West Atlantic Sweden CRJ2 near Akkajaure on Jan 8th 2016, lost height after emergency call

Last Update: December 12, 2016 / 16:45:30 GMT/Zulu time

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Incident Facts

Date of incident
Jan 8, 2016

Classification
Crash

Aircraft Registration
SE-DUX

ICAO Type Designator
CRJ2

On Dec 12th 2016 Sweden's SHK released their final report concluding the probable causes of the crash were:

Factors as to cause and contributing factors

The accident was caused by insufficient operational prerequisites for the management of a failure in a redundant system.

Contributing factors were:

- The absence of an effective system for communication in abnormal and emergency situations.

- The flight instrument system provided insufficient guidance about malfunctions that occurred.

- The initial manoeuver that resulted in negative G-loads probably affected the pilots' ability to manage the situation in a rational manner.

Factors as to risk

The fact that fault descriptions regarding aircraft and its components are reported in a less detailed manner might imply that the faults will not be identified and corrected in an efficient way. This can in turn lead to a flight safety issue as, for instance, intermittent faults cannot always be detected by general tests.

The SHK reported that the captain (42, ATPL, 3,365 hours total, 2,208 hours on type), pilot flying, initiated the approach briefing prior to the upset, the first officer (33, CPL, 3,232 hours total, 1,064 hours on type) was pilot monitoring. The autopilot and yaw damper were engaged, the aircraft had been in stable flight since levelling off at FL330 at 275 KIAS and a speed over ground of 422 knots.

During the approach briefing the flight data recorder recorded an increasing pitch from the Inertial Reference System #1, initially at 1 degrees/second, increasing to 1.7 degrees/second and subsequently at 6 degrees/second.

After the briefing was completed the captain exclaimed a strong expression "What (!)" - according to the flight data recorder the Primary Flight Display showed a pitch angle of 15 degrees nose up with speed and altitude remaining unchanged, the FDR also recorded the Angle of Attack had remained unchanged, the AoA however is not being displayed to the crew. Immediately afterwards the cavalry sounds of the autopilot being disconnected were recorded by the cockpit voice recorder, most likely the autopilot disconnected automatically due to disagreement of the pitch servo commands. The Cavalry charge remained active for 18 seconds.

The elevators moved into a position to command a nose down, the AoA vanes recorded the AoA becoming negative. In addition, nose down trim commands originating from the left control column were recorded over the next 19 seconds. The recorded pitch position reached 30 degrees nose up. The aircraft began to experience negative G-Load reaching -1G, engine oil pressure indications for both engines followed causing by the negative G-load, the audio signal for high trim speed movement over 3 seconds - caused by manual trim input - was activated, a bank angle warning activated, Vmo and Mmo were exceeded, the overspeed warning activated, descending through FL240 the aircraft exceeded 400 KIAS, the pitch trim began to roll nose up and reached 0.3 degrees nose down. The last recorded airspeed was 508 KIAS at a vertical acceleration of now +3G. The aircraft collided with the ground at position N67.7167 E16.9000 at an elevation of 722 meters/2370 feet 80 seconds after the begin of the upset.

The SHK reported: "Concerning the PFD-units, SHK has not found any descriptions in the operator’s manuals regarding the functions unusual attitude, declutter (non-essential information is removed from the display) or the chevron symbols (red arrows indicating direction of recovery). However, these items are described in the manufacturer’s Pilot Reference Manuals (PRM). The operator did not have access to PRM and the operator’s training organization did not use the manuals."

The SHK analysed that the aircraft did not break up in flight supported by the fact that all control surfaces and the aircraft's all four corners were found at the crash site. The motions of the aircraft as recorded were coherent with the movement of the control surfaces. The SHK analysed there was no cargo shift.

The SHK anlaysed the scenario immediately prior to the upset:

Immediately before the beginning of the event the crew was pro-ceeding with the briefing for the approach to Tromsø. The fact that the manoeuvring of the aircraft was not handed over to the PM may have contributed to a less than optimal instrument monitoring during the briefing. However this has probably not affected the course of the event since the pitch angle deviation was detected within seconds.

The SHK analysed at the time of the begin of the upset:

The approach briefing was in progress, which probably meant that the pilots partly focused their attention on the approach charts. Since the maps must have been illuminated to be read, and other cockpit lighting could have been lit, the pilots' night vision had probably deteriorated. This meant that external visual references were virtually non-existent. The pilots were therefore entirely dependent on the aircraft's attitude indicators.

The pilot in command’s first strong expression "What (*)" was recorded after approximately two seconds. The expression is inter-preted as a surprise effect due to the rising pitch angle displayed on PFD 1. It also indicates that the attitude indicator was monitored by the pilot in command at this stage.

SHK’s opinion is that the pilot in command at this moment was exposed to a surprise effect because of the difference between what was expected and what was displayed. As the left PFD displayed information that was not consistent with the aircraft's actual movements and external visual references were absent the pilot in command was subjected to a degradation of his spatial orientation.

The SHK analysed the rapid evolution of the upset:

Almost simultaneously the audio warning Single Chime was activated with a delay due to the autopilot disconnect warning that had priority. No verbal acknowledgement from either pilot was heard on the CVR recordings. Both elevators moved towards nose down while the left control wheel trim switch was activated indicating that the pilot in command, who was the PF, was manoeuvring the aircraft.

The action is probably due to several factors. Pilots have learned since basic instrument training to rely on their instruments, which may ex-plain the pilot in command’s actions. The fact that the pitch angle dis-played on the left PFD was high and increased rapidly in combination with the display of the red chevrons requesting pitch down inputs probably contributed to the pilot’s instinctive reaction to act according to the displayed unusual attitude.

...

The situation indicates that the pilots initially became communica-tively isolated from each other. A contributing factor to this was the lack of regular training of procedures for unusual attitudes. Nor were there any clear rule-based behaviour to fall back upon. Therefore, the situation evolved into problem solving and improvisation, thus a knowledge-based behaviour.

...

By this time, the pilots probably had different perceptions of the situation because of differences in the display on the respective attitude indicator. A basic prerequisite for the crew to jointly cope with the situation was sharing the same perception, or mental model of the situation. In order to achieve a common perception, or mental model, one needs to communicate with each other.

...

At t9 (9 seconds after the captain's "What" exclamation) the co-pilot exclaimed a strong expression which was his first recorded verbal reaction since the beginning of the event. This was answered by the pilot in command with the same expression.

The flight crew communications during the flight, up to the beginning of the event, indicate an open dialogue with mutual forgiveness and exchange of information. SHK therefore concludes that the lack of communication until now in the course of events was not based on any hierarchical conditions that impaired the communication. However, the silence of the crew is a clear indication of a lack of understanding of the current situation and an inability to verbally communicate to troubleshoot the abnormal situation. Variations in G-load probably also affected communication ability.

...

At approximately t13 (13 seconds after the captain's exclamation) the Pitch Angle on the right side PFD reached minus 20 degrees which meant that the declutter mode now was activated on this side (see figure 28 below). The co-pilot exclaimed the first operational callout “Come up”. At about the same time the warning for stabilizer movement, Stab Trim Clacker, was activated and another Triple Chime sounded which was cut-out by two Bank Angle warnings.

The situation at this time meant that the crew were presented with two contradictory attitude indicators with red chevrons pointing in oppo-site directions. At the same time none of the instruments displayed any comparator caution.

None of the pilots verbally referred to the standby horizon. This can be explained by the complex situation facing the flight crew due to variations in G-load and a great number of audio and visual cautions and warnings. This probably further contributed to cognitive tunnel vision and focus on each on-side attitude indicator.

...

The aircraft left its flight envelope at t17 when VMO was exceeded, which activated the overspeed warning. Recorded vertical G-load now turned to positive rates. At the same time irregular sounds were recorded once again. SHK finds it probable that these sounds were due to loose objects falling back down towards the cockpit floor.

The SHK analysed the automated "decluttering mode" of the Primary Flight Displays:

The declutter function means that only roll and pitch angle is displayed on the attitude indicator part of the PFD units during unusual attitudes. This meant that the comparator monitor indication disappeared from the PFD 1 and PFD 2 at an early stage of the sequence.

The purpose of clearing the PFD units from unnecessary information, and thereby providing the pilots with a better display of the situation during unusual attitudes, is easy to understand. It is however more difficult to understand why indications related to instrument errors are removed.

It is possible that such an indication could have helped the pilots to identify the erroneous PFD display. Furthermore, there is a delay of more than 1 second between the caution message and the associated single chime. In case of multiple cautions and warnings, the audio alerts may be desynchronized with the visual messages, causing confusion in the flight crew’s troubleshooting.

Furthermore, as the system does not know which PFD displays the correct parameters when EFIS COMP MON triggers, no declutter function should be automatically performed in this case to avoid the removal of information useful to troubleshoot the situation.

SHK considers that the decluttering of the caution indications on the PFD displays during unusual attitudes is a weakness in the system design.

With respect to the Inertial Reference Unit #1 fault the SHK analysed:

The only plausible explanation to the erroneous data recorded by DFDR is an internal malfunction of IRU 1.

SHK and the manufacturer of the unit have searched for similar events without finding any similar IRU malfunctions.

The manufacturer of the unit performed physical tests and software tests without being able to reproduce the scenario.

Figure 23 shows that only the pitch angle information was erroneous until the aeroplane was changing its roll angle. Until that time, only the pitch gyro was needed to calculate the pitch angle. When the aeroplane started to turn, both the gyros for pitch and yaw were needed to calculate pitch angle and heading. The roll angle will change if a movement in yaw is introduced when the pitch angle is other than zero.

This means that, upon manoeuvring in several dimensions simultane-ously, the roll and heading information will become erroneous if the pitch gyro provides erroneous information. The errors in ground speed can be explained by the erroneous calculation of the heading angle.

The internal continuous self-test accepted the attitude parameters as valid thereby providing the erroneous information to both PFD 1 and DFDR.

When SSM (Sign Status Matrix) sets an error on the attitude parameters from IRU 1 the system shall display a failure flag on PFD 1. This means that the attitude information is removed and replaced by a red “ATT” flag. The failure of the self-test to discover the error can be logical if the failure consisted of an error from a single gyro unit within the system’s limitations.

The aeroplane’s two IRUs had different part numbers and thereby different software. Such a setup is not approved, which meant that the installation of IRU 1 was not a correct maintenance action. However, this is not considered to have had any impact on the sequence of the event.

The SHK analysed in summary:

The malfunction occurred when the crew was performing the approach briefing, which meant that attention was divided between two simultaneous tasks. This probably contributed to the surprise effect.

This meant that the pilot in command’s PFD indicated a sharp increase in pitch angle although the aeroplane was in level flight. Moreover, this led to the automatic disconnection of the autopilot.

At the same time, the co-pilot's PFD displayed information which was consistent with the aeroplane's actual attitude.

The aircraft was equipped with three independent attitude indicators, one of which indicated incorrect values. Thus there were two working attitude indicators that could give the crew the correct attitude information to operate the aircraft safely. However, this requires that the crew has the ability to identify the malfunction and to rationally evaluate the situation.

The PIT miscompare indication on the PFD displays, supposed to in-form the crew about the miscompare between PFD 1 and PFD 2 was probably activated, but was displayed on PFD 1 for a very short pe-riod of time. By design, the miscompare indication, along with other information considered secondary, disappears at unusual attitudes to allow the crew to focus on a more limited set of information.

The crew was trained to rely on their instruments, in the absence of external visual references. This may explain the reflexive manoeu-vring that induced the rapid descent of the aeroplane when the red chevrons appeared.

The captain's and first officer's instrument readings at the captain's exclamation "What (!)" (Photo: SHK):


The captain's and first officer's instrument readings 9 seconds after the captain's exclamation "What (!
Incident Facts

Date of incident
Jan 8, 2016

Classification
Crash

Aircraft Registration
SE-DUX

ICAO Type Designator
CRJ2

This article is published under license from Avherald.com. © of text by Avherald.com.
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