Decision Altitude (DA) is a specific height during an instrument approach at which the pilot must either see the runway environment or immediately begin a go-around. It is expressed as altitude above mean sea level (MSL).
How It Works#
During an instrument approach in low visibility, the pilot flies a precise, guided descent called an instrument landing system (ILS) approach or similar precision approach. The aircraft descends along a defined vertical path toward the runway. DA marks the lowest point the pilot may descend to while still relying solely on instruments.
At DA, the pilot has one job: look outside and decide. If the pilot sees the runway environment (the runway, approach lights, or other specified visual references), the approach can continue. If not, the pilot must execute a missed approach, also called a go-around, immediately.
DA is defined by the approach procedure itself. Designers calculate it based on obstacle clearance, terrain, and navigation signal accuracy. A typical ILS Category I approach has a DA of 200 feet above the touchdown zone elevation, paired with a required visibility of 2,400 feet (or half a statute mile).
It is worth noting the difference between DA and a related term: Minimum Descent Altitude (MDA). MDA applies to non-precision approaches, where the pilot levels off and searches for the runway. DA applies to precision and approach procedures with vertical guidance, where the descent continues right to the decision point without leveling off.
Example in Aviation#
A regional airline crew is flying an ILS approach into a fog-covered airport. Visibility is reported at just over half a mile. As the aircraft passes through 200 feet above the touchdown zone, the captain calls "decision altitude." The first officer looks up from the instruments, spots the approach lighting system through the mist, and calls "runway in sight." The captain continues the landing.
Had neither pilot seen the required visual references at that moment, the crew would have immediately applied full power and followed the published missed approach procedure, climbing away from terrain.
Why It Matters#
DA is a hard boundary, not a suggestion. Descending below DA without the required visual references is a serious regulatory violation under 14 CFR §91.175 and a genuine safety threat. Controlled flight into terrain (CFIT) accidents have resulted from pilots pressing below DA in hopes of finding the runway.
Understanding DA also helps student pilots grasp how precision approaches are designed around safety margins. Every foot of altitude has been calculated to keep the aircraft clear of obstacles. Respecting DA is one of the most discipline-critical habits in instrument flying.
Key Takeaways#
- DA is the lowest altitude a pilot may reach before committing to land or go around.
- The pilot must see the runway environment at or before reaching DA.
- DA applies to precision approaches and approaches with vertical guidance.
- Descending below DA without visual contact is a regulatory violation and a safety hazard.
- DA is expressed in altitude MSL; the related term MDA applies to non-precision approaches.