Instrument Approach Procedure Loading and Execution via FMS

Modern transport-category aircraft rely on Flight Management Systems (FMS) to load, sequence, and execute instrument approach procedures with a precision and consistency that far exceeds manual navigation. Yet automation does not replace pilot judgment—it amplifies it. An incorrect FMS entry, a missed constraint, or a misunderstood mode transition can cascade into a serious deviation. Understanding exactly how the FMS loads and flies an approach, and where the human role remains indispensable, is a cornerstone of Airline Transport Pilot proficiency.

This article walks through the complete process: selecting and loading the approach from the navigation database, verifying its integrity, arming and activating approach modes, monitoring vertical and lateral guidance, and executing or responding to a missed approach—all viewed through the lens of FAA guidance, 14 CFR requirements, and the operationally realistic cockpit environment.

How the FMS Loads an Instrument Approach

Every certified FMS draws approach data from a navigation database compiled and updated on a 28-day AIRAC cycle. The database encodes each published instrument approach procedure as a sequence of waypoints, legs, altitude constraints, speed constraints, and course information extracted directly from the government-published approach plate. When a crew selects an approach—for example, the ILS RWY 28L at a major hub—the FMS populates the flight plan's arrival segment with all of those coded elements automatically.

The loading process typically follows this sequence in the FMS ARRIVALS or APPROACH page: first, the crew selects the destination airport; second, they choose the runway; third, they select the specific approach type and identifier (e.g., ILS Z RWY 28L vs. ILS Y RWY 28L, which may carry different minima or transition altitudes); and fourth, they select the appropriate transition—either a published feeder fix transition, a vectors-to-final option, or a full procedure with a procedure turn or hold-in-lieu-of-procedure-turn (HILPT). Each choice chains together a specific set of coded leg types defined by ARINC 424 standards, such as course-to-fix (CF), track-to-fix (TF), initial fix (IF), and final approach fix (FAF).

A critical pilot responsibility at this stage is to cross-check the FMS-loaded procedure against the paper or electronic approach chart. The database is presumed current and accurate, but its cycle must be verified as current and the crew must confirm that the loaded waypoint sequence, inbound course, and any altitude constraints visible on the FMS match the published plate. Any discrepancy must be resolved—never simply accepted—before continuing. The Instrument Procedures Handbook emphasizes that pilots must not rely on automation to self-validate; independent chart verification is mandatory.

Transition Options: Vectors, Full Procedure, and RNAV Arrivals

How the crew reaches the initial approach fix (IAF) affects how the FMS sequences the approach. With a vectors-to-final selection, the FMS suspends automatic sequencing at the FAF or a designated intercept point, awaiting a heading interrupt from the crew. ATC vectors the aircraft to intercept the final approach course, and the crew activates approach mode at an appropriate point. The FMS will display a discontinuity or dashed path until the crew manually activates the leg or sequences to the FAF.

With a full procedure transition, the FMS sequences automatically from the transition fix through the procedure turn or HILPT, then to the FAF and beyond. This requires the crew to verify that the aircraft will be at the appropriate altitude and speed at each constraint and that no conflicting ATC clearance has altered the profile without a corresponding FMS update.

On RNAV (GPS) and RNP approaches, the FMS transitions through specific phases: terminal (typically RNP/accuracy of ±1 NM), approach/LNAV (typically ±0.3 NM), and where applicable, RNP AR final (accuracies as tight as 0.1 NM or less). These are standard RNP containment values associated with each phase rather than a universally fixed figure for every procedure, so the crew must confirm the FMS-displayed RNP against the value published for that specific approach. The crew must confirm that the required navigation performance (RNP) value displayed by the FMS is equal to or less than the published RNP for the procedure, and that no alerts exist indicating the system cannot meet that value.

Arming and Activating Approach Modes

For ILS-coupled approaches, the crew typically arms both lateral (LOC or APP) and vertical (G/S) modes on the Flight Mode Annunciator (FMA) prior to intercepting the localizer. The FMS may feed lateral guidance to the autopilot through its internal LOC receiver interface or by providing course deviation for the flight director. Once the localizer is captured, the FMS lateral guidance typically hands off to the raw localizer signal, and the glideslope capture follows automatically as the aircraft intercepts the glideslope from below.

On RNAV/RNP approaches without ILS, the FMS itself provides both lateral and vertical (VNAV or LNAV/VNAV) guidance to the autopilot. The crew arms LNAV and VNAV (or the equivalent approach mode), and the FMS drives the flight director and autopilot along the coded path, respecting coded step-down fixes, FAF altitude constraints, and the final approach glidepath angle (commonly around 3 degrees as a general convention, but the actual angle is published on each individual plate and can vary). The pilot must confirm approach mode is active, not just armed, before crossing the FAF—a common automation trap.

Monitoring During the Approach

Automation does not reduce the monitoring workload—it changes its character. Key monitoring responsibilities during FMS-driven approaches include:

FMA verification at each mode transition: Confirm each arm-to-capture transition (e.g., LOC armed → LOC captured → G/S armed → G/S captured) is annunciated correctly and matches intent.
Cross-checking raw data: Even on a fully coupled approach, both pilots should cross-check raw ILS needles or GPS deviation against the FMS-driven flight director. Unexplained disagreement must be called out and resolved immediately.
Altitude constraint compliance: The FMS displays crossing restrictions at step-down fixes. The pilot monitoring (PM) should call altitude constraints as the aircraft approaches them and confirm the aircraft is on the correct profile.
Speed management: Many FMS implementations will honor approach speed constraints or target VREF + additive automatically when in managed speed mode; the crew must verify the target speed is correct and not defaulting to a stale or incorrect value.
Automation mode awareness: The Instrument Flying Handbook stresses that a fundamental error in instrument operations is trusting the automation implicitly. Pilots must know what mode the FMS and autopilot are in at every point, why they are in that mode, and what the next mode transition will be.

The Missed Approach in the FMS Context

The missed approach procedure is also encoded in the navigation database and automatically appended to the approach sequence. When the crew executes a missed approach, proper technique involves: simultaneously applying go-around thrust, selecting the appropriate go-around (GA) mode on the autopilot/flight director, and confirming the FMS sequences forward past the missed approach point (MAP) to the first missed approach waypoint.

A critical risk here is the suspended sequencing condition. On many FMS platforms, sequencing is suspended at or near the MAP, particularly on non-precision approaches, to prevent automatic sequencing past the runway threshold. The crew must manually sequence past the MAP after initiating the go-around, or the FMS will continue to display the MAP as the active waypoint, potentially commanding a turn back toward the runway. Crews must know their specific aircraft's FMS behavior at this point.

Additionally, if ATC issues an amended missed approach or a radar vector, the crew must enter the amendment into the FMS immediately to prevent the autopilot from following the coded missed approach track against the ATC clearance. Amendments to the published missed approach require FMS re-programming, which should be accomplished by the pilot not currently flying while the other maintains positive aircraft control.

Key Numbers and Rules

Navigation database update cycle: 28 days (AIRAC cycle); under 14 CFR 91.503 and related operator procedures/operations specifications, use of an expired database is governed by applicable regulation and ops specs, though Part 91 non-commercial operators have more flexibility and may use an expired database if data currency is independently verified against current charts.
RNAV (GPS) approach lateral accuracy: ±0.3 NM in approach (LNAV) mode; RNP AR may require as little as 0.1 NM or less.
Decision Altitude (DA) applies to precision and APV approaches; Decision Height (DH) is referenced to height above the runway threshold elevation for ILS Cat II/III (distinct from the charted threshold crossing height, or TCH, which is a separate glidepath obstacle-clearance value).
At or before the FAF, approach mode (not merely armed) must be confirmed active before descending on the final approach segment.
14 CFR 91.175 governs IFR approach requirements: the aircraft must be continuously in a position from which a descent to a landing can be made before descending below MDA/DA, and flight visibility must meet published minimums.
Currency for flying instrument approaches: six instrument approaches within the preceding six calendar months under 14 CFR 61.57(c), or an Instrument Proficiency Check (IPC) to restore currency.

Common Test Traps

Confusing armed vs. active: The ATP knowledge test and practical test both probe whether candidates understand that a mode being armed (annunciated in white on many FMAs) does not mean the aircraft is yet flying that guidance. Approach mode must be active before the FAF.
Assuming the database is always correct: The FMS database is presumed accurate but must be verified against current charts. The FAA expects pilots to detect and report database errors, not simply follow them.
Forgetting to sequence past the MAP: On systems that suspend sequencing at the MAP, failure to manually sequence results in the FMS commanding a turn toward the runway rather than the missed approach track—a trap in simulator evaluations.
Confusing LNAV, LNAV/VNAV, and LPV minima: These are distinct lines of minima on an RNAV (GPS) chart with different authorizations and aircraft equipment requirements. The FMS approach mode must match the minima the crew intends to fly; selecting LNAV/VNAV minima requires either a certified baro-VNAV system or SBAS (WAAS) capability meeting the applicable TSO and RNP approach approval requirements to provide the coded vertical guidance.
Vectors-to-final discontinuity: When vectors is selected, a flight plan discontinuity appears in the FMS. Candidates sometimes believe the FMS will automatically resolve this when ATC turns the aircraft toward final—it will not. The crew must activate the approach leg at the appropriate time.