Monitoring of Navigation Facilities
VOR, VORTAC, and instrument landing system (ILS) facilities, as well as most NDBs and marker beacons installed by the FAA, are provided with an internal monitoring feature. Internal monitoring is provided at the facility through the use of equipment that causes a facility shutdown if performance deteriorates below established tolerances. A remote status indicator also may be provided through the use of a signal-sampling receiver, microwave link, or telephone circuit. Older FAA NDBs and some non-Federal NDBs do not have the internal feature, and monitoring is accomplished by manually checking the operation at least once each hour. FAA facilities, such as automated flight service stations (AFSSs) and ARTCCs/sectors, are usually the control point for NAVAID facility status. Pilots can query the appropriate FAA facility if they have questions in flight regarding NAVAID status, in addition to checking NOTAMs prior to flight, since NAVAIDs and associated monitoring equipment are continuously changing.
A navigational course guidance gap, referred to as an MEA gap, describes a distance along an airway or route segment where a gap in navigational signal coverage exists. The navigational gap may not exceed a specific distance that varies directly with altitude, from 0 NM at sea level to 65 NM at 45,000 feet MSL and not more than one gap may exist in the airspace structure for the airway or route segment. Additionally, a gap usually does not occur at any airway or route turning point. To help ensure the maximum amount of continuous positive course guidance available when flying, there are established en route criteria for both straight and turning segments. Where large gaps exist that require altitude changes, MEA “steps” may be established at increments of not less than 2,000 feet below 18,000 feet MSL, or not less than 4,000 feet at 18,000 MSL and above, provided that a total gap does not exist for the entire segment within the airspace structure. MEA steps are limited to one step between any two facilities to eliminate continuous or repeated changes of altitude in problem areas. The allowable navigational gaps pilots can expect to see are determined, in part, by reference to the graph depicted in Figure 2-49. Notice the en route chart excerpt depicting that the MEA is established with a gap in navigation signal coverage northwest of the Carbon VOR/DME on V134. At the MEA of 13,000, the allowable navigation course guidance gap is approximately 18.5 NM, as depicted in Figure 2-49. The navigation gap area is not identified on the chart by distances from the navigation facilities. Proper flight planning will help pilots prepare for MEA gaps by insuring that appropriate maps are available as they may need to dead reckon through the gap. Calculating the ground track (with adjustments for winds) before and after the gap will also help to stay on course when navigational course guidance is not available.
NAVAID Accuracy Check
The CFRs and good judgment dictate that the equipment of aircraft flying under IFR be within a specified tolerance before taking off. When approved procedures are available, they should be used for all equipment inspections.
VOR accuracy can be checked by using any of the following methods: VOR test facility signal (VOT), VOR checkpoint signs, dual VOR check, or airborne VOR check.
The VOT is an approved test signal and is located on an airport. This enables the pilot to check the VOR accuracy from the flight deck before takeoff. Listed below are the steps used for a VOT:
- Tune the VOR receiver to the VOT frequency. VOT frequencies can be found in the CS. [Figure 2-50] These frequencies are coded with a series of Morse code dots or a continuous 1020-cycle tone.
- On the VOR, set the course selector to 0° and the track bar (TB) indicator should read center. The TOFROM indicator should read FROM.
- Set the course selector to 180° and the TO-FROM indicator should read TO and the TB should then be centered.
Note: Determining the exact error in the receiver is done by turning the track selector until the TB is centered and noting the degrees difference between 180° or 0°. The maximum bearing error with the VOT system check is plus or minus 4° and apparent errors greater than 4° indicate that the VOR receiver is beyond acceptable tolerance.
VOR Checkpoint Signs
Many aerodromes have VOR checkpoint signs that are located beside the taxiways. [Figure 2-51] These signs indicate the exact point on the aerodrome that there is sufficient signal strength from a VOR to check the aircraft’s VOR receiver against the radial designated on the sign. Listed below are the steps to use at a VOR checkpoint:
- Tune the proper VOR frequency.
- Identify the VOR frequency.
- Set the published radial on the course deviation lindicator (CDI).
- Confirm that the TB is centered.
- Check the needle sensitivity by changing the omnibearing select (OBS) 10° each way.
- Set the reciprocal of the radial and check the TO-FROM flag change.
- The maximum permissible difference between aircraft equipment and the designated radial is 4° and 0.5 NM of the posted distance.
Dual VOR Check
If a VOT or VOR checkpoint is not available and the aircraft is equipped with dual VORs, the equipment may be checked against one another by tuning both sets to the VOR facility at the same time and noting the indicated bearings to that station. [Figure 2-52] A difference greater than 4° between the two VORs indicates that one of the receivers may be out of tolerance.
Airborne VOR Check
OR equipment can also be checked for accuracy while in flight by flying over a fix or landmark located on a published radial and noting the indicated radial. Variances of more than 6° from the published radial should be considered out of tolerance and not be used for IFR navigation.
NDB Accuracy Check
The pilot must identify an NDB before using it for navigation, and continuously monitor it while using it or an instrument approach. The lack of an IDENT may indicate that the NDB is out of service, even though it may still be transmitting (for instance for maintenance or test purposes). If an incorrect IDENT is heard, then the NDB should not be used.
RNAV Accuracy Check
RNAV accuracy checks may differ depending on the different type of equipment and manufacturer. When available, all written procedures should be followed.
Below is a list of generic checks that should be used when checking the accuracy of the system prior to flight.
- System initialization—pilots should confirm that the navigation database is current and verify that the aircrafts present position has been entered correctly.
- Active flight plan check—the active flight plan should be checked by comparing the aeronautical charts, departure and arrival procedures, and other applicable documents with the map display.
- Prior to takeoff—ensure that the RNAV system is available. If possible, check to see that the system is updating when aircraft position is changing.
Note: While in flight, continue to verify system accuracy by displaying bearing/range to a VOR/DME on the RNAV system and compare it to the actual RMI reading of that particular NAVAID.