Modern airliners over 5, 700 kg are fitted with ice protection systems that allow operations in areas of known icing. As you are probably aware by recent accidents such as that of an ATR 72 in America, these systems cannot always protect against particularly severe icing conditions, and so you should treat ice with the considerable respect it deserves.

In this mini editorial we will deal with the types of ice protection systems used on turbojet aircraft.

De-Ice
These are turned on after a certain amount of ice has been allowed to build up. An example being the inflatable rubber boots found on the leading edge of turboprop aircraft wings. The boots are cycled periodically after ice has accumulated sufficiently. This has the effect of cracking the ice, which departs into the slip-stream. Cycling the boots too soon will allow the ice to build up in a shape that effectively encases the boot within an ice shell, becoming impossible to move thereafter.

Anti-ice
These systems are turned on whenever ice formation is likely, and before it occurs. Obviously you would turn them on when planning to penetrate clouds in temperatures at or below freezing.

Electric anti-ice
Some light business jet aircraft use electrical power to heat the leading edges, though larger jet aircraft use hot bleed air from the N2 engine compressor to accomplish this. The parts of the aircraft that are normally electrically heated are the pitot/static vents, cockpit windscreen, Angle of Attack (AoA) vane, outside air temperature probe, and the radar nacelle (nose cone). Intake EPR probes and fan intake cones are sometimes electrically heated, and sometimes by compressor bleed air, dependant on engine type.

Thermal Anti-Ice
Hot engine compressor air is fed to the leading edges of the wings, tailplane, vertical fin, and engine intake nacelles. Due to the large surface areas associated with wings and tailplanes, bleed air is cycled through each of these automatically in turn. This reduces the bleed air demand at any given moment. Bleeding compressor air for this purpose, in addition to the normal requirements for cabin pressurisation and air conditioning can markedly reduce the engine thrust output.

This in turn reduces aircraft acceleration, takeoff performance, speed, climb ability, and altitude capability.

Wing anti-ice causes a larger performance reduction than that caused by heating of the relatively small engine nacelle areas.

It should be noted that engine intake icing can occur well above zero degrees centigrade, as air entering the engine accelerates and cools due to the convergent duct formed at that point. Typically aircraft flight manuals will specify the use of anti-ice on these surfaces when operating in the presence of visible moisture, with outside air temperatures of +10C or less. For those aircraft such as the Fokker F28 whose engines feature inlet nozzle guide vanes, these to will be supplied hot air to prevent ice build up. If ice is allowed to build up in the engine intakes, then is dislodged, it can cause engine damage.

The fuel tanks are NOT heated as the freezing point of Jet A1 is approximately -50C. Fuel enroute to the engines is heated by way of a fuel/oil heat exchanger, which serves the double purpose of cooling engine and hydraulic oil also.

Fluid de-ice
This is NOT used much these days. It was more common on propeller driven aircraft in the 50’ and 60’s. The penalty in using fluid de-ice is that such a large weight of fluid is required to be carried to de-ice wings etc. Fluid de-ice is still a feature of some turboprop aircraft, where it is used to de-ice propellers. More modern turboprops use electrically heated rubber boots on the propeller leading edges.

Effect of icing on Top of Descent point
With jet aircraft, forecasts of icing on descent will see flight crews starting the descent EARLIER. This is because rather than descending at flight idle, the engine speed, (and hence thrust) will be higher to afford sufficient bleed air, meaning a flatter descent gradient is achieved. Aircraft Flight Manuals (AFM) will specify the TopD correction for icing conditions on descent.

I hope that this mini training editorial helps you in your studies. Further Airline Pilot training texts such as these are available at most pilot supply shops around Australia, or through the secure ONLINE SHOP at this website for those of you with plastic money. A list of approved distributors is available within the Avfacts site. ATPL training course information is also available within the Avfacts website.