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AIM

10/12/17

4

−4−3

ATC Clearances and Aircraft Separation

action necessary to avoid possible confliction
between aircraft. Clearances will require that a flight
“hold” or change altitude prior to reaching the point
where standard separation from other IFR traffic
would no longer exist.

NOTE

Some pilots have questioned this action and requested
“traffic information” and were at a loss when the reply
indicated “no traffic report.” In such cases the controller
has taken action to prevent a traffic confliction which
would have occurred at a distant point.

b.

A pilot may wish an explanation of the handling

of the flight at the time of occurrence; however,
controllers are not able to take time from their
immediate control duties nor can they afford to
overload the ATC communications channels to
furnish explanations. Pilots may obtain an explana-
tion by directing a letter or telephone call to the chief
controller of the facility involved.

c.

Pilots have the privilege of requesting a

different clearance from that which has been issued
by ATC if they feel that they have information which
would make another course of action more
practicable or if aircraft equipment limitations or
company procedures forbid compliance with the
clearance issued.

4

−4−5. Coded Departure Route (CDR)

a.

CDRs provide air traffic control a rapid means

to reroute departing aircraft when the filed route is
constrained by either weather or congestion.

b.

CDRs consist of an eight

−character designator

that represents a route of flight. The first three
alphanumeric characters represent the departure
airport, characters four through six represent the
arrival airport, and the last two characters are chosen
by the overlying ARTCC. For example, PITORDN1
is an alternate route from Pittsburgh to Chicago.
Participating aircrews may then be re

−cleared by air

traffic control via the CDR abbreviated clearance,
PITORDN1.

c.

CDRs are updated on the 56 day charting cycle.

Participating aircrews must ensure that their CDR is
current.

d.

Traditionally, CDRs have been used by air

transport companies that have signed a Memorandum
of Agreement with the local air traffic control facility.
General aviation customers who wish to participate in

the program may now enter “CDR Capable” in the
remarks section of their flight plan.

e.

When “CDR Capable” is entered into the

remarks section of the flight plan the general aviation
customer communicates to ATC the ability to decode
the current CDR into a flight plan route and the
willingness to fly a different route than that which
was filed.

4

−4−6. Special VFR Clearances

a.

An ATC clearance must be obtained prior to

operating within a Class B, Class C, Class D, or
Class E surface area when the weather is less than that
required for VFR flight. A VFR pilot may request and
be given a clearance to enter, leave, or operate within
most Class D and Class E surface areas and some
Class B and Class C surface areas in special VFR
conditions, traffic permitting, and providing such
flight will not delay IFR operations. All special VFR
flights must remain clear of clouds. The visibility
requirements for special VFR aircraft (other than
helicopters) are:

1.

At least 1 statute mile flight visibility for

operations within Class B, Class C, Class D, and
Class E surface areas.

2.

At least 1 statute mile ground visibility if

taking off or landing. If ground visibility is not
reported at that airport, the flight visibility must be at
least 1 statute mile.

3.

The restrictions in subparagraphs 1 and 2 do

not apply to helicopters. Helicopters must remain
clear of clouds and may operate in Class B, Class C,
Class D, and Class E surface areas with less than
1 statute mile visibility.

b.

When a control tower is located within the

Class B, Class C, or Class D surface area, requests for
clearances should be to the tower. In a Class E surface
area, a clearance may be obtained from the nearest
tower, FSS, or center.

c.

It is not necessary to file a complete flight plan

with the request for clearance, but pilots should state
their intentions in sufficient detail to permit ATC to
fit their flight into the traffic flow. The clearance will
not contain a specific altitude as the pilot must remain
clear of clouds. The controller may require the pilot
to fly at or below a certain altitude due to other traffic,
but the altitude specified will permit flight at or above
the minimum safe altitude. In addition, at radar