AINsight: Six feet from the disaster | Wbactive

Pilots have benefited from it RNAV (Geographic Positioning System) approaches using vertical guidance for more than two decades. These approach procedures provide constant vertical guidance to a decision height (THERE) and provide a tremendous safety leap by eliminating the hazards associated with “dive-and-drive” non-precision approach procedures.

undoubtedly, RNAV (Geographic Positioning System) Approaches have reduced the risk of controlled flight into terrain (CFIT) accident, but if not flown properly it can be just as dangerous.

Case study: In May, the pilots of an Airbus A320 fly in RNAV (Geographic Positioning System) Approach came within six feet – based on radio altimeter (RA) – when hitting terrain 0.8 nm before the runway in Paris Charles de Gaulle (LFPG) Airport. French agency for the investigation of civil aviation accidents BEA immediately launched an investigation and classified the incident as a serious incident.

The approach flown was stable and procedurally correct with one exception – the crew could not detect an air traffic controller error.

Most alarming is the fact that during the approach, despite the aircraft being 280 feet below the actual mapped profile and vertical flight angle (VPA). Other pilot actions like an altitude versus distance crosscheck or the TAWS Too low terrain alert would not have prevented the incident.

So what happened?

First, let’s examine the main differences of the most common RNAV (Geographic Positioning System) Approaches. It is extremely important for pilots to understand the nuances of each approach type and the source of its altitude reference.

For this discussion, a RNAV (Geographic Positioning System) approach is the FAA Equivalent to a ICAO RNP approach.

Most common today RNAV (Geographic Positioning System) Approaches are localization performance with vertical guidance (LPV) and lateral navigation/vertical navigation (LNAV/VNAV) Approaches. According to that FAABoth methods offer high-quality vertical navigation functions that improve safety and reduce risks CFIT.

LPV Approaches use the high-precision wide-area augmentation system (WHAT) for both lateral and vertical guidance to allow an approach flown to a THEREsimilar to a category I ILS. The design of a LPV Approach involves angular guidance with increasing sensitivity as the aircraft approaches the runway.

LNAV/VNAV Approaches provide both horizontal and approved vertical approach guidance. VNAV returns an internally generated slideway or VPA– nominally set at three degrees – based on both WHAT or barometric-VNAV (or baro-VNAV) systems. LNAV/VNAV Minimum values ​​are published as a THERE. baroVNAV Systems are common on aircraft not equipped with a WHAT-enabled system or if WHAT is dismantled.

As the name suggests, Baro-VNAV Systems rely on an accurate barometric setting for their altitude reference – inadvertently selecting the wrong barometric setting in the aircraft’s altitude measurement system can provide a vertical path that is lower or higher than published altitudes on instrument approach procedures (IAP). It is imperative that pilots and air traffic controllers understand them VNAV Capabilities of an aircraft and the importance of using the correct barometric setting and applying temperature corrections.

High to low, look down

In the investigations into LFPG vicinity-CFIT It was found that the pilots of the event A320 planned to fly RNAV (Geographic Positioning System) Approach LNAV/VNAV At least based on the ILS was out of order and the plane was not WHAT fitted.

When descending into LFPGthe crew copied those ATIS as 1,500 ft broken, 10 km visibility and QNH 1001 hectopascals (hPa). It was later determined that there were rain showers in the terminal area.

Upon arrival in the terminal area ATC the flight crew cleared to 6,000 feet and stated: “QNH 1011 hPa.” Two minutes later another descent clearance to 5,000 feet was given QNH 1011 hPa. The flight crew confirmed each descent clearance and repeated the wrong thing twice QNH from 1011 hPa.

The difference from 1011 hPa to 1001 hPa corresponded to an error of 280 feet (1 hPa corresponds to 28 feet). The equivalent error using inches of mercury would be 29.85 and 29.56 inches, respectively.

For the remainder of the approach, the aircraft was approximately 280 feet below the correct approach altitude and trajectory. At the final descent point, the aircraft began its descent in final approach guidance mode with the autopilot and autothrottle engaged. The conditions during the approach were IMC with rain showers.

At 1,000 feet above field elevation, the aircraft was fully configured for landing and “at speed” in Vapp. Flight deck displays indicated the aircraft was on its expected horizontal and vertical path, although the aircraft remained 280 feet below and parallel to the actual VPA.

At 200 feet RA (1.53 nm from the runway) the air traffic controller received a warning for the minimum safe altitude (MSAW). Moments later, the air traffic controller transmitted a warning to the flight crew MSAW Alarmed and asked the flight crew to confirm that the runway was in sight – at this point the aircraft was at an altitude of approximately 122 feet RA.

Six seconds later at 52 feet RAthe captain applied pitch-up inputs and selected TOGA. The lowest point recorded during the first approach was six feet RA.

During the go-around, the air traffic controller cleared the flight crew to 5,000 feet with a QNH of 1001 hPa, but the pilot repeated 1011 hPa – the wrong barometric setting.

The second approach was also flown with the wrong barometric setting. During this approach another MSAW The alarm sounded, but the crew was able to establish visual contact with the runway, disengage the autopilot, and adjust the aircraft’s flight path to line up with the runway PAPI indication and land without further problems.

After this incident, Airbus published an article highlighting the dangers of flying RNAV (Geographic Positioning System) Approach LNAV/VNAV Minima with an incorrect barometric setting. The manufacturer warned: “Using the wrong barometric setting (or QNH) may cause the aircraft to fly lower than the published approach path when vertical guidance and flight path deviations use the barometric reference.”

The article explained the possible consequences of using the wrong barometric reference and gave pilots guidance on how to spot an error to: a CFIT Accident at night or when visibility is poor.

Airbus reported that an incorrect entry on the QNH affects all final approach guidance modes that use a barometric reference.

Accordingly, the article states: “The FMS uses the aircraft’s barometric altitude to calculate the deviation of the aircraft’s flight path from the calculated final descent path. If an incorrect altitude is used, the aircraft will follow a flight path that is parallel to the published path, but offset either above or below it.” Flight deck displays show that the aircraft is on the correct flight path even when it is not .

Incorrect barometric adjustment will negate the effectiveness of an altitude versus distance check by the flight crew. According to Airbus, “These checks use the indicated barometric altitude based on the erroneous barometric setting. The effect is that the flight crew will find they are at the expected altitude for any range value, even if the aircraft is flying above or below the published trajectory.”

During the event there was an absence of TAWS warnings. This was due to the closeness of the actual flight path to the published path and because it stayed outside the terrain clearance alert envelope.

Airbus has identified two ways for pilots to detect a discrepancy in barometric settings: one during descent and the other during final approach.

When descending, when ATC provides a descent clearance, pilots should look for a barometric reference that differs significantly from that ATIS (or other weather source) used during approach preparation. A difference is a symptom of barometric reference error and the flight crew should then compare and verify the correct barometric reference from all available sources.

Unexpected low on final approach RA Labels above the field elevation are an indication that the aircraft may be too low on the flight path – this can be caused by an incorrect barometric setting. This may be less reliable due to underlying terrain under the approach path (VPA).

RNAV (Geographic Positioning System) Vertical guidance approaches have the risk of a CFIT Accident by eliminating the level segment – at MDA— a non-ILS approach. The pilot must understand the altitude references of each type of approach and be aware of the pitfalls such as: B. incorrect barometric settings, which can potentially lead to a disaster.

The opinions expressed in this column are those of the author and are not necessarily endorsed by him AIN media group.

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