What is the difference between Decision Altitude and Minimum Descent Altitude on an instrument approach?

Decision Altitude (DA) is used on precision approaches and vertical navigation approaches, and it is the altitude at which the pilot must decide to either continue the approach if the required visual references are in sight or immediately execute a missed approach. Minimum Descent Altitude (MDA) is used on non-precision approaches that lack an electronic glidepath, and the pilot may not descend below it unless the runway environment is in sight and a normal landing can be made. Unlike DA, where the missed approach is initiated at the altitude itself, MDA requires the pilot to level off and fly at that altitude until the runway is visible or the missed approach point is reached, as explained in the Instrument Flying Handbook.

What visual references must a pilot have before descending below DA or MDA?

Under 14 CFR 91.175, a pilot may not descend below DA or MDA unless flight visibility meets the minimums prescribed for the approach and at least one of the required visual references for the intended runway is distinctly visible, such as the approach lights, threshold, touchdown zone, or runway markings. Simply seeing the glow of approach lights through clouds or fog is not sufficient if the runway environment cannot be identified well enough to make a safe landing. If these conditions are not met at or before reaching the missed approach point, the pilot must execute the published missed approach procedure without delay.

Why do precision approaches use Decision Altitude instead of Minimum Descent Altitude?

Precision approaches, such as ILS approaches, provide both lateral course guidance and a vertical glidepath, allowing the aircraft to continuously descend along a defined angle all the way to the DA. Because the aircraft is always on a controlled descent, there is no need to level off and wait at a fixed altitude — the decision to land or go missed is made at a single specific point, which is why the term Decision Altitude is used rather than Minimum Descent Altitude. The Instrument Flying Handbook notes that this design supports a more stabilized approach and reduces the risk associated with flying level at low altitude in instrument conditions, which is the scenario encountered on non-precision approaches using MDA.

IFR Approach Minimums and Decision Altitude vs Minimum Descent Altitude

Every instrument approach procedure published by the FAA includes a floor — a specific altitude below which you may not descend unless you have the required visual references in sight. But not all approaches use the same type of floor. Precision approaches use a Decision Altitude (DA), while non-precision approaches use a Minimum Descent Altitude (MDA). These two concepts are often confused by instrument students, yet they represent fundamentally different philosophies of how an approach is flown and when the critical go/no-go decision is made. Mastering this distinction is essential not only for the FAA Instrument Rating knowledge test but for every approach you will ever fly in actual IMC.

This article breaks down exactly what DA and MDA are, how they differ mechanically and operationally, what the regulations require, and how to think clearly about them in the cockpit under pressure.

Decision Altitude (DA): The Precision Approach Floor

A Decision Altitude is used on precision approaches — those that provide both lateral and vertical electronic guidance. The most common example is the ILS (Instrument Landing System). The DA is expressed as a mean sea level (MSL) altitude, and it represents the point at which you must make an immediate decision: land or go around.

The critical word in DA is decision. You do not level off at the DA, wait, look around, and then decide. At the DA, the decision must already be in progress. 14 CFR Part 91.175 states that a pilot may not operate an aircraft below DA unless the aircraft is continuously in a position from which a descent to a landing on the intended runway can be made at a normal rate using normal maneuvers, and that one of the specified visual references for the intended runway is distinctly visible and identifiable by the pilot. If those conditions are not met at DA, you must immediately execute the published missed approach.

The reason precision approaches use a DA rather than an MDA is the vertical guidance itself. Because the ILS glideslope (or WAAS-based LPV glidepath) continuously drives you down a precise 3-degree (approximately) path, you are always descending on approach. You never level off. The DA is simply the altitude where, if you don't see what you need to see, you begin the go-around — but note that your aircraft will continue to descend slightly below DA during the transition to climb attitude. The system accounts for this; the obstacle clearance surfaces are designed with this brief descent in mind.

Common DA values: a standard ILS CAT I approach typically has a DA of 200 feet above the touchdown zone elevation (TDZE), with a visibility requirement of 1/2 statute mile (or 2,400 RVR). These are minimums — your specific approach plate will show the actual published minimums for that runway and aircraft category.

Minimum Descent Altitude (MDA): The Non-Precision Approach Floor

A Minimum Descent Altitude is used on non-precision approaches — those that provide lateral guidance only, without an electronic glideslope. Examples include the VOR approach, NDB approach, LOC (localizer only) approach, and LNAV minima on an RNAV approach. The MDA is also expressed in MSL, but operationally it works very differently from a DA.

On an approach to an MDA, the pilot descends from the final approach fix (FAF) to the MDA and then levels off at that altitude. You fly level at the MDA until either: (1) you acquire the required visual references and can safely complete a landing, or (2) you reach the missed approach point (MAP) — at which point you must execute the missed approach if landing is not assured. The MAP on a non-precision approach is typically defined by a specific navaid, a distance fix (DME), or a timing calculation from the FAF based on your groundspeed.

The leveling-off at MDA is a key mechanical difference. Because there is no electronic vertical guidance keeping you on a precise path, you must arrest your descent at MDA and then transition to a visual descent — but only if you have the runway environment in sight. If you reach the MAP still at MDA with no visual contact, you go missed. You may never descend below the MDA during the approach, even if you briefly catch sight of the runway lights and then lose them again.

The MDA provides obstacle clearance, but only in the horizontal sense — it ensures a minimum of 300 feet of clearance above the highest obstacle in the final approach segment. Because there is no glideslope to guide the descent from MDA to touchdown, pilots must visually judge that descent, which is one reason non-precision approaches carry a higher accident rate than precision approaches in low-visibility conditions.

LNAV/VNAV and LPV: The In-Between Cases

Modern RNAV (GPS) approaches introduce intermediate categories that blend these two concepts. LNAV/VNAV minima provide advisory vertical guidance (not certified precision guidance) and use a DA — because you are flying a computed vertical path, the decision model applies. LPV (Localizer Performance with Vertical Guidance) minima use WAAS-based precision-like guidance and also use a DA, often with minimums approaching or matching ILS CAT I. LNAV minima (lateral navigation only) use an MDA, just like a traditional non-precision approach. Your approach plate will clearly show which type of minimum applies to each set of minima — always read it carefully before beginning the approach.

Why It Matters: Safety and Legal Implications

The distinction between DA and MDA is not merely academic. Busting below MDA — descending below the floor before the required visual references are established — is a serious violation of 14 CFR 91.175 and has been a causal factor in controlled flight into terrain (CFIT) accidents. The MDA exists because the obstacle environment below it has not been evaluated. Similarly, continuing below DA in IMC when you cannot see the required visual references is a path toward flying into the ground in the final moments of approach.

Remember: seeing the approach lights alone does not automatically authorize you to descend below MDA on a non-precision approach. The regulations specify that seeing the approach light system (ALS) allows you to descend to 100 feet above the TDZE — but not all the way to the runway surface — unless you can also see one of the other specified visual references (runway threshold, threshold markings, threshold lights, REIL, VASI/PAPI, touchdown zone, touchdown zone markings, touchdown zone lights, or the runway or runway markings or runway lights).

Key Numbers and Rules

DA is used for precision approaches (ILS, LPV, LNAV/VNAV); the pilot must go missed if visual references are not established at DA.
MDA is used for non-precision approaches (VOR, NDB, LOC, LNAV); the pilot levels off at MDA and flies to the MAP before going missed if no visual contact.
Standard ILS CAT I minimums: DA of 200 feet HAT (height above TDZE), visibility 1/2 SM or 2,400 RVR.
Non-precision obstacle clearance: minimum 300 feet above the highest obstacle in the final approach segment.
Approach light system (ALS) sighting allows descent to 100 feet above TDZE on a non-precision approach — not to the surface.
Visual references required (14 CFR 91.175): at least one element of the approach lighting system, or runway environment, must be clearly visible to continue below DA/MDA.
Aircraft must be continuously in a position to make a normal descent and landing — no dive-and-drive maneuvers below MDA intended to scoop up a landing.
The missed approach must be executed immediately upon reaching DA without required visual references, or upon reaching the MAP at MDA without required visual references.

Memory Aid

"D" for Decision, "M" for Minimum" — think of it this way: at a Decision Altitude you are still Descending and must Decide right now. At a Minimum Descent Altitude you are at your Minimum — level off, Maintain altitude, and look for the runway to the Missed approach point. The approach type tells you the behavior: precision = decide on the way down; non-precision = stop, look, then go missed or land.

Common Test Traps

Confusing DA with MDA on approach plates: The FAA knowledge test may show a plate and ask which type of minimum applies. LPV and ILS minima show DA; LNAV shows MDA. Check the label carefully — "DA" or "MDA" will appear explicitly on the plate.
Thinking MDA allows a continued descent once the runway is in sight: You may not dive steeply from MDA to the runway. The descent from MDA must be at a normal rate from a position where landing can be made with normal maneuvers.
Approach lights = cleared to land: Seeing only the ALS does NOT authorize you to descend all the way to the runway. It permits descent to 100 feet above TDZE only. You need an additional visual reference to continue to the surface.
MAP timing errors: On a non-precision approach without a DME or GPS, the MAP is often determined by timing from the FAF. If you fly faster than planned, you will reach the MAP sooner than your timer indicates — always compute timing for your actual groundspeed.
Assuming LNAV/VNAV is a precision approach: LNAV/VNAV uses a DA, which feels like a precision approach, but it is officially a non-precision approach with advisory vertical guidance — obstacle clearance standards differ from a true precision approach like the ILS.