LOC (Localizer)

The Localizer transmitter is sited at the far end of the runway from the approach direction, broadcasting two overlapping VHF directional patterns — typically modulated at 90 Hz on one side of the centreline and 150 Hz on the other — that the aircraft's Localizer receiver compares to produce a left/right course deviation indication. When the aircraft is on the runway extended centreline, the two modulation depths are balanced and the course deviation indicator (CDI) reads centred; deviation off centreline produces a CDI deflection proportional to the angular displacement, with the deflection sensitivity being approximately four times sharper than a typical VOR course (full-scale deflection at approximately 2.5 degrees on either side of centreline for a Localizer versus 10 degrees for a VOR). This higher sensitivity is what makes Localizer guidance suitable for precision-approach lateral tracking down to decision altitude.

The operational distinction between an ILS approach and a LOC approach is significant for both training and operational use. An ILS approach provides both Localizer (lateral) and Glideslope (vertical) guidance, supporting precision approach minima at Category I (typically 200 ft decision height and 1/2-mile visibility), Category II (100 ft DH, RVR 1200), and Category III (DH below 100 ft, RVR as low as 0 for autoland-equipped Category IIIb/c operations). A LOC approach, by contrast, provides only the Localizer lateral guidance with no electronic vertical guidance; the pilot uses published step-down altitudes between the Final Approach Fix and the Missed Approach Point to manage the vertical profile, and the approach is categorised as a non-precision approach with higher minima — typically a Minimum Descent Altitude (MDA) rather than a Decision Height (DH), and visibility minima around 3/4 mile or higher depending on aircraft category and approach procedure design.

LOC approaches arise in two principal contexts. First, where an airport has installed a Localizer transmitter but no Glideslope — either because the runway terrain or approach obstruction environment prevents a standard Glideslope installation, or because the airport authority elected to install LOC-only capability for cost or operational reasons. Second, where an ILS is published as the primary procedure but the Glideslope is reported out of service or unreliable — in which case ATC may issue "glideslope unusable" or the NOTAM may indicate the glideslope is out, and the pilot flies the procedure as a LOC-only approach using the published LOC minima.

LOC back course (LOC BC) approaches are a distinct procedure type using the Localizer signal in the opposite direction from the standard LOC approach. The Localizer transmits a directional pattern; the strong front-course lobe supports normal LOC approaches, while the weaker back-course lobe (transmitted toward the opposite runway end) can support an approach to the opposite runway threshold where published. LOC BC approaches present a specific pilot-skill challenge: the course deviation indicator may operate "reverse-sensed" (deflection opposite to the corrective input required) depending on the aircraft's avionics implementation, and pilot recognition of which sense applies is a standard IR-syllabus training point. Modern HSI (Horizontal Situation Indicator) and EFIS displays typically auto-resolve the reverse-sense issue when the inbound course is correctly set, but in older avionics the pilot must apply the reverse-sense convention manually.

The global trend over the past two decades has been steady replacement of LOC and ILS infrastructure with GNSS-based approaches (RNAV/RNP) at airports where the older ground-based aids would otherwise have required reinvestment. RNAV approaches with LPV minima (under WAAS in the United States) provide equivalent or better lateral and vertical guidance than ILS Category I without the cost of ground transmitter maintenance, and many airports worldwide have decommissioned LOC-only approaches when an RNAV equivalent became available. However, ILS and LOC infrastructure remains dominant at airline-serving airports, retains its primary-status in many national instrument procedure portfolios, and continues to be a core element of contemporary IR syllabi.

For flight schools running Instrument Rating syllabi, the LOC and ILS approach distinction is a foundational training topic that recurs throughout the IR module and into the practical test. Students must be able to recognise from the approach chart whether the procedure provides Glideslope guidance (ILS with vertical bar in the profile view) or LOC-only guidance (no glideslope bar; published step-down altitudes only), brief the approach with the correct minima category (DH for ILS, MDA for LOC), and execute the approach with the appropriate vertical profile management technique. Schools that treat ILS and LOC as the same procedure with different minima typically produce students who struggle on practical tests when examiners specifically test the LOC-only scenario or the inadvertent glideslope-failure recognition during an ILS.

For IR training equipment, the LOC and ILS receiver capability in the training aircraft fleet is generally co-located in the same NAV receiver (the so-called "VOR/LOC" receiver), but the Glideslope receiver is a separate component with its own serviceability state. Schools should track Glideslope receiver serviceability separately from LOC and VOR receiver serviceability — a Glideslope failure removes ILS-precision-approach capability from that aircraft without affecting its LOC and VOR navigation utility, and student progression decisions need this granular fleet status.

Aviatize's training management module supports the LOC and ILS approach distinction at the lesson-plan level. The IR syllabus can specify required exposure to both procedure types — ILS approaches with full guidance, LOC approaches with no glideslope, and the inadvertent-glideslope-failure scenario where a planned ILS reverts to LOC mid-approach — with competency-based assessment captured against each. The post-lesson record captures which procedure types the student has flown, supporting the chief instructor's confidence that the student has the breadth of approach experience the practical test expects.

The maintenance control module tracks Glideslope receiver serviceability separately from LOC and VOR receiver status, surfacing the affected aircraft's ILS-precision-approach capability to instructors during pre-dispatch review. The smart planning and booking module uses this serviceability data to suggest appropriate training aircraft when the lesson plan specifies an ILS-required exercise versus a LOC-acceptable exercise, supporting fleet utilisation across the IR cohort.