Instrument approach

Instrument approach

An instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft under instrument flight rules (IFR) from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually. Instrument approaches are designed to safely guide aircraft to airports during conditions of low visibility, such as in fog, rain, or nighttime operations.

Types of Instrument Approaches

Instrument approaches are classified based on the level of navigation accuracy and the equipment required:

Precision Approaches

Precision approaches provide both lateral and vertical guidance to the runway.

• ILS (Instrument Landing System): Provides precise lateral (localizer) and vertical (glideslope) guidance.
• GBAS (Ground-Based Augmentation System): A satellite-based system that enhances GPS signals to provide precision approach capability.

Approaches with Vertical Guidance (APV)

APVs provide vertical guidance but are not classified as precision approaches.

• LNAV/VNAV (Lateral Navigation/Vertical Navigation)
• LPV (Localizer Performance with Vertical guidance)

Non-Precision Approaches

Non-precision approaches provide only lateral guidance.

• VOR (VHF Omnidirectional Range)
• NDB (Non-Directional Beacon)
• RNAV (Area Navigation) - LNAV only
• LOC (Localizer only, without glideslope)

Approach Segments

Each instrument approach procedure consists of several segments:

• Initial Approach Segment: Aligns the aircraft with the intermediate or final approach segment.
• Intermediate Segment: Positions the aircraft for the final descent.
• Final Approach Segment: Guides the aircraft from the final approach fix to the runway or missed approach point.
• Missed Approach Segment: Outlines the procedure if the pilot cannot land.

Minimums

Each approach has minimum descent altitudes (MDA) or decision altitudes (DA), depending on the type:

• DA (Decision Altitude): Used with precision and APV approaches.
• MDA (Minimum Descent Altitude): Used with non-precision approaches.

Charts and Procedure Plates

Pilots use approach charts to conduct instrument approaches. These include:

• Airport and runway information
• Frequencies for navigation aids
• Missed approach instructions
• Minimums for various approach categories (A, B, C, D)

Category of Operations

Instrument approaches are classified into three categories based on visibility and decision height:

Category	Decision Height (DH)	Runway Visual Range (RVR)
CAT I	≥ 200 ft	≥ 550 m (1800 ft)
CAT II	≤ 200 ft, ≥ 100 ft	≥ 300 m (1000 ft)
CAT IIIa	< 100 ft	≥ 200 m (700 ft)
CAT IIIb	< 50 ft	≥ 75 m (250 ft)
CAT IIIc	0 ft	0 m (not yet implemented operationally)

Autoland and HUD

In low-visibility conditions, many modern aircraft use autoland systems or head-up displays (HUDs) to conduct approaches down to CAT III minimums.

References

• FAA Terminal Procedures
• SKYbrary - Instrument Approach