History of the Flight
On 14 August 2020, a Cessna 525A registered N222NF departed from Paris-Le Bourget airport for Deauville-Normandie. The pilot and three passengers were on board. The start-up and before-take-off checks were normal.
Immediately after lift-off at 09:27 UTC, the left primary flight display (PFD) screen went blank. The pilot followed the procedure for this failure and transferred the PFD information to the central multifunction display (MFD).
After overflying the opposite runway threshold, the controller instructed the pilot to change frequency and contact Paris-Charles de Gaulle control. Seconds later, the autopilot disengaged, and visual and aural warnings for overspeed and pull-up activated. Between 800 and 900 ft, the pilot noticed the altitude on the MFD increase from 800 ft to 2,900 ft. Comparing the MFD with the right PFD, he observed altitude differences up to 1,500 ft. He also noted airspeed differences between the two screens. Concluding the altitude and speed data were unreliable, he announced a problem with the autopilot and requested clearance to return to Le Bourget. The controller asked if he was declaring an emergency, and the pilot confirmed.
The Le Bourget controller informed Paris-Charles de Gaulle control, and a controller was assigned to provide radar vectors for the return. Meanwhile, a Minimum Safe Altitude Warning (MSAW) triggered at the Paris-Charles de Gaulle control center.
At 09:30, the pilot informed the Paris-Charles de Gaulle controller of multiple failures and that he would attempt to return in manual control. Over the next several minutes, the pilot and controller exchanged altitude and speed information, noting discrepancies between the aircraft's instruments and radar data. At 09:35, the pilot stated he had lost the left screen, was using the MFD, and would rely on the screen matching the controller's radar altitude. The controller provided vectors and altitude instructions.
At 09:38, the pilot reported alternating VMC and IMC and entered clouds. He later indicated he had no ILS approach information and hoped for a visual approach. The controller provided radar altitudes and descent instructions. At 09:45, the Enhanced Ground Proximity Warning System (EGPWS) "Too Low Terrain" warning activated. The pilot then switched the transponder to use data from air data computer 2 (ADC2), causing the displayed altitude on the controller's radar to change from 2,850 ft to 500 ft, triggering a second MSAW warning.
At 09:46, the pilot reported the runway in sight and made a visual landing on runway 27 at Le Bourget at 09:48.
Information on the Aeroplane and Its Systems
The N222NF was a Cessna 525A equipped with a Rockwell Collins Pro Line 21 avionics suite, including two PFDs and one MFD. The aircraft was not fitted with a flight recorder, which was not a regulatory requirement.
Examination found that air data system 1 (ADC1) was faulty. The altitude and speed information displayed on the left PFD and subsequently on the MFD were erroneous. The transponder selected was supplied by ADC1 and transmitted incorrect altitude information to air navigation service radars. The right PFD and standby altimeter, supplied by ADC2, displayed the actual altitude.
Consequences and Damage
There were no consequences or damage to the aircraft or persons on board.