Cessna 172 accident report. Carb Icing or something else?
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Cessna 172 accident report. Carb Icing or something else?
The Australian Transport Safety Bureau has released its final report into the wirestrike and collision with terrain of a Cessna 175RG at Parafield Airport, South Australia, on 3 July 2018.
See: Wirestrike and collision with terrain involving Cessna 172RG, VH-LCZ, Parafield Airport, South Australia, on 3 July 2018
Report summary:
At about 6.00 pm, while descending through 450 ft for landing, the aircraft’s propeller speed reduced to 1,300 rpm. The pilot turned on the carburettor heat and switched the fuel tanks selection from BOTH to LEFT, however the engine did not respond. The pilot realised there was not enough altitude to reach the runway and that a forced landing was necessary.The pilot turned the aircraft towards an unlit area and positioned the aircraft to achieve its maximum glide distance. As it descended, the aircraft struck treetops. The nose wheel then struck a power line and the aircraft collided with terrain. The pilot received minor injuries and the aircraft was substantially damaged. The ATSB found the power loss was likely the result of carburettor icing.
Carburettor icing and suspected somatogravic illusion are two fall-back probable causes often used in ATSB general aviation aircraft accident reports where the cause of an accident cannot be proved because of absence of definite evidence.
In the case of the above accident, it seems the engine of the Cessna 172 lost power while approaching to land and the aircraft forced landed short of the runway. Conditions favoured the presence of carburettor icing. As the pilot failed to use carburettor heat during the period before the apparent engine failure, the ATSB felt that the power loss was likely the result of carburettor icing. And they may well be right.
Over the years there have been many instances where faulty engine idling settings have caused engines to stop. Typically this happened during slow flight at idle power approaching the stall or even while gliding. It was a relatively common occurrence in Tiger Moths. These aircraft did not have a starter motor and the only way to get the propeller to turn after a stoppage in flight was to dive to obtain sufficient airflow over the propeller to get it to rotate. The same procedure could be used in aircraft such as the Cessna 150 and 172, if for some reason, the starter motor could not turn the propeller. Where slow idling RPM has caused a propeller to stop turning in flight, it may not be possible in the short time permitting, to say for certain if it was carburettor icing or very low idling that caused the engine to stop.
This writer had an incident some years ago where the propeller of his Cessna 152 stopped turning during practice gliding at 2000 feet. The starter motor was tried but the engine did not catch. The aircraft was close to the shore of a bay and there was no suitable forced landing area within gliding distance. The decision was made to ditch the aircraft next to moored yachts. A Mayday call was made to ATC as well as the transponder switched to 7700. With the aircraft down to 200 feet above the water, I reached across the student pilot in the left seat, and actuated the primer pump with two full strokes. The starter motor was again tried. Fortunately the engine fired and we were able to climb away and cancelled the Mayday.
After landing, the defect was recorded in the aircraft maintenance release. It turned out that during a stalling exercise, another flying instructor in the same aircraft at 3000 feet on the day before, had the same problem. On this occasion the propeller stopped turning as the aircraft stalled. Turning the propeller with the starter motor was unsuccessful so the instructor dived the Cessna 152 until the propeller started to turn. The engine started immediately and the aircraft landed safely at base.
The instructor should have recorded the defect in the maintenance release but deliberately chose not to do so. Unaware of its history, I flew the same Cessna 152 on the following day and came close to getting wet feet. Ironically, on a previous flight a few weeks earlier I had written up in the maintenance release that the primer pump shaft was jamming and could not be moved. The defect was rectified by an LAME. The primer pump was therefore serviceable when I needed it most in an emergency. Depending on fuel system design, and the circumstances of an engine stopping in flight, pilots should be aware that if an attempt is made to re-start an engine and it doesn’t work, then use of the primer should be considered.
Having said that, one thing is for sure. There is no valid excuse for failure to record the defect in the maintenance release immediately after landing. The next pilot to fly that aircraft could be caught unawares with an un-airworthy aircraft.
See: Wirestrike and collision with terrain involving Cessna 172RG, VH-LCZ, Parafield Airport, South Australia, on 3 July 2018
Report summary:
At about 6.00 pm, while descending through 450 ft for landing, the aircraft’s propeller speed reduced to 1,300 rpm. The pilot turned on the carburettor heat and switched the fuel tanks selection from BOTH to LEFT, however the engine did not respond. The pilot realised there was not enough altitude to reach the runway and that a forced landing was necessary.The pilot turned the aircraft towards an unlit area and positioned the aircraft to achieve its maximum glide distance. As it descended, the aircraft struck treetops. The nose wheel then struck a power line and the aircraft collided with terrain. The pilot received minor injuries and the aircraft was substantially damaged. The ATSB found the power loss was likely the result of carburettor icing.
Carburettor icing and suspected somatogravic illusion are two fall-back probable causes often used in ATSB general aviation aircraft accident reports where the cause of an accident cannot be proved because of absence of definite evidence.
In the case of the above accident, it seems the engine of the Cessna 172 lost power while approaching to land and the aircraft forced landed short of the runway. Conditions favoured the presence of carburettor icing. As the pilot failed to use carburettor heat during the period before the apparent engine failure, the ATSB felt that the power loss was likely the result of carburettor icing. And they may well be right.
Over the years there have been many instances where faulty engine idling settings have caused engines to stop. Typically this happened during slow flight at idle power approaching the stall or even while gliding. It was a relatively common occurrence in Tiger Moths. These aircraft did not have a starter motor and the only way to get the propeller to turn after a stoppage in flight was to dive to obtain sufficient airflow over the propeller to get it to rotate. The same procedure could be used in aircraft such as the Cessna 150 and 172, if for some reason, the starter motor could not turn the propeller. Where slow idling RPM has caused a propeller to stop turning in flight, it may not be possible in the short time permitting, to say for certain if it was carburettor icing or very low idling that caused the engine to stop.
This writer had an incident some years ago where the propeller of his Cessna 152 stopped turning during practice gliding at 2000 feet. The starter motor was tried but the engine did not catch. The aircraft was close to the shore of a bay and there was no suitable forced landing area within gliding distance. The decision was made to ditch the aircraft next to moored yachts. A Mayday call was made to ATC as well as the transponder switched to 7700. With the aircraft down to 200 feet above the water, I reached across the student pilot in the left seat, and actuated the primer pump with two full strokes. The starter motor was again tried. Fortunately the engine fired and we were able to climb away and cancelled the Mayday.
After landing, the defect was recorded in the aircraft maintenance release. It turned out that during a stalling exercise, another flying instructor in the same aircraft at 3000 feet on the day before, had the same problem. On this occasion the propeller stopped turning as the aircraft stalled. Turning the propeller with the starter motor was unsuccessful so the instructor dived the Cessna 152 until the propeller started to turn. The engine started immediately and the aircraft landed safely at base.
The instructor should have recorded the defect in the maintenance release but deliberately chose not to do so. Unaware of its history, I flew the same Cessna 152 on the following day and came close to getting wet feet. Ironically, on a previous flight a few weeks earlier I had written up in the maintenance release that the primer pump shaft was jamming and could not be moved. The defect was rectified by an LAME. The primer pump was therefore serviceable when I needed it most in an emergency. Depending on fuel system design, and the circumstances of an engine stopping in flight, pilots should be aware that if an attempt is made to re-start an engine and it doesn’t work, then use of the primer should be considered.
Having said that, one thing is for sure. There is no valid excuse for failure to record the defect in the maintenance release immediately after landing. The next pilot to fly that aircraft could be caught unawares with an un-airworthy aircraft.
Last edited by Centaurus; 17th Dec 2018 at 07:06.
