What is the difference between MDA and MDH in aviation?

The distinction between Minimum Descent Altitude (MDA) and Minimum Descent Height (MDH) is critical for understanding instrument flight procedures. MDA is defined as an altitude expressed in Mean Sea Level (MSL) that a pilot may descend to during an approach without having visual reference to the runway environment. This altitude is primarily used in non-precision approaches, where the pilot must maintain a certain altitude until visual cues are available to continue the approach safely.

On the other hand, MDH is an altitude expressed in Above Ground Level (AGL), specifically used in the context of approaches that provide a precise vertical guidance system, such as an ILS (Instrument Landing System). The MDH is used to signify the minimal height above the ground that a pilot must maintain while completing an approach.

Understanding this difference is essential as it impacts the decision-making process during an approach and affects the safety and compliance with instrument approach procedures. Recognizing that MDA provides a reference from a level above sea, while MDH reflects the distance above the ground, allows pilots to maintain proper situational awareness and altitude during descents in varying geographic conditions.