Definition
A low-power non-directional beacon (NDB) installed at the outer marker position of an instrument landing system (ILS) approach. It transmits a continuous radio signal that the aircraft's automatic direction finder (ADF) can home in on, allowing the pilot to navigate to the final approach course and identify the outer marker fix.
Plain English
A small radio beacon placed at the start of the final approach path on an ILS. The aircraft's direction-finding equipment can point straight at it, helping the pilot line up with the runway and confirm they have reached the outer marker.
Context Anchor
Seen in instrument approach equipment discussions, especially older ILS approaches that use marker beacons or compass locators to identify approach points.
Derivation
Locator' because it helps the aircraft locate the final approach course. 'Outer marker' because it sits at the outer marker position on the ILS — the point where the glideslope is normally intercepted, several miles from the runway threshold.
Why Pilots Care
It confirms the aircraft has reached the correct point to begin descent on the glide slope and provides reliable position information even when DME or GPS is unavailable.
Grounding Statement
Picture the airplane flying inbound on an ILS approach and receiving a radio signal from a small transmitter on the ground that marks a specific point ahead of the runway.
Intuition Check
Do not read “locator outer marker” as just any outside marker. In instrument flying, it means a specific radio aid tied to the outer marker point on an ILS approach.
Example Sentence 1
Cleared for the ILS, the pilot tuned the ADF to the locator outer marker and tracked inbound to intercept the final approach course.
Example Sentence 2
On the approach plate the locator outer marker was shown 4.5 miles from the runway threshold.