Aircraft Electrical Systems: A320/A321 Briefing

 

Briefing Document: Risks and Mitigation of Erroneous Barometric Settings During Approach

Date: October 26, 2023

Source: Excerpts from "use-the-correct-baro-setting-for-approach.pdf" (Airbus, Safety first, 2022)

Executive Summary: Using an incorrect barometric reference (QNH/QFE) during an aircraft approach, particularly when vertical guidance relies on this reference, poses a significant risk of the aircraft flying below the published approach path. This can lead to controlled flight into terrain (CFIT), especially in poor visibility or at night. This briefing document outlines the causes and consequences of such errors, details a real-world case study, and describes both operational considerations for flight crews and system enhancements designed to detect and alert crews to erroneous barometric settings. A 1 hPa difference in QNH/QFE results in an approximate 28 ft shift in displayed barometric altitude.

Main Themes and Key Ideas:

1. Risk of Controlled Flight Into Terrain (CFIT) Due to Erroneous Barometric Setting:

• Core Danger: "Using an erroneous barometric reference setting during approach may cause the aircraft to fly lower than the published approach path... This can lead to a risk of controlled flight into terrain in poor visibility conditions or at night."
• Mechanism: When vertical guidance (e.g., LNAV/VNAV) and trajectory deviations use the barometric reference, an incorrect setting will cause the aircraft's instruments to display a correct flight path even if the actual altitude is too low.

1. Case Study: A320 Low Approach Event:

• Event Description: An A320 flight crew received and acknowledged an incorrect QNH of 1011 hPa, which was 10 hPa higher than the actual airport QNH of 1001 hPa.
• Consequences:The aircraft leveled off approximately 280 ft below the intended 5000 ft altitude. (A 10 hPa error led to a 280 ft shift, confirming the 28 ft/hPa rule: 10 hPa * 28 ft/hPa = 280 ft).
• The aircraft commenced its final descent using FINAL APP guidance mode "280 ft below the published approach."
• Despite being significantly low, "the ND and PFD indicated that it was on its expected horizontal and vertical flight path."
• ATC issued a Minimum Safety Altitude Warning (MSAW) at 1.53 NM from the runway when the aircraft was at 891 ft indicated altitude.
• The Pilot Flying initiated a go-around at 735 ft indicated altitude, but the aircraft's radio altitude subsequently "indicated a descent to 6ft during the go-around maneuver."
• A second approach was attempted with the same erroneous QNH, triggering another MSAW. The crew eventually established visual contact, disconnected the autopilot, and corrected the trajectory using PAPI indications for a manual landing.
• Reasons for Non-Detection by Flight Crew:"The vertical deviation symbol was centered."
• "Altitude vs. distance checks were correct" (due to the erroneous barometric reference being used for the checks).
• "There was no Terrain Avoidance Warning System (TAWS) alert."
• The runway approach lights were not activated during the first attempt, hindering visual detection.

1. Effects of an Erroneous Barometric Setting:

• Barometric Altitude Shift: "A 1 hPa difference in the QNH/QFE value creates a 28 ft shift of the barometric altitude displayed on the PFD."
• Impact on Final Approach Guidance Modes (Barometric Reference Dependent):Modes like FINAL APP (A320 family), VGP, P.DES/P.APP, F-G/S, APP-DES, and DES are affected.
• "If an erroneous barometric altitude is used, the aircraft will follow a flight path that is parallel to the published path but is shifted either above or below it."
• Crucially, "The vertical deviation symbol, or the FLS symbol, will indicate that the aircraft is on the correct flight path even if it is not the case."
• Impact on Altitude-vs-Distance Checks: These checks become ineffective because they rely on the same erroneous displayed barometric altitude, leading crews to believe they are on track. "The effect is the flight crew will observe that they are at the expected altitude for each distance value, even if the aircraft is flying above or below the published flight path."
• Potential Absence of TAWS Alert: The Honeywell EGPWS TOO LOW TERRAIN alert may not trigger because the erroneous path, though low, might remain outside the Terrain Clearance Floor (TCF) alert envelope. ACSS T2CAS and T3CAS Premature Descent Alerts (PDA) may also not be triggered.
• Modes Not Affected: "The final approach path of approaches using ILS, GLS, or SLS guidance are not affected, because the G/S guidance mode uses the ILS signal or a beam computed with an augmented GPS altitude."

1. Operational Considerations for Flight Crews (Detection and Crosscheck):

• During Descent: Flight crews should "crosscheck the barometric reference" by comparing the ATC-provided setting with the ATIS barometric reference used for approach preparation. A significant difference warrants confirmation from all available sources.
• During Final Approach: "Unexpected low RA callouts in final approach" while the barometric altitude is still high above airfield elevation can be a clue of a barometric reference discrepancy. However, terrain profiles can influence RA callouts, so this may not always be present.

1. System Enhancements (ALTimeter Setting Monitoring - ALTSM):

• Functionality: The ALTSM function (available on some Honeywell EGPWS standards) "compares the barometric altitude on the captain side with the GPS altitude. If the difference exceeds a threshold, the EGPWS emits an 'ALTIMETER SETTING' alert."
• Current Availability: A first step (audio-only alert) is available for activation via Service Bulletin (SB) on some A320 and A330 aircraft with specific Honeywell EGPWS standards.
• Future Enhancements (ALTSM v2): Airbus is developing an update to include a "flashing QNH/QFE value on the PFD in addition to the audio alert." This "second step will be included in the Landing Surveillance package, to be incrementally certified from 2023 to 2024," and will be available on production aircraft and for retrofit (EGPWS and T3CAS computers).
• A350 Integration: A similar function is planned for the next A350 surveillance computer standard, expected in 2027 for newly produced aircraft.

Conclusion: The "use-the-correct-baro-setting-for-approach.pdf" article clearly articulates the critical safety implications of an erroneous barometric setting, particularly during approaches relying on barometric vertical guidance. The detailed case study highlights how multiple layers of protection (visual, instrument cross-checks, TAWS) can be circumvented, making the error exceptionally difficult to detect without external cues or dedicated monitoring systems. Airbus emphasizes the need for diligent cross-checking by flight crews and is actively implementing advanced ALTimeter Setting Monitoring (ALTSM) functions to provide both audio and visual alerts, significantly enhancing safety by addressing this insidious risk.