Definition
An ATC altitude instruction directing the pilot to hold the assigned altitude during radar vectors to the final approach course, and to continue holding that altitude until the aircraft is established on the localizer. Once established on the localizer, the pilot may then descend in accordance with the published approach procedure (typically to the next published altitude or glideslope intercept).
Plain English
Stay at the altitude ATC just gave you. Don't start descending on the approach until your aircraft is actually tracking the localizer course inbound. Once you're on the localizer, you can begin the approach descent.
Context Anchor
Heard during radar vectors to an instrument approach, commonly in a clearance such as, “Maintain 3,000 until established on the localizer, cleared ILS Runway 27 approach.”
Derivation
Maintain comes from older words meaning “to hold.” Localizer comes from “localize,” meaning to place or fix something in a location. Together, the phrase means to hold the assigned condition until the aircraft is fixed on the runway-alignment guidance.
Why Pilots Care
Prevents premature descent or course changes that could compromise separation or place the aircraft outside protected airspace.
Intuition Check
Do not assume “cleared for the approach” automatically means you may descend right away. If ATC says “maintain until established on the localizer,” stay with the assigned condition until the aircraft is actually following the localizer course.
Example Sentence 1
The controller cleared us for the ILS Runway 27 approach with the instruction, "maintain three thousand until established on the localizer."
Example Sentence 2
The controller cleared us to maintain 3000 until established on the localizer so we would not descend below the segment minimums too soon.