347 

Federal Aviation Administration, DOT 

§ 25.1331 

from providing guidance or control to 
an unsafe speed. 

(i) The flight guidance system func-

tions, controls, indications, and alerts 
must be designed to minimize 
flightcrew errors and confusion con-
cerning the behavior and operation of 
the flight guidance system. Means 
must be provided to indicate the cur-
rent mode of operation, including any 
armed modes, transitions, and rever-
sions. Selector switch position is not 
an acceptable means of indication. The 
controls and indications must be 
grouped and presented in a logical and 
consistent manner. The indications 
must be visible to each pilot under all 
expected lighting conditions. 

(j) Following disengagement of the 

autopilot, a warning (visual and audi-
tory) must be provided to each pilot 
and be timely and distinct from all 
other cockpit warnings. 

(k) Following disengagement of the 

autothrust function, a caution must be 
provided to each pilot. 

(l) The autopilot may not create a 

potential hazard when the flightcrew 
applies an override force to the flight 
controls. 

(m) During autothrust operation, it 

must be possible for the flightcrew to 
move the thrust levers without requir-
ing excessive force. The autothrust 
may not create a potential hazard 
when the flightcrew applies an override 
force to the thrust levers. 

(n) For purposes of this section, a 

transient is a disturbance in the con-
trol or flight path of the airplane that 
is not consistent with response to 
flightcrew inputs or environmental 
conditions. 

(1) A minor transient would not sig-

nificantly reduce safety margins and 
would involve flightcrew actions that 
are well within their capabilities. A 
minor transient may involve a slight 
increase in flightcrew workload or 
some physical discomfort to passengers 
or cabin crew. 

(2) A significant transient may lead 

to a significant reduction in safety 
margins, an increase in flightcrew 
workload, discomfort to the flightcrew, 
or physical distress to the passengers 
or cabin crew, possibly including non- 
fatal injuries. Significant transients do 
not require, in order to remain within 

or recover to the normal flight enve-
lope, any of the following: 

(i) Exceptional piloting skill, alert-

ness, or strength. 

(ii) Forces applied by the pilot which 

are greater than those specified in 
§ 25.143(c). 

(iii) Accelerations or attitudes in the 

airplane that might result in further 
hazard to secured or non-secured occu-
pants. 

[Doc. No. FAA–2004–18775, 71 FR 18191, Apr. 
11, 2006] 

§ 25.1331

Instruments using a power 

supply. 

(a) For each instrument required by 

§ 25.1303(b) that uses a power supply, 
the following apply: 

(1) Each instrument must have a vis-

ual means integral with, the instru-
ment, to indicate when power adequate 
to sustain proper instrument perform-
ance is not being supplied. The power 
must be measured at or near the point 
where it enters the instruments. For 
electric instruments, the power is con-
sidered to be adequate when the volt-
age is within approved limits. 

(2) Each instrument must, in the 

event of the failure of one power 
source, be supplied by another power 
source. This may be accomplished 
automatically or by manual means. 

(3) If an instrument presenting navi-

gation data receives information from 
sources external to that instrument 
and loss of that information would 
render the presented data unreliable, 
the instrument must incorporate a vis-
ual means to warn the crew, when such 
loss of information occurs, that the 
presented data should not be relied 
upon. 

(b) As used in this section, ‘‘instru-

ment’’ includes devices that are phys-
ically contained in one unit, and de-
vices that are composed of two or more 
physically separate units or compo-
nents connected together (such as a re-
mote indicating gyroscopic direction 
indicator that includes a magnetic 
sensing element, a gyroscopic unit, an 
amplifier and an indicator connected 
together). 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–41, 42 FR 36970, July 18, 
1977] 

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