Aircraft Electrical Systems: A320/A321 Briefing

 

Mastering Barometric Reference Settings for Approach Safety

Study Guide

This study guide is designed to review your understanding of the provided source material regarding the importance of correct barometric reference settings during aircraft approach.

I. Core Concepts & Definitions

• Barometric Reference Setting (BARO Setting): The atmospheric pressure value set in an aircraft's altimeter, used to display barometric altitude.
• QNH: The barometric pressure setting that, when set on an aircraft's altimeter, will cause the altimeter to read airfield elevation when the aircraft is on the ground at that airfield. Used for approaches to display altitude above mean sea level.
• QFE: The barometric pressure setting that, when set on an aircraft's altimeter, will cause the altimeter to read zero when the aircraft is on the ground at that airfield. Used for approaches to display height above the airfield.
• Controlled Flight Into Terrain (CFIT): An accident in which an airworthy aircraft under the control of the pilot is inadvertently flown into terrain, obstacles, or water. An erroneous BARO setting can increase this risk.
• Final Approach Guidance Modes (Barometric Reference Dependent): Aircraft systems that use barometric altitude to compute and display the aircraft's deviation from the intended final descent path (e.g., FINAL APP, VGP, P.DES, F-G/S, APP-DES, FPA, V/SPD, PITCH, V/S).
• Minimum Safety Altitude Warning (MSAW): An air traffic control (ATC) system that alerts controllers when an aircraft is flying below a predefined minimum safe altitude for its position.
• Decision Altitude (DA): A specified altitude in a precision approach or approach with vertical guidance at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.
• Radio Altimeter (RA): An instrument that measures the aircraft's actual height above the terrain directly below it by transmitting radio waves and measuring the time it takes for them to reflect back. Unlike a barometric altimeter, it is not affected by barometric pressure settings.
• Terrain Avoidance Warning System (TAWS)/Enhanced Ground Proximity Warning System (EGPWS): Systems designed to prevent CFIT by providing pilots with timely warnings of dangerous proximity to terrain. The effectiveness can be limited by erroneous BARO settings in certain scenarios.
• Altimeter Setting Monitoring (ALTSM) function: A system enhancement that compares the barometric altitude with GPS altitude and provides an alert if a significant discrepancy is detected, indicating a potential erroneous barometric setting.
• ILS/GLS/SLS Guidance Modes: Instrument Landing System, GNSS Landing System, and Satellite-based Landing System. These modes are not affected by erroneous barometric settings for their final approach path because they use radio beams or augmented GPS altitudes, not barometric references, for vertical guidance.

II. Key Principles & Relationships

• Impact of Erroneous BARO Setting: An incorrect BARO setting leads to an inaccurate barometric altitude display. If the setting is too high, the aircraft will fly lower than intended; if too low, it will fly higher.
• Altitude Shift Calculation: A 1 hPa difference in QNH/QFE causes a 28 ft shift in the displayed barometric altitude.
• Misleading Indications: When an erroneous BARO setting is used with barometric-dependent guidance modes, the vertical deviation symbol will appear centered, and altitude-vs-distance checks will seem correct, despite the aircraft being off its true flight path.
• Limitations of Safety Systems: TAWS/EGPWS may not always detect a too-low flight path caused by an erroneous BARO setting because the path might remain outside specific alert envelopes (e.g., Terrain Clearance Floor).
• Unaffected Guidance Modes: ILS, GLS, and SLS approaches provide robust vertical guidance independent of barometric settings, as they rely on external signals or augmented GPS.

III. Detection and Mitigation Strategies

• Crosschecking Barometric Reference: Flight crews should compare the ATC-provided barometric reference during descent with the ATIS reference used for approach preparation. Significant discrepancies warrant further investigation.
• Unexpected Low RA Callouts: Abnormally low Radio Altimeter (RA) callouts while the barometric altimeter still indicates a high altitude above the airfield can be a strong clue of an erroneous barometric setting, although terrain profile can influence RA callouts.
• ALTSM Function: This system enhancement (currently audio-only, with visual updates planned) provides an alert by comparing barometric and GPS altitudes, serving as an independent check.
• Visual Confirmation: In good visibility, visual cues can help detect an incorrect trajectory. However, poor visibility exacerbates the danger of an erroneous BARO setting.
• ATC Warnings: MSAW alerts from ATC can provide an external warning of a dangerously low altitude.

IV. Case Study Analysis

• Scenario: A320 performing an RNP approach with LNAV/VNAV minima.
• Error: Flight crew acknowledged and set an ATC-provided QNH of 1011 hPa, which was 10 hPa above the actual airport QNH of 1001 hPa.
• Consequence: The aircraft flew approximately 280 ft below the intended altitude (10 hPa * 28 ft/hPa = 280 ft).
• Lack of Detection:Vertical deviation symbol appeared centered.
• Altitude-vs-distance checks appeared correct.
• No TAWS alert triggered (due to proximity to published path, remaining outside TCF).
• Poor visibility and unlit runway approach lights hindered visual detection.
• Go-around Initiation: Initiated only after an MSAW alert from ATC and the pilot flying (PF) crossing the Decision Altitude (DA) plus airline policy margin. Aircraft descended to 6ft RA during the maneuver.
• Second Approach: Repeated the same QNH error. Visual contact with the runway (after lights were turned on) allowed for manual correction using PAPI.

Quiz: Erroneous Barometric Settings

Instructions: Answer each question in 2-3 sentences.

1. Explain how an erroneous barometric reference setting during an approach can lead to a risk of controlled flight into terrain (CFIT).
2. What is the approximate altitude shift for every 1 hPa difference in the QNH/QFE value, and in which direction does the aircraft fly if the set QNH is higher than the actual QNH?
3. Why might the vertical deviation symbol appear centered and altitude-vs-distance checks seem correct even when an aircraft is flying on an incorrect path due to an erroneous BARO setting?
4. Describe one specific reason why a Terrain Avoidance Warning System (TAWS) or Enhanced Ground Proximity Warning System (EGPWS) might not trigger an alert for a too-low flight path caused by an erroneous BARO setting.
5. Which specific final approach guidance modes are not affected by an erroneous barometric setting, and why are they immune to this issue?
6. What is the primary method recommended for flight crews to detect a potential barometric reference setting discrepancy during descent?
7. How can abnormal Radio Altimeter (RA) callouts serve as a clue for flight crews that the aircraft may be too low on its final approach path due to a barometric reference discrepancy?
8. Briefly describe the function of the ALTimeter Setting Monitoring (ALTSM) system enhancement.
9. In the A320 case study, the flight crew set a QNH of 1011 hPa, while the actual airport QNH was 1001 hPa. How did this 10 hPa difference affect the aircraft's actual altitude relative to its intended altitude?
10. During the first approach in the case study, what factor significantly hindered the flight crew's ability to visually detect the runway and correct their trajectory, despite flying too low?

Answer Key - Quiz

1. An erroneous barometric reference setting can cause the aircraft to fly lower than the published approach path, especially when vertical guidance uses this reference. In poor visibility or at night, without accurate visual cues, this lower-than-intended path significantly increases the risk of the aircraft making unintentional contact with terrain, leading to CFIT.
2. A 1 hPa difference in QNH/QFE creates an approximate 28 ft shift in the barometric altitude displayed on the PFD. If the set QNH is higher than the actual QNH, the aircraft will fly lower than its intended altitude.
3. Barometric-dependent guidance modes and altitude-vs-distance checks rely on the aircraft's displayed barometric altitude, which itself is based on the erroneous setting. Consequently, the system computes deviations and displays altitudes relative to this incorrect reference, making it appear as if the aircraft is on the correct path.
4. The relative proximity of the actual (but too low) flight path to the published path can prevent TAWS/EGPWS alerts like "TOO LOW TERRAIN" from triggering. This happens because the aircraft's position may remain outside the specific Terrain Clearance Floor (TCF) alert envelope, even though it is dangerously low.
5. ILS, GLS, and SLS guidance modes are not affected by an erroneous barometric setting. This is because their final approach paths use external signals (ILS beam) or augmented GPS altitude, rather than the aircraft's barometric reference, for vertical guidance.
6. Flight crews should crosscheck the barometric reference by comparing the QNH/QFE value provided by Air Traffic Control (ATC) during the first altitude clearance in descent with the value obtained from the ATIS during approach preparation. A significant discrepancy between these values should prompt further investigation.
7. Abnormally decreasing Radio Altimeter (RA) audio callouts, occurring while the barometric altimeter still indicates a relatively high altitude above airfield elevation, suggest that the aircraft is actually much closer to the ground than indicated by the barometric altitude. This disparity can strongly imply a barometric reference discrepancy causing the aircraft to fly too low.
8. The ALTimeter Setting Monitoring (ALTSM) function compares the barometric altitude from the captain's side altimeter with the GPS altitude. If the difference between these two values exceeds a predefined threshold, the EGPWS emits an "ALTIMETER SETTING" audio alert, indicating a potential error in the barometric setting.
9. With a 10 hPa difference where the set QNH was higher (1011 hPa) than the actual (1001 hPa), the aircraft flew approximately 280 ft (10 hPa * 28 ft/hPa) below its intended altitude of 5000 ft. This meant it was actually at about 4720 ft.
10. The runway approach lights were not turned ON for their first approach attempt. This made it extremely difficult for the flight crew to establish visual contact with the runway in poor weather conditions, preventing them from visually detecting their too-low trajectory and correcting it.

Essay Questions

1. Discuss the multifactorial nature of safety incidents caused by erroneous barometric settings, drawing on the A320 case study to illustrate how technological indications, human factors, and environmental conditions can converge to create a hazardous situation.
2. Compare and contrast the effectiveness of flight crew detection methods (crosschecking ATIS/ATC, RA callouts) with system enhancements (ALTSM function) in identifying an erroneous barometric reference setting during approach. Include considerations of their respective strengths and limitations.
3. Analyze the statement: "An undetected erroneous BARO setting can cause an aircraft to fly above or below the published final approach flight path when following approach guidance that uses a barometric reference." Elaborate on how this can deceive flight crews, explaining the misleading indications from flight instruments and standard checks, and why this poses a significant risk.
4. Explain why ILS, GLS, and SLS approaches are immune to errors caused by incorrect barometric settings, contrasting their vertical guidance principles with those of barometric-dependent modes. Discuss the implications of this difference for approach safety and procedure design.
5. Propose a comprehensive strategy for preventing and detecting erroneous barometric settings during aircraft approaches, integrating both procedural and technological solutions mentioned in the source material. Justify each component of your strategy with reference to the potential consequences of such errors.

Glossary of Key Terms

• ALTimeter Setting Monitoring (ALTSM) function: A system enhancement, primarily on Honeywell EGPWS, that compares the barometric altitude with GPS altitude and alerts the flight crew if a significant difference, indicating a potential incorrect barometric setting, is detected.
• Altitude-vs-distance checks: A standard flight crew procedure to verify the aircraft's vertical position against its distance from the runway, using displayed barometric altitude.
• Approach Path: The intended trajectory an aircraft follows during its final descent to a runway.
• ATIS (Automatic Terminal Information Service): A continuous broadcast of recorded aeronautical information in busier airports, including current weather, active runways, and pertinent NOTAMs (Notices to Airmen), often including the airport's current QNH.
• Autothrust: An automatic system that manages engine thrust to maintain a selected airspeed or Mach number, or to achieve a specific thrust setting.
• Autopilot: An automatic flight control system that maintains the aircraft's attitude, heading, and/or altitude without manual input from the pilot.
• Barometric Altitude: Altitude determined by measuring atmospheric pressure and referencing it against a standard atmosphere model. This is the altitude displayed on the aircraft's primary flight display (PFD) when a BARO setting is applied.
• BARO Setting (Barometric Reference Setting): The atmospheric pressure value entered into an aircraft's altimeter to establish the reference for barometric altitude.
• Controlled Flight Into Terrain (CFIT): An accident where an airworthy aircraft, under the control of the flight crew, is inadvertently flown into terrain, obstacles, or water.
• Decision Altitude (DA): A specified altitude in a precision approach or approach with vertical guidance, at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.
• Enhanced Ground Proximity Warning System (EGPWS): An advanced terrain avoidance warning system (TAWS) that provides pilots with timely warnings of dangerous proximity to terrain, often with look-ahead capabilities.
• Final Approach Guidance Modes: Automatic or selected flight control modes that assist the flight crew in following the correct vertical and/or lateral path during the final segment of an approach (e.g., FINAL APP, VGP, F-G/S).
• Flight Level (FL): A nominal altitude of an aircraft, expressed in hundreds of feet, usually referenced to a standard pressure setting of 1013.25 hPa or 29.92 inHg, typically used above a transition altitude.
• Flight Management System (FMS): An integrated computer system that provides navigation, flight planning, and performance management capabilities to the flight crew.
• GLS (GNSS Landing System): A satellite-based landing system that uses GNSS (Global Navigation Satellite System) for lateral and vertical guidance during approach.
• Go-around: A procedure where an aircraft abandons an approach to landing and initiates a climb to regain altitude, typically due to an unstable approach, an unconfirmed landing clearance, or an unsafe situation.
• hPa (hectopascal): A unit of pressure, commonly used in aviation for barometric settings, equivalent to one millibar (mb).
• ILS (Instrument Landing System): A ground-based radio navigation system that provides precise lateral and vertical guidance to an aircraft approaching a runway, often used for precision approaches.
• LNAV/VNAV (Lateral Navigation/Vertical Navigation): An RNAV (Area Navigation) approach minima allowing for both lateral and vertical guidance, often utilizing the FMS and barometric altitude for vertical guidance.
• Minimum Safety Altitude Warning (MSAW): An Air Traffic Control (ATC) system that alerts controllers when an aircraft deviates below a predetermined minimum safe altitude for a given geographical area.
• ND (Navigation Display): A primary flight display component that shows navigational information such as aircraft position, track, heading, and waypoints.
• PFD (Primary Flight Display): A flight instrument display that combines critical flight information (airspeed, altitude, attitude, heading, vertical speed) into a single, integrated screen.
• QFE: A barometric pressure setting that, when set on an aircraft's altimeter, will cause the altimeter to read zero when the aircraft is on the ground at that airfield, displaying height above the airfield.
• QNH: A barometric pressure setting that, when set on an aircraft's altimeter, will cause the altimeter to read the airfield elevation when the aircraft is on the ground at that airfield, displaying altitude above mean sea level.
• Radio Altimeter (RA): An electronic instrument that measures the actual height of the aircraft above the terrain directly beneath it by transmitting radio waves and measuring the time for their return.
• RNP (Required Navigation Performance): A type of area navigation (RNAV) that requires onboard monitoring and alerting to ensure the aircraft remains within a specified navigation performance accuracy.
• Runway Threshold: The beginning of the portion of the runway available for landing.
• SLS (Satellite-based Landing System): A generic term for landing systems that use satellite navigation, similar to GLS.
• Stabilized Approach: An approach in which the aircraft's configuration, speed, descent rate, and flight path are within predefined parameters by a specified altitude, ensuring a safe landing.
• TAWS (Terrain Avoidance Warning System): An onboard system designed to prevent Controlled Flight Into Terrain (CFIT) by providing alerts when the aircraft is at an unsafe proximity to terrain.
• TCF (Terrain Clearance Floor): A component of EGPWS that defines a "floor" below which terrain alerts will be triggered, helping to prevent impact with terrain during approaches.
• T2CAS/T3CAS: Advanced Traffic Collision Avoidance System (TCAS) variants with additional surveillance and safety features, including the Premature Descent Alert (PDA).
• Vertical Deviation Symbol: An indication on the PFD or navigation display that shows the aircraft's vertical position relative to the desired flight path.
• Vapp: Approach speed, typically a specific speed related to the aircraft's stall speed in the landing configuration.
• Visual Reference: The ability of the flight crew to see and identify the runway environment and associated approach lights, crucial for continuing an approach below Decision Altitude.