1-7 fix without attaining the MRA. On AeroNav Products charts,  the MRA is indicated by the symbol  and the altitude  preceded by “MRA” (e.g., “MRA 9300”).  [Figure 1-2] The minimum crossing altitude (MCA) is charted when  a higher MEA route segment is approached. The MCA is  usually indicated when a pilot is approaching steeply rising  terrain and obstacle clearance and/or signal reception is  compromised. In this case, the pilot is required to initiate a  climb so the MCA is reached by the time the intersection is  crossed. On AeroNav Products charts, the MCA is indicated  by the symbol  , and the Victor airway number, altitude,  and the direction to which it applies (e.g. “V24 8000 SE”). The maximum authorized altitude (MAA) is the highest  altitude at which the airway can be flown with assurance  of receiving adequate navigation signals. Chart depictions  appear as “MAA-15000.” When an MEA, MOCA, and/or MAA change on a segment  other than at a NAVAID, a sideways “T” ( ) is depicted  on the chart. If there is an airway break without the symbol,  one can assume the altitudes have not changed (see the upper  left area of  Figure 1-2 ). When a change of MEA to a higher  MEA is required, the climb may commence at the break,  ensuring obstacle clearance.  [Figure 1-4] Very high frequency omnidirectional ranges (VORs) are the  principal NAVAIDs that support the Victor and Jet airways.  Many other navigation tools are also available to the pilot.  For example, nondirectional beacons (NDBs) can broadcast  signals accurate enough to provide stand-alone approaches,  and DME allows the pilot to pinpoint a reporting point on the  airway. Though primarily navigation tools, these NAVAIDs  can also transmit voice broadcasts. Tactical air navigation (TACAN) channels are represented  as the two- or three-digit numbers following the three-letter  identifier in the NAVAID boxes. The AeroNav Products  terminal procedures provide a frequency-pairing table for  the TACAN-only sites. On AeroNav Products charts, very- high frequencies and ultra-high frequencies (VHF/UHF)  NAVAIDs (e.g., VORs) are depicted in black, while low  frequencies and medium frequencies (LF/MF) are depicted  as brown.  [Figure 1-5] Intersections along the airway route are established by a variety  of NAVAIDs. An open triangle   indicates the location of an  ATC reporting point at an intersection. If the triangle is solid  , a report is compulsory.  [Figure 1-4]  NDBs, localizers,  Navigation Features Types of NAVAIDs Identifying Intersections Figure 8-3. En route airport legend.  Figure 1-3.  En route airport legend.