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capabilities of the aircraft that will use the procedures
must be balanced to create airspace that combines functionality with safety. Although it is not necessary for
pilots to have an in-depth knowledge of how airspace is
protected, it is useful to understand some of the terms
used.
Required Obstacle Clearance (ROC) is the minimum
vertical clearance required between the aircraft and
ground obstructions over a specific point in an instrument procedure. Procedure designers apply the ROC
when designing instrument approach procedures. On
the initial segment, the ROC is approximately 1,000
feet, and it is at least 500 feet on the intermediate segment. Obviously, an imaginary surface 1,000 feet
above the actual terrain and obstacles would be as
rough and irregular as the surface below, so for practical reasons, airspace planners create smooth planes
above the highest ground features and obstructions.
These are called obstacle clearance surfaces (OCSs).
Procedure designers use both level and sloping obstacle clearance surfaces when designing approaches.
Fix Displacement Area (FDA) is an area created by
combining the permissible angular errors from the two
VOR or nondirectional beacon (NDB) NAVAIDs that
define the fix. When the NAVAIDs are close together
and the angle that defines the fix is near 90 degrees, the
B-3
FDA is relatively small. At greater distances or less
favorable angles, the FDA is larger. Airspace planners
use the FDA to define the limits of protected airspace.
[Figure B-2]
Fix Displacement Tolerance (FDT) is an area that
applies to area navigation (RNAV) and equates to a
FDA for VOR or NDB NAVAIDs. The FDT has an
Along Track (ATRK) tolerance and a Cross Track
(XTRK) tolerance.
Flight Technical Error (FTE) is the measure of the pilot
or autopilot’s ability to control the aircraft so that its
indicated position matches the desired position. For
example, FTE increases as the CDI swings further from
center. If the cockpit instruments show the airplane to
be exactly where you want it, the FTE is essentially
zero.
Navigation System Error (NSE) is the error attributable
to the navigation system in use. It includes the navigation sensor error, receiver error, and path definition
error. NSE combines with FTE to produce the Total
System Error (TSE). TSE is the difference between
true position of the aircraft and the desired position.
It combines the flight technical errors and the navigation system tracking errors.
Actual navigation performance (ANP) is an estimate of
confidence in the current navigation system’s performance. ANP computations consider accuracy, availability, continuity, and integrity of navigation performance
at a given moment in time. Required Navigation
Performance (RNP) necessitates the aircraft navigation
system monitor the ANP and ensures the ANP does not
exceed the RNP value required for the operation. The
navigation system must also provide the pilot an alert
in the primary field of view when ANP exceeds RNP.
[Figure B-3 on page B-4]
While you may have thought of protected airspace as
static and existing at all times whether aircraft are present or not, protection from conflicts with other aircraft
is dynamic and constantly changing as aircraft move
through the airspace. With continuous increases in air
traffic, some routes have become extremely congested.
Fortunately, the accuracy and integrity of aircraft navigation systems has also increased, making it possible to
reduce the separation between aircraft routes without
compromising safety. RNP is a standard for the navigation performance necessary to accurately keep an
aircraft within a specific block of airspace.
Containment is a term central to the basic concept of
RNP. This is the idea that the aircraft will remain within
a certain distance of its intended position (the stated
RNP value) at least 95 percent of the time on any flight.
The FDA is smallest when
the NAVAIDs are close to
the fix and the angle
defining the fix is
90 degrees.
At greater distances
from the NAVAIDs,
the FDA is larger.
At less favorable angles,
the FDA is larger.
Fix Displacement Area (FDA)
Facility
Facility
Facility
Figure B-2. The size of the protected airspace depends on where the terrestrial NAVAIDs that define it are located.
B-4
This is a very high percentage, but it would
not be enough to ensure the required level of
safety without another layer of protection outside the basic containment area. This larger
area has dimensions that are twice the RNP
value, giving the aircraft two times the lateral
　
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