305 

Federal Aviation Administration, DOT 

§ 23.1337 

integrates signals from auxiliary con-
trols or furnishes signals for operation 
of other equipment, positive interlocks 
and sequencing of engagement to pre-
vent improper operation are required. 

(g) There must be protection against 

adverse interaction of integrated com-
ponents, resulting from a malfunction. 

(h) If the automatic pilot system can 

be coupled to airborne navigation 
equipment, means must be provided to 
indicate to the flight crew the current 
mode of operation. Selector switch po-
sition is not acceptable as a means of 
indication. 

[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964; 30 
FR 258, Jan. 9, 1965, as amended by Amdt. 23– 
23, 43 FR 50593, Oct. 30, 1978; Amdt. 23–43, 58 
FR 18976, Apr. 9, 1993; Amdt. 23–49, 61 FR 5169, 
Feb. 9, 1996] 

§ 23.1331

Instruments using a power 

source. 

For each instrument that uses a 

power source, the following apply: 

(a) Each instrument must have an in-

tegral visual power annunciator or sep-
arate power indicator to indicate when 
power is not adequate to sustain proper 
instrument performance. If a separate 
indicator is used, it must be located so 
that the pilot using the instruments 
can monitor the indicator with min-
imum head and eye movement. The 
power must be sensed at or near the 
point where it enters the instrument. 
For electric and vacuum/pressure in-
struments, the power is considered to 
be adequate when the voltage or the 
vacuum/pressure, respectively, is with-
in approved limits. 

(b) The installation and power supply 

systems must be designed so that— 

(1) The failure of one instrument will 

not interfere with the proper supply of 
energy to the remaining instrument; 
and 

(2) The failure of the energy supply 

from one source will not interfere with 
the proper supply of energy from any 
other source. 

(c) For certification for Instrument 

Flight Rules (IFR) operations and for 
the heading, altitude, airspeed, and at-
titude, there must be at least: 

(1) Two independent sources of power 

(not driven by the same engine on mul-
tiengine airplanes), and a manual or an 

automatic means to select each power 
source; or 

(2) A separate display of parameters 

for heading, altitude, airspeed, and at-
titude that has a power source inde-
pendent from the airplane’s primary 
electrical power system. 

[Doc. No. 26344, 58 FR 18976, Apr. 9, 1993, as 
amended by Amdt. 23–62, 76 FR 75761, Dec. 2, 
2011] 

§ 23.1335

Flight director systems. 

If a flight director system is in-

stalled, means must be provided to in-
dicate to the flight crew its current 
mode of operation. Selector switch po-
sition is not acceptable as a means of 
indication. 

[Amdt. 23–20, 42 FR 36969, July 18, 1977] 

§ 23.1337

Powerplant instruments in-

stallation. 

(a) 

Instruments and instrument lines. 

(1) Each powerplant and auxiliary 
power unit instrument line must meet 
the requirements of § 23.993. 

(2) Each line carrying flammable 

fluids under pressure must— 

(i) Have restricting orifices or other 

safety devices at the source of pressure 
to prevent the escape of excessive fluid 
if the line fails; and 

(ii) Be installed and located so that 

the escape of fluids would not create a 
hazard. 

(3) Each powerplant and auxiliary 

power unit instrument that utilizes 
flammable fluids must be installed and 
located so that the escape of fluid 
would not create a hazard. 

(b) 

Fuel quantity indication. There 

must be a means to indicate to the 
flightcrew members the quantity of us-
able fuel in each tank during flight. An 
indicator calibrated in appropriate 
units and clearly marked to indicate 
those units must be used. In addition: 

(1) Each fuel quantity indicator must 

be calibrated to read ‘‘zero’’ during 
level flight when the quantity of fuel 
remaining in the tank is equal to the 
unusable fuel supply determined under 
§ 23.959(a); 

(2) Each exposed sight gauge used as 

a fuel quantity indicator must be pro-
tected against damage; 

(3) Each sight gauge that forms a 

trap in which water can collect and 

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