Created Tuesday, Nov 23rd 2021 17:54Z, last updated Wednesday, Feb 2nd 2022 17:27Z
A Royal Air Maroc Boeing 737-800, registration CN-ROJ performing flight AT-780
from Rabat (Morocco) to Paris Orly (France), was on an Cat III ILS approach to Paris Orly when the crew decided to go around from about 1400 feet due to a technical fault and to divert to Lyon (France). The aircraft climbed to FL250 and was on an ILS approach to Lyon's runway 35R when the aircraft went through the localizer without intercepting the localizer. The crew opted to perform a manual approach to runway 35R but descended resulting in Minimum Safe Altitude Warnings at the ATC desk and EGPWS alerts in the cockpit. The crew climbed to 5000 feet, was vectored for another approach to runway 35R and landed without further incident.
France's BEA rated the occurrence a serious incident, labelling the type of occurrence as "unstabilized approach, descent below minimum safe altitude, MSAW and EGPWS alerts" and as controlled flight into terrain, and opened an investigation.
On Nov 23rd 2021 the BEA released their final report in French only concluding the probable causes of the serious incident were:
- Two concurrent independent failures of two different aircraft systems, the origin and consequences of which were difficult to establish for the crew without adequate information in the operational documentation and insufficiently salient alarms triggered by the aircraft systems
- Gradual loss of confidence into the aircraft systems by the crew during the course of the flight causing significant stress onto the crew who focussed their attention on the remaining fuel levels
- The degradation of CRM in times of high workload, that became particularly high due to the need of managing an IRS FAULT during the go around
- The operating logic of the Flight Control Computer (FCC), which in accordance with its specifications does not provide monitoring inertial data of the ADIRUs except when both autopilots are engaged during approaches.
- The logic of internal ADIRU monitoring with respect to verifying validity of data transmitted to other systems. The criteria for drift angle failure, which caused the IRS FAULT alarm, can appear late in relation to the beginning of the ADIRU Inertial Reference System malfunction
- Unsystematic reporting of technical malfunctions by crews like the disappearance of F/D, ADIRU malfunctions and automatic disconnections of the autopilot
- The persistence of intermittent failures
As the BEA have pledged to release all reports in English in due time, The Aviation Herald will summarize the final report in more details as soon as the English version has been released.
On Feb 2nd 2022 the BEA released their final report in English translating the conclusion as follows:
- The concomitance of two independent failures within two separate systems where the cause of the failures, the absence of any link and the consequences were difficult for the crew to determine, without appropriate information in the operational documentation or sufficiently salient warnings emitted by the aircraft systems.
- The crew’s progressive loss of confidence in the plane’s systems as the flight continued, causing substantial stress and the crew to focus their attention on the level of remaining fuel.
- The deterioration in CRM by the crew when the workload became very high due, in particular, to having to manage the IRS FAULT during the missed approach phase.
- The operating logic of the FCC which does not monitor the inertial data provided by the ADIRU, except for approaches with the two A/P engaged. The FCC was not designed to, nor was it required for certification, to monitor the ADIRU inputs.
- The ADIRU internal monitoring logic with respect to the validity of the inertial data transmitted to other systems. The activation criteria of the “Drift Angle” fault, which in turn activates the IRS FAULT warning, can cause the latter to appear at a late stage with respect to the start of the ADIRU IR module malfunction.
- The crews’ non-systematic reporting of technical malfunctions in the TLB such as the disappearance of the F/D, the ADIRU malfunctions and the A/P automatically disconnecting.
ˆ- The persistence of intermittent faults.
The BEA analysed the scenario:
En route FMC alert messages (left IRS)
Around one hour after take-off, the Flight Management Computer (FMC) detected inconsistent aeroplane speed and position data coming from the left Inertial Reference System (IRS). The “IRS-L DRIFT” alert message was displayed on the screen of the Control Display Unit (CDU). Unable to find the procedure associated with this message in the onboard documentation, the crew carried out actions from memory, which consisted of checking the position given by the two IRS. It appeared that the left IRS was indicating erroneous and fluctuating position and speed values. The crew observed that the problem seemed to be resolving itself.
During the descent to Paris-Orly, a second alert message appeared on the CDU (“VERIFY POS: IRS-IRS”). Given that there was no Master Caution and IRS FAULT warnings, the crew considered that the IRS was operating and that the values would become consistent again before arriving at Paris-Orly.
The crew decided to continue to Paris-Orly on a CAT III ILS 06 approach and to divert to Lyon if there was a problem during the approach.
CAT III approach to Paris-Orly and impossibility to engage both A/P
On intercepting the LOC signal, the A/P commanded an S-shaped path around the LOC path which led the Approach controller to ask the crew if they had indeed intercepted the LOC. The crew did not reply.
The flight was transferred to the Tower without being given an approach clearance. The APP mode was thus not armed. As a consequence, the crew’s attempt to engage both A/P at this point failed.
After receiving the approach clearance from the Tower and arming the APP mode, the crew twice tried to engage the second A/P (A/P B). This time, it was the Flight Control Computer (FCC B) not receiving RA 2 data which prevented A/P B from being engaged and also caused A/P A to disengage. The CAT III ILS approach could not be carried out with this malfunction.
The crew re-engaged A/P A, informed the controller that they could not perform a CAT III ILS, and requested to fly a missed approach and divert to Lyon-Saint Exupéry airport.
Conditions at Lyon permitted a CAT I ILS approach, which could be carried out with only one A/P functioning.
On carrying out the missed approach at Paris-Orly to head to Lyon, the quantity of fuel on board permitted an estimated holding time of 27 minutes at Lyon(42), before carrying out the approach and landing at the alternate airport with a final fuel quantity of 1,100 kg. Taking into account the trip fuel of 2,100 kg between Orly and Lyon (including approach to Lyon) and a holding fuel consumption of 40 kg/mn.
Diversion to Lyon
During the diversion and the preparation for the ILS 35R approach to Lyon-Saint Exupéry, the crew indicated that they tried to arm, without success, the automatic braking system and observed that the amber AUTOBRAKE DISARM light came on.
The non-operation of the automatic braking system was possibly linked to the malfunction of the left IRS.
As the manufacturer’s documentation did not indicate the possible consequences of the FMC “IRS-L DRIFT” and “VERIFY POS: IRS-IRS” messages on the automatic braking system, and as the IRS FAULT light was not illuminated, the crew had no way of anticipating the nonarming of the automatic braking system and of being informed of the left IRS malfunction which seemed to be nevertheless present at this moment.
The erroneous selection of the LSE VOR combined with the indication on the CDU that the LSE VOR was situated at an improbable distance then made the crew doubt the position of the plane given by the FMS.
First missed approach at Lyon and IRS FAULT warning
When the A/P changed to intercept mode and the Localizer radio signal was captured (green VOR/ LOC mode), the aeroplane suddenly veered to the right with a bank angle of 30°, crossed the Localizer path and then departed from it on a clearly divergent heading (heading 70°) without any onboard warning being activated. This untimely right turn was commanded by the A/P due to the erroneous data from the IRS module of the left ADIRU.
The captain thought that this untimely manoeuvre was linked to the reception of a false LOC signal.
The invalidity of the data supplied by the left ADIRU next led to the disengagement of A/P A. The captain then manually piloted the plane with the F/D displayed on the PFD.
The crew first tried to return to the path. As the deviation from the path compromised continuing the approach, they flew a missed approach. The Tower controller cleared them to climb to 5,000 ft.
The crew selected the target altitude of 5,000 ft on the MCP on climbing through 4,600 ft. The F/D command bars were not followed by the captain who was flying manually. The captain unintentionally exceeded the altitude of 5,000 ft, and engaged A/P B when the aeroplane flew through 5,650 ft still in climb. The aeroplane temporarily stopped climbing and then made a short descent to the selected altitude of 5,000 ft.
The aeroplane had just been transferred to the Approach again when the L IRS FAULT warning was activated causing the disengagement of A/P B, and the disappearance of the pitch, roll and heading data along with the F/D bars from the left PFD. The captain again flew manually while carrying out from memory one of the actions of the check-list, namely IRS Transfer Switch - BOTH ON R. The two FCC thus used the data supplied by the right ADIRU. The pitch, roll and heading were displayed again on the left PFD and the F/D bars reappeared.
Twenty seconds after his action on the IRS Transfer Switch, the captain engaged the A/T and A/P B. The aeroplane which had climbed to an altitude of around 6,500 ft started descending in order to return to 5,000 ft.
The captain transferred control of the aircraft to the co-pilot and then tried to use the ACARS system in order to obtain the latest weather information for a possible diversion to Marseilles. The system displayed the “ACARS NO COM” message.
The captain, who said that he was worried about the fuel during this phase, decided to carry out a second approach to Lyon-Saint Exupéry.
At this point, the alternate fuel of 2,100 kg had been used as provided for and the quantity of fuel on board permitted the plane to hold for 27 minutes before making a new approach to Lyon(43) and landing with at least the final fuel quantity of 1,100 kg.
During this phase, the crew did not carry out the “IRS FAULT” check-list in the QRH, as they considered that they lacked time, which meant that they were not aware of the limitations concerning the use of the A/P in approach imposed by the manufacturer.
Second approach to Lyon and non-stabilization of approach
During the Localizer interception phase, the captain asked for a radar vector, indicating that they were having positioning problems. The controller vectored them to the ILS.
The crew carried out a “fast” approach with respect to the (speed, flaps, landing gear) configurations recommended by the ILS approach procedure described in the FCOM. A/P B was kept engaged during the approach.
On capturing the Glide, A/P B automatically disengaged due to FCC B not receiving data from RA 2. The F/D then disappeared from both sides. The co-pilot transferred control of the aircraft to the captain. As the A/P had disconnected before starting the final descent, the captain thus had to manage both taking back control of the aircraft and an immediate initiation of descent in order to intercept the approach slope.
This interception was made all the more difficult by the captain not having had the time to calibrate his inputs on the stick, i.e. to adjust them in terms of the force and amplitude required to correct the path deviations, as is the case when the disconnection of the A/P is anticipated and is made by the pilot beforehand.
The captain continued to manage the radio communications until being transferred to the Tower controller and twice informed the Approach controller that he was continuing the approach in manual.
The approach, in manual and without the F/D, was not stabilized with respect to either the path and slope or the aeroplane’s speed and configuration. Several EGPWS “SINK RATE”, “GLIDE SLOPE” and “TOO LOW TERRAIN” warnings were activated on final.
The go-around called out by the co-pilot was not followed as according to the captain, it was concomitant with sight of the runway. The co-pilot then announced to the controller, in a voice revealing hesitation and substantial stress, that they were continuing the manoeuvre after all.
The captain would specify later that he was aware that the aeroplane was not stabilized but that he preferred correcting the path and landing due to the stress level generated by the two missed approaches already made.
The BEA analysed the left ADIRU's inertial reference system suffered internal problems and produced erroneous ground speed and position data, which was detected by the FMS. The FMS stopped accepting the left IRS data.
On approach to Lyon the flight control computer FCC A however used the data without further checks resulting in 30 degrees of roll and a heading divergency of 70 degrees, the ADIRU declared the route data invalid causing A/P A to disengage. During the go around invalid roll data prompted autopilot B to disengage, too.
In the subsequent analysis the BEA states:
The analysis of the QAR data and fault messages recorded in the FCC BITE brought to light that the RAM crews did not systematically report the technical malfunctions in the CN-ROJ TLB.
The crews being used to the phenomenon of the F/D disappearing, explained above, may be one of the reasons for this.
However, it is more difficult to explain the absence of reports concerning the automatic disengagements of an A/P or the malfunctions of an ADIRU shown by the appearance of an IR FAULT warning or FMC alert messages.
With respect to maintenance the BEA analysed:
The examination of the FCC carried out after the incident showed that RA 2 of CN-ROJ had been encountering malfunctions since 26 June 2016, i.e. around six months before the occurrence. These malfunctions, shown by the RADIO ALT-2 (J1B-B04, A04) fault message in the BITE of FCC B, were due to the FCC B not receiving valid data from RA 2. These fault messages had consequences on the automated systems: disappearance of the F/D display and/or disengagement of the A/P during the approach phase.
In the six months up until the day before the occurrence, this fault linked to RA 2 was recorded 35 times, including 16 times since 9 December. During the occurrence flight, this same fault, RADIO ALT-2 (J1B-B04, A04) was recorded seven times in FCC B.
Although not systematically reported by the crews in the TLB, these malfunctions were indicated after flights carried out on 10, 12 and 25 December. In the TLB of these flights, the maintenance operation steps were not all systematically indicated which meant that it was not possible for the BEA to follow the complete sequence of each maintenance action. RAM indicated to the BEA that the reply in the TLB was the final/major maintenance action carried out on entering the engineering shop (45). The reading of the maintenance documents suggests that the fault did not reappear during the ground tests, thereby indicating, in accordance with the FIM, that either it was intermittent or it only appeared in flight conditions.
The Boeing procedure indicates that, in this case, the maintenance technicians must comply with the operator’s policy for processing intermittent faults, use their judgement and the operator’s maintenance history and specifically monitor the aeroplane in question.
Following the crew’s report of a problem linked to RA 2 on 10 December 2016, RAM replaced it. However, the fault occurred again on 12 December 2016. The cause of the fault was therefore not removed. It then reoccurred on 25 December 2016 after 54 problemfree flights. FCC B was replaced three days after the occurrence, on 2 January 2017, without the problem disappearing. It was therefore highly likely that the malfunction was due to neither RA 2 or FCC B, but due to the connection between the two systems.
Although it appeared on several occasions and its consequences were notified by the crews, the base maintenance centre was not able to solve this fault, resulting, on the day of the event, in a degraded aircraft, presenting automated system problems mainly in the approach phase.
LFLL 301230Z VRB03KT 4000 BR BKN007 01/M00 Q1037 NOSIG
LFLL 301200Z VRB02KT 4000 BR OVC006 01/M01 Q1037 NOSIG
LFLL 301130Z 36004KT 4000 BR OVC006 00/M01 Q1037 NOSIG
LFLL 301100Z VRB02KT 4000 BR OVC005 00/M01 Q1038 NOSIG
LFLL 301030Z VRB02KT 3000 BR OVC004 M00/M01 Q1038 NOSIG
LFLL 301000Z 00000KT 3000 BR OVC004 M00/M01 Q1038 NOSIG
LFLL 300930Z 36002KT 2000 BR OVC003 M01/M01 Q1038 NOSIG
LFLL 300900Z VRB02KT 3000 BR OVC002 M01/M01 Q1038 NOSIG
LFLL 300830Z VRB03KT 2500 0800S R35L/P2000 R17R/P2000 R35R/P2000 R17L/P2000 BCFG OVC002 M01/M01 Q1038 NOSIG
LFLL 300800Z VRB02KT 2500 0800S R35L/1700U R17R/P2000 R35R/1700U R17L/P2000 BCFG OVC001 M01/M02 Q1038 NOSIG
Metars Paris Orly:
LFPO 301200Z VRB02KT 0300 R02/0800N R26/0500N R08/0550N R24/0400N R06/1100D FZFG VV/// M02/M02 Q1038 TEMPO 0100 FZFG VV///
LFPO 301130Z VRB03KT 0300 R02/0550U R26/0325N R08/0900U R24/0500N R06/0600U FZFG VV/// M02/M03 Q1038 TEMPO 0100 FZFG VV///
LFPO 301100Z VRB02KT 0250 R02/0400N R26/0325N R08/0650D R24/0700N R06/0400N FZFG VV/// M02/M03 Q1038 TEMPO 0100 FZFG VV///
LFPO 301030Z VRB03KT 0250 R02/0500U R26/0300N R08/0700U R24/0650U R06/0450N FZFG VV/// M02/M03 Q1038 TEMPO 0100 FZFG VV///
LFPO 301000Z VRB02KT 0200 R02/0350N R26/0300N R08/0400N R24/0400N R06/0400N FZFG VV/// M02/M03 Q1038 TEMPO 0100 FZFG VV///
LFPO 300930Z VRB02KT 0200 R02/0350N R26/0250N R08/0600U R24/0325N R06/0400N FZFG VV/// M02/M03 Q1038 TEMPO 0100 FZFG VV///
LFPO 300900Z VRB02KT 0150 R02/0300N R26/0225N R08/0375N R24/0350N R06/0400N FZFG VV/// M02/M03 Q1038 TEMPO 0100 FZFG VV///
LFPO 300830Z VRB01KT 0200 R02/0375N R26/0225N R08/0325N R24/0350N R06/0400N FZFG VV/// M02/M02 Q1038 TEMPO 0100 FZFG VV///
LFPO 300800Z VRB01KT 0250 R02/0350N R26/0275N R08/0400N R24/0400N R06/0450N FZFG VV/// M02/M02 Q1038 TEMPO 0100 FZFG VV///
Related Flight: AT780
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