Serious incident to the Cessna 525 registered F-HGPG operated by Valljet and to the Embraer ERJ170 reistered F-HBXG operated by HOP on 12/01/2022 en route

The crew of the Cessna 525 CJ registered F-HGPG were carrying out a flight between Paris-Le Bourget airport (Seine-Saint-Denis) and Geneva airport (Switzerland).

During the climb, following a sudden variation in the nose-up attitude with the autopilot engaged in IAS mode, the crew observed erratic speeds on the system 1 airspeed indicator. After a short manual flight phase, the climb was continued with the autopilot in VS mode. Later, when approaching the en-route level, the crew realised that there was a difference in altitude between the two altimeters (system 1 and system 2). The cross-check with the help of the controller who had the flight level transmitted by the aeroplane’s transponder displayed on his radar screen did not enable the crew to identify that the system 1 altimeter indications were erroneous. The climb was continued to the en-route level based on an erroneous altitude.

En route, after having observed that the left and right altimeters were giving different indications, the crew informed the controller of the onboard altimeter fault. The latter then informed the crew of converging traffic (the Embraer 170 registered F- HBXG) at a distance of 2 NM, in theory 1,000 ft higher than them. In reality, the traffic was lower than them (the minimum separation was estimated at 665 ft and 1.5 NM). No collision avoidance system warning, whether it be on the ground or onboard the Embraer 170 was emitted, as the systems had analysed erroneous data from the Cessna 525. Subsequently, the controller asked the crew to deactivate the transponder Mode C, he coordinated with the Swiss control services and the flight continued to Geneva, its destination.

The head of the control centre room attempted to determine the actual altitude of the aeroplane with the help of the National air operations centre (CNOA), however, the latter did not have additional altitude information. However, another parameter, the aeroplane’s altimeter setting, which the CNOA shared with the controller proved to be erroneous. The investigation was not able to determine the cause of this difference.

The investigation showed that the fault on air data system 1 (altimeter and airspeed indicator on captain’s side) had already occurred three times on this aeroplane in 2017, 2019 and 2021.

The BEA has issued six safety recommendations concerning five topics:

• maintenance documentation published by Textron Aviation;
• notification of technical faults at the operator, Valljet;
• DSNA quick reference card when a pilot has a doubt about the altitude of his flight;
• information transmitted by CNOA;
• EASA’s analysis of the risk posed by an air data system fault.