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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
to the location of the NAVAID is a primary factor in determining OSV range.
Route 1. Your aircraft navigating from A to B is conducting Class I navigation because you remain within the OSV
of ICAO standard NAVAIDs during your entire flight.
Route 2. Your aircraft navigating from A to B is conducting Class I navigation while within the OSV of the NAVAIDs. You are
conducting Class II navigation during the portion of your route outside the OSV of the NAVAIDs. Because the duration of the
Class II navigation is 1 hour or less, long-range navigation equipment or a flight navigator may not be required.
Route 3. Your aircraft navigating from A to B is conducting Class I navigation while within the OSV of the NAVAIDs. You are
conducting Class II navigation when outside the OSV of the NAVAIDs. The duration of the Class II navigation is more than
1 hour. Therefore, long-range navigation equipment or a flight navigator is required.
4-14
4-15
conventional NAVAIDs. Class II navigation includes
transoceanic operations and operations in desolate
and remote land areas such as the Arctic. The primary
types of specialized navigational systems approved
for Class II operations include inertial navigation system (INS), Doppler, and global positioning system
(GPS). Figure 4-13 provides several examples of
Class I and II navigation.
A typical limitations entry in a commercial operator’s
pilot handbook states, “The area navigation system used
for IFR Class I navigation meets the performance/accuracy criteria of AC 20-130A for en route and terminal
　
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