ILS approaches: one-page summary
FAA procedures (AIM, 14 CFR 91.175). Explanations, worked examples and practice: pilotscafe.com/Understanding-ILS/
The parts and their numbers
| Localizer | 108.10–111.95 MHz, odd tenths. ID: I + three letters. 700 ft wide at the threshold (about 5°, full scale at 2.5°). Usable to 18 NM within 10°, 10 NM within 35°. The OBS doesn't move the needle. |
| Glideslope | 329.15–335.00 MHz, paired with the localizer. Usually 3°; 1.4° wide (full scale ±0.7°). Usable to 10 NM. |
| Range | An outer marker or a substitute (DME, GPS, radar, a VOR or NDB fix, a compass locator) marks the LOC final approach fix. A middle marker is no longer required. |
| Lights | 2,400–3,000 ft for precision runways. Red bars: ALSF-1, SALS and SALSF (terminating bars); ALSF-2 (side row bars). |
Markers
| Marker | Light | Tone | Where |
|---|---|---|---|
| OM | Blue | Dashes, 400 Hz | 4–7 mi; glideslope intercept |
| MM | Amber | Dot-dash, 1,300 Hz | ~3,500 ft; ~200 ft on the GS |
| IM | White | Dots, 3,000 Hz | CAT II decision height |
| BC | White | Dot pairs, 3,000 Hz | Back course final fix |
Categories (AIM 1-1-9)
| DH | RVR | |
|---|---|---|
| CAT I | 200 ft | 2,400 (1,800 with TDZ and CL lights, or FD, AP or HUD) |
| SA CAT I | 150 ft | 1,400, HUD to DH |
| CAT II | 100 ft | 1,200 (1,000 with autoland or HUD to touchdown, if authorized) |
| CAT IIIa / b / c | <100 / <50 / none | 700+ / 150 to <700 / none |
CAT I is a DA (MSL); CAT II and III a DH on the radar altimeter. CAT II and III: a PIC with a CAT II or III authorization, an instrument-rated SIC, the equipment and an approved manual (61.3, 91.189, 91.191).
At the DA (91.175(c))
Go below the DA only with all three:
- Continuously in position to land with a normal descent and normal maneuvers.
- Flight visibility at least what the approach requires.
- At least one reference distinctly visible: the approach lights, but no lower than 100 ft above the TDZE unless the red terminating or side row bars are also visible; the threshold, its markings or lights; REIL; a PAPI or VASI; the touchdown zone, its markings or lights; the runway, its markings or lights.
Not met at the DA, or lost below it before touchdown: go missed (91.175(e)).
When something is out
- Localizer: no ILS approach.
- Glideslope: fly the LOC line: an MDA, and the step-down fixes now apply.
- Approach lights: ILS line +¼ mi; on RVR 1,800/2,000/2,200 ILS lines, ALSF, MALSR or SSALR out → RVR 4,000/4,500, TDZ or CL lights out → RVR 2,400. Other straight-in lines +½ mi (ALSF, MALSR, SSALR) or +¼ mi (MALS, MALSF, SSALS and similar). Chart notes override the table; circling unaffected.
- "DME required for LOC": no DME, no LOC line, unless an IFR GPS stands in (AIM 1-2-3); the localizer still gives the final course.
- A marker: use a listed substitute.
Traps
- False glideslopes lie above the real one (9°–12°): intercept from below at the lightning-bolt altitude and check your altitude at the fix.
- Plain CDI reverses inbound on the back course and outbound on the front course. An HSI set to the front course reads correctly. Ignore the glideslope on a back course.
- Step-down fixes on final apply to the LOC line only.
- Critical areas are protected only by an operating tower, below an 800 ft ceiling or 2 mi visibility; never at an uncontrolled airport.
- CAT I signals aren't flight-checked below DA − 100 ft.
Descent rate
ft/NM × groundspeed ÷ 60. 3.0° = 318 ft/NM (478 fpm at 90 kt); 3.1° = 329 ft/NM (494 fpm).