IFR
ILS glideslope, localizer & DA/DH
Glideslope
- Standard/nominal ILS glideslope angle: 3° above horizontal.
- Steeper glideslopes (commonly up to about 4°–4.5°, and higher still on some dedicated helicopter or steep/noise-abatement approach procedures) may be published where obstacles or noise abatement require it — always fly the published angle, never assume 3°.
Localizer
- Provides lateral guidance aligned with the runway centreline.
- Course width is designed so that full-scale deflection corresponds to a roughly fixed distance either side of the runway centreline at the threshold — the angular width therefore varies with runway length rather than being one fixed number of degrees for every installation.
Decision altitude/height (DA/DH)
- The altitude/height on a precision or APV approach at which a missed approach must be initiated if the required visual reference has not been established.
- Contrast with MDA/MDH (minimum descent altitude/height), used on non-precision approaches — a floor you may level off at and continue toward, rather than a single go/no-go instant.
Source: ICAO Annex 10 / Doc 8168 – ILS characteristics & approach criteria