Decision Height (DH): Aviation Glossary – Comprehensive Guide

Definition

Decision Height (DH) is a critical concept in instrument approach procedures, especially on precision approaches like ILS, MLS, and PAR. Defined by ICAO Annex 6 and the FAA AIM, DH is the specific height above the runway touchdown zone elevation (TDZE) where the pilot must decide—based solely on visual references and visibility conditions—whether to land or execute a missed approach. If required runway environment cues (like threshold lights, markings, or approach lighting) and visibility are present at DH, the approach can be continued. If not, a missed approach is mandatory, with no delay or further descent.

DH is always referenced to the highest point in the first 3,000 feet of the runway surface (TDZE) and expressed in feet above ground level (AGL). This ensures the pilot’s vertical position is directly relevant to the landing threshold, maximizing safety. DH is more than a number: it is the legal and operational boundary in instrument approaches, meticulously established during procedure design for obstacle clearance and navigational integrity.

Regulatory Context and Source References

The regulatory framework for DH is robust and harmonized globally. In the U.S., 14 CFR §91.175(c) outlines the conditions for descent below DH:

1. The aircraft must reach DH/DA.
2. Required flight visibility as published must be met.
3. At least one of ten specific runway environment visual references must be visible.

ICAO’s Annex 6 and Doc 8168 mirror these requirements, mandating that DH is the lowest height a missed approach must be initiated unless the runway environment is visible and the aircraft is in a safe position to land. EASA CS-OPS and other national authorities follow ICAO’s definitions. Approach charts (FAA, Jeppesen, etc.) always display DH or DA(H) for each precision approach, giving pilots clear regulatory guidance.

Operational guidance from the FAA, ICAO, and national bodies stresses that the missed approach must be executed immediately at DH if visual cues are not present. This strict regulatory context underlines DH’s safety-critical nature.

How Decision Height Is Used in Aviation Operations

In all forms of instrument flight, DH is the operational pivot for precision approaches. Its purpose is to provide a standardized, objective point for pilots operating in IMC to make the critical land/go missed decision.

During a precision approach:

• The aircraft descends along a defined glide path.
• The pilot monitors altimeter readings and approach indications, cross-checking with published minimums.
• On reaching DH, the pilot must immediately transition from instrument to visual cues and decide whether to land or initiate a missed approach.

In multi-crew cockpits, the pilot monitoring (PM) calls out “Minimums” at DH, prompting the pilot flying (PF) to confirm visual cues or call for a missed approach. Modern avionics provide aural and visual “minimums” alerts, reducing error risk. Airlines may also impose higher “operational minimums” than those published, based on crew experience or aircraft equipment.

DH ensures worldwide uniformity: regardless of airport, pilots can rely on a safe, standardized procedure for the landing commitment moment.

Decision Height vs. Related Terms (DA, MDA, MAP)

Understanding DH’s relationship to Decision Altitude (DA), Minimum Descent Altitude (MDA), and Missed Approach Point (MAP) is crucial:

• Decision Height (DH): Height above TDZE on precision approaches (ILS, MLS, PAR) with vertical/lateral guidance. Expressed in feet AGL.
• Decision Altitude (DA): Equivalent to DH but expressed as altitude above mean sea level (MSL). Many charts list both (e.g., DA(H) 216 (200)).
• Minimum Descent Altitude (MDA): For non-precision approaches (e.g., VOR, NDB), with no vertical guidance. Aircraft must level off at MDA until the MAP.
• Missed Approach Point (MAP): On a precision approach, coincides with DH/DA; on non-precision approaches, it is a defined location (by DME, time, or fix) where the land/go missed decision is made.

Misunderstanding these definitions can result in regulatory violations and safety risks. ICAO, FAA, and EASA harmonize these terms for global consistency.

Determining and Interpreting Decision Height on Approach Charts

Correctly identifying DH on approach charts is essential:

• Minimums Section: For precision approaches, the minimums line is labeled “DA(H)” or “DA/DH.” The first value is DA (MSL), followed by DH (AGL), e.g., “DA(H) 216 (200).”
• Multiple Lines: Charts list separate minimums for different approach types or aircraft categories.
• Cross-Check: Always verify DH against briefing material, airport diagrams, and NOTAMs for temporary changes.

Approach designers set DH for each procedure based on obstacle clearance, nav aid performance, and visual segment availability. The goal: guarantee that at DH, a safe landing or missed approach is always possible.

Operational Procedure at Decision Height

Reaching DH is a moment of maximum workload and critical decision-making:

1. Assess Flight Visibility: Is it at or above the published minimum from the cockpit?
2. Visual Reference: Is at least one published runway environment cue distinctly visible and identifiable?
3. Decision: If both are met, continue to land. If not, execute a missed approach immediately.

If only the approach lighting system is visible, descent is permitted only to 100 feet above TDZE (see “100-Foot Rule”). In multi-crew operations, SOPs require clear callouts at DH. In single-pilot ops, pre-brief and use of autopilot minimums alerts help manage workload.

Visual and Environmental Cues at DH

The successful transition from instruments to visual at DH depends on detecting the correct runway environment features:

• Runway threshold, markings, or lights
• Approach lighting system
• Runway End Identifier Lights (REIL)
• Touchdown zone or its markings/lights
• Runway lights
• Precision approach path indicators (VASI, PAPI)

ICAO and FAA specify these ten cues in regulations. Approach lighting systems are especially critical in poor visibility, with “decision bars” providing extra assurance near the runway.

Pilots must review the lighting system at their destination via approach charts and NOTAMs, as not all airports have full visual aids. Poor weather or lighting failures may necessitate a missed approach even if DH is reached.

Approach Lighting Systems and the “100-Foot Rule”

Approach Lighting Systems (ALS), detailed in ICAO Annex 14 and FAA AC 150/5340-30, are crucial in low-visibility approaches. The “100-Foot Rule” allows pilots who see only the ALS at DH to continue descending, but only to 100 feet above the TDZE. By that point, additional visual cues (threshold, markings, or lights) must be visible to land.

If the ALS includes red terminating or side row bars (e.g., ALSF-2), and these are visible, descent and landing can proceed. Pilots must know their destination’s ALS configuration and not mistake approach lights for runway cues.

Worked Examples and Use Cases

Example 1: Category I ILS Approach

A pilot flies an ILS to Runway 09, DA(H) 220 (200), visibility 1/2 mile. At 200 feet AGL, only the approach lights are visible. The pilot may descend to 100 feet above TDZE but must go missed if no other visual cues appear.

Example 2: ALSF-2 Lighting with Red Bars

On a foggy approach, the pilot sees red terminating bars at 100 feet above TDZE. Because these are approved cues, descent and landing may continue.

Example 3: Non-Precision (MDA/MAP)

On a VOR approach, MDA is 540 feet MSL (400 AGL) and MAP is at the threshold. The pilot must maintain MDA until MAP; if visual cues are lacking at MAP, a missed approach is required.

Example 4: RNAV (GPS) Approach (LPV)

On an RNAV approach with DA(H) 340 (250), the land/go missed decision is made at 250 feet above TDZE as per DH/DA logic.

Tips, Pitfalls, and Safety Considerations

Tips:

• Brief the approach lighting and missed approach procedure in advance.
• Use avionics “minimums” alerts.
• In multi-crew ops, clarify callouts and division of duties.

Pitfalls:

• Descending below DH without required cues or visibility is a regulatory violation and major safety risk.
• Confusing approach lights with runway lights, or misunderstanding the “100-Foot Rule,” can lead to controlled flight into terrain (CFIT).

Safety Considerations:

• Consider personal minimums above published values for challenging situations.
• Always check NOTAMs and lighting system status before approach.
• Never “duck under” minimums—even if the runway seems close.

Summary Table: DH Quick Reference

Conclusion

Decision Height (DH) is a cornerstone of global aviation safety, providing a standardized, regulated point for pilots to commit to landing or execute a missed approach based on visual references and visibility. Understanding DH, its regulatory context, operational procedures, and its relation to DA, MDA, and MAP is essential for safe, compliant instrument flight operations.

For more on instrument procedures, approach charts, or regulatory guidance, consult FAA AIM, ICAO Doc 8168, and your national aviation authority.

References

• FAA Aeronautical Information Manual (AIM)
• 14 CFR §91.175
• ICAO Annex 6, Annex 14, Doc 8168
• EASA CS-OPS
• FAA AC 150/5340-30
• Jeppesen and FAA Approach Charts
• FAASafety.gov