A Cessna 172 aircraft conducting powerline inspections near Canberra stalled and entered a spin at a height too low for recovery before it collided with the ground, an Australian Transport Safety Bureau investigation details.
In the early afternoon of 13 April 2021, the Cessna R172K departed Canberra Airport to conduct powerline surveying to the north of Sutton township, NSW. On board was a crew of two comprising a pilot and an observer.
About three hours into the flight, while manoeuvring to inspect a powerline adjacent to Tallagandra Lane, nearby witnesses observed the aircraft flying low above the trees before it commenced a left turn that continued into a steep descent before colliding with the ground.
The pilot and the observer were fatally injured in the accident, and the aircraft was destroyed.
During the accident flight, according to recorded data and witness accounts, the Cessna transitioned from a level, right turn to the north-north-east into a tighter, possibly climbing, left turn.
From the ATSB’s analysis of the turns conducted by the pilot earlier in the flight, it was estimated that the final turn was likely conducted at a comparatively high angle of bank and closer to the stall speed of the aircraft.
As the manoeuvre continued, the aircraft likely exceeded the critical angle of attack for the wing, causing the wing to aerodynamically stall.
“This investigation reinforces to pilots the importance of managing airspeed and bank angle to minimise the risk of stalling,” ATSB Director Transport Safety Stuart Macleod said.
“This is particularly important when operating in close proximity to the ground, such as conducting low-level air work, as well as during take-off and landing, as recovery may not be possible.”
Mr MacLeod noted the Pilot’s Operating Handbooks for most light aircraft, including the accident Cessna R172K’s, provides stall speed guidelines to avoid a wings level stall.
However, pilots should be cognisant of the raised stall speed when operating turns.
“In a bank the vertical lift component is reduced, and so pilots must pull back on the control yoke to maintain altitude,” noted Mr Macleod.
“This increases the angle of attack of the wing, and if the angle of attack reaches a critical angle, loss of lift and increased drag occurs, and the wing will aerodynamically stall.”
Following the accident, the operator amended the training and checking section of its Operations Manual to incorporate Threat and Error Management and Situational Awareness training modules for powerline low-level survey operations. The amendments enhanced existing topics in the operator’s crew resource management training and stipulated learning outcomes and assessment criteria specific to Threat and Error Management and Situational Awareness.
“The operator also provided detail of intended additions to its low-level procedures to implement an airspeed ‘manoeuvre margin’ that will take into account the increased stall speed associated with steep turns,” Mr Macleod said.
Further, the operator plans to modify its aircraft to include an angle of attack indicator and a g-meter with recording and data download capability.
“These will not only supplement the aircraft’s stall warning device by providing additional warning of an impending stall, but will allow for a record of the maximum and minimum in-flight readings to be downloaded post flight for review,” Mr Macleod said.