. 
runway length available.

Mandatory Missed Approach
On all instrument approaches, execute an immediate missed approach:
. 
if a navigation radio or flight instrument failure occurs which affects the
ability to safely complete the approach in instrument conditions

. 
when on ILS final approach, in instrument conditions, and either the
localizer and/or glide slope indicator shows full deflection

. 
when the navigation instruments show significant disagreement and
visual contact with the runway has not been made

. 
when on an RNP based approach, and an FMC alerting message indicates
that ANP exceeds RNP

. 
when on a radar approach and radio communication is lost.

Landing Minima
Most regulatory authorities require visibility for landing minima. Ceilings are not required. There are limits on how far an aircraft can descend without visual contact with the runway environment when making an approach. Descent limits are based on a decision altitude/height DA(H) for approaches using a glide slope or certain approaches using a VNAV path; or a MDA(H) for approaches that do not use vertical guidance, or where a DA(H) is not authorized for use. Most agencies do not require specific visual references below AH.
Approach charts use the abbreviation DA(H) or MDA(H). DA(H) applies to Category I, II, and certain fail passive Category III operations. A decision altitude “DA” or minimum descent altitude “MDA” is referenced to MSL and the parenthetical height “(H)” is referenced to TDZE or threshold elevation. Example: A DA(H) of 1,440’ (200’) is a DA of 1,440’ with a corresponding height above TDZ of 200’.

When RVR is reported for the landing runway, it typically is used in lieu of the reported meteorological visibility.

Radio Altimeter (RA)
A radio altimeter is normally used to determine DH when a DA(H) is specified for Category II or Category III approaches, or to determine alert height (AH) for Category III approaches. Procedures at airports with irregular terrain may use a marker beacon instead of a DH to determine the missed approach point. The radio altimeter may also be used to cross check the primary altimeter over known terrain in the terminal area. However, unless specifically authorized, the radio altimeter is not used for determining MDA(H) on instrument approaches. It should also not be used for approaches where use of the radio altimeter is not authorized (RA NOT AUTHORIZED). However, if the radio altimeter is used as a safety backup, it should be discussed in the approach briefing.

Missed Approach Point (MAP)
A missed approach point is a point where a missed approach must be initiated if suitable visual references are not available to make a safe landing or the airplane is not is a position to make a safe landing.

Determination of a MAP
For approaches such as ILS or GLS, the DA(H) in conjunction with the glide slope is used to determine the MAP. For non-ILS or G/S out approaches, two methods for determining the MAP are acceptable in lieu of timing due to the accuracy of FMC positioning:
. 
when arriving at the MDA(H) or DA(H) in conjunction with a VNAV
path

. 
if not using a VNAV path, use of the map display to determine when the
airplane has reached the VDP or the MAP. The approach legs along with
distance and time to the missed approach waypoint are displayed on the
map.

Timing During Approaches
Since FMC use is appropriate for instrument approach navigation, timing is not the primary means to determine the missed approach point. The probability of multiple failures that would result in timing being the only method of determining the missed approach point is remote. However, some regulatory agencies may still require the use of timing for approaches. The timing table, when included, shows the distance from the final approach fix to the MAP.
For non-RNP capable FMCs, timing for instrument approaches is not necessary when DME-DME updating of FMC position is active.

For RNP capable FMCs, timing for instrument approaches is not necessary as long as there is no unable RNP alert displayed.

Instrument Landing System (ILS)
Arrival at the MAP is determined by reference to an altimeter. DA is determined by reference to the barometric altimeter, while DH is determined by reference to the radio altimeter.

Precision Approach Radar (PAR)
The MAP for a PAR is the geographic point where the glide path intersects the DA(H). Arrival at the MAP is determined by the pilot using the altimeter or as observed by the radar controller, whichever occurs first.

Instrument Approach using VNAV
When specifically authorized by the instrument procedure and regulatory authority, approaches may be flown to the following minima:
　
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