AINsight: Six Feet from Disaster

 - November 23, 2022, 8:30 AM

Pilots have benefited from RNAV (GPS) approaches with vertical guidance for more than two decades. These approach procedures provide constant vertical guidance to a decision altitude (DA) and provide a huge leap in safety by eliminating the hazards associated with “dive-and-drive” non-precision approach procedures.

Undoubtedly, RNAV (GPS) approaches have reduced the threat of a controlled flight into terrain (CFIT) accident, but if not flown properly can be just as dangerous. 

Case-in-point: in May, the pilots of an Airbus A320 flying an RNAV (GPS) approach came within six feet—based on the radio altimeter (RA)—of hitting terrain 0.8 nm short of the runway at Paris Charles de Gaulle (LFPG) Airport. French civil aviation accident investigation agency BEA immediately commenced an investigation and classified the event as a serious incident.  

The approach, as flown, was stable and procedurally correct with one exception—the crew failed to catch an error by an air traffic controller.

Most alarming is the fact that during the approach, the flight deck displays indicated that the aircraft was on its expected lateral and vertical path even though the aircraft was 280 feet below the actual charted profile and vertical path angle (VPA). Other pilot actions such as an altitude-versus-distance cross-check or the TAWS “Too Low Terrain” alert would not have prevented the incident.

So, what happened? 

First, let's explore the major differences of the most common RNAV (GPS) approaches. For pilots, understanding the nuances of each approach type and the source of its altitude reference are extremely important.

For this discussion, an RNAV (GPS) approach is the FAA equivalent of an ICAO RNP approach.

Today, the most common RNAV (GPS) approaches are localizer performance with vertical guidance (LPV) and lateral navigation/vertical navigation (LNAV/VNAV) approaches. According to the FAA, both procedures provide high-quality vertical navigation capabilities that improve safety and reduce the risks of CFIT.

LPV approaches take advantage of the highly accurate wide-area augmentation system (WAAS) for both lateral and vertical guidance to provide an approach that is flown to a DA, much like a Category I ILS. The design of an LPV approach incorporates angular guidance with increasing sensitivity as the aircraft gets closer to the runway.

LNAV/VNAV approaches provide both horizontal and approved vertical approach guidance. VNAV provides an internally generated glideslope or VPA—nominally set at three degrees—based on either WAAS or barometric-VNAV (or baro-VNAV) systems. LNAV/VNAV minimums are published as a DA. Baro-VNAV systems are common on aircraft not equipped with a WAAS-enabled system or if WAAS is degraded.

As the name implies, Baro-VNAV systems rely on an accurate barometric setting for its altitude reference—inadvertently selecting the wrong barometric setting in the aircraft’s altimetry system can provide a vertical path that is lower or higher than the published altitudes on the instrument approach procedure (IAP). It is imperative that pilots and air traffic controllers understand the VNAV capabilities of an aircraft and the importance of using the correct barometric setting and applying any temperature corrections.

High to Low, Look Out Below

During the investigation into the LFPG near-CFIT event, it was determined that the pilots of the A320 planned to fly the RNAV (GPS) approach to LNAV/VNAV minima based on the ILS being out of service and the aircraft was not WAAS equipped. 

During the descent into LFPG, the crew copied down the ATIS as 1,500 ft broken, 10 km visibility, and QNH 1001 hectopascals (hPa). Later it was determined that there were rain showers in the terminal area. 

Upon arrival in the terminal area, ATC cleared the flight crew to 6,000 feet and stated “QNH 1011 hPa.” Two minutes later a further descent clearance was issued to 5,000 feet with QNH 1011 hPa. The flight crew acknowledged each descent clearance and twice repeated the wrong QNH of 1011 hPa.

The difference of 1011 hPa to 1001 hPa equated to a 280-foot error (1 hPa equals 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 path. At the final descent point, with autopilot and autothrust on, the aircraft began its descent in the final approach guidance mode. 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 “on speed” at Vapp. Flight deck displays indicated that the aircraft was on its expected horizontal and vertical path, even though the aircraft remained 280 feet below and paralleling the actual VPA.

At 200 feet RA (1.53 nm from the runway), the controller received a minimum safety altitude warning (MSAW). Moments later, the controller transmitted a warning to the flight crew about the MSAW alert and asked the flight crew to confirm that the runway was in sight—at this point, the aircraft was at approximately 122 feet RA.

Six seconds later at 52 feet RA, the 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 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 using the incorrect barometric setting. During this approach, another MSAW alert sounded but the crew was able to establish visual contact with the runway, disconnect the autopilot, adjust the aircraft’s trajectory to align with the PAPI indication, and land without further issues.

Following this incident, Airbus published an article that highlighted the dangers of flying an RNAV (GPS) approach to LNAV/VNAV minima using an erroneous barometric setting. The manufacturer cautioned saying “using the wrong barometric setting (or QNH) may cause the aircraft to fly lower than the published approach path when the vertical guidance and trajectory deviations use the barometric reference.” 

The article explained the potential consequences of using the wrong barometric reference and provided guidance to pilots on how to detect an error to prevent a CFIT accident at night or in poor visibility.

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

Accordingly, the article states, “The FMS uses the aircraft barometric altitude to compute the deviation of the aircraft trajectory with the computed final descent path. If an erroneous 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.” Flight deck displays will indicate that the aircraft is on the correct flight path even if that is not the case. 

An erroneous barometric setting will negate the effectiveness of an altitude-versus-distance check by the flight crew. According to Airbus, “These checks use the displayed barometric altitude, which is based on the erroneous barometric setting. The effect is that 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.”

During the event, there was an absence of TAWS alerts. This was due to the proximity of the actual flight path to the published path and remaining outside of the terrain clearance floor alert envelope. 

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

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

During final approach, unexpected low RA callouts above field elevation are a clue that the aircraft may be too low on the flight path—this may be caused by an erroneous barometric setting. This may be less reliable due to the underlying terrain beneath the approach path (VPA). 

RNAV (GPS) approaches with vertical guidance have greatly reduced the threat of a CFIT accident by eliminating the level segment—at MDA—of a non-ILS approach. The pilot must understand the altitude references of each type of approach and be aware of the gotchas, such as erroneous barometric settings, that can potentially lead to disaster.

The opinions expressed in this column are those of the author and not necessarily endorsed by AIN Media Group.