center point of the MSA, the applicable radius of the MSA,  a depiction of the sector(s), and the minimum altitudes  above mean sea level which provide obstacle clearance.  For conventional navigation systems, the MSA is normally  based on the primary omnidirectional facility on which the  IAP is predicated, but may be based on the airport reference  point (ARP) if no suitable facility is available. For RNAV  approaches, the MSA is based on an RNAV waypoint. MSAs  normally have a 25 NM radius; however, for conventional  navigation systems, this radius may be expanded to 30 NM  if necessary to encompass the airport landing surfaces.  Depicted on the Plan View of approach charts, a single  sector altitude is normally established. However when it is  necessary to obtain obstacle clearance, an MSA area may  be further divided with up to four sectors.  Another important altitude that should be briefed during  an IAP briefing is the FAF altitude, designated by the cross  on a non-precision approach, and the lightning bolt symbol  designating the glideslope/glidepath intercept altitude on  a precision approach. Adherence and cross-check of this  altitude can have a direct effect on the success and safety  of an approach.  Proper airspeed, altitude, and configuration, when crossing  the FAF of a non-precision approach, are extremely  important no matter what type of aircraft is being flown.  The stabilized approach concept, implemented by the  FAA within the SOPs of each air carrier, suggests that  crossing the FAF at the published altitude is often a critical  component of a successful non-precision approach,  especially in a large turbojet aircraft.  The glideslope intercept altitude of a precision approach  should also be included in the IAP briefing. Awareness of  this altitude when intercepting the glideslope can ensure  the flight crew that a “false glideslope” or other erroneous  indication is not inadvertently followed. Many air carriers  include a standard callout when the aircraft passes over the  FAF of the non-precision approach underlying the ILS. The  PM states the name of the fix and the charted glideslope  altitude, thus allowing both pilots to cross-check their  respective altimeters and verify the correct indications.  MDA—the lowest altitude, expressed in feet MSL, to which  descent is authorized on final approach or during circle-to ­ land maneuvering in execution of a standard instrument  approach procedure (SIAP) where no electronic glideslope  is provided.  Minimum Safe Altitudes are published for emergency use  on IAP charts. MSAs provide 1,000 feet of clearance over  all obstacles but do not necessarily assure acceptable  navigation signal coverage. The MSA depiction on the plan  view of an approach chart contains the identifier of the  Note: Pilots are cautioned to adhere to altitudes as  prescribed because, in certain instances, they may be used  as the basis for vertical separation of aircraft by ATC. If a  depicted altitude is specified in the ATC clearance, that  altitude becomes mandatory as defined above.  Approach waypoints, except for the missed approach  waypoint (MAWP) and the missed approach holding  waypoint (MAHWP), are normally FB WPs. Notice that in the  plan view in Figure 4-9, there are four FB WPs, but only the  circled WP symbol at PRINO is a FO WP. If flying manually to  a selected RNAV WP, pilots should anticipate the turn at a FB  WP to ensure a smooth transition and avoid overshooting  the next flight segment. Alternatively, for a FO WP, no turn  is accomplished until the aircraft passes the WP.  There are circumstances when a WP may be coded into the  database as both a FB WP and a FO WP, depending on how  the WPs are sequenced during the approach procedure. For  example, a WP that serves as an IAF may be coded as a FB  WP for the approach and as a FO WP when it also serves  as the MAWP for the missed approach procedure (MAP).  This is just one reason why instrument approaches should  be loaded in their entirety from the FMS and not manually  built or modified.  Prescribed altitudes may be depicted in four different  configurations: minimum, maximum, recommended, and  mandatory. The U.S. Government distributes approach  charts produced by the FAA. Altitudes are depicted on  these charts in the profile view with an underscore or  overscore, or both to identify them as minimum, maximum,  or mandatory, respectively.  4-19 Minimum Descent Altitude (MDA), Decision  Altitude (DA), And Decision Height (DH)  Minimum Safe/Sector Altitude  Final Approach Fix Altitude  Altitudes  • Recommended altitudes are depicted without an underscore or overscore. • Mandatory altitudes are depicted with the altitude value  both underscored and overscored. Aircraft are required  to maintain altitude at the depicted value (e.g., 5500).  • Maximum altitudes are depicted with the altitude value  overscored. Aircraft are required to maintain altitude  at or below the depicted value (e.g., 4800). • Minimum altitudes are depicted with the altitude value  underscored. Aircraft are required to maintain altitude  at or above the depicted value (e.g., 3000).