Altitude Alerting System Operation and FAR Requirements

When a flight crew selects a target altitude and then finds themselves drifting away from it — whether through inattention, turbulence, or distraction — the consequences can range from an altitude bust that triggers a controller call to a serious controlled-flight-into-terrain event. The altitude alerting system (AAS) exists specifically to interrupt that chain of events with a timely, unmistakable warning. For the aviation maintenance technician working on turbine-powered aircraft, understanding how altitude alerting systems work, how to verify they meet regulatory requirements, and how to troubleshoot common failures is an essential skill set.

This article covers the operating principles of altitude alerting systems, the specific Federal Aviation Regulation that governs their use, what constitutes an acceptable system, and the practical maintenance and inspection considerations an AMT must know.

How an Altitude Alerting System Works

At its heart, an altitude alerting system is a comparator circuit. It continuously compares the aircraft's current pressure altitude — as measured by an encoding altimeter or an air data computer — against a target altitude that the crew dials into a selector, typically called the altitude select or altitude pre-select knob. When the two values converge within a defined threshold window, the system provides a positive alert. If the aircraft subsequently deviates beyond a second threshold, a deviation alert is issued.

The Two Alert Phases

Most altitude alerting systems generate alerts in two distinct phases:

• Approaching alert: When the aircraft is within approximately 300 to 1,000 feet of the selected altitude (specific values vary by aircraft and system design), an aural tone, a visual light, or both are triggered. This gives the crew advance notice to prepare for level-off. The alert typically cancels automatically once the aircraft reaches and stabilizes at the selected altitude.
• Deviation alert: After the aircraft has captured the selected altitude, if it subsequently departs by a threshold amount — commonly 300 feet or more — the system re-alerts. This is the "bust" warning. The deviation alert remains active until the crew either corrects back to within tolerance or re-selects a new altitude on the alerting panel.

Signal Sources

The altitude data used by an alerting system almost always comes from an encoding altimeter or, on more modern aircraft, from the air data computer (ADC) or air data reference unit. These devices produce a digital representation of pressure altitude (in Gillham code or via an ARINC 429 data bus, depending on the generation of avionics). A standalone altitude alerter processes this signal electronically. In glass-cockpit or fully integrated avionics suites, the alerting function may be embedded within the flight management system or the primary flight display system, but the regulatory requirement for its function remains the same.

Alert Output Methods

Regulatory requirements specify that an acceptable altitude alerting system must provide at least an aural signal. Most systems also provide a visual cue — typically a flashing or illuminated light on the glareshield, annunciator panel, or integrated into the primary flight display. Some older systems use a steady tone with a light; more modern systems may use a synthesized voice alert or a distinctive chime. The aural component is critical because it commands attention even when a pilot is heads-down or looking outside the cockpit.

The Governing Regulation: 14 CFR Part 91, Section 91.219

The specific FAR that mandates altitude alerting systems is 14 CFR §91.219. It is worth knowing this section number precisely because FAA knowledge tests regularly reference it. The regulation applies to civil turbine-powered airplanes of U.S. registry. Here is what the rule requires in plain terms:

• No person may operate a civil turbine-powered airplane of U.S. registry unless it is equipped with an altitude alerting system or device that meets the performance standards of the regulation.
• The system must be operative — meaning a known inoperative altitude alerter is not an acceptable dispatch condition under this section unless a specific aircraft-level exemption or minimum equipment list (MEL) provision applies.
• The system must be capable of alerting the flight crew upon approaching a preselected altitude AND upon deviation from that altitude by a defined amount.
• The aural alert is required; a visual alert may accompany it but does not substitute for it.

There are exceptions carved out in §91.219 for specific aircraft types and operations. For instance, certain aircraft originally type-certificated without any electrical system provision for altitude alerting may be excepted, and some operations conducted under waivers or specific exemptions may differ. The AMT must be familiar with the applicable aircraft flight manual (AFM) and the approved MEL to understand exactly which exceptions apply to a given aircraft in their care.

Why It Matters: Safety and Regulatory Compliance

Altitude deviations are one of the most common pilot deviations reported in the National Airspace System. An aircraft drifting even a few hundred feet off its assigned altitude can create a traffic conflict in busy en route airspace, trigger a traffic collision avoidance system (TCAS) resolution advisory on an adjacent aircraft, or result in violation of an altitude restriction that exists to protect terrain or conflicting traffic.

From a maintenance standpoint, an inoperative altitude alerting system must be addressed before flight under most circumstances. Because §91.219 makes the system mandatory equipment for covered aircraft, a failure discovered during a preflight check or maintenance inspection effectively grounds the aircraft unless the operator has an FAA-approved MEL that permits continued operation with a specified crew procedure in lieu of the system. The MEL provision, if one exists, will typically require placarding and may impose operational limitations such as requiring additional crew vigilance procedures or altitude cross-checks.

Ensuring the system works properly is not simply a paperwork exercise — it directly reduces the probability of an altitude deviation incident, which in turn reduces risk to all aircraft sharing that airspace.

Key Numbers, Rules, and Testable Facts

• Governing regulation: 14 CFR §91.219 — mandatory for civil turbine-powered airplanes of U.S. registry.
• Required alert types: Approaching preselected altitude AND deviation from preselected altitude — both must be annunciated.
• Aural alert: Specifically required by the regulation; a visual-only system does not satisfy §91.219.
• Typical approach alert window: Commonly 300–1,000 feet from selected altitude (exact value is system-specific and set during design/certification).
• Typical deviation alert threshold: Commonly 300 feet from selected altitude (system-specific).
• Altitude data source: Encoding altimeter or air data computer providing pressure altitude; the same source that feeds the transponder Mode C function.
• MEL applicability: An approved MEL may allow dispatch with an inoperative alerter under specific conditions; absent an MEL provision, the aircraft is not legal to fly under §91.219.
• Aircraft type applicability: Civil turbine-powered airplanes of U.S. registry — does NOT apply to piston-engine aircraft, rotorcraft, or gliders.

Maintenance and Inspection Considerations for the AMT

When performing an inspection or maintenance on an altitude alerting system, the AMT should approach the task methodically:

1. Verify the signal source: Confirm that the encoding altimeter or ADC feeding the alerter is serviceable and properly calibrated. An inaccurate altitude source will produce alerts at incorrect altitudes, which is potentially more dangerous than no alert at all.
2. Functional test: Most aircraft maintenance manuals provide a specific ground test procedure. Typically, the technician sets a target altitude on the alert selector, then simulates a change in pressure altitude using a pitot-static tester or by referencing the system's built-in test function. Verify that the aural and visual alerts trigger at the correct thresholds.
3. Check wiring and connectors: Altitude alerting systems use low-signal-level data lines (especially in older Gillham code installations) that are sensitive to connector corrosion, chafing, or improper shielding. Inspect wiring harnesses in the area of the encoding altimeter and the alerting unit.
4. Review the alert thresholds: Some systems allow adjustment of the approach and deviation thresholds via internal adjustments or software configuration. Confirm the thresholds are set to the values specified in the approved data for that aircraft.
5. Document the work: All testing and findings must be properly documented in the aircraft maintenance records per 14 CFR §43.9 and §43.11, and the aircraft's return to service must be authorized by an appropriately certificated individual.

Common Test Traps

• Confusing the alert types: Some test questions try to get you to say only one type of alert (approaching or deviation) is required. Both are required by §91.219 — know both.
• Visual-only alert: A visual alert alone does not satisfy the regulation. The aural component is specifically mandated. An AMT who clears an aircraft as airworthy with only a flashing light and a failed aural horn has made a legal — and safety — error.
• Applicability to piston aircraft: §91.219 applies to civil turbine-powered airplanes of U.S. registry. It does NOT apply to piston-engine general aviation aircraft. Test questions sometimes try to extend it too broadly.
• Inoperative system and MEL: Without an FAA-approved MEL provision, an inoperative altitude alerting system on a covered aircraft is a no-go item. Technicians sometimes assume any avionics failure can be deferred — altitude alerters require specific MEL authorization.
• Data source confusion: The altitude alerter uses pressure altitude from the encoding altimeter or ADC — the same pressure altitude used for Mode C transponder reporting. It is NOT driven by GPS altitude or radio altimeter data, which measure geometric or radar height respectively.