Really interesting article outlining Boeing's electrical pressurization system in the 787. Not only do they say that they save up to 35% energy from a conventional system, but that the weight and the maintenance of the pressurization ducting is a big saving (didn't know it had to be titanium either).

https://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_02_1.html

I am skeptical the bleed air system on Boeings use a large amount of titanium. The pressures involved are not that great, could be handled by other materials, and titanium is ridiculously expensive.

I would want further explanation before I believed what was said.

Mike C.

Pressures aren't very high, the temperatures are often on the order of 500 deg F.

I am interested in the electrical anti-icing system and the energy that it requires.

Aluminum melts at 1200 F.

Stainless steel melts at 2750 F.

I have both materials in parts of my bleed air system, as well as various silicone caskets and couplers.

Mike C.

I am skeptical the bleed air system on Boeings use a large amount of titanium. The pressures involved are not that great, could be handled by other materials, and titanium is ridiculously expensive.

I would want further explanation before I believed what was said.

Mike C.

Increased fuel efficiency by eliminating most of the bleed air draw from the engines isn't a free lunch, as some of the functions of bleed air are shifted to electrical power which is produced by engine-driven generators with the engines taking up the increased load of generating more electrical power for compressors and pumps, wing heat, previously powered by bleed air. Considering that the generators must also produce a lot power to run several gallies with ovens in addition to items converted .

The 747 using the traditional bleed air system had 4-90 KVA generators, one on each engine, so it seems that the 787 with two engines must have generators of humongous output; enough to carry the load with an engine out; probably cancelling meal service though.

They tout a 35% increase in efficiency but only a 3% or less reduction in fuel burns, although over the total hours of the fleet, that's a lot of fuel, and as they say, simplifies maintenance. One of the advantages I can see with electric pressurization is that it eliminates the occasional smoke and fumes in the cabin from failed engine seals.

I presume the 35% number is the increase in efficiency (reduction in power) of the cabin pressurization system, not the engines themselves (which would so large as to be bogus).

3% fuel savings is really large. And this is considering the 787 has higher pressurization than most airplanes.

A key advantage to the electric pressurization is that you don't start with high pressure hugely hot air that you have to lower in pressure and then lower in temperature. You can, with electric compression, run the system at the exact operating point you need, not making the air any hotter or higher pressure than you need. This saves power in conditioning the air before putting it into the cabin.

The 787 has four 250 KVA generators, two on each engine, plus 2 more on the APU (??? KVA) for a total of 6. So they normally have about 1 MW of electrical power. That's about 1000 amps per generator, so some weight is in the wires but they are smaller than bleed ducts and don't need insulation and other stuff those ducts do.

There was a software bug related to the 787 GCUs (generator control units). Under certain conditions, the GCUs on all four engine generators would shutdown spontaneously without cause potentially disabling all power aboard the airplane. A plane in Japan did this, happily while it was on the ground.

Turns out the GCUs have software defined fault monitoring which uses time in various functions. The time is a 32 bit integer that counts at 100 Hz. The bug is that the timer should be an UNSIGNED INT and the GCUs used it as a SIGNED INT. When the timer got to 248.55 days, the timer would clock over from 2^31 - 1 to 2^31. In UNSIGNED INT, that is 2,147,483,648. If treated as an SIGNED INT, that is -2,147,483,648. Time went backwards. That caused all the fault functions that rely on time to blow up and think the system had been overloaded and shutdown the generators. Since all the timers started about the same time, when power was first applied to the airplane, they all shutdown about the same time. Synchronized failure.

The Japan airplane had been continuously powered for 248+ days when this happened. It just happened to be on the ground at the time.

The FAA issued an AD for this condition, AD 2015-09-07. The solution? Every 120 days, disconnect all power (including main battery) for 5 minutes, then reconnect. The good old power on reboot. Why 120 days? For redundancy. If someone missed one cycle, you had another one before you reached 248 days.

Boeing has since patched this bug, of course.

The mind blowing thing to me is not that the bug existed, but that a 787 spent 248 days continuously powered.

Mike C.

Aluminum melts at 1200 F.

Stainless steel melts at 2750 F.

I have both materials in parts of my bleed air system, as well as various silicone caskets and couplers.

Mike C.

I presume the 35% number is the increase in efficiency (reduction in power) of the cabin pressurization system, not the engines themselves (which would so large as to be bogus).

3% fuel savings is really large. And this is considering the 787 has higher pressurization than most airplanes.

A key advantage to the electric pressurization is that you don't start with high pressure hugely hot air that you have to lower in pressure and then lower in temperature. You can, with electric compression, run the system at the exact operating point you need, not making the air any hotter or higher pressure than you need. This saves power in conditioning the air before putting it into the cabin.

The 787 has four 250 KVA generators, two on each engine, plus 2 more on the APU (??? KVA) for a total of 6. So they normally have about 1 MW of electrical power. That's about 1000 amps per generator, so some weight is in the wires but they are smaller than bleed ducts and don't need insulation and other stuff those ducts do.

There was a software bug related to the 787 GCUs (generator control units). Under certain conditions, the GCUs on all four engine generators would shutdown spontaneously without cause potentially disabling all power aboard the airplane. A plane in Japan did this, happily while it was on the ground.

Turns out the GCUs have software defined fault monitoring which uses time in various functions. The time is a 32 bit integer that counts at 100 Hz. The bug is that the timer should be an UNSIGNED INT and the GCUs used it as a SIGNED INT. When the timer got to 248.55 days, the timer would clock over from 2^31 - 1 to 2^31. In UNSIGNED INT, that is 2,147,483,648. If treated as an SIGNED INT, that is -2,147,483,648. Time went backwards. That caused all the fault functions that rely on time to blow up and think the system had been overloaded and shutdown the generators. Since all the timers started about the same time, when power was first applied to the airplane, they all shutdown about the same time. Synchronized failure.

The Japan airplane had been continuously powered for 248+ days when this happened. It just happened to be on the ground at the time.

The FAA issued an AD for this condition, AD 2015-09-07. The solution? Every 120 days, disconnect all power (including main battery) for 5 minutes, then reconnect. The good old power on reboot. Why 120 days? For redundancy. If someone missed one cycle, you had another one before you reached 248 days.

Boeing has since patched this bug, of course.

The mind blowing thing to me is not that the bug existed, but that a 787 spent 248 days continuously powered.

Mike C.

I am skeptical the bleed air system on Boeings use a large amount of titanium. The pressures involved are not that great, could be handled by other materials, and titanium is ridiculously expensive.

I would want further explanation before I believed what was said.

Mike C.