Minimum Descent Height (MDH) – Aviation Glossary

Minimum Descent Height (MDH): Comprehensive Definition

Minimum Descent Height (MDH) is a critical safety parameter in instrument approach procedures, specifically within non-precision and circling approaches. MDH is defined as the lowest height above aerodrome elevation (or above runway threshold elevation if the threshold is more than 2 meters [7 feet] below aerodrome elevation), to which an aircraft is authorized to descend without the required visual reference. MDH is a protective barrier that ensures obstacle clearance during the final stages of an approach when vertical guidance is not provided by electronic means.

MDH is established through procedural analysis, considering aerodrome elevation, local terrain, obstacles in the approach area, and the capabilities of the aircraft and crew. Regulatory authorities such as ICAO, EASA, and FAA require that MDH be established through robust obstacle assessment and procedural design. For circling approaches, MDH is always referenced to aerodrome elevation, ensuring a standard obstacle clearance surface regardless of runway threshold position.

MDH is expressed in feet or meters above local reference and is always paired with Minimum Descent Altitude (MDA), which is referenced to mean sea level (MSL). The distinction is fundamental: height (MDH) is above a local reference (aerodrome/threshold), while altitude (MDA) is above sea level.

This minimum is published on approach charts and must be strictly observed. Descending below MDH without required visual cues (such as runway lights, markings, or the environment) is a breach of procedure and threatens safety. A pilot may only continue descent below MDH if the required visual reference is acquired, confirming a safe position to land. Otherwise, the approach must be discontinued at or before the missed approach point (MAP).

ICAO: “Minimum Descent Height (MDH): The lowest height, above the aerodrome elevation or above the runway threshold elevation if that is more than 2m below the aerodrome elevation, to which descent is authorized on final approach, or during circle-to-land manoeuvres, in execution of a non-precision approach or circling approach without visual reference.”

Where is MDH Used?
MDH governs non-precision approaches (such as VOR, NDB, or RNAV approaches lacking vertical guidance) and circling approaches. It does not apply to precision approaches (ILS, GLS, LPV, etc.), which use Decision Altitude/Height (DA/DH).

MDH in Operational Practice

When and How MDH Is Used in Flight

MDH becomes critical during the final stages of a non-precision or circling approach. After navigating the initial and intermediate segments, the aircraft is configured for landing and follows the published lateral guidance to the runway, but vertical guidance is managed manually or with reference to barometric altitude. The MDH marks the lowest permissible height at which the aircraft may be flown during this phase, unless the required visual references are acquired.

The approach chart for a given procedure specifies the MDH (often alongside MDA), ensuring pilots have a clear barrier for descent. As the aircraft approaches the missed approach point (MAP)—the location where a decision to land or execute a missed approach must be made—the pilot must maintain MDH unless sufficient visual cues are gained to continue descent.

Key Operational Steps:

• Begin descent from the final approach fix (FAF) and follow the published profile to the MDH.
• Upon reaching MDH, level off. Continuous descent below MDH is strictly prohibited unless the runway environment or specified visual references become visible.
• If visual reference is obtained before the MAP, the pilot may continue descent and land.
• If the MAP is reached and visual reference is not established, initiate the published missed approach procedure immediately.

Visual Reference Criteria:
ICAO and national regulations specify the required visual reference must include elements like the approach lighting system, runway threshold, markings, or runway lights, sufficient to confirm lateral and vertical position for a safe landing.

Example Scenario:
A pilot conducting a VOR approach is cleared to descend to an MDH of 460 ft above aerodrome elevation. At this height, the pilot levels off, searching for runway cues. If the runway or approach lighting is seen before the MAP, descent for landing is permitted. If not, a missed approach is performed.

Safety Margin:
MDH ensures minimum obstacle clearance, typically 300 feet (92 meters) for the final approach segment, with even greater margins for circling procedures, as outlined in ICAO PANS-OPS (Doc 8168). This clearance accounts for navigation errors, barometric inaccuracies, and altimeter setting variations.

Distinctions: MDH, MDA, DA, and DH

Understanding the Differences and Practical Applications

Aviation approach procedures employ several types of minima, each tailored to the nature of the approach. The main terms are MDH, MDA, DA (Decision Altitude), and DH (Decision Height).

MDH/MDA:
In non-precision approaches, the aircraft descends to the MDH/MDA and must level off. There is no “decision point” as with DA/DH; the pilot maintains the minimum until the MAP, at which point a decision is made based on visual cues.

DA/DH:
Used in precision approaches, DA/DH is the precise point where a decision is made to land or execute a missed approach. There is no level-off at DA/DH; the approach is flown as a continuous descent.

Regulatory Guidance:
ICAO Doc 8168, EASA, and FAA procedures require clear publication of reference datum and minima values on all approach charts to avoid ambiguity.

Regulatory and Measurement Considerations

How MDH Is Determined and Applied

The establishment and depiction of MDH are subject to strict regulatory oversight and procedural design, primarily governed by ICAO Annex 6, ICAO Doc 8168 (PANS-OPS), EASA CS-ADR-DSN, and the FAA’s instrument procedures handbooks.

Reference Datum:

• For straight-in non-precision approaches, MDH may be referenced to threshold elevation if the threshold is more than 2 m (7 ft) below aerodrome elevation.
• For circling approaches, MDH is always referenced to aerodrome elevation to ensure consistent clearance.

Obstacle Clearance:
MDH ensures obstacle clearance, typically 300 feet (straight-in) or up to 400 feet or more (circling), depending on category and local terrain.

Visual Reference Requirements:
The required visual reference must be sufficient for the pilot to assess/control the aircraft’s position and rate of change laterally and vertically (approach lighting, threshold, markings, runway lights, etc.).

Missed Approach Execution:
If visual reference is not acquired at the MAP, the missed approach procedure must be initiated immediately.

Charting Standards:
Approach charts from authorities like Jeppesen, Lido, or national AIPs display MDH in both feet and meters, always in conjunction with MDA, derived from detailed surveys for procedural safety.

MDH in Pilot Procedures and Flight Operations

Step-by-Step Application in Cockpit Operations

Use of MDH is embedded in cockpit procedures, checklists, and decision-making processes to maximize safety.

Approach Preparation:
Pilots brief the procedure, noting MDH, approach type, MAP, and missed approach instructions. MDH is set on the altimeter or flight guidance system as a safeguard.

Descent to MDH:
After intercepting the final approach course, descent begins at the FAF. Upon reaching MDH, the pilot must arrest the descent, flying level until the MAP.

Visual Reference Acquisition:
Pilots scan for visual cues—runway lights, markings, approach lighting. If sighted and in a safe position to land, descent below MDH is permitted.

Missed Approach Procedures:
If no visual reference is available at the MAP, the pilot executes the missed approach.

Automation and Human Factors:
Modern aircraft may allow MDH/MDA to be set in the flight guidance system, providing alerts as the aircraft reaches this value.

Training and Proficiency:
Pilots undergo training and simulator sessions to ensure strict MDH compliance. Non-adherence can result in regulatory violations and safety incidents.

MDH in Circling Approaches

Unique Considerations and Maneuvering Requirements

Circling approaches require maneuvering visually at low altitude, usually due to wind, runway length, or obstacle constraints.

MDH Reference:
Circling MDH is always referenced to aerodrome elevation, ensuring consistent clearance in the circling area.

Maneuvering Area:
Defined by aircraft category (A, B, C, D, or E); the MDH provides protection for the entire circling area.

Pilot Technique:
The pilot must keep the runway environment in sight while maneuvering. Loss of visual contact requires immediate missed approach initiation from the current position.

Safety Margins:
Obstacle clearance for circling MDH is greater than straight-in approaches, reflecting the increased risk and complexity.

MDH Example Scenarios

Scenario 1: Non-Precision Approach

A Boeing 737 is flying a VOR approach with an MDH of 520 feet. Weather is marginal, with a ceiling at 600 feet and visibility at 2,000 meters. The pilot descends to 520 feet, levels off, and maintains this height until MAP. With no visual cues, the missed approach is initiated.

Scenario 2: Circling Approach

A regional turboprop completes an RNAV approach to runway 09, but must land on runway 27. Circling MDH is 640 feet above aerodrome elevation. The pilot maintains 640 feet, maneuvering visually for a safe landing on 27, descending further only when the runway is in sight.

Scenario 3: Early Visual Reference

A pilot flying an NDB approach detects the runway at 700 feet, well above the MDH of 500 feet. The pilot may continue descent, but must maintain obstacle clearance until stabilized on final approach.

Scenario 4: Loss of Visual Reference

During a circling approach, the pilot loses sight of the runway in a rain shower and must immediately begin the missed approach.

Regulatory Framework for MDH

ICAO, EASA, and FAA Provisions

ICAO:
Annex 6 and Doc 8168 establish MDH standards, adopted worldwide.

EASA:
Mandates ICAO minima, with additional design and training requirements.

FAA:
Uses MDA and MDH interchangeably in context. U.S. charts show MDA (MSL) with MDH (AGL) in parentheses. FAA handbooks provide detailed guidance.

Charting:
MDH/MDA always published. Circling MDH is clearly labeled. DA/DH used only for approaches with vertical guidance.

Legal Responsibility:
The pilot-in-command must strictly adhere to published minima. Operators must ensure training, SOPs, and checklists support correct MDH use.

Glossary of Related Terms

References

• ICAO Doc 8168 – Procedures for Air Navigation Services – Aircraft Operations (PANS-OPS)
• ICAO Annex 6 – Operation of Aircraft
• EASA Air OPS and CS-ADR-DSN
• FAA Instrument Procedures Handbook
• Jeppesen and national AIP approach charts

For more in-depth technical and regulatory guidance, consult national aviation authorities and ICAO documentation.