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.  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:  A navigational course guidance gap, referred to as an  MEA gap, describes a distance along an airway or route  message instructing to divert to Las Vegas, requiring a  change in the flight plan as highlighted on the depicted  chart excerpt.  During the flight deck review of the high and low altitude  en route charts, the pilot determines that the best course  of action is to fly direct to the MIRAJ waypoint, 28 DME  northeast of the Las Vegas VORTAC on the 045° radial. This  places the aircraft 193 NM out on a 259° magnetic course  inbound, and may help to avoid diverting north, allowing to  bypass the more distant originating and intermediate fixes  feeding into Las Vegas. The pilot requests an RNAV random  route clearance direct MIRAJ to expedite the flight. Denver  Center comes back with the following amended flight plan  and initial clearance into Las Vegas:  “Marathon five sixty four, turn right heading two six zero,  descend and maintain one six thousand, cleared present  position direct MIRAJ.”  The latitude and longitude coordinates of the aircraft’s  present position on the high altitude chart is N36 19.10  and W110 40.24 as the course is changed. Notice the GPS  moving map (upper left), the FMS control display unit  (below the GPS), and FMS map mode navigation displays  (to the right of the GPS) as the flight is rerouted to Las Vegas.  For SA, the pilot makes note that the altitude is well above  any of the OROCAs on the direct route as the flight arrives  in the Las Vegas area using the low altitude chart.  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.  2-29  1. 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.  Navigational Gaps  VOT  VOR Accuracy  NAVAID Accuracy Check  Monitoring of Navigation Facilities 