NFC Learning

Obstacle. All fixed (whether temporary or permanent) and mobile objects, or parts thereof, that:
a) are located on an area intended for the surface movement of aircraft; or
b) extend above a defined surface intended to protect aircraft in flight; or
c) stand outside those defined surfaces and that have been assessed as being a hazard to air navigation.

Note.— The term obstacle is used in this Annex solely for the purpose of specifying the charting of objects that are considered
a potential hazard to the safe passage of aircraft in the type of operation for which the individual chart series is
designed.

“obstacle clearance altitude (OCA) or obstacle clearance height (OCH)” means the
lowest altitude or lowest height above the elevation of the relevant runway threshold
or the aerodrome elevation as applicable, used in establishing compliance with
appropriate obstacle clearance criteria;

“obstacle clearance limit (OCL)” means the height above the elevation of an
aerodrome below which the minimum required vertical clearance cannot be
maintained on approach or in the event of a missed approach;

Obstacle clearance altitude (OCA) or obstacle clearance height (OCH). The lowest altitude or the lowest height above the
elevation of the relevant runway threshold or the aerodrome elevation as applicable, used in establishing compliance with
appropriate obstacle clearance criteria.

Note 1.— Obstacle clearance altitude is referenced to mean sea level and obstacle clearance height is referenced to the
threshold elevation or in the case of non-precision approaches to the aerodrome elevation or the threshold elevation if that is
more than 2 m (7 ft) below the aerodrome elevation. An obstacle clearance height for a circling approach is referenced to the
aerodrome elevation.

Note 2.— For convenience when both expressions are used they may be written in the form “obstacle clearance
altitude/height” and abbreviated “OCA/H”.

Note 3.— See Procedures for Air Navigation Services — Aircraft Operations (Doc 8168), Volume I, Part I, Section 4,
Chapter 1, 1.5, and Volume II, Part I, Section 4, Chapter 5, 5.4, for specific applications of this definition.

ODALS:   Omni directional Approach Lighting System

‘OEI’ means one engine inoperative

Offset frequency simplex. A variation of single channel
simplex wherein telecommunication between two stations
is effected by using in each direction frequencies that are intentionally slightly different but contained within a
portion of the spectrum allotted for the operation.

Obstacle free zone (OFZ). The airspace above the inner approach surface, inner transitional surfaces, and balked landing
surface and that portion of the strip bounded by these surfaces, which is not penetrated by any fixed obstacle other than a
low-mass and frangibly mounted one required for air navigation purposes.

Ohm (Ω). The electric resistance between two points of a conductor when a constant difference of potential of 1 volt, applied
between these two points, produces in this conductor a current of 1 ampere, this conductor not being the source of any
electromotive force.

OIS - Onboard Information Systems