(b)
Airborne and ground check points consist of certified radials that should be received at specific points on the airport surface or over specific landmarks while airborne in the immediate vicinity of the airport. Should an error in excess of ±4 degrees be indicated through use of the ground check or ±6 degrees using the airborne check, IFR flight should not be attempted.

(c)
TACAN receiver check point tolerances: Military bases normally designate a specific ground point for checking the accuracy of aircraft TACAN receivers. The tolerances for the ground check are similar to the VOR within ±4 degrees of the designated radial and within one-half mile or 3 percent of the distance to the facility, whichever is greater.

(4)
The list of VOR Airborne Check Points is published in the Area Planning Documents (AP1, AP2, AP3 and AP4) under country listing.

ENROUTE

6-12 AIRWAY/ROUTES INCLUDING CONDITIONAL ROUTE INFORMATION
a. ENROUTE DESCENT
(1) An enroute descent is a descent from an enroute altitude to the final approach of an established procedure without execution of the entire Instrument Approach Procedure prescribed in the FLIP Terminal publication. The type of final approach to be flown (Precision Approach Radar, Airport Surveillance Radar, Instrument Landing System, TACAN, etc.) should be understood by the pilot and controller prior to commencing descent. An enroute descent may be requested as follows: "REQUEST ENROUTE DESCENT TO (destination airport)". This service may also be initiated by the controller, in which case he must advise the pilot of his intention to provide this service. The pilot may refuse the service in favor of a published Instrument Approach Procedure, either High or Low Altitude. Controllers are not to authorize an enroute descent if other than normal vectoring delays are anticipated. When an enroute descent is authorized, the controller may not terminate it without the consent of the pilot, except as required by a Radar outage or other emergency situations.
(2)
It is the pilot’s responsibility to request a High Altitude approach if an enroute descent is not desired.

(3)
An enroute descent may be issued in a nonradar environment. However, Radar capability should exist which will permit the controller to vector the aircraft to the final approach course of a published High Altitude Instrument Approach Procedure or Precision Approach Radar/Airport Surveillance Radar approach. This procedure will not be used if other than normal vectoring delays are anticipated.

(4)
Prior to issuance of a descent clearance below the highest Initial Approach Fix altitude established for any High Altitude Instrument Approach Procedure for the destination airport, the controller will inform the pilot:

(a)  Type of approach to expect.

(b)
Radar vectors will be provided to the final approach course.

(c)
Current weather whenever the ceiling is below either 1,000 feet (1,500 feet for Air Force controllers) or the highest circling minimum (whichever is greater), or when the visibility is less than 3 miles.

b.
SFA-UHF SINGLE FREQUENCY APPROACH - This service is provided to single-piloted jet aircraft on an IFR flight plan during the hours of darkness or when the aircraft is in instrument weather conditions. The abbreviation "SFA" will be shown after the heading "COMMUNICATIONS" in the Enroute Supplement when procedures have been implemented by that station. Pilots receiving this service will not be required to change frequency from beginning of penetration to touchdown, except that pilots conducting an enroute descent are required to change frequency when control is transferred from the Air Route Traffic Control Center to the Terminal facility. Controllers may discontinue the service to all pilots who cancel IFR flight plans during daylight hours and to those pilots in level flight who cancel IFR flight plans at night.
　
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