Aviation Safety Letter 3/2003
Night VFR approach strikes again
On October 11, 2001, a Fairchild SA226TC (Metroliner), with two pilots and a flight nurse on board, departed Gods Lake Narrows, Manitoba, at approximately 23:00 central daylight time, on a flight to Shamattawa. Approaching Shamattawa, the crew began a descent to the 100 NM minimum safe altitude of 2 300 ft ASL and attempted a night, visual approach to Runway 01. The aircraft was too high and too fast on final approach and the crew elected to carry out a "missed approach." Approximately 30 seconds after the power was increased, the aircraft flew into trees slightly to the left of the runway centreline and about 2 600 ft from the departure end of the runway. The aircraft broke apart along a wreckage trail of about 850 ft. The captain and first officer were fatally injured on impact and the flight nurse was seriously injured. This synopsis is based on the Transportation Safety Board of Canada (TSB) Final Report A01C0236.
The aircraft was equipped with a cockpit voice recorder (CVR) that indicated the aircraft was under controlled flight and the crew did not express any concern prior to impact. The aircraft weight and centre of gravity (C of G) were within limits throughout the flight and site examination, along with the CVR information, revealed no indication of any system malfunction or failure prior to impact. The captain and first officer were both properly qualified and experienced, and had also completed controlled flight into terrain (CFIT) training in December 2000.
Shamattawa is a small community 400 NM northeast of Winnipeg. It is served by a certified airport with a 4 000-ft long gravel runway with low-intensity runway lighting. Each end of the runway has green threshold and red edge lighting, and all were functioning on the evening of the occurrence. Neither runway at the airport was served by a ground-based, visual approach slope indicator. There were no ground lights beyond the end of the runway in the direction that the missed approach was conducted. The absence of any celestial light due to overcast conditions meant the missed approach was being carried out in total darkness.
The crew used a global positioning system (GPS) for the initial descent and became visual at about 3 000 ft ASL about 5 NM from the airfield. They flew a left hand visual approach. At 3 NM, they were about 700 ft above the desired approach path. They completed the final landing check, and the airspeed and altitude were still too high. Both pilots concurred that a missed approach was necessary, and the captain initiated it by calling for maximum power. The aircraft was seen over the threshold of the runway at about the height of the trees that were parallel to the runway along the airport boundary. During the missed approach, the aircraft's nose moved upwards initially, but the aircraft did not climb away, staying at the approximate height of the trees along the airport boundary.
As the first officer was setting the engine power, the captain called positive rate and gear up. The first officer raised the landing gear, retracted the flaps, and set the engine torque for the missed approach. Approximately 20 seconds after starting the missed approach, and 7 seconds before impact, the captain indicated that he would climb to 1 300 ft ASL and go around left hand. Two seconds later, the aircraft struck the trees.
What makes this CFIT accident particularly painful is its resemblance to a similar CFIT accident in 1989 involving a Metroliner aircraft at Terrace, British Columbia (TSB File A89H0007). The following explanation of two relevant flight illusions, somatogravic and somatogyral, was presented in the Terrace report.
Errors in the perception of attitude can occur when aircrew are exposed to force environments that differ significantly from those experienced during normal activity on the surface of the earth where the force of gravity is a stable reference and is regarded as the vertical. The acceleration of gravity is the same physical phenomenon as an imposed acceleration, and hence, in certain circumstances, one may not be easily distinguishable from the other.
When the imposed acceleration is of short duration such as the bounce of a car or the motion of a swing, one can separate perceptually the imposed motion from that of gravity. When the imposed acceleration is sustained, however, such as the prolonged acceleration of an aircraft along its flight path, the human perceptual mechanism is unable to distinguish the imposed acceleration from that of gravity. The body senses the sum of these two accelerations, and this resultant sum becomes the reference acceleration, which is regarded as the vertical. Illusions of attitude occur almost exclusively when there are no outside visual references to provide a true horizon.
In the absence of visual cues, the perception of motion and position is sensed primarily by the vestibular organs, and hence the term vestibular illusion is used to describe the circumstances where these organs do not correctly sense motion and/or position. Experiments have shown that there are large individual differences in the magnitude of such illusions and in the time required for the illusions to develop.
If one considers an aircraft flying straight and level and accelerating along the direction of flight because of an increase in power, for example, then the direction of the inertial force due to the acceleration is to the rear of the aircraft and, for the purposes of this discussion, can be assumed to be along the longitudinal axis of the aircraft. This inertial force combines with the force of gravity to produce a resultant which is inclined to the rear of the aircraft. If this resultant is then used by the pilot as the vertical reference, then the pilot will incorrectly sense that the aircraft is in a nose-up attitude. If the pilot then trims or eases forward on the control column to correct for this nose-up perception, the nose of the aircraft will drop and the airspeed will increase. This change in attitude will change the direction of the resultant force vector in such a manner as to maintain and perhaps magnify the illusory perception of a nose-up attitude.
Significant errors in perception can develop within the first few seconds of a change in the force environment. Experiments carried out in flight have shown that there is little lag in the onset of the illusion and that there is a relatively rapid increase in its magnitude during the initial six to eight seconds. This illusion is known as the somatogravic illusion, and it is particularly dangerous when it occurs on takeoff or when overshooting, especially at night or in poor visibility. An aircraft deceleration will result in the opposite effect, that is, a perceived nose-down attitude.
Analysis - Although reference is made to the term "missed approach," the crew were conducting a visual approach and overshoot. After the rejected landing, the crew intended to fly a 1 000-ft AGL circuit for another landing attempt. However, given the absence of any celestial or ground lights in the area, the aircraft had to be flown with reference to the flight instruments.
The descent was started late, which led to the aircraft being high and fast on approach. The absence of ground-based approach slope indicators made the determination of the approach angle more difficult for the crew. The presence of an approach slope indicator would have enabled the crew to take earlier, more positive corrective action to avoid the missed approach.
The ground-based observation, that the aircraft did not climb, indicates that the required 8 to 10° pitch attitude was likely not set by the captain. Neither pilot revealed any awareness or concern that the aircraft was not in a climbing attitude. This lack of concern is an indication that the captain, at least, lost situational awareness after the missed approach was initiated, and that the first officer was either not monitoring the flight or he also lost situational awareness.
The loss of visual references and the aircraft's acceleration forces were ideal for the onset of somatogravic illusion. Even 7 seconds prior to impact, the captain believed that he was climbing to 1 000 ft AGL. The captain's performance was consistent with his being unable to distinguish the imposed acceleration as the aircraft speed increased from that of gravity and, although he probably thought the aircraft was climbing, it was not.
The first officer may also have been influenced by the somatogravic illusion. During the 30 seconds of the missed approach, his tasks were to react to the captain's commands and to monitor the instruments. Apparently the first officer did not observe anything remarkable or he would have alerted the captain that the aircraft was not climbing. The TSB noted that the non-directional beacon (NDB) receiver was turned off just prior to impact, and since the control head is on the first officer's side of the cockpit, it was likely he who turned the NDB off. Given the short duration of the overshoot and the tasks that the first officer was performing, it is probable that he had a false perception that the aircraft was climbing.
Even though the conditions were present for the crew to be affected by somatogravic illusions, these illusions could have been overcome by at least one of the crew. During the visual approach, the pilots were able to fly with visual reference to the surface. However, pilots are required to transition to instruments when entering, or about to enter, weather or environmental conditions where visual flight conditions do not prevail, as was the case when the overshoot was initiated. Had this transition been made, the fact that the aircraft was not climbing would have been evident.
Following the accident, the operator made changes to their procedures and increased crew training. Among those, the standard operating procedures (SOPs) were amended to include a "three positive rates of climb" call to be made by the pilot flying in response to the "positive rate" call made by the pilot not flying. A new section was added to specify missed approach procedures in detail. Crew training has increased the emphasis on missed approaches and the similarities between northern night flying and instrument flight. The company has also introduced crew evaluations in a generic simulator during semi-annual recurrent training.
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