Step-Down Fix / Minimum Altitude
Vertical profile with crossing fixes and restrictions that control the descent.
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The profile reads like the profile view of an approach chart: fixes from left to right toward the runway, each with its charted altitude. An underlined value is a minimum (at or above), an overlined value a maximum (at or below), a value with both lines is mandatory, and two stacked values form a block (window). The purple stepped line is the lowest altitude allowed: after the FAF you may descend to the next step-down fix altitude, but not below it until you have passed that fix — then to the MDA.
Pick a technique and watch the amber path. Dive and drive descends to each altitude and levels off, ending in a long level segment at the MDA close to the ground. One constant rate is smooth but can cut below a step-down altitude. The two CDFA techniques fly the vertical descent angle: start at the FAF (when a step-down fix controls the angle, first a shallower rate to that fix, then the VDA rate) or start at a computed point past the FAF. A CDFA reaches the MDA without levelling: land if you have the required references, otherwise go around from there without descending below the MDA.
Press Practice to fly the final yourself. Set the descent rate on the V/S tape (drag it, or use the arrow keys on it), with the slider, or press Level off; violations are circled as they happen, and the readout tells you where you reached the MDA compared with the visual descent point (V). The fix row shows how each fix is identified — DME from a station or the navigator’s distance to the runway; if you cannot identify a step-down fix, the higher MDA applies.
Assumption: schematic approaches — fictional, never real procedures and never for navigation (one is built on the worked example of AC 120-108A, an obsolete procedure, with an illustrative MDA). Still air, constant groundspeed, instant changes of descent rate; in Practice the aircraft levels off at the MDA by itself (as with the altitude preselected) but not at step-down altitudes; altitudes are held exactly (real tolerances are not modelled); the VDA uses a flat earth (the design formula adds the earth’s curvature, a difference far below 0.01° here).
Charted altitudes (AIM 5-4-5 b): minimum altitude underscored — at or above; maximum overscored — at or below; mandatory both — at; recommended neither — for descent planning; the TPP legend adds the mandatory block altitude. Step-down altitudes in the final segment apply only to lines of minimums with an MDA; outside the final segment they apply whatever line you fly (AIM 5-4-5 b 5). When a fix in the final segment cannot be identified, a second, higher set of minimums may be published (AIM 5-4-20 e).
Vertical descent angle (AC 120-108A 3.3, 3.3.1; Order 8260.3G 2-6-4): from the FAF altitude to the threshold crossing height, or from a step-down fix altitude when the path from the FAF would cross that fix too low; optimum 3.00°, minimum 2.75° for straight-in minimums, maximum by category (table 2-6-1: CAT A 6.40° at 80 kt or less and 5.70° at 81–90 kt, B 4.20°, C 3.77°, D 3.50°). Gradient = 6,076 ft × tan(angle) per NM; descent rate = gradient × groundspeed ÷ 60 (the same descent calculation as the IFR Trainer’s Descent module). The VDA is advisory, not vertical guidance (AIM 5-4-5 k).
Techniques: dive and drive (AC 120-108A 2.1); CDFA = a continuous descent from at or above the FAF to about 50 ft above the threshold with no level-off (2.4); descent at the FAF to the step-down fix, then the VDA rate (3.5.1); descent point past the FAF = (FAF altitude − TDZE − TCH) ÷ the VDA gradient from the runway (3.5.2, 3.5.3: 2,000 − 81 − 46 = 1,873 ft ÷ 316 ft/NM = 5.9 NM = 2.8 DME in the AC example, reproduced in scenario 6); never below the MDA on a go-around from a CDFA (3.7). VDP (AIM 5-4-5 h): where the VDA path from the TCH meets the MDA.
Checks while flying: below the lowest altitude allowed before passing the fix that sets it; a maximum, mandatory or block altitude not met at the crossing; still above the MDA at the MAP; a descent rate above 1,000 ft/min within 1,000 ft of the runway (IPH FAA-H-8083-16B ch. 4 “Maximum Acceptable Descent Rates”).
- Aeronautical Information Manual (effective 9 July 2026), para. 5-4-5 b (minimum altitudes underscored, maximum overscored, mandatory both, recommended none), b 5 (step-down fix altitudes in the final segment apply to lines of minima identified as an MDA; outside the final segment on every line), h (visual descent point), k (vertical descent angle: advisory, VDA/TCH, Visual Segment – Obstacles)
- Aeronautical Information Manual, para. 5-4-20 e (two sets of minimums when a fix in the final segment can or cannot be identified)
- FAA Advisory Circular 120-108A, Continuous Descent Final Approach (2023): 2.1 dive and drive, 2.4 definition of CDFA, 3.3 and 3.3.1 VDA from the FAF or a controlling step-down fix (optimum 3.0°, minimum 2.75°), 3.5 descent at the FAF and descent point past the FAF with the worked example (185 ft/NM = 370 fpm; 2.98° = 316 ft/NM = 632 fpm at 120 kt; 1,873 ft ÷ 316 ft/NM = 5.9 NM = 2.8 DME), 3.7 derived decision altitude, 3.8 decision approaching the MDA
- FAA Instrument Procedures Handbook, FAA-H-8083-16B (2017), ch. 4: Vertical Navigation (the VDA is advisory; respect every step-down fix and the MDA), Descent Rates and Glidepaths for Nonprecision Approaches (more than about 1,000 fpm unacceptable below 1,000 ft AGL)
- FAA Order 8260.3G (2024-07-01), para. 2-6-4 (VDA from the PFAF or step-down fix to TCH; optimum 3.00°, minimum 2.75°) and table 2-6-1 (maximum GPA/VDA by category)
- U.S. Terminal Procedures Publication legend, cycle effective 1 October 2026: profile view (VDA and TCH, altitude restrictions at step-down fixes, Maltese cross FAF, visual descent point, mandatory block altitude)
- Cross-tool: the descent gradient and rate are the same calculation as the IFR Trainer's Descent module, identical results checked
- Model choices of this tool (stated on the page): fictional schematic approaches (one built on the AC 120-108A example with an illustrative MDA); still air and constant groundspeed; altitudes held exactly; flat-earth VDA