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ASN Wikibase Occurrence # 192821
Last updated: 24 November 2021
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Type:Silhouette image of generic B738 model; specific model in this crash may look slightly different
Boeing 737-85P (WL)
Owner/operator:Royal Air Maroc
Registration: CN-ROJ
MSN: 33979/1963
Fatalities:Fatalities: 0 / Occupants: 155
Other fatalities:0
Aircraft damage: None
Category:Serious incident
Location:near Lyon Saint-Exupéry Airport (LYS/LFLL) -   France
Phase: Approach
Nature:Passenger - Scheduled
Departure airport:Rabat-Sale Airport (RBA/GMME)
Destination airport:Paris-Orly Airport (ORY/LFPO)
Investigating agency: BEA
The crew of Royal Air Maroc flight AT780, a Boeing 737-800, operated on a scheduled service from Rabat-Sale Airport in Morocco to Paris-Orly Airport, France.
During the cruise, the crew received alerts about the drift of the left inertial unit. The crew observed that the problem had resolved and continued the flight to Paris-Orly. During the ILS CAT III approach for runway 06 at Paris-Orly, the aircraft captured the Localizer for runway 06 by oscillating, probably due to the reception of erroneous data from the left ADIRU. Then the crew was unable to engage the two autopilots simultaneously, which made the ILS CAT III approach unfeasible, because the flight control computer (FCC B) did not receive the RA (Radio Altimeter) 2 data.
The crew did a go-around and requested a clearance to the Lyon-Saint Exupéry aerodrome where the weather conditions allowed a CAT I approach.
During the interception of the ILS 35R at Lyon, the aircraft made an untimely right turn, while in Localizer capture in VOR/LOC green mode, because of erroneous data provided by the left ADIRU. No alarm was produced on board at this moment. The autopilot A then disengaged. The crew requested a new approach and then engaged autopilot B. A few moments later, the L IRS FAULT alarm activated and caused autopilot B to disengage. The captain set the IRS Transfer Switch to BOTH ON R, one of the items on the IRS FAULT checklist, and then re-engaged autopilot B. One of the next checklist items states that the autopilot cannot be used during the approach. While vectoring to the Localizer, the captain indicated to the controller that he was having positioning problems. At the time of the capture of the glide slope, the autopilot B disengaged because of the non-reception by the FCC B of the data from the radio altimeter 2. No alarm was triggered on board. The F/Ds disappeared on the Pilot Flying and Pilot Monitoring sides. The controls were transferred to the captain by the first officer and the captain decided to continue the approach in manual flight, without autopilot, nor F/D. During the approach, flown in IMC, EGPWS alerts "SINK RATE", "GLIDE SLOPE" and "TOO LOW TERRAIN" followed one another.
The aircraft, not stabilized at 1000 ft AGL, exited the cloud layer, at 400 ft, between the two runway axes. The EGPWS "TOO LOW TERRAIN" and "SINK RATE" alerts continued to sound on board until the landing. The aircraft landed approximately 600 m from the threshold of runway 35R.

Contributing factors
Contributed to this serious incident:
- The simultaneous occurrence of two independent failures in two different systems whose origin, lack of connection and consequences were difficult to establish for the crew without adequate information in the operational documentation or sufficiently salient alarms issued by the aircraft systems.
- A progressive loss of confidence of the crew in the aircraft systems over the course of the flight, generating significant stress and a focus of the crew's attention on the remaining fuel level.
- The degradation of the CRM within the crew during a workload that has become very high, due in particular to the need to manage the IRS FAULT failure during the approach abort phase.
- The operating logic of the FCC, which does not, in accordance with its specifications, provide for monitoring of the inertial data supplied by the ADIRUs, except in the case of approaches with both A/Ps engaged.
- The internal monitoring logic of the ADIRUs with regard to the validity of the inertial data transmitted to the other systems. The criteria for triggering the Drift Angle failure, the factor that triggers the IRS FAULT alarm, can lead to the alarm appearing late in relation to the start of the malfunction of the ADIRU's IR module.
- The non-systematic reporting by the crews of technical malfunctions on the TLB such as the disappearance of the F/D, ADIRU malfunctions and automatic disconnections of the A/P.
- The persistence of intermittent failures


Accident investigation:
Investigating agency: BEA
Status: Investigation completed
Duration: 4 years and 11 months
Download report: Final report


Photo of CN-ROJ courtesy

Brussels - National / Melsbroek (EBBR / EBMB / BRU)
7 July 2021; (c) LRS747

Revision history:

13-Jan-2017 20:10 harro Added
13-Jan-2017 20:12 harro Updated [Source]
13-Jan-2017 20:15 harro Updated [Narrative]
24-Nov-2021 16:14 harro Updated [Accident report]
24-Nov-2021 16:47 harro Updated [Total occupants, Narrative]

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