Era DH8A near Anchorage on Sep 5th 2012, uncommanded left roll and loss of altitude
Last Update: July 9, 2020 / 21:52:39 GMT/Zulu time
Incident Facts
Date of incident
Sep 5, 2012
Classification
Incident
Airline
Era Aviation
Flight number
7H-874
Departure
Anchorage, United States
Destination
Homer, United States
Aircraft Registration
N886EA
Aircraft Type
De Havilland Dash 8 (100)
ICAO Type Designator
DH8A
The NTSB reported the incident occurred in instrument meteorological conditions, an investigation has been opened.
On Jul 9th 2020 the NTSB released their final report concluding the probable cause of the incident was:
an in-flight loss of control due to the flight crew's inattention to airspeed, pitch attitude, and engine power during the climb leading to an aerodynamic stall. Contributing to the incident was the flight crew's failure to recognize and properly recover from an aerodynamic stall in a timely manner.
The NTSB complained that the CVR present on board of the aircraft did not comply with the regulations (requiring 2 hours of recording), the FAA had been unaware of this. The lack of CVR recording impaired the investigation and did not permit to determine, whether the stick shaker had activated during the event as the FDR did not record the parameter of stick shaker activation. The crew reported the stick shaker did not activate during the event.
The NTSB described the sequence of events:
During the climb to 10,000 feet, FDR data show that the flight maintained about 90 percent torque and 150 knots, which was consistent with company procedures. The captain stated that the airplane encountered icing conditions between about 7,000 and 8,000 feet, at which time, he turned on all of the deicing equipment, which both flight crewmembers stated was working properly. When the captain leveled the airplane at 10,000 feet, the planned cruise altitude, he reduced power from a climb setting of 90 percent of maximum torque to about 70 percent of maximum torque. At the time, the IAS was 170 knots. Shortly thereafter, the flight crew requested and received a clearance from ATC to maintain a block altitude of 10,000 to 14,000 feet to avoid continued icing conditions.
The captain stated that he did not adjust the power when he began the climb, which FDR data confirmed. The captain believed he engaged the IAS mode of the autopilot when he began the climb; however, if the captain had used the IAS mode at the reduced power setting, the airplane would likely not have climbed. Both pilots stated in interviews that climbing in the VS mode if the autopilot was engaged was not recommended and that they did not recall selecting the autopilot VS mode. Specifically, the Era Aviation FOTP stated that the VS mode should not be used for sustained climb if the autopilot was engaged since the airspeed was not protected and a stall may occur, but that it may be used to establish initial climb pitch attitude. Despite this guidance and the flight crew's apparent awareness of it, data showed that the flight began a steady climb of 850 feet per minute, which was consistent with the captain selecting the VS mode.
Because the airplane began to climb steadily without the addition of power, the pitch attitude began to increase, and the indicated airspeed began to decrease. Recorded data showed that, over the same interval, the airplane pitch attitude increased from 3° airplane nose up at 10,000 feet to 14° airplane nose up at 12,000 feet and that the airspeed decreased from 170 to 103 knots, the speed at which the airplane stalled.
As noted previously, in addition to airspeed indicators, orange speed control indicators providing fast/slow indications were installed in the captain's and first officer's ADIs on the instrument panel directly in front of both pilots. The speed control indicators depicted airspeed relative to 1.3 Vstall and would have moved into the "slow" region of the indicator as the airspeed dropped below about 116 knots. Given that the pitch attitude is a primary control indication and airspeed is a primary performance indication, both pilots (PF and PM) should have been cross checking (that is, continuously and logically observing the instruments for attitude and performance information) both indications frequently. However, both pilots reported that they were not aware of the changes in pitch attitude or airspeed.
The captain stated that, during the climb out of 10,000 feet, he was monitoring the icing situation outside the airplane. He described the icing conditions as "the high end of moderate" and he stated they needed to avoid those conditions to continue the flight. When the airplane began to break out of the clouds at an altitude about 11,500 feet, the captain decided to level off at 12,000 feet, and he began monitoring the autopilot as it captured that altitude. He stated that he thought the airspeed was 150 to 160 knots, but he did not recall looking at the airspeed indicator or the fast-slow indication on the ADI.
The first officer stated that normally the PF would specify the climb speed, but he did not recall if the captain did so and he could not remember what the airspeed was in the climb. He said he was busy taking care of paperwork and charts, preparing to communicate with the destination station, looking outside, focusing on the icing conditions, making sure the de-ice boots were inflating, and seeing whether the airplane was shedding ice or not. As the PM, the first officer was responsible for watching the primary instrument indications and ensuring that the airplane was maintaining the appropriate climb airspeed. However, he stated that he could not recall what the indications were on the instrument panel before the stall but that he thought that the airspeed was about 150 knots before the upset.
The pilots allowed the airspeed to drop to stall speed because the captain failed to set climb power when he inappropriately selected the autopilot VS mode for climb and both pilots were preoccupied with other duties and were not watching their airspeed or attitude indicators during the climb.
As the airplane began to level off at 12,000 feet, the airplane began to shudder. The flight crew stated that they attributed the shudder to an unbalanced condition of the propellers due to uneven ice shedding. Both pilots stated that they had experienced similar ice shedding on their previous flight, and neither pilot associated the vibrations they felt as the approach to an aerodynamic stall. FDR data indicate that the airplane was experiencing an aerodynamic prestall buffet; however, neither pilot recognized the buffet as an indication that the airplane was about to stall.
No recorded data were available to confirm a stall warning, but, according to the pilots, the stick shaker did not activate at the time the airplane began to lose lift, which according to FDR data and the airplane performance study, occurred at 1041:18 as the airplane was climbing through 12,192 feet at 103 knots. The FDR data showed the control column moving aft from 3° to 8.5° beginning at the time of the loss of lift consistent with autopilot control. This was followed by autopilot disconnection and continued rapid aft movement of the control column to 33° within the next 3 seconds. Aerodynamic stall recovery requires the pilot to reduce the airplane's AOA by pushing the nose down so that proper air flow across the wing and control surfaces can be restored. Therefore, the captain's aft movement of the control column was an inappropriate response to the stall and impeded its recovery.
The FDR also showed that the airplane began a left roll 5 seconds after the initial loss of lift and that the roll coincided with the autopilot disconnection. Following the left roll, pitch decreased from 20° nose up to 37° nose down. The captain stated that he attempted to control the airplane by rolling it to wings level and pulling nose up, but he was unable to regain control. FDR data indicated that the captain held the control column aft to more than 33? for 16 seconds during the descent and that he did not attempt to push the nose over for another 7 seconds after releasing back pressure.
The captain stated that, during the descent, he made a combination of control and power inputs, pushing the yoke and power forward and back. The captain's statements were confirmed by FDR data. Following the initial roll, engine torque on both engines decreased to about 30 percent, and subsequently increased to over 100 percent twice, exceeding the torque limitations on the engines. The captain stated he did not recall seeing any speed during the event as he "never once" looked at the airspeed indicator. As the airspeed increased above 160 knots during the descent, pitch began to increase, and the airplane leveled off at an altitude of about 7,072 feet.
Era Aviation's stall recovery procedure stated that the first action was to start recovery at the earliest warning (stick shaker). However, as noted, the captain stated that he did not think the stick shaker had activated and that, at the time, he did not realize they were experiencing an aerodynamic stall.
The second action listed in the stall recovery procedure was to advance the power levers and call "max power." However, the captain initially reduced power, and, only later, as the airplane was descending, did he advance the power to maximum. Once the airplane was fully stalled and descending, the addition of maximum power without also reducing the AOA was not effective in the stall recovery.
The third action listed in the stall recovery procedure was to reduce back pressure to stop the shaker and minimize altitude loss; however, the captain did not reduce back pressure. Rather, he increased and maintained back pressure significantly. Further, since the airplane was fully stalled, it was not possible to minimize altitude loss.
In summary, the captain's response to the aerodynamic stall delayed the recovery of the airplane. Pulling back on the column and reducing engine power kept the airplane from achieving the necessary AOA for air flow and lift to be restored. Without awareness of the airplane's airspeed and pitch attitude as the airplane approached the stall, the captain did not recognize the prestall buffet when it occurred and, once the airplane was fully stalled, he held inappropriate nose-up pitch control and reduced power, actions which exacerbated the stall and contributed to the flight's significant altitude loss.
The first officer stated that, when the airplane pitched over, his hands were not on the controls, and he did not know what control inputs the captain made or whether he increased or decreased power. He said he first thought they should get the nose down, but then thought if they had a tail icing event that pushing forward would be wrong. The first officer stated in subsequent interviews that he did not think they had tail icing, but his initial confusion about whether the flight was experiencing an aerodynamic stall or a tail stall may have caused him to hesitate in responding.
When asked as PM, what instrument he should monitor to assist the PF in recovering from a stall, he replied "airspeed," but he could not recall what the airspeed was during the event. He also could not recall the position of the fast/slow speed control indicator. As the PM, the first officer could have called out airspeed and the position of the fast/slow speed control indicator, and he could have directed the captain to apply and maintain nose-down pitch to aid in the stall recovery; however, the first officer was surprised by the airplane's loss of control and did not provide any useful assistance to the captain during the recovery.
Incident Facts
Date of incident
Sep 5, 2012
Classification
Incident
Airline
Era Aviation
Flight number
7H-874
Departure
Anchorage, United States
Destination
Homer, United States
Aircraft Registration
N886EA
Aircraft Type
De Havilland Dash 8 (100)
ICAO Type Designator
DH8A
This article is published under license from Avherald.com. © of text by Avherald.com.
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