A system of standard calls should be developed to provide commercial flight crews with initial guidance for handling abnormal and unexpected occurrences in flight, says the Swedish Accident Investigation Authority (SHK). The SHK believes such guidance might help to prevent accidents similar to the one that befell a Bombardier CRJ200 the night of Jan. 8, 2016.
Investigators found that there was no immediate communication or coordination between the CRJ pilots when the pilot-in-command’s (PIC’s) primary flight display (PFD) indicated that the airplane’s pitch attitude was increasing rapidly. The PIC’s surprised reaction to the pitch indication led to an upset from which recovery was not accomplished. The pitch indication was found to have been erroneous and precipitated by a malfunctioning inertial reference unit (IRU).
The accident occurred in Oajevágge, Sweden, during a cargo flight carrying mail and packages from Oslo to Tromso, both in Norway. The airplane was being operated as Air Sweden Flight 294 by West Atlantic Sweden AB.
No Significant Weather
The flight crew had reported for duty at 1810 local time and were flying the second round-trip flight between Oslo and Tromso that evening in the same airplane.
The PIC, 42, held an airline transport pilot license and had 3,365 flight hours, including 2,208 hours in type. He had received his basic flight training in Spain and had flown as a CRJ900 pilot with another operator before being employed by Air Sweden. The PIC served as the pilot flying during the accident flight.
The copilot, 33, had a commercial pilot license and 3,232 flight hours, including 1,064 hours in type. He had received his basic flight training in France and had begun his commercial flight career with Air Sweden as a BAe Jetstream 61 pilot.
Light snow was falling at Oslo, but no significant weather was forecast for the flight to Tromso. Estimated flight time was 1 hour and 43 minutes, and the planned departure time was 2300. However, the departure was delayed about 9 minutes while the crew had the airplane deiced. The airplane was a CRJ200-PF (package freighter), a short- to medium-range cargo transport.
“The takeoff, departure and climb to the cleared flight level, FL 330 [approximately 33,000 ft], were performed according to normal procedures,” the report said. “The autopilot was engaged during the climb at approximately FL 180. At 2337, the aeroplane was established in level flight at FL 330. … All recorded DFDR [digital flight data recorder] parameters were stable with normal values from the point in time when the aeroplane first leveled out at the cruise altitude.”
Cockpit voice recorder (CVR) data indicated that after establishing cruise flight at FL 330, the pilots engaged in some private conversations. “No language barriers were observed, which indicates that the communication between the pilots was not hampered or deteriorated for reasons of language,” the report said.
‘Dependent on Instruments’
The CRJ was cruising at an indicated airspeed of 275 kt in dark night visual meteorological conditions. “The lack of external visual references meant that the pilots were totally dependent on their instruments which, inter alia, consisted of three independent attitude indicators,” the report said.
Attitude information is provided by PFDs on the left and right sides of the instrument panel, and by a standby attitude indicator in the center of the panel. Attitude data for the PFDs is generated by an inertial navigation system comprising two IRUs.
“Each IRU consists of three ring laser gyros (RLGs), a three-axis accelerometer and the computing section,” the report said. “An RLG senses angular changes around its axis by measuring frequency differences between the two counter-rotating laser beams. The accelerometers sense acceleration along the same axis. … Hence, the IRU calculates the three-dimensional trajectory and the aeroplane’s angles in pitch, roll and yaw axis.”
The information provided on each PFD is monitored by a comparator system, which flashes warnings on the flight displays if the information varies from prescribed limits — for example, if the pitch or roll information provided on the PFDs varies by more than 4 degrees.
The “miscompare” warnings and other information are removed from the flight displays if the airplane enters an unusual attitude — that is, if the pitch attitude shown in either PFD exceeds 30 degrees nose-up or 20 degrees nose-down, or when the displayed roll angle exceeds 65 degrees. This function is called “decluttering” and is designed to help pilots focus on remaining information deemed pertinent in recovering from the unusual attitude.
The flight path from Oslo to Tromso took the airplane into Swedish airspace controlled by Norwegian air traffic control (ATC). The crew was told to expect clearance for a circling approach to the Tromso airport.
The pilots were conducting an approach briefing at 0020 when the erroneous pitch indication first appeared on the PIC’s flight display. From what had been a constant indication of about 1 degree nose-up in cruise flight, the indicated pitch attitude increased to 1.7 degrees momentarily, then to 36 degrees at a rate of 6 degrees per second. The airplane, however, remained in level flight. “The recorded altitude, speed and angle-of-attack remained unchanged,” the report said.
The PIC’s display briefly flashed an amber “PIT” miscompare warning, indicating that the pitch information did not compare with that shown on the copilot’s PFD. Although the PIC’s flight display indicated an increasing nose-up pitch attitude and a flight director command to lower the nose, the copilot’s display showed the airplane in level flight and a flight director command to maintain the indicated pitch attitude.
As the indicated pitch attitude increased through 15 degrees on the left PFD (see Figure 1), an exclamation by the PIC — “What!” — was captured by the CVR. “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,” the report said. “As the left PFD displayed information that was not consistent with the aircraft’s actual movement and external visual references were absent, the pilot-in-command [also] was subjected to a degradation of his spatial orientation.”
Figure 1 — Inconsistent Readings
Source: Statens haverikomission
The crew then received an aural warning that the autopilot had disengaged. “According to the aeroplane’s manufacturer, the autopilot was most likely automatically disconnected due to differences in the pitch servo commands [generated by the IRUs],” the report said. “The aural warning remained active for the next 18 seconds.”
Reacting by Instinct
Neither pilot commented on what was happening. “The lack of a prescribed procedure and standard callouts for automatic autopilot disconnection might explain why this was not commented upon or acknowledged by the crew,” the report said. “Furthermore, it was not made clear verbally that any of the pilots had assumed manual control of the aeroplane.”
Reacting to the erroneous indications on his PFD, the PIC pushed his control column forward and applied nose-down trim to reduce the airplane’s pitch attitude. Recorded flight data showed that “both elevators moved towards nose-down and [that] nose-down stabilizer trim was gradually activated from the left control wheel trim switch,” the report said. “The aeroplane started to descend, the angle-of-attack and G-loads became negative. Both pilots exclaimed strong expressions.”
The SHK determined that the PIC’s reaction was instinctive and consistent with his training. “Pilots have learned since basic instrument training to rely on their instruments,” the report said. “The fact that the pitch angle displayed on the left PFD was high and increasing rapidly in combination with the [flight director] display requesting pitch-down inputs probably contributed to the pilot’s instinctive reaction to act according to the displayed unusual attitude.”
When the pitch attitude shown on the PIC’s flight display increased through 30 degrees, the PFD went into the declutter mode. Among the secondary information removed from the PFD was the pitch miscompare warning. The PFD also generated an additional, and prominent, steering command — large red chevrons prompting the PIC to decrease the airplane’s pitch attitude (see Figure 2).
Figure 2 — Declutter
Source: Statens haverikomission
‘Cognitive Tunnel Vision’
The substantial negative G-load that resulted from the PIC’s nose-down pitch inputs and the large number of audio and visual warnings that were being presented to the pilots likely caused them to experience “cognitive tunnel vision,” the report said. They likely focused solely on their individual flight displays and disregarded, or were unable to assimilate, other information, such as that provided by the standby flight instruments.
“By this time, the pilots probably had different perceptions of the situation because of differences in the display on the respective attitude indicator,” the report said. “A basic prerequisite for the crew to jointly cope with the situation was sharing the same perception, or mental model, of the situation. [Communication is necessary] to achieve a common perception, or mental model.”
The SHK concluded that if the pitch miscompare warning had been retained after the PIC’s flight display went into declutter mode, the pilots eventually might have detected the erroneous indications on the PIC’s display. “It is … difficult to understand why indications related to instrument errors are removed,” the report said. “The decluttering of the caution indications on the PFD displays during unusual attitudes is a weakness in the system design.”
The CVR recorded sounds similar to loose objects striking the cockpit roof due to the negative G-loads being imposed on the airplane, as well as an aural warning of low engine oil caused by loads imposed on the engines and several exclamations by the pilots as the upset progressed.
Nine seconds after the upset began, the airplane started to bank left. This likely resulted when the copilot grabbed the control yoke for support while being pushed upward by the negative G-load, the report said. A few seconds later, the enhanced ground-proximity warning system (EGPWS) sounded a warning that the bank angle had reached at least 40 degrees.
The declutter mode by now had activated in the copilot’s PFD, which showed indications of a steep left bank and a 20-degree nose-down pitch attitude, as well as red chevrons providing a nose-up steering command. The copilot likely was reacting to these indication when he shouted “come up” and then “turn right.” The PIC said, “Come on, help me, help me, help me.” The report noted that this was the first time since the start of the event that the pilots attempted to communicate with each other.
The PIC’s display was still showing a steep nose-up pitch attitude and a nose-down steering command. “The situation at this time meant that the crew were presented with two contradictory attitude indicators with red chevrons pointed in opposite directions,” the report said. Spatially disoriented and focusing solely on their PFDs, neither pilot made any verbal reference to the standby attitude indicator.
‘Mayday, Mayday, Mayday’
An aural warning (a “clacker”) sounded when the indicated airspeed exceeded the maximum operating speed of 0.85 Mach. “The pilot-in-command asked for help again, which was answered by the copilot by saying, ‘Yes, I am trying,’” the report said.
The pilots still had not realized that their PFDs were providing conflicting information. “The dialogue between the pilots consisted mainly of different perceptions regarding turn directions,” the report said. “The efforts to regain control were not based on rational decisions or communication, but probably [were] the result of trained flight control inputs guided by the erroneous information.”
Airspeed had reached 0.91 Mach when the copilot radioed, “Mayday, mayday, mayday, Air Sweden two niner four.” Shortly after ATC acknowledged the call, the copilot said, “We [will] call you back.” The crew received, but did not acknowledge two more ATC radio transmissions as the upset continued.
The report said that recorded flight data became unreliable for analysis about 24 seconds into the upset. Although CVR data indicated that the crew continued their efforts to recover from the upset, the report said that the possibility of regaining control of the airplane by this time was limited.
The CRJ was in an inverted attitude when it struck terrain in a valley at 2040. The impact occurred 80 seconds after the airplane began the descent from FL 330. The pilots were killed, and the airplane was destroyed. There was no indication that an in-flight breakup had occurred.
The SHK concluded that the erroneous attitude indications on the PIC’s flight display had been caused by an internal malfunction of the no. 1 IRU. However, the specific cause of the IRU malfunction was not determined. The report said that there was no record of a similar malfunction and that tests by the manufacturer of the system components and software were inconclusive.
The report noted that although a miscompare warning was presented briefly on both PFDs, there was no specific indication to the crew that the no. 1 IRU had failed.
Investigators found no information in the manuals available to the crew about the removal of miscompare warnings when flight displays are decluttered. Moreover, the investigation revealed that the miscompare warnings presented to the crew during the upset were different from what they had seen during training. “The pitch and roll comparator indications of the PFDs were removed when the attitude indicators displayed unusual attitudes [during the upset],” the report said. “In the simulator in which the crew had trained, the corresponding indications were not removed.”
The SHK was unable to determine conclusively if fatigue might have been a factor in the accident. Although the investigation revealed no specific findings that fatigue might have impaired the crew’s performance, the report said that the event began “at a time when performance deterioration can occur due to fatigue” and that unexpected events such as the abnormal pitch indication “increase the demands on cognitive ability.”
“The investigation has found deficiencies in the pilots’ communication and difficulties in handling the situation,” the report said. “This type of difficulty of cognitive character can be seen during fatigue. … The pilots’ duty hours did not exceed the flight time limitations; however, there is no information available about the crew’s actual sleep time during the days preceding the accident.”
The report said that the pilots were “communicatively isolated” from each other at the beginning of the upset. For the first 12 seconds, the CVR recorded only expressions of surprise.
Although immediate action items and specific callouts typically are prescribed for emergency procedures and required to be memorized by pilots, few manufacturers and aircraft operators provide similar guidance for abnormal and unusual situations. Because the CRJ pilots did not have such guidance to respond to the abnormal pitch attitude shown on the no. 1 PFD, “the situation evolved into problem-solving and improvisation,” the report said.
Based on these findings, the SHK concluded that the accident was caused by “insufficient operational prerequisites for the management of a failure in redundant systems” and that a contributing factor was “the absence of an effective system for communication in abnormal and emergency situations.”
“SHK considers that clear and distinct communication between crewmembers is essential to maintain situation awareness and thereby optimize flight safety,” the report said. “The authorities and organizations publishing regulations in the matter should therefore ensure that a general system of initial standard calls is introduced in commercial aviation for clear, precise and bidirectional communication between crewmembers in abnormal and emergency, as well as unusual and unexpected, situations.”
The SHK also concluded that among the factors contributing to the accident was the absence of a specific warning to the crew about the IRU malfunction. Another factor was the negative G-loads experienced during the upset, which “probably affected the pilots’ ability to manage the situation in a rational manner,” the report said.
Based on the findings of the investigation, the SHK recommended to the International Civil Aviation Organization and several regional and national aviation authorities that they “ensure that a general system of initial standard calls for the handling of abnormal and emergency procedures and also for unusual and unexpected situations [should be] implemented throughout the commercial air transport industry.”
The SHK also called on aviation authorities to “ensure that the design criteria of PFD units are improved in such a way that pertinent cautions are not removed during unusual attitude or declutter modes.”
This article is based on Statens haverikomission (Swedish Accident Investigation Authority) Final Report RL 2016:11e, “Accident in Oajevágge, Norrbotten County, Sweden on 8 January 2016 involving the aeroplane SE-DUX of the model CL-600-2B19, operated by West Atlantic Sweden AB.”
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