Instrument approach
An instrument approach, or instrument approach procedure (IAP), is a series of predetermined maneuvers executed by reference to the aircraft's flight instruments, providing specified protection from obstacles from the initial approach fix—or, where applicable, from the start of a defined arrival route—to a point from which a landing can be completed, and thereafter, if landing is not achieved, to a position where holding or en-route obstacle clearance criteria apply.[1] These procedures enable pilots flying under instrument flight rules (IFR) to safely transition from the en route phase of flight to the terminal environment and landing at an airport, particularly in low-visibility conditions where visual references are insufficient.[2] In the United States, standard instrument approach procedures are prescribed by the Federal Aviation Administration (FAA) under 14 CFR Part 97 for civil airports, encompassing the procedures themselves, obstacle departure procedures (ODPs) for certain airports, and the weather minimums applicable to IFR landings and takeoffs.[3] These procedures are developed in accordance with FAA Order 8260.3 (Terminal Instrument Procedures, or TERPS), which ensures obstacle clearance and safe navigation, and are depicted on official aeronautical charts for use by pilots operating under 14 CFR Parts 91, 121, 125, or 135.[2] Internationally, similar standards are outlined by the International Civil Aviation Organization (ICAO) in Annex 6 and Doc 8168, emphasizing precision in maneuvers to facilitate efficient airspace use and safety.[1] Instrument approaches are categorized by the level of guidance provided: precision approaches, such as the Instrument Landing System (ILS), offer both lateral and vertical (glideslope) guidance meeting ICAO Annex 10 accuracy standards; non-precision approaches, like VHF Omnidirectional Range (VOR) or Non-Directional Beacon (NDB), provide only lateral course deviation information, requiring pilots to descend to a minimum descent altitude (MDA); and approaches with vertical guidance (APV), including Localizer Performance with Vertical Guidance (LPV) using RNAV/GPS, deliver course and approximate glideslope information but not to full precision levels.[2] Each type includes defined segments—initial approach (from the initial fix to the intermediate fix), intermediate (alignment for final descent), final (descent to decision altitude or MDA), and missed approach (contingency if visual references are not acquired)—with specific equipment requirements, such as dual VHF radios for precision runway monitor (PRM) approaches and temperature compensations for barometric vertical navigation (Baro-VNAV) below -11°C or above 49°C.[2] Pilots must conduct pre-approach planning 100–200 nautical miles from the destination, including weather assessment via sources like Automatic Terminal Information Service (ATIS), performance calculations, navigation setup, and a formal briefing, while adhering to air traffic control (ATC) clearances and minimums to ensure compliance with currency and training requirements under FAA regulations.[2] As of August 2025, the U.S. National Airspace System features approximately 6,600 public RNAV (GPS) approach procedures, 1,564 ILS/MLS procedures, 585 VOR facilities under the Minimum Operational Network (MON), and 144 public NDB approach procedures, reflecting a shift toward satellite-based navigation for enhanced accuracy and capacity.[4][5] Special variants, such as circling approaches for non-aligned runways or converging ILS for angled runways (15°–100° divergence, with minimums like 600-foot ceilings and 1¼–2 mile visibility), further adapt procedures to diverse airport configurations.[2]Introduction
Definition and Purpose
An instrument approach procedure is a standardized series of predetermined maneuvers that enables an aircraft operating under instrument flight rules to transition safely from the en route phase of flight to either a landing at the destination airport, initiation of a missed approach, or entry into a holding pattern. These procedures rely on a combination of onboard avionics and ground-based or satellite navigation aids to provide lateral and, in some cases, vertical guidance, ensuring obstacle clearance and precise alignment with the runway environment.[6] The primary purpose of an instrument approach is to deliver pilots with reliable navigational guidance for aligning the aircraft with the runway and descending in a controlled manner when visual references, such as runway lights or terrain features, are obscured by low visibility, clouds, or other adverse weather conditions. By establishing defined paths and minimum altitudes, these procedures mitigate the risks associated with instrument meteorological conditions, allowing operations to continue safely without relying on visual flight rules. This guidance is essential for maintaining spatial orientation and preventing deviations that could lead to collisions with terrain or other obstacles.[6][7] Instrument approaches offer key benefits by enhancing airport accessibility during poor weather, thereby minimizing flight delays and cancellations while supporting all-weather operations for commercial, general, and military aviation. Since their widespread adoption in the 1940s, these procedures have contributed to substantial reductions in approach-related incidents; for instance, overall U.S. commercial aviation fatal accident rates dropped from approximately 40 per million departures in 1959 to less than 2 per million by 1969, with approach and landing phases—historically accounting for a significant portion of accidents—benefiting from improved precision and stabilized descent techniques. Further safety gains were realized through initiatives like the International Civil Aviation Organization's Approach and Landing Accident Reduction (ALAR) program, which addressed data from 287 fatal approach-and-landing accidents between 1980 and 1996 (averaging 17 per year) and led to enhanced training and procedural standards that continue to lower risks today.[6][8][9] Within the broader context of instrument flight rules (IFR), instrument approaches serve as the critical final phase of operations conducted in controlled airspace under instrument meteorological conditions, where pilots must rely exclusively on flight instruments rather than external visual cues. IFR encompasses the entire flight from departure through en route navigation to arrival, with approaches integrating seamlessly to ensure compliance with air traffic control clearances and regulatory minimums, thereby upholding the safety and efficiency of the global airspace system.[10][11]Historical Development
The origins of instrument approaches trace back to the late 1920s, when aviation pioneers sought ways to enable safe flight in poor visibility without visual references. On September 24, 1929, U.S. Army Lieutenant James H. "Jimmy" Doolittle achieved the first completely blind instrument flight, taking off, navigating a predetermined course, and landing solely using gyroscopic instruments such as an artificial horizon and directional gyro, under a hood that blocked external views; this demonstration at Mitchel Field, New York, marked a pivotal step toward reliable all-weather operations.[12] Building on this, the U.S. Department of Commerce's Aeronautics Branch developed low-frequency radio ranges (LFRs), also known as four-course radio ranges, starting in the late 1920s; by 1929, the system was standardized with seven operational beacons providing directional guidance along airways, and throughout the 1930s, over 200 such stations were installed to support instrument navigation amid expanding commercial air routes.[13] Post-World War II advancements accelerated the adoption of more precise systems, driven by wartime innovations and growing air traffic demands. The Instrument Landing System (ILS), a precision approach aid providing both lateral and vertical guidance, was authorized for civil installation by the Civil Aeronautics Administration (CAA, predecessor to the FAA) in 1941, following the first scheduled civil use in 1938; initial installations enabled landings in visibility as low as half a mile, significantly enhancing safety at major airports.[14] Concurrently, the VHF Omnidirectional Range (VOR) emerged as a superior en route navigation tool, with the first VOR stations commissioned in 1947 and the establishment of 4,380 miles of VOR airways by 1950; by 1952, VOR networks spanned 45,000 miles, replacing older radio ranges and enabling more flexible routing through the 1950s and 1960s.[14] Key international and regulatory milestones further shaped instrument approaches. The International Civil Aviation Organization (ICAO), established by the 1944 Chicago Convention, began standardizing global instrument procedures and navigation aids to ensure interoperability among nations.[15] In the U.S., the FAA formalized standards through 14 CFR Part 97 in 1967, prescribing instrument approach procedures and minimums for civil airports to support safe IFR operations.[3] The 1990s saw a shift to satellite-based area navigation (RNAV), bolstered by GPS integration, allowing aircraft to fly user-defined paths rather than ground-based tracks; this evolution culminated in FAA's GPS overlay approaches by the late 1990s, reducing reliance on vulnerable terrestrial infrastructure.[16] Entering the 2000s, Required Navigation Performance (RNP) enhancements introduced performance-based criteria for greater accuracy and onboard monitoring, with FAA authorizations expanding from oceanic routes to terminal procedures.[17] Regulatory updates, including Wide Area Augmentation System (WAAS) certification in 2003, enabled precision-like approaches with vertical guidance down to 200 feet, marking the transition to resilient space-based systems that minimize ground aid dependencies.[18]Basic Concepts
Approach Segments
An instrument approach procedure is divided into standardized segments to ensure orderly transition from en route flight to landing, providing pilots with defined paths, altitudes, and navigation guidance. These segments typically include the initial, intermediate, final, and missed approach phases, each with specific purposes related to aircraft positioning, descent, and safety.[2][3] The initial approach segment begins at the initial approach fix (IAF) and extends to the intermediate fix (IF) or the point where the aircraft is established inbound on the intermediate or final approach course. Its primary purpose is to align the aircraft with the subsequent segments while permitting a descent from en route altitudes, often via feeder routes or terminal transitions that connect from arrival procedures. Variations in entry at the IAF may include straight-in, teardrop, or holding-pattern configurations, depending on the procedure design to accommodate different arrival directions. Obstacle clearance in this segment is protected at a minimum of 1,000 feet above terrain (or 2,000 feet in mountainous areas).[2] The intermediate approach segment starts at the IF and continues to the final approach fix (FAF), serving to position the aircraft for entry onto the final approach course from a track aligned within 30 degrees of the final inbound heading. This phase allows further descent and configuration for landing, with minimum altitudes provided to ensure terrain clearance of at least 500 feet. It acts as a bridge between the initial alignment and the precision of the final descent, often involving straight segments or arcs to refine course guidance.[2] The final approach segment commences at the FAF and extends to the missed approach point (MAP) for non-precision approaches or to the decision height (DH) for precision approaches, focusing on a stabilized descent to meet landing minimums. Here, the aircraft follows the published course and glidepath (if applicable), with distances and timing from the FAF critical for speed and configuration management; obstacle protection is reduced to 250 feet or less near the runway threshold. This segment demands the highest level of precision, as it directly leads to visual acquisition of the runway or initiation of the missed approach if required.[2] The missed approach segment begins at the MAP or DH if a landing cannot be safely completed, directing the aircraft into a climb and departure path that provides obstacle clearance and returns it to the en route structure or a holding fix. Initiation criteria include failure to acquire the runway environment by the MAP/DH or any deviation beyond stabilized approach parameters; the procedure typically specifies immediate climb to a safe altitude followed by specific routing.[2] Procedural charts, such as those published by the Federal Aviation Administration (FAA) or Jeppesen, depict these segments in plan and profile views, marking fixes like the IAF, IF, FAF, and MAP with symbols (e.g., Maltese crosses for FAF on FAA charts), along with associated courses, distances, minimum altitudes (underscored), maximum altitudes (overscored), and descent gradients where applicable. The plan view illustrates the horizontal path and transitions, while the profile view shows vertical progression, enabling pilots to visualize the sequence from initial entry to missed approach.[2] These segments presuppose a transition from en route arrival routes, such as Standard Terminal Arrival Routes (STARs), to the approach fixes, requiring pilots to have verified aircraft equipment, weather conditions, and alternate plans prior to commencement.[2]Decision Altitude/Height and Minimum Descent Altitude
In instrument approaches, the decision altitude (DA) applies to precision approaches that provide vertical guidance, such as an Instrument Landing System (ILS), and is defined as the specified altitude, referenced to mean sea level (MSL), at which the pilot must initiate a missed approach if the required visual references to the runway environment are not acquired.[2] The decision height (DH) is similarly used in precision approaches but is expressed as a height above the touchdown zone elevation (TDZE), typically measured by radio altimeter, allowing for more precise low-altitude decisions in categories such as Category II or III ILS operations.[2] In contrast, the minimum descent altitude (MDA) is the lowest authorized altitude, also referenced to MSL, for non-precision approaches lacking vertical guidance, below which descent is not permitted without establishing the necessary visual references.[2] The calculation of DA and DH is primarily tied to the precision approach's glide path, ensuring compliance with obstacle clearance surfaces as defined in the U.S. Standard for Terminal Instrument Procedures (TERPS); for instance, DA incorporates the height above touchdown (HAT) plus TDZE, with minimum HAT values such as 200 feet for Category I approaches at a 3-degree glide path angle.[19] MDA calculations, applied to non-precision approaches, are based on TERPS criteria for obstacle clearance and visibility minimums, incorporating a required obstacle clearance (ROC) of 250 to 500 feet above the highest obstacle in the final approach segment, adjusted for terrain and approach geometry to maintain a safe margin during level flight at the missed approach point.[19] Visibility requirements, such as runway visual range (RVR), are integrated into these minima to ensure safe landing visibility, with credits applied for runway lighting systems that reduce the required RVR.[19] During an approach, at DA or DH on a precision procedure, the pilot must have acquired visual references—such as the runway threshold, approach lights, or runway markings—to continue the descent to landing; otherwise, a missed approach is required immediately, as stipulated in 14 CFR § 91.175.[2] For non-precision approaches at MDA, descent below this altitude is prohibited unless a straight-in landing can be assured with the required visual references in sight, emphasizing level flight maintenance until the runway environment is confirmed.[2] These thresholds ensure safe transition from instrument to visual flight conditions. Factors influencing DA, DH, and MDA include the aircraft's approach category (A through E, based on landing speed), which determines specific minimums and visibility allowances; for example, higher categories like E require steeper glide path adjustments and potentially higher HAT values.[19] Runway lighting, such as an Approach Lighting System with Sequenced Flashing Lights (ALSF-2), can lower visibility minimums by up to 50%, reducing RVR from 2,400 feet to 1,800 feet for a Category I approach.[19] Typical values include a DA of 200 feet HAT for a standard Category I ILS, while an MDA for a non-precision approach might be 400 to 600 feet above airport elevation, depending on local obstacles.[2] Regulatory standards differ between the FAA and ICAO; while both define DA(H) for precision approaches and MDA(H) for non-precision in similar terms under ICAO PANS-OPS (Doc 8168), FAA TERPS criteria often permit lower obstacle clearances (e.g., 250 feet ROC in final segments) compared to ICAO's more conservative margins, and the FAA mandates the highest applicable minimum from procedure, aircraft, or operator specifications.[19][20] Additionally, FAA procedures allow both constant-angle (stabilized) and dive-and-drive techniques for non-precision approaches at MDA, whereas ICAO emphasizes constant descent final approaches to enhance stability.[21][22]Rate of Descent Formula
The rate of descent (ROD) during an instrument approach is calculated to maintain a stabilized descent, typically targeting a 3° glide path in precision approaches or a constant descent angle in non-precision procedures. A widely used rule of thumb for a 3° glide path is ROD in feet per minute (fpm) ≈ ground speed (GS) in knots × 5.[23][24] This approximation provides pilots with a quick mental calculation to ensure the aircraft descends at the appropriate vertical speed from the final approach fix (FAF) onward.[25] The formula derives from basic trigonometry, where the vertical component of the descent is determined by the tangent of the glide path angle. Specifically, \text{[ROD](/page/Rod) (fpm)} = \text{GS (knots)} \times \tan([\theta](/page/Theta)) \times \frac{6076}{60} Here, \theta is the glide path angle in degrees, 6076 feet approximates 1 nautical mile, and division by 60 converts from feet per hour to feet per minute. For a standard 3° glide path, \tan(3^\circ) \approx 0.0524, yielding a factor of approximately 5.3, so ROD ≈ GS × 5.3 fpm; the ×5 rule of thumb simplifies this for practical use while remaining sufficiently accurate for most scenarios.[26] This equates to a vertical descent of about 318 feet per nautical mile traveled for a 3° path.[27] Pilots apply this formula starting at the FAF to compute the required vertical speed, ensuring a stabilized approach by maintaining the calculated ROD along the final approach segment for either precision glideslope tracking or non-precision constant descent.[6] For example, at a GS of 140 knots on a 3° glide path, the required ROD is approximately 700–740 fpm using the rule of thumb or precise calculation, respectively.[23][27] Adjustments to the formula account for wind effects, which influence GS (headwinds decrease GS and thus ROD, while tailwinds increase it), higher groundspeeds requiring proportionally higher ROD to maintain the glide angle, and non-standard glides steeper than 3° (e.g., a 3.5° path uses a factor of about 6.2).[25][28] Since GS inherently incorporates wind, pilots monitor indicated airspeed and crosswind components to refine the calculation dynamically during the approach.[2] Modern tools facilitate real-time computation, including flight directors that provide vertical guidance cues based on the formula, autopilots that automate ROD adjustments to track the desired path, and electronic flight bag (EFB) applications that integrate GS, wind data, and glide angle for instant ROD displays.[6] Historically, before digital aids, pilots relied on manual slide rules or circular flight computers, such as the E6B, to compute descent rates and vertical profiles during approach planning. Maintaining the calculated ROD is critical for safety, as it ensures adequate obstacle clearance, proper energy management, and a stabilized approach profile; unstabilized approaches, often resulting from improper ROD, are a leading factor in controlled flight into terrain (CFIT) incidents, contributing to over 60% of approach-and-landing accidents according to safety analyses.[29][27]Approach Procedures
Straight-In Approaches
A straight-in approach is an instrument approach procedure (IAP) in which the final approach course aligns with the runway centerline, enabling a continuous descent from the final approach fix (FAF) to the runway threshold without requiring a course reversal or procedure turn.[30] This alignment allows pilots to maintain a stabilized descent using the published navigation aid, such as a radial or GPS track, typically at configured power settings to achieve a constant rate of descent.[2] Straight-in approaches offer several advantages, including reduced pilot workload due to the absence of maneuvering, faster execution times, and increased airport capacity in high-traffic environments where the arrival route is directly oriented toward the runway.[2] They are particularly beneficial when weather conditions permit visual acquisition near the minimum descent altitude (MDA) or decision altitude (DA), minimizing the risk associated with complex turns.[30] The procedure is authorized when the final approach course is aligned within 30 degrees of the extended runway centerline, ensuring a normal descent gradient without excessive bank angles or obstacle conflicts that would necessitate a reversal.[30] Minimums for straight-in landings are based on the specific IAP type, such as MDA for non-precision approaches or DA for precision approaches with vertical guidance, and require no intervening obstacles penetrating the protected airspace.[2] For RNAV approaches, the threshold is tightened to 15 degrees to account for GPS precision.[30] Execution of a straight-in approach follows these key steps:- Obtain ATC clearance for the specific IAP and intercept the final approach course at or above the FAF crossing altitude, often via radar vectors.[30]
- Configure the aircraft for a stabilized descent from the FAF, monitoring groundspeed and altitude to maintain the required rate of descent while tracking the centerline.[2]
- Continue descent to the DA or MDA, acquiring required visual references (such as the runway environment or approach lights) to transition to a landing under 14 CFR § 91.175.
- If visual references are not acquired at minimums, execute the published missed approach procedure immediately.[30]