Nonprecision Approach

8.nonprecision-approach. Nonprecision Approach

A nonprecision approach (NPA) is an instrument approach procedure that provides lateral course guidance but does not include precise vertical (glidepath) guidance certified to ILS standards. Examples include the VOR, NDB, LOC (localizer-only), LOC-BC (back course), LDA, SDF, GPS/RNAV (LNAV), and VOR/DME approaches. By contrast, precision approaches (ILS, PAR, GLS) and approaches with vertical guidance (LPV, LNAV/VNAV) provide an electronic glidepath.

Segments of the Approach

Every instrument approach is divided into five segments:

• Feeder route — transitions from the en route structure to the IAF.
• Initial approach segment — begins at the Initial Approach Fix (IAF) and positions the aircraft for the intermediate segment.
• Intermediate segment — begins at the Intermediate Fix (IF); aircraft is configured and slowed.
• Final approach segment — begins at the Final Approach Fix (FAF), depicted on nonprecision charts as a Maltese cross. Descent to the MDA occurs here.
• Missed approach segment — begins at the Missed Approach Point (MAP).

Minimums: MDA vs. DA

The defining characteristic of a nonprecision approach is the use of a Minimum Descent Altitude (MDA) rather than a Decision Altitude (DA). The pilot may descend to, but not below, the MDA until the requirements of 14 CFR 91.175(c) are met — namely, the aircraft is in a position to land using normal maneuvers, flight visibility meets the published minimum, and at least one of ten specified visual references (approach lights, threshold, REIL, VASI, runway, etc.) is distinctly visible and identifiable.

If those conditions are not met when the MAP is reached, the published missed approach must be flown.

Descent Techniques

Two descent profiles are common:

• Dive-and-drive — descend at or near Vdes to MDA promptly after the FAF, then level off and fly to the MAP. Increases workload and CFIT risk; less common in modern operations.
• Constant Descent Final Approach (CDFA) — a stabilized, continuous descent from the FAF to a derived Decision Altitude (DDA) typically 50 ft above the MDA, treating the approach much like a precision approach. CDFA is now the FAA-recommended technique because it reduces accident risk.

A target descent rate is computed from the FAF altitude, MDA, distance, and groundspeed. Rule of thumb:

• Required descent gradient (ft/NM) = (FAF alt − TDZE) ÷ FAF-to-threshold distance
• Vertical speed (fpm) ≈ groundspeed (kt) × descent gradient (ft/NM) ÷ 60

For a typical 318 ft/NM gradient at 90 kt: 90 × 318 / 60 ≈ 477 fpm.

Identifying the FAF and MAP

• The FAF is the Maltese cross on the profile view.
• The MAP may be defined by time from the FAF (using a published time/groundspeed table), by a DME fix, by a GPS waypoint, or by station passage on a VOR/NDB final.
• When timing the MAP, start the clock at the FAF and use groundspeed (corrected for wind), not airspeed.

Step-Down Fixes

Many NPAs include step-down fixes between the FAF and MAP that allow lower altitudes once the fix is identified. Without the equipment to identify the step-down fix, the higher minimum applies. The chart will list separate minimums (e.g., "S-LOC 24" vs. a higher MDA without DME).

Visibility and the 91.175 "Dive Below" Rule

Under 91.175(c)(3)(ii), once visual references are acquired, descent below MDA may continue only if the aircraft remains in a position from which a normal descent to landing on the intended runway can be made. Approach lights alone allow descent only to 100 ft above TDZE, unless the red terminating bars or red side row bars are also distinctly visible.

Procedure Summary

1. Brief the approach: frequencies, courses, FAF, MDA, MAP, missed approach, minimum safe altitudes.
2. Cross the IAF; complete the procedure turn or course reversal if required.
3. Configure (gear, flaps, approach speed) on the intermediate segment.
4. At the FAF, start the clock, begin descent, and report as required.
5. Cross step-down fixes at or above charted altitudes; descend to MDA (or DDA for CDFA).
6. At the MAP, if visual references and 91.175 criteria are met, land; otherwise, execute the missed approach immediately — climb on runway heading or as published, then proceed to the holding fix.

Common Errors

• Descending below MDA without required visual references.
• Failing to start timing at the FAF.
• Using indicated airspeed instead of groundspeed for MAP timing.
• Beginning the missed approach climb before reaching the MAP, which can cause obstacle clearance issues since the missed approach surface assumes the climb begins at the MAP.