VOR & Non-Precision Approaches
Non-precision approaches don't have vertical guidance — you fly to a Minimum Descent Altitude (MDA) and decide based on visual references. The most common is the VOR approach. Variants include LDA (offset localizer-type), SDF (simplified directional), and LOC back-course. The 30-day VOR receiver check (VODGA / PEDS) is required for VOR-based IFR ops.
VOR — Very High Frequency Omnidirectional Range
Ground-based radio nav using two signals (a 360° rotating signal + a fixed reference signal) to determine the radial the aircraft is on relative to the station. Accurate to ±1°.
Approach types:
- VOR off-airport facility: MAP identified by timing from FAF or DME distance. Approach plate has a timing table at the bottom.
- VOR on-airport facility: MAP identified by station passage. No FAF required. Higher accuracy.
VOR MON (Minimum Operational Network): backup network of VORs ensuring any VOR-equipped aircraft will receive a usable signal at or above 5,000 ft AGL anywhere in the contiguous US, with a MON airport (ILS or VOR approach) within 100 NM. Designed as a redundancy for GPS outages.
VOR receiver checks — VODGA / PEDS
For VOR-equipped aircraft used for IFR, a VOR equipment check must be performed within the preceding 30 days. Acceptable methods (mnemonic VODGA):
- V — VOT (VOR Test Facility, 108.0): 360° from / 180° to. Limit ±4°.
- O — Own check: On a Victor airway above a significant fix at least 20 NM from the VOR. Limit ±6°.
- D — Dual check: Two receivers tuned to the same VOR. Compared to each other. Limit ±4° between them.
- G — Ground check: At a designated airport spot per the Chart Supplement. Limit ±4°.
- A — Airborne check: Over a designated fix at a designated radial per the Chart Supplement. Limit ±6°.
All checks must be recorded with Place, Error, Date, Signature (PEDS) in the logbook.
DME — Distance Measuring Equipment
- Transponder-based device measuring slant range (not ground distance) from aircraft to ground station.
- UHF band (960-1,211 MHz), range up to 199 NM.
- Aircraft transmits interrogation on 1030 MHz; ground station replies on 1090 MHz; equipment times the round trip and computes distance.
- Slant range error: Directly over the station, DME displays the aircraft's altitude above the station rather than zero.
- Common combinations: VOR/DME (civilian), VORTAC (military, identical functionality).
- GPS may substitute for DME per AIM 1-2-3 if the GPS database is current.
Other localizer-type approaches
- LOC (Localizer-only): Non-precision. Lateral guidance only, no glideslope. Same accuracy as the localizer portion of an ILS.
- LDA — Localizer-type Directional Aid: Comparable accuracy to a localizer, but not aligned with the runway. Course width 3°-6°. Some have glideslopes. Identifier "I" + 3 letters. If misalignment ≤ 30°, straight-in minimums published; > 30°, only circling minimums.
- SDF — Simplified Directional Facility: Similar to LOC but wider beam (fixed at 6° or 12°). May be offset from runway centerline by up to 3°. 3-letter ID transmitted on the SDF frequency.
- LOC BC — Localizer Back Course: Uses the localizer signal in the opposite direction. Reverse sensing on a non-HSI CDI — "fly away from the needle." Never use the glideslope on a back course; disregard any GS indication. Service volume: 10° on either side to 18 NM, narrowing to 10 NM at 35°.
HSIs eliminate reverse sensing on back-course localizers because they rotate the CDI with the OBS-set course.