Serious incident to the Cessna 525A registered N222NF on 14/08/2020 near Le Bourget

Investigation progression Closed
Progress: 100%

As soon as the wheels left the ground, the pilot was confronted with a failure on his left PFD. He complied with the procedure relating to the failure of a PFD and switched the information display from his left PFD to the MFD. He then noticed inconsistencies between the altitude and speed information displayed on the MFD and the right PFD.

Note: the investigation was unable to reproduce the failure of the left PFD and to understand what caused it. No link could be established between the left PFD failure and the inconsistencies in the displayed information.

The pilot informed the Le Bourget controller with whom he was still in contact, of the technical difficulties encountered. Prompted by the controller, he declared an emergency situation. He was then transferred to the Paris-Charles de Gaulle control service frequency.

The pilot found himself in a difficult situation where he was flying:

• as sole pilot in a high performance aeroplane;
• without the autopilot and with aural warnings sounding, in particular the overspeed warning;
• in meteorological conditions that did not allow him to acquire external visual references at all times.

The pilot gave priority to the piloting of his aircraft by using the pitch and roll indications and by selecting a thrust compatible with the aircraft configuration.

As he was being radar vectored, he did not comply with the procedure dedicated to inconsistent altitude and speed information. He tried to clarify the situation with the help of the controller.

However, while vectoring the aeroplane, based on erroneous altitude information indicating that the aeroplane was higher than its actual altitude, the controller asked the pilot to descend several times. Between 09:34 and 09:42, the controller was provided with altitude information indicating that the aircraft was stable at around 3,000 ft whereas in reality it had descended from 5,000 to 1,600 ft.

When the pilot had the Precision Approach Path Indicator (PAPI) and the runway in sight, he considered that he was able to carry out a visual landing. The landing took place without any particular difficulties.

The examination of the air data system found that the cause of the inconsistent altitude and speed information was the contamination of the line connected to the right static port.

Transponder 1 of the aeroplane, associated with the faulty ADC1 system, provided the controller with an erroneous altitude. This erroneous information was used by the pilot and the controller and resulted in a confirmation bias regarding the aeroplane's altitude. This bias could have significantly increased the risk of collision with the terrain.

The possibility of a confirmation bias in a situation of this type was highlighted by the BEA's investigation into an event in 2010. An operational directive was issued to the air traffic control services after the 2010 event, and incorporated in the Operations Manuals. However, the controller on duty did not recall this directive and the associated emergency procedure.

The separation of the aeroplane with other possibly conflicting traffic was ensured but the other items of the emergency procedure were not carried out. The pilot was not informed that the controllers could not help him clarify his altitude and he was not asked to switch off the mode C of his transponder. The radar vectors given by the controller made it possible for the pilot to reach Le Bourget airport.


The BEA issues 3 safety recommendations:

- Recommendation FRAN-2022-012 / Manufacturers’ procedures in cases of doubtful or erroneous altitude situations:

The BEA recommends that:

- whereas the "AMBER ROLL, PIT, ATT, HDG, ALT or IAS (Comparator monitor alerts)” procedure is common to all Cessna Citation C525s whatever their equipment;

- whereas the "AMBER ROLL, PIT, ATT, HDG, ALT or IAS (Comparator monitor alerts)" procedure is both incomplete and not adapted to a Cessna Citation 525-A equipped with the option of a third PFD and a second ADC: in the event of a fault on air data system 2, the procedure leads to the pilot using an erroneous air data source;

- whereas the Cessna procedure omits to mention the selection of the transponder supplied with information from the air data system identified as valid;

the FAA ensure that the procedure in the Cessna Citation 525 flight manual is updated to provide pilots with a specific procedure for processing inconsistent air data information, adapted to the configuration of the aeroplane concerned.

- Recommendation FRAN-2022-013 / Manufacturers’ procedures in cases of doubtful or erroneous altitude situations:

The BEA recommends that:

- whereas the investigation carried out ten years later shows that there are still incomplete procedures on what to do in the event of inconsistent altitude information;

EASA, in liaison with the primary airworthiness authorities of the aeroplanes, implement the recommendation by not limiting itself to the particular case of the Pilatus PC12.

- Recommandation FRAN-2022-016 / Emergency procedures for air navigation services linked to doubtful or erroneous altitude situations:

The BEA recommends that:

- whereas a certain number of controllers are not aware that the altitude information they see on their control screens comes solely from the aeroplane's transponder;

the DSNA ensure, in addition to the teaching of the emergency procedure to be applied when the pilot announces a doubt about the altitude of his flight, that all active controllers have correct knowledge of how the air data information, available to them on their screens, is obtained.

The recommendations are being processed

The status of the recommendations is available at SRIS2: click here


Note: in accordance with the provisions of Article 17.3 of Regulation No 996/2010 of the European Parliament and of the Council of 20 October 2010 on the investigation and prevention of accidents and incidents in civil aviation, a safety recommendation in no case creates a presumption of fault or liability in an accident, serious incident or incident. The recipients of safety recommendations report to the issuing authority in charge of safety investigations, on the measures taken or being studied for their implementation, as provided for in Article 18 of the aforementioned regulation.