Initial approach altitude: Domestic and supplemental operations

This regulation sets minimum altitudes for Part 121 aircraft beginning instrument approaches.

Standard rule: When flying to a radio navigation facility under IFR, you cannot descend below the published minimum altitude for initial approach until you've definitively established your position over that facility. This prevents premature descent when you're not certain of your location.

Special case: When conducting approaches under §121.657(d) (which involves certain alternate airport scenarios), stricter rules apply. You cannot even begin the instrument approach until you've confirmed arrival over the facility. Once established, you must stay at whichever is *lower*: 1,000 feet above the cloud tops or the Administrator's published minimum altitude for that approach segment.

The key practical point is confirming your position before descending. The regulation prevents flying into terrain or obstacles by ensuring positive navigation fixes before leaving safe altitudes, with extra caution required in the alternate airport situation.

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*