Carburretors and icing

The recent release of the ATSB accident report on a Piper PA32-260 Cherokee Six * should be a must-read for all pilots – old, bold or new.

There are a number of operational elements in that accident the ATSB report touched on. In brief, the aircraft took off from runway 24L at Jandakot and at 700ft the engine lost power. The pilot elected to try and make for runway 30, but at an indicated airspeed of 40 kts. Carburettor heat was not applied. No flap was used in the landing. The aircraft landed in trees short of runway 30 and was written off. There were minor injuries to one of the three occupants.

There had been no pre-takeoff safety briefing that included reviewing emergency airspeeds and actions. No defects were found in the subsequent examination of the engine or airframe. The pilot had not had a flight review in the last two years.

VH-TSZ was a Piper PA32-260 Cherokee Six with a carburetted Lycoming 0-540 engine of 192 kW (260 hp). The stall warning is via a light on the panel.

The ATSB report found the most likely cause of the engine power loss was carburettor icing, something the owner pilots had experienced before on climb and with descent power settings.

The atmospheric conditions on the day were known to be conducive to possible carburettor icing. The outside air temperature was 19.8C and the dewpoint was 11.2C. The carburettor icing-probability chart found in the Visual Flight Rules Guide puts this combination into the moderate icing range – cruise power, serious icing – descent power.

It is interesting to note that these figures are very close to those found in the investigation into the fatal accident of Cessna 172G VH-RPI at Trefoil Island, Tasmania on March 14 2003. As with the case of VH-TMZ there were other factors involved in the accident (CG, take-off weight) that occurred soon after takeoff. The ATSB found the most likely cause of the engine failure in VH-RPI was also carburettor icing.

The checklist held in the Piper advised maintain safe airspeed for the after-takeoff scenario then to 87kts glide speed inflight to 77 kts for a power-off landing. The pilot targeted a speed around 40 kts and maintained that speed for the duration of the glide. In contrast, the owner’s handbook listed a flaps-up stall speed of 61 kts. To maintain a speed of 40 kts would have given the elevator little ability to effect a round-out for the touchdown. The use of flaps would have helped decrease the ground speed for the landing.

For all pilots the pre-takeoff safety briefing is a good time to remind one’s self of what speeds and actions they need to apply in the unlikely event of an engine failure during or immediately after takeoff, as well as the Vy or best rate of climb speed.

It’s best to do the briefing in the run-up bay with no taxying distractions.

If the temperature on the day is such that carby icing may occur, not only should the aircraft be taxied with a lean mixture setting, but to apply and hold carby heat at the holding point before line up. It could even be the case to push the carby heat in/off with throttle application. ‘Carby heat off, mixture rich, throttle up’.

A flight review, if properly conducted, should include at least one practise forced landing. This would give the conducting instructor the ability to see the candidate execute a forced landing and if need be recommend better speeds for the power-off glide. If the speed is too high or too low the gliding aircraft will not obtain the optimum glide distance.

Carburettor icing has been around since powered flight began, but the same rules apply. If an aero engine suffers inflight carby icing the pilot must maintain control, apply carby heat and consider the best options for a possible landing. Those options include flying the aircraft at the recommended glide speed and use flap to assist with the lowest possible ground speed on touch down. A pre-takeoff safety briefing including climb and glide speeds will assist the pilot should an engine failure occur during the takeoff and climb out.

It is worth noting that the incidence of carby icing events may actually increase in the next few years as the new breed of fuel injection-trained pilots gravitate to carburetor-equipped aircraft.

“A pre-takeoff safety briefing… will assist the pilot should an engine failure occur during the takeoff and climb-out.”

For all pilots, know what the conditions are that cause carburettor icing and how to combat it. And know your V speeds and glide speed!

* ATSB Transport Safety Report AO-2012-158. Piper Cherokee Six, VH-TSZ, Jandakot Airport, WA, 23 November 2012.