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ARRIVAL NAVIGATION CONCEPTS
Today, the most significant and demanding navigational
requirement is the need to safely separate aircraft. In a
nonradar environment, ATC does not have an independent means to separate air traffic and must depend
entirely on information relayed from flight crews to
determine the actual geographic position and altitude. In
this situation, precise navigation is critical to ATC’s ability to provide separation.
Even in a radar environment, precise navigation and position reports, when required, are still the primary means
of providing separation. In most situations, ATC does not
have the capability or the responsibility for navigating an
aircraft. Because they rely on precise navigation by the
flight crew, flight safety in all IFR operations depends
directly on your ability to achieve and maintain certain
levels of navigational performance. ATC uses radar to
monitor navigational performance, detect possible navigational errors, and expedite traffic flow. In a nonradar
environment, ATC has no independent knowledge of the
actual position of your aircraft or its relationship to other
aircraft in adjacent airspace. Therefore, ATC’s ability to
detect a navigational error and resolve collision hazards
is seriously degraded when a deviation from a clearance
occurs.
Figure 4-12. Altitude Management When Cleared Direct.
"....cleared present
position direct....."
"I need to check my
altitude requirement."
4-13
The concept of navigation performance, previously discussed in this book, involves the precision that must be
maintained for both the assigned route and altitude.
Required levels of navigation performance vary from
area to area depending on traffic density and complexity
of the routes flown. The level of navigation performance must be more precise in domestic airspace than
in oceanic and remote land areas since air traffic
density in domestic airspace is much greater. For
example, there are two million flight operations conducted within Chicago Center’s airspace each year.
The minimum lateral distance permitted between
co-altitude aircraft in Chicago Center’s airspace is 8
NM (3 NM when radar is used). The route ATC
assigns an aircraft has protected airspace on both sides
of the centerline, equal to one-half of the lateral separation minimum standard. For example, the overall
level of lateral navigation performance necessary for
flight safety must be better than 4 NM in Center airspace. When STARs are reviewed subsequently in this
chapter, you will see how the navigational requirements become more restrictive in the arrival phase of
flight where air traffic density increases and procedural
design and obstacle clearance become more limiting.
The concept of navigational performance is fundamental to the code of federal regulations, and is best
defined in Parts 121.103 and 121.121, which state that
each aircraft must be navigated to the degree of accuracy required for air traffic control. The requirements
of Part 91.123 related to compliance with ATC clearances and instructions also reflect this fundamental
concept. Commercial operators must comply with their
Operations Specifications (OpsSpecs) and understand
the categories of navigational operations and be able
to navigate to the degree of accuracy required for the
control of air traffic. In the broad concept of air navigation, there are two major categories of navigational
operations consisting of Class I navigation and Class
II navigation. Class I navigation is any en route flight
operation conducted in controlled or uncontrolled
airspace that is entirely within operational service volumes of ICAO standard NAVAIDs (VOR, VOR/DME,
NDB). Class II navigation is any en route operation
that is not categorized as Class I navigation and
includes any operation or portion of an operation that
takes place outside the operational service volumes of
ICAO standard NAVAIDs. For example, your aircraft
equipped only with VORs conducts Class II navigation when your flight operates in an area outside the
operational service volumes of federal VORs. Class II
navigation does not automatically require the use of
long-range, specialized navigational systems if special navigational techniques are used to supplement
Figure 4-13. Class I and II Navigation.
CLASS II
1 HOUR OR LESS
CLASS I CLASS I
CLASS I CLASS I
CLASS I CLASS I
CLASS II
MORE THAN 1 HOUR
ROUTE 1
ROUTE 2
ROUTE 3
A B
NOTE: The area encompassed by the cylinders represents the volume of airspace within the
operational service volume (OSV) of ICAO standard NAVAIDs. The altitude of your aircraft with respect
　
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