Sec. 23.955 - Fuel flow.
(a) General. The ability of the
fuel system to provide fuel at the rates specified in this section and at
a pressure sufficient for proper engine operation must be shown in the
attitude that is most critical with respect to fuel feed and quantity of
unusable fuel. These conditions may be simulated in a suitable mockup. In
(1) The quantity of fuel in the tank may
not exceed the amount established as the unusable fuel supply for that
tank under §23.959(a) plus that quantity necessary to show compliance with
(2) If there is a fuel flowmeter, it
must be blocked during the flow test and the fuel must flow through the
meter or its bypass.
(3) If there is a flowmeter without a
bypass, it must not have any probable failure mode that would restrict
fuel flow below the level required for this fuel demonstration.
(4) The fuel flow must include that flow
necessary for vapor return flow, jet pump drive flow, and for all other
purposes for which fuel is used.
(b) Gravity systems. The fuel
flow rate for gravity systems (main and reserve supply) must be 150
percent of the takeoff fuel consumption of the engine.
(c) Pump systems. The fuel flow
rate for each pump system (main and reserve supply) for each reciprocating
engine must be 125 percent of the fuel flow required by the engine at the
maximum takeoff power approved under this part.
(1) This flow rate is required for each
main pump and each emergency pump, and must be available when the pump is
operating as it would during takeoff;
(2) For each hand-operated pump, this
rate must occur at not more than 60 complete cycles (120 single strokes)
(3) The fuel pressure, with main and
emergency pumps operating simultaneously, must not exceed the fuel inlet
pressure limits of the engine unless it can be shown that no adverse
(d) Auxiliary fuel systems and fuel
transfer systems. Paragraphs (b), (c), and (f) of this section apply
to each auxiliary and transfer system, except that --
(1) The required fuel flow rate must be
established upon the basis of maximum continuous power and engine
rotational speed, instead of takeoff power and fuel consumption; and
(2) If there is a placard providing
operating instructions, a lesser flow rate may be used for transferring
fuel from any auxiliary tank into a larger main tank. This lesser flow
rate must be adequate to maintain engine maximum continuous power but the
flow rate must not overfill the main tank at lower engine powers.
(e) Multiple fuel tanks. For
reciprocating engines that are supplied with fuel from more than one tank,
if engine power loss becomes apparent due to fuel depletion from the tank
selected, it must be possible after switching to any full tank, in level
flight, to obtain 75 percent maximum continuous power on that engine in
not more than --
(1) 10 seconds for naturally aspirated
(2) 20 seconds for turbocharged
single-engine airplanes, provided that 75 percent maximum continuous
naturally aspirated power is regained within 10 seconds; or
(3) 20 seconds for multiengine
(f) Turbine engine fuel systems.
Each turbine engine fuel system must provide at least 100 percent of the
fuel flow required by the engine under each intended operation condition
and maneuver. The conditions may be simulated in a suitable mockup. This
flow must --
(1) Be shown with the airplane in the
most adverse fuel feed condition (with respect to altitudes, attitudes,
and other conditions) that is expected in operation; and
(2) For multiengine airplanes,
notwithstanding the lower flow rate allowed by paragraph (d) of this
section, be automatically uninterrupted with respect to any engine until
all the fuel scheduled for use by that engine has been consumed. In
(i) For the purposes of this section,
"fuel scheduled for use by that engine" means all fuel in any tank
intended for use by a specific engine.
(ii) The fuel system design must clearly
indicate the engine for which fuel in any tank is scheduled.
(iii) Compliance with this paragraph
must require no pilot action after completion of the engine starting phase
(3) For single-engine airplanes, require
no pilot action after completion of the engine starting phase of
operations unless means are provided that unmistakenly alert the pilot to
take any needed action at least five minutes prior to the needed action;
such pilot action must not cause any change in engine operation; and such
pilot action must not distract pilot attention from essential flight
duties during any phase of operations for which the airplane is approved.
[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as
amended by Amdt. 23-7, 34 FR 13093, Aug. 13, 1969; Amdt. 23-43, 58 FR
18971, Apr. 9, 1993; Amdt. 23-51, 61 FR 5136, Feb. 9, 1996]