Definition
One of the legs of a standard Terminal Arrival Area (TAA) at an RNAV (GPS) approach, positioned to the left of the final approach course when viewed from above. The left base area provides a defined sector within which an arriving aircraft can transition from the en route environment onto the approach without needing radar vectors, with published minimum altitudes that guarantee terrain and obstacle clearance.
Plain English
A pie-slice-shaped arrival area on the left side of the approach path. If you arrive into the airport from this sector, you have a published safe altitude and a defined route onto the final approach.
Context Anchor
Seen on instrument approach charts that use a terminal arrival area, especially in the plan view around the approach fixes.
Derivation
From the traffic pattern term 'base leg' — the leg flown perpendicular to the runway just before turning final. In a TAA, the 'left base' sector occupies a similar position relative to the final approach course, just on a much larger scale.
Why Pilots Care
It provides a protected area for descent and alignment so the pilot can reach the final approach fix at the proper altitude and speed.
Intuition Check
Left base does not mean the left side of the runway surface or a parking area. Here it means the left-side arrival sector used to join an instrument approach from the side.
Example Sentence 1
Arriving from the northwest, the pilot identified the aircraft's position as being within the left base sector of the TAA and descended to the published minimum altitude.
Example Sentence 2
Flying the left base of the TAA allowed a continuous descent to the FAF without leveling off.