Why is the B787 one of the few aircraft with electro-thermal wing anti-/de-icing?

Question

While I was searching for a listing of aircraft with different anti-/de-icing methods - especially for electro-thermal wing anti-ice - I couldn't find any information about other aircraft except the B787. Is there any reason, why other methods like TKS or using bleed air were prefered in the past? Are there any studies related to the pro's and con's of the different anti-/de-icing systems?

Answer 1

Partially because 787 has no bleed air available for any purpose. Its engines do not draw bleed air traditionally. Any pneumatic system draws air from other pumps. Hence electro-thermal wing anti-ice system is needed since no bleed air is available.

Answer 2

Traditional anti-ice systems use engine bleed air through the pneumatic system. Engine bleed air is air that is compressed by the engine compressor to high pressure. Due to thermodynamic effects the air also warms to fairly high temperatures. This air is then bled off the engine through valves to supply the aircraft's pneumatic system. The pneumatic air is then sprayed on the inside of the leading edge of the wing or engine cowl to prevent ice accumulation.

Engine bled is a very expensive source of energy on an aircraft because compressing air isn't that efficient, the engine core has to be slightly larger to accommodate the increased airflow, and the pneumatic system itself is typically one of the least reliable systems on the aircraft. The system also uses many large ducts that are heavy and take up a lot of space, which dictates larger passages in structure, which is more weight again. However, bleed air is available making it an attractive source of energy.

The 787, however, shed the vast majority of the pneumatic system (see the link Sports Racer posted above http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_06/article_04_3.html). This decreases the specific fuel consumption of the engines quite a bit. Reduces weight, and removes an very unreliable system from the aircraft.

The 787 is able to get away with this because it has very advanced electrical starter/generators. The generators can produce about 500 KW each, significantly more than any other Boeing aircraft. Since there is so much power available, electronic heater blankets for anti-ice are feasible, no other Boeing aircraft has the excess power to use electric anti-ice on the wings. Besides being much lighter, the electric blankets will use the energy more efficiently as well.

Although the generators still pull power off the engines, increasing fuel burn, the process is much more efficient than using bleed air.

FYI: The generators are also the engine starters, which eliminates another major pneumatic component on other aircraft.

Answer 3

The reason is that is is new. The B787 is one of the few recent examples of an aircraft where the engineers were allowed to go back to the drawing board and really innovate. It is therefore one of the only examples of a truly new generation of aircraft. Most of the time when an aerospace engineer is asked to work on a new aircraft they are required, for many reasons, to draw heavily from “heritage” systems that have already been flight proven. It is only when there is a demanding business case for entirely new designs, that an engineer is allowed innovate. The B787 was intended to be a long range aircraft that would compete with larger aircraft (like the A380) that seat more people. The idea was to offer more flights with the same or better per capita cost. In order to make that vision a reality, the 787 had to be significantly more fuel efficient. Which is why Boeing went back to the drawing board and redesigned the engine and the wing shape and relied heavily on composite materials so that the aircraft would be exceptionally light.

There are two huge changes that necessitate an electrothermal icing protection system: 1) the use of composites and 2) elimination of the bleed air system many others have mentioned. Bleed air is, by definition, an inefficiency. The system poaches hot air from the jet engines in order to prevent ice formation on the wings. All of the energy used by the bleed air system would have otherwise been used to produce thrust. And that hot air is much hotter than it really needs to be in order to protect the aircraft from ice accretion. There is a big desire across the aviation industry to get rid of inefficient bleed air systems, however, these systems are integrated into the aircraft structure so they really can't be eliminated until a particular aircraft model is completely re-designed. The B787 system is said to be 50% more efficient than a bleed air system would be (if on the 787).

The other factor I mentioned was composite structures. The B787 is the first major airliner to be made largely of composite materials. Composites make the aircraft lighter and therefore more fuel efficient. As I mentioned before, bleed air is hotter than it needs to be to protect the aircraft from ice. It is also hotter than most carbon-fiber composites can tolerate. The hot bleed air is piped from the engine through the wings to the leading edges of the wings so there is a concern with composite aircraft of damaging the aircraft structure with extremely hot air. Electrothermal systems eliminate this concern by applying heat only where and when it is needed and only with the necessary energy. There are high-temperature composites but they are more expensive and given that bleed-air is inefficient it’s better to go with electrothermal.

We will see all major aircraft turn to electrically based ice protection in the future. There have, indeed, been other “new” aircraft models since the 787 but it is all a matter of whether the business case for those models was strong enough to warrant a costly re-engineering and re-certification effort (e.g. with the FAA). For example, a smaller 737 aircraft that typically travels within the same continent has less of a driving need for these efficiencies. That said, Boeing has plans for all their models to be re-designed in the image of the ’87 when the time is right. And other aircraft OEMs are making similar changes.