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Federal Aviation Administration, DOT 

§ 91.177 

specifications authorize use of EFVS; 
and 

(7) The aircraft is equipped with, and 

the pilot uses, an enhanced flight vi-
sion system, the display of which is 
suitable for maneuvering the aircraft 
and has either an FAA type design ap-
proval or, for a foreign-registered air-
craft, the EFVS complies with all of 
the EFVS requirements of this chapter. 

(m) For purposes of this section, 

‘‘enhanced flight vision system’’ 
(EFVS) is an installed airborne system 
comprised of the following features and 
characteristics: 

(1) An electronic means to provide a 

display of the forward external scene 
topography (the natural or manmade 
features of a place or region especially 
in a way to show their relative posi-
tions and elevation) through the use of 
imaging sensors, such as a forward- 
looking infrared, millimeter wave 
radiometry, millimeter wave radar, 
and low-light level image intensifying; 

(2) The EFVS sensor imagery and air-

craft flight symbology (

i.e.,  at least 

airspeed, vertical speed, aircraft atti-
tude, heading, altitude, command guid-
ance as appropriate for the approach to 
be flown, path deviation indications, 
and flight path vector, and flight path 
angle reference cue) are presented on a 
head-up display, or an equivalent dis-
play, so that they are clearly visible to 
the pilot flying in his or her normal po-
sition and line of vision and looking 
forward along the flight path, to in-
clude: 

(i) The displayed EFVS imagery, at-

titude symbology, flight path vector, 
and flight path angle reference cue, and 
other cues, which are referenced to this 
imagery and external scene topog-
raphy, must be presented so that they 
are aligned with and scaled to the ex-
ternal view; and 

(ii) The flight path angle reference 

cue must be displayed with the pitch 
scale, selectable by the pilot to the de-
sired descent angle for the approach, 
and suitable for monitoring the 
vertical flight path of the aircraft on 
approaches without vertical guidance; 
and 

(iii) The displayed imagery and air-

craft flight symbology do not adversely 
obscure the pilot’s outside view or field 
of view through the cockpit window; 

(3) The EFVS includes the display 

element, sensors, computers and power 
supplies, indications, and controls. It 
may receive inputs from an airborne 
navigation system or flight guidance 
system; and 

(4) The display characteristics and 

dynamics are suitable for manual con-
trol of the aircraft. 

[Doc. No. 18334, 54 FR 34294, Aug. 18, 1989, as 
amended by Amdt. 91–267, 66 FR 21066, Apr. 
27, 2001; Amdt. 91–281, 69 FR 1640, Jan. 9, 2004; 
Amdt. 91–296, 72 FR 31678, June 7, 2007; Amdt. 
91–306, 74 FR 20205, May 1, 2009] 

§ 91.177

Minimum altitudes for IFR op-

erations. 

(a) 

Operation of aircraft at minimum al-

titudes.  Except when necessary for 
takeoff or landing, or unless otherwise 
authorized by the FAA, no person may 
operate an aircraft under IFR below— 

(1) The applicable minimum altitudes 

prescribed in parts 95 and 97 of this 
chapter. However, if both a MEA and a 
MOCA are prescribed for a particular 
route or route segment, a person may 
operate an aircraft below the MEA 
down to, but not below, the MOCA, pro-
vided the applicable navigation signals 
are available. For aircraft using VOR 
for navigation, this applies only when 
the aircraft is within 22 nautical miles 
of that VOR (based on the reasonable 
estimate by the pilot operating the air-
craft of that distance); or 

(2) If no applicable minimum altitude 

is prescribed in parts 95 and 97 of this 
chapter, then— 

(i) In the case of operations over an 

area designated as a mountainous area 
in part 95 of this chapter, an altitude of 
2,000 feet above the highest obstacle 
within a horizontal distance of 4 nau-
tical miles from the course to be flown; 
or 

(ii) In any other case, an altitude of 

1,000 feet above the highest obstacle 
within a horizontal distance of 4 nau-
tical miles from the course to be flown. 

(b) 

Climb. Climb to a higher minimum 

IFR altitude shall begin immediately 
after passing the point beyond which 
that minimum altitude applies, except 
that when ground obstructions inter-
vene, the point beyond which that 
higher minimum altitude applies shall 

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