Aviation Safety Letter 1/2003
Loss of Control - Collision with Terrain
On May 25, 2001, a Cessna T 310Q was working as a birddog aircraft on a forest fire about 33 NM northeast of Red Earth Creek, Alberta. During a turning manoeuvre at low altitude, in preparation to lead the tanker group's Douglas B26 water bombers to a drop zone, the aircraft descended into the trees and crashed. The aircraft was destroyed by fire. Both occupants - the pilot and the air attack officer - were fatally injured. This synopsis is based on the Transportation Safety Board of Canada (TSB) Final Report A01W0118.
At 13:34, the aircraft was dispatched as a member of a group tasked to conduct an initial attack on a forest fire located 94 NM east of Manning, Alberta. This was the first operational flight of the season for this aircraft and its pilot.
A birddog pilot's role is to transport the Land and Forest Service air attack officer (AAO) to the scene of a fire, help the AAO plan and coordinate the airborne attack, and manage the restricted airspace near the fire. The pilot assists in the planning and checking of routes to and from the drop zone and in leading the water bombers into their bombing runs. These activities usually involve extensive manoeuvring of the birddog aircraft through a number of circuits at low altitude and low airspeed. It is common for birddog pilots in the Cessna 310 to conduct their low-level operations at about 120 to 140 mph, with 15° of flap and landing gear retracted. Birddogs regularly achieve bank angles of 40° to 60° in turns, as confirmed by measurement of previous recordings from forward-looking infrared cameras mounted on various birddog aircraft.
When the group arrived on location, the fire was spreading south with a light northerly wind. The smoke column was well defined, with good visibility along the flanks. After flying clockwise reconnaissance circuits around the fire, the aircraft was observed making steep left-hand turns east of the fire. This was consistent with the crew's communication that they intended to have the tankers lay retardant on the east flank of the fire in a line from north to south. The circuit in which the accident occurred was to be a "dummy run" where the Cessna 310 would demonstrate the desired flight path and zone for the first retardant drop. The aircraft was last observed in a left turn about 200 ft above ground level (AGL) and about 0.7 SM from the east flank of the fire, as it entered the downwind leg of the dummy-run circuit.
The accident occurred 1 911 ft above sea level (ASL), in relatively level, obstacle-free, forested terrain with trees from 20 to 30 ft tall.
The wreckage was examined on site, to the extent possible, because of destruction by impact and fire. No pre-existing defects could be found. The main wreckage trail was about 100 ft long, preceded by a 40-ft long by 10-ft wide slash through trees at an angle of 42° from horizontal.
The pilot held an airline transport rating and had about 10 000 hr of total flying time, with 368 hr on type, including about 85 hr in 2000 in his first season as a birddog pilot. In April 2001, he completed 3 hr of supervised flight training and 4 hr of recurrent ground training, which met the company's annual training requirement for birddog pilots of at least 3 hr of recurrent flight training and 3 hr of recurrent ground training.
Good visual flight rules (VFR) weather conditions prevailed throughout the area. At the time of the accident, the weather on location was observed to be generally high cloud, visibility greater than 15 SM, and wind from the north at about 13 mph, with no turbulence.
The weight of the aircraft was within the maximum gross weight limit of 5 500 lb. The calculated centre of gravity (C of G) was 36.8 in. aft of the datum, which is at the forward limit of the C of G envelope for a weight of 5 200 lb.
In Flight Training Manual, Transport Canada defines an aerodynamic aircraft stall as a loss of lift and an increase in drag that occurs when an aircraft is flown at an angle of attack greater than the angle for maximum lift. The stalling speed increases in manoeuvring flight, such as turns or abrupt changes in the aircraft's flight path; the steeper the turn, the higher the stalling speed. The manufacturer calculates the power-off stalling speed of the Cessna T 310Q at 5 200 lb, in straight and level flight, with landing gear retracted and flaps at 15°, to be 84 mph indicated airspeed. Under the same conditions at 45° of bank, the stalling speed increases to 100 mph, and at 60° of bank to 119 mph. A forward C of G will normally increase the stalling speed. When an aircraft stalls during a level or descending turn, the inside wing normally stalls first and the aircraft will roll to the inside of the turn. During a climbing turn, the higher wing normally stalls first and drops abruptly.
Analysis - Several factors were involved during the manoeuvring for the dummy run: low relative airspeed, steep left turns, and forward C of G position, which would have increased the stall speed and decreased the margin between airspeed and stall. The aircraft likely stalled in a climbing attitude. This would result in a sharp roll to the right. With the aircraft's low altitude, recovery before ground impact would be difficult. Tree-strike evidence indicates that at impact the aircraft was in a 42° nose-down attitude in a right bank of about 105°. On ground contact, the aircraft cartwheeled and tumbled.
Examination of the aircraft wreckage revealed no defects that could have led to the accident. Damage to the two propellers indicated that both engines were producing power on impact. The TSB concluded that the aircraft likely entered a stall during a low-level turning manoeuvre from which recovery was not possible.
Lesson learned - While birddog flying is a highly specialized activity, the above occurrence clearly reminds us of the aerodynamic limits of flying an airplane - any airplane.
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