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6/17/21 

AIM 

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. 

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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. 

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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 

ATC Clearances and Aircraft Separation 

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