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I was reading the comments, on the Gizmodo post, about the engine start video (at the bottom). A lot of people didn’t really realize the significance of this, so I said I would write a short blurb about it.
Most turbine (not turbo-prop) aircraft start their engines from bleed air from the Auxiliary Power Unit (APU) mounted in the tail of the plane. This unit makes electricity to power aircraft systems, and bleed air is used for air conditioning and engine starts. Usually once the engines are started, or in some cases once airborne, the APU is shut down for the remainder of the flight. At that point the engine bleed air (for air conditioning, anti-ice, and God forbid engine restarts) and generators (electrical systems) are used for the remainder of the flight.
The problem with bleed air is that is HOT, damn hot, around 800C on some aircraft. You have to route that air to the air conditioning systems (usually at the belly), and to the wings for anti-ice. In the process of routing it your taking a risk that one of the ducts could leak, or rupture, which could cause a fire very quickly. Not only is there threat of fire, but the air is taken from the compression cycle, so you’re taking power from the engines.
So to counteract this Boeing designed a lot of electrical systems, where bleed air is normally used. They not only moved to electrical engine starts, and air conditioning, but even the brakes are electrically actuated, instead of hydraulically powered. In theory it is a lot of increased safety by engineering new systems. The problem with new systems is working out the bugs, and getting them certified. Ideally this will increase the reliability over the long run. The starters in the engine alone are supposed to increase 300% in the number of cycles it can perform.
Here is the original Gizmodo post:
Remember the Boeing 787 Dreamliner? You know, the eternally-delayed wonderplane? Well, rejoice because it has started the engines in a huge puff of smoke. All on its own, which is a first according to Boing:
The occasion marks the first all-electric start of a commercial jetliner engine on a twin-aisle commercial jetliner; the engines had been started electronically in test facilities earlier.
OK, Boeing, if you say so, I believe you. They used the Auxiliary Power System to power the start of its two Rolls-Royce Trent 1000 engines.
It’s not just that the bleed air on other jet aircraft is hot, but the energy it took to get the air that hot. Then, when it is used for air conditioning and pressurization, that heat get’s thrown away in both a pre-cooler and air conditioning packs.
The 787 still pulls energy off of the engine, to turn the generator, and then uses some of that energy to turn a cabin air compressor (located behind the fancy looking scoop at near the front of the wing root). But the cabin air compressors (CAC) produces less heat than typical bleed air – they even have a circuit (if needed) to put some CAC air back into the front of it to get it hot enough ( for cabin heating).