Airhub 4311

The crew of the Airbus A320, performing scheduled flight NSZ4311, took off on 23 May around 09:30 from Stockholm Arlanda airport (Sweden) bound for Paris-Charles de Gaulle airport (CDG). The captain was the PF, the co-pilot was the PM.

Before descent, the flight crew prepared for a RNP approach with LNAV/VNAV minima to CDG runway 27R. The meteorological conditions indicated in the ATIS Q used by the flight crew when preparing the approach were the following: transition level 70, wind 280 / 10 kt, visibility 10 km, broken clouds at 1,500 ft, few cumulonimbus (CB) at 5,000 ft, temperature 19 °C, dew point 14 °C, QNH 1001.

The crew stated that during all the approach they remained in clouds, without visual references. They experienced moderate turbulence and flew through heavy rain, using the wipers at high speed.

At 11:32:24, on approach to CDG, the Intermediate (ITM) controller instructed the flight crew to descend to 6,000 ft with an incorrect QNH (1011 instead of 1001 valid at the time), “Red Nose 4 3 1 1, descend . . . descend 6,000 ft, 1 0 1 1.”

The PM read back with this QNH, “6,000 ft, 1 0 1 1 . . . 1 . . . 0 1 1, Red Nose 4 3 1 1.”

At 11:34:28, she instructed the crew to descend to 5,000 ft, repeating the incorrect QNH, “Red Nose 4 3 1 1, descend 5,000 ft, 1 0 1 1, cleared full RNP 2 7 R.”

The PM read back the information, “Descend 5,000 ft, QNH 1 0 1 1, cleared full RNP approach 2 7 R, Red Nose 4 3 1 1.”

Source: BEA, §1

At 11:35:37, the ITM instructed an easyJet flight crew to descend to 5,000 ft with the same incorrect QNH, “Easy 7 5 Mike Alpha direct Papa Golf 6 5 0 . . . and descend 5,000 ft 1 0 1 1, cleared RNP approach 2 7 R.” The easyJet crew read back giving the valid QNH, namely 1001, without asking for confirmation of the QNH value, “Direct to Papa Golf 6 5 0 descent 5,000 ft QNH 1 0 0 1 Easy 7 5 Victor Alpha.” This was not noticed by the controller.

At 11:36:04, the ITM instructed an Air France flight crew, in French, to descend to 5,000 ft, with the correct QNH 1001. The Air France crew read back with the correct QNH 1001.

Source: BEA, §1

At 11:36:55, the crew of the NSZ4311 flight reached the final descent point (FDP) at an indicated altitude of 4,889 ft QNH 1011 (4,623 ft QNH 1001), and at 14.3 NM from the runway threshold. The indicated airspeed was 185 kt and the aircraft was in configuration CONF 2.

At 11:37:56, the ITM instructed the flight crew to contact the north tower (LOC N controller).

NOTE: the flight crew stated that they conducted the altitude-distance checks every mile during the approach, which led them to believe that they were on the descent profile. However, in such a situation where the altimeter setting is incorrect, during a RNP approach with LNAV/VNAV minima, altitude-distance checks do not detect that the flight path has deviated.

Due to the incorrect QNH setting (1011 instead of 1001), the altitude value displayed on the aircraft instruments was around 280 ft above the real aircraft altitude. The flight crew were thus conducting a RNP approach with LNAV/VNAV minima around 280 ft below the published approach descent profile.

At 11:38:44, the aircraft was descending through a Radio-Altimeter (RA) height of 2,500 ft. The RA height values were now displayed on the primary flight displays (PFD).

At 11:38:09, the flight crew contacted the LOC N who replied, “Bonjour Red Nose 4 3 1 1, you are number 1, wind 2 6 0°, 12 kt, runway 2 7 R, cleared to land.” The crew correctly read back the clearance.

At 11:40:49, at an indicated altitude of about 1,392 ft QNH 1011 (1,123 ft QNH 1001, 837 ft RA), corresponding to the stabilization altitude for the crew (1,000 ft above aerodrome), and at 3.1 NM from the runway threshold, the aircraft was configured for landing (CONF FULL) at a speed of 139 kt IAS (Vapp was 139 kt) and with a vertical speed of -738 ft/min.

NOTE: aside from the fact that the aircraft’s flight path was not on the correct descent profile, the approach could be considered as stabilized as per standard criteria for the crew.

First approach profile, BEA, fig. 1

At 11:41:32, the ground Minimum Safe Altitude Warning (MSAW) was triggered (Point 1 Figure 1). The aircraft was at an indicated altitude of 891 ft QNH 1011 (617 ft QNH 1001, 200 ft RA), at 1.53 NM from the runway threshold.

NOTE: the controllers stated that the visibility and ceiling at the airport were sufficient not to implement LVP, but that locally in the east of the airport, the visibility was poorer due to clouds and rainfall. The controllers in the tower could not see the aircraft on short final because of the bad weather.

At 11:41:41, at 1.2 NM from the runway threshold, and with a vertical speed of -717 ft/min, the aircraft passed the indicated altitude of 802 ft QNH 1011 (537 ft QNH 1001, 122 ft RA), which corresponded to the Decision Altitude (DA) for the crew (Point 2 Figure 1).

The airline policy is to add 50 ft to the published minima for RNP approach with LNAV/VNAV minima. Therefore, according to the NavBlue chart used by the crew, their DA was 802 ft (752 ft + 50 ft).

Source: BEA, §1

The flight crew stated that arriving at the minima, they did not acquire visual references and consequently performed a go-around.

At the same time (11:41:41), nine seconds after the MSAW was triggered, the LOC N advised the crew, “Red Nose 4 3 1 1, I just had a ground proximity alert, are you okay? Do you see the runway?” (Point 2 Figure 1).

The crew stated they did not hear this radio-communication. During the message from the controller, a second MSAW was triggered.

At 11:41:47 and during the controller’s message, at an indicated altitude of 735 ft QNH 1011 (461 ft QNH 1001, 52 ft RA), and at 1 NM from the runway threshold, the AP was disengaged, and the captain pitched up (Point 3 Figure 1).

Three seconds later, at 11:41:50, at an indicated altitude of 679 ft QNH 1011 (405 ft QNH 1001), the minimum radio-altimeter height was recorded at 6 ft above the ground. The aircraft was 0.8 NM from the runway threshold. At the same time, the captain moved the thrust levers forward into the TOGA detent (Point 4 Figure 1).

No Terrain warning (TAWS) was recorded during the approach.

The two pilots stated that they did not hear any radio-altimeter Auto-Callouts other than the callouts at 2,500 ft and 1,000 ft.

At 11:42:00, the flight crew announced to the controller that they were going around (Point 5 Figure 1). He replied, “Roger Red Nose 4 3 1 1, turn right on heading 3 6 0 and climb altitude 5,000 ft, 1 0 0 1.” The crew read back with the previous incorrect QNH, which was not noticed by the LOC N, “3 6 0 and climb 5,000 ft on 1 0 1 1, Red Nose 4 3 1 1” (Point 6 Figure 1).

During these exchanges, the controllers in the tower could not yet see the aircraft. After a few seconds, they saw the aircraft coming out of the clouds, at low height, with a pitch up attitude.

At 11:42:05, the AP was reengaged at an indicated altitude of 1,203 ft QNH 1011 (930 ft QNH 1001, 593 ft RA), and at 0.3 NM before the runway threshold.

At the same time, and while the LOC N was transmitting the message to the flight crew, the LOC-S assistant advised the LOC-N assistant that they had not switched ON the approach lights.

At 11:42:27 the LOC N controller switched ON the approach lights. Following the MSAW and the omission of switching ON the approach lights, the LOC N was replaced by his LOC-N assistant and a new LOC-N assistant took over.

At 11:44:42 and after giving further radar vectors, the LOC N instructed the crew to contact the arrival (ITM controller) for the second approach. The aircraft was on the downwind leg for runway 27R.

At 11:45:00, the ITM controller indicated to the flight crew to expect RNP 27R.

At 11:49:09 and after further vectors, the ITM cleared the flight crew for a second RNP 27R approach.

At 11:53:40 and after being contacted by the crew, the LOC N cleared the crew for landing. The crew read back and asked if the approach lights were ON, which was confirmed by the controller.

NOTE: following the clearance to climb after the go-around and until landing, the QNH was given neither to the flight crew nor to another aircraft in contact with ATC on the same radio frequency as the crew.

The aircraft continued to fly with an incorrect altimeter setting. Neither the crew nor the controllers were aware of the incorrect QNH setting which again put the aircraft around 280 ft below the published descent profile.

At 11:55:43, a new MSAW was triggered. The aircraft was at an indicated altitude of 1,403 ft QNH 1011 (1,131 ft QNH 1001, 842 ft RA), and at 3.1 NM from the runway threshold.

Four seconds later, the LOC N controller (who was the LOC-N assistant for the first approach) advised the crew of the MSAW warning, “Red Nose 4 3 1 1, I just got a . . . a terrain alert, are you okay?”

The crew told the BEA that they did not understand the reason for this message. The PM answered, “Red Nose 4 3 1 1, we are established on path and we have visual now.”

At 11:56:00 at an indicated altitude of 1,227 ft QNH 1011 (954 ft QNH 1001, 572 ft RA), and at 2.54 NM from the runway threshold, the PF applied a nose up input on his sidestick. Simultaneously, the AP disconnected. Flight Directors (FD) were switched off eight seconds later.

The crew stated that this time they had visual contact with the runway well above the minima. They described the PAPI lights as follows, “One white, one pink, and two reds. Maybe three reds but not four.”

The PF corrected the flight path on final and then landed without further incident.

Source: BEA, §1

The QNH given to the flight crew differed by 10 hPa from the valid QNH at the time and included in the ATIS information, leading to an approach carried out around 280 ft below the descent profile.

The Air Traffic Control operational procedures require the controller to give the QNH to the flight crew at the first altitude clearance, but there is no obligation to repeat the QNH before landing. Initial interviews conducted as part of the investigation suggest that the importance of the QNH for approaches using the baro-VNAV function, with respect to the risk of CFIT, may be underestimated and not clearly understood by controllers.

In the case of a RNP approach with LNAV/VNAV minima, vertical guidance uses the baro-VNAV function. Therefore, in this case, QNH setting errors are critical. These errors cannot be detected by altitude-distance cross-checks against values provided on the approach chart. In addition, there is no dedicated item requiring a specific cross-check of the QNH with another source of information such as the ATIS, METAR or confirmation of the QNH with ATC.

The BEA recommends:

• That Paris-Charles de Gaulle Air Traffic Services: Ensure without delay, that controllers are aware of the importance of the QNH for approaches using the baro-VNAV function, with respect to the risk CFIT.
• Ensure without delay, that controllers are aware of the importance of checking that the information read back by flight crews is correct.
• That Airhub Airlines: Ensure without delay, that the flight crews are made aware of the importance of the QNH setting for approaches using the baro-VNAV function, with respect to the risk of CFIT.
• Implement without delay, a procedure to mitigate the risks of an incorrect QNH setting affecting both altimeters during approaches using the baro-VNAV function, possibly by crosschecking the QNH with another source of information, in particular with the ATIS information when available or by asking the controller for confirmation of the QNH.

Source: BEA, §4

In the case of a RNP approach with LNAV/VNAV minima, vertical guidance uses the baro-VNAV function. Therefore, in this case, QNH setting errors are critical. These errors cannot be detected by altitude-distance cross-checks against values provided on the approach chart. In addition, there is no dedicated item requiring a specific cross-check of the QNH with another source of information such as the ATIS, METAR or confirmation of the QNH with ATC.

Source: BEA, §4