The preliminary analysis of Saratov Airlines flight 6W706 accident has shown that during the accidental flight, the heating of all the three Pitot probes were in the OFF position.
As for all the other lights recorded by the FDR (15 more flights), the Pitot probes’ heating had been switched to ON before takeoff, at the lineup.
Saratov Airlines flight 6W706 started its takeoff at approximately 11:21 (here and further the time is given in the UTC format).
After the liftoff, at the height of 130-150 m (here and further the height above the runway level is given), the autopilot was engaged. In the autopilot pitch channel, the desired altitude capture mode was being provided, in the autopilot yaw channel, the lateral navigation mode was being engaged. At the height of 550 m, the flaps retraction was completed.
The emergency situation on Saratov Airlines flight 6W706 started to develop about 2 min 30 s after the liftoff, at the height of approximately 1300 m and with the IAS of 465-470 km/h, when the disagreement in the values of speed received fr om the air data module (МВП1) at the left pilot’s station (ППД1) and from the standby air data module МВП3 (ППД3) started to occur.
The speed received from the МВП2 at the right pilot’s station (ППД2) is not to be recorded by the FDR. The disagreement in the height readings from the same sources (МВП1 and МВП3) was not significant.
About 25 s later, the disagreement in speed readings reached the value of about 30 km/h (the speed value received from МВП1 was higher), and the single signal “MATCH IAS” (“V приборная – СРАВНИ”) was generated (annunciation for the flight crew). The recorded single signal was ON for approximately 10 s and then disappeared.
Approximately 50 s later, at the height of about 2000 m, the same single signal was recorded once more; this time, the speed received from МВП3 was higher and continued to increase, however the speed received from МВП1 continued to decrease.
After the same single signal (annunciation) was generated once more, the flight crew disengaged the autopilot. The rest of the accidental flight was conducted in the manual mode.
The reading of the speed received from МВП1 continued to decrease, and 34 s after the autopilot disengagement, the reading reached zero (0). However, the reading of the speed received from МВП3 showed about 540-560 km/h.
For about 50 s after the autopilot disengagement, the flight of Saratov Airlines flight 6W706 was conducted at the height of 1700-1900 m, with vertical G variations of between 1.5 and 0.5.
Further, while the МВП1 readings continued to show 0 km/h, the МВП3 readings started to decrease aggressively (down to 200 km/h and lower).
Further on, the aircraft was put into the aggressive descending with pitch angles of about 30-35 degrees and vertical G down to 0.
The collision with the ground occurred at about 11:27:05. Prior to the impact, the МВП 3 readings started to increase aggressively, and at the moment of the impact, they reached about 800 km/h. The МВП1 readings continued to show“0”.
At the instant of the collision, the pitch (nose-down) angle was about 30 degrees; 4 or 5 s before the collision, the aircraft right roll started to develop and reached 25 degrees.
The Investigation team continues to analyze the available flight data.
Based on the analysis of the recorded data and the studies of the similar accidents in past, it could be concluded that the inflight emergency situation might have been caused by the incorrect speed readings shown on the cockpit instrument panel, which in their turn could be related to the Pitot probes’ iced condition with the heating system set to OFF.
For the purpose of detecting the reasons of the three Pitot probes’ heating systems being in OFF position, the Investigation team plans to conduct the following works:
- the CVR data readout in order to receive the information on the flight crew’s actions, their performing of the Standard Operating Procedures (SOP) and responding to the cockpit annunciations;
- studying of the SOP related to the Pitot probes heating system, including the cockpit instrument panel indications;
- layout of the available fragments of the Pitot probes heating system.
The members of the Investigation team continue their work at the accident site, wh ere the field sketches are being completed, as well as the recovering the aircraft fragments for further examination.
For the purpose of preventing the similar accidents and developing immediate safety recommendations, today the briefing was held at the Interstate Aviation Committee with participation of the representatives of the Ministry of Defense, the Ministry of Emergency Situations of Russia, the Russian CAA (Rosaviatsia), Rostransnadzor, and the airlines who operate the said type of the aircraft. During the briefing, the IAC experts presented the preliminary results of the investigation in order to take the immediate safety actions related to flight operations.