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AINsight: Six Toes from Catastrophe

Pilots have benefited from RNAV (GPS) approaches with vertical steering for greater than 20 years. These method procedures present fixed vertical steering to a call altitude (GIVES) and supply an enormous leap in security by eliminating the hazards related to “dive-and-drive” non-precision method procedures.

Undoubtedly, RNAV (GPS) approaches have decreased the specter of a managed flight into terrain (CFIT) accident, but when not flown correctly will be simply as harmful.

Case-in-point: in Could, the pilots of an Airbus A320 flying an RNAV (GPS) method got here inside six ft—based mostly on the radio altimeter (RA)—of hitting terrain 0.8 nm wanting the runway at Paris Charles de Gaulle (LFPG) Airport. French civil aviation accident investigation company BEA instantly began an investigation and categorized the occasion as a severe incident.

The method, as flown, was secure and procedurally appropriate with one exception—the crew did not catch an error by an air visitors controller.

Most alarming is the truth that throughout the method, the flight deck shows indicated that the plane was on its anticipated lateral and vertical path although the plane was 280 ft under the precise charted profile and vertical path angle (VPA). Different pilot actions similar to an altitude-versus-distance cross-check or the TAWS “Too Low Terrain” alert wouldn’t have prevented the incident.

So, what occurred?

First, let’s discover the largest variations of the most typical RNAV (GPS) approaches. For pilots, understanding the nuances of every method sort and the supply of its altitude reference are extraordinarily essential.

For this dialogue, an RNAV (GPS) method is the FAA equal of an ICAO RNP method.

At the moment, the most typical RNAV (GPS) approaches are localizer efficiency with vertical steering (LPV) and lateral navigation/vertical navigation (LNAV/VNAV) approaches. In keeping with the FAAeach procedures present high-quality vertical navigation capabilities that enhance security and scale back the dangers of CFIT.

LPV approaches benefit from the extremely correct wide-area augmentation system (WAAS) for each lateral and vertical steering to supply an method that’s flown to a GIVESvery similar to a Class I ILS. The design of an LPV method incorporates angular steering with rising sensitivity because the plane will get nearer to the runway.

LNAV/VNAV Approaches present each horizontal and authorized vertical method steering. VNAV offers an internally generated glideslope or VPA—nominally set at three levels—based mostly on both WAAS or barometric-VNAV (or baro-VNAV) methods. LNAV/VNAV minimums are printed as a GIVES. Baro-VNAV methods are widespread on plane not geared up with a WAAS-enabled system or if WAAS is degraded.

Because the identify implies, Baro-VNAV Programs depend on an correct barometric setting for its altitude reference—inadvertently choosing the fallacious barometric setting within the plane’s altimetry system can present a vertical path that’s decrease or larger than the printed altitudes on the instrument method process (IAP). It’s crucial that pilots and air visitors controllers perceive the VNAV capabilities of an plane and the significance of utilizing the right barometric setting and making use of any temperature corrections.

Excessive to Low, Look Out Under

In the course of the investigation into the LFPG near-CFIT occasion, it was decided that the pilots of the A320 deliberate to fly the RNAV (GPS) method to LNAV/VNAV based mostly on the ILS being out of service and the plane was not WAAS geared up.

In the course of the descent into LFPGthe crew copied down the ATIS as 1,500 ft damaged, 10 km visibility, and QNH 1001 hectopascals (hPa). Later it was decided that there have been rain showers within the terminal space.

Upon arrival within the terminal space, ATC cleared the flight crew to six,000 ft and said “QNH 1011 hPa.” Two minutes later an extra descent clearance was issued to five,000 ft with QNH 1011 hPa. The flight crew acknowledged every descent clearance and twice repeated the fallacious QNH of 1011 hPa.

The distinction of 1011 hPa to 1001 hPa equated to a 280-foot error (1 hPa equals 28 ft). The equal error utilizing inches of mercury could be 29.85 and 29.56 inches, respectively.

For the rest of the method, the plane was roughly 280 ft under the right method altitude and path. On the ultimate descent level, with autopilot and autothrust on, the plane started its descent within the ultimate method steering mode. The situations throughout the method had been BMI with rain showers.

At 1,000 ft above subject elevation, the plane was totally configured for touchdown and “on velocity” at Vapp. Flight deck shows indicated that the plane was on its anticipated horizontal and vertical path, although the plane remained 280 ft under and paralleling the precise VPA.

At 200 ft RA (1.53 nm from the runway), the controller obtained a minimal security altitude warning (MSAW). Moments later, the controller transmitted a warning to the flight crew in regards to the MSAW alerted and requested the flight crew to verify that the runway was in sight—at this level, the plane was at roughly 122 ft RA.

Six seconds later at 52 ft RAthe captain utilized pitch-up inputs and chosen TOGA. The bottom level recorded throughout the first method was six ft RA.

In the course of the go-around, the controller cleared the flight crew to five,000 ft with a QNH of 1001 hPa, however the pilot repeated 1011 hPa—the fallacious barometric setting.

The second method was additionally flown utilizing the wrong barometric setting. Throughout this method, one other MSAW alert sounded however the crew was in a position to set up visible contact with the runway, disconnect the autopilot, regulate the plane’s trajectory to align with the DADDY indication, and land with out additional points.

Following this incident, Airbus printed an article that highlighted the hazards of flying an RNAV (GPS) method to LNAV/VNAV minima utilizing an faulty barometric setting. The producer cautioned saying “utilizing the fallacious barometric setting (or QNH) might trigger the plane to fly decrease than the printed method path when the vertical steering and trajectory deviations use the barometric reference.”

The article defined the potential penalties of utilizing the fallacious barometric reference and supplied steering to pilots on the best way to detect an error to stop a CFIT accident at night time or in poor visibility.

Airbus reported that an faulty entry on the QNH selector impacts all ultimate method steering modes that use a barometric reference.

Accordingly, the article states, “The FMS makes use of the plane barometric altitude to compute the deviation of the plane trajectory with the computed ultimate descent path. If an faulty altitude is used, the plane will observe a flight path that’s parallel to the printed path however is shifted both above or under it.” Flight deck shows will point out that the plane is on the right flight path even when that’s not the case.

An faulty barometric setting will negate the effectiveness of an altitude-versus-distance verify by the flight crew. In keeping with Airbus, “These checks use the displayed barometric altitude, which is predicated on the faulty barometric setting. The impact is that the flight crew will observe that they’re on the anticipated altitude for every distance worth, even when the plane is flying above or under the printed flight path.”

In the course of the occasion, there was an absence of TAWS alerts. This was as a result of proximity of the particular flight path to the printed path and remaining outdoors of the terrain clearance flooring alert envelope.

Airbus has recognized two alternatives for pilots to detect a discrepancy in barometric settings: one throughout descent and the opposite throughout ultimate method.

Throughout descent, when ATC offers a descent clearance, pilots ought to take note of a barometric reference that’s considerably completely different from the ATIS (or different climate supply) used throughout method preparation. A distinction is a symptom of a barometric reference error, and the flight crew ought to then examine and confirm the right barometric reference from all out there sources.

Throughout ultimate method, sudden low RA Callouts above subject elevation are a clue that the plane could also be too low on the flight path—this can be brought on by an faulty barometric setting. This can be much less dependable as a result of underlying terrain beneath the method path (VPA).

RNAV (GPS) approaches with vertical steering have enormously decreased the specter of a CFIT accident by eliminating the extent phase—at MDA—of a non-ILS method. The pilot should perceive the altitude references of every sort of method and pay attention to the gotchas, similar to faulty barometric settings, that may probably result in catastrophe.

The opinions expressed on this column are these of the writer and never essentially endorsed by AIN MediaGroup.


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