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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