drop to 200 pounds/30
gallons or 300 pounds/45 gallons, depending on certain serial number ranges.
Another big gotcha strikes when fuel
bladders need to be replaced. The new
bladders have to be heated in warm water
to make them pliable enough to be folded,
carefully fed through their smallish access
panels, and then unfolded and connected
to the neighboring tanks. This is a job
done mostly by feel and can rack up labor
charges-especially if more than one bladder is involved.
AOPA
EMAIL tom.horne@aopa.org

AOPA Turbine Pilot Magazine - April 2019

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