FAA Grounds 787s

[Hi_Tech]

The Boeing 787 could be the last all electric design for an aircraft

The following is a bit long, but if you are a pilot or engineer you might appreciate this discussion.
Design decisions are not given out by aircraft manufacturers. But if you know complete engineering details of several aircraft, like in my case, one can see through the tangle and guess what are the problems Boeing is facing. I am not a design engineer, but one who was trained and worked on the last 5 Boeing jets produced and the last 5 airbus including A380.
On all aircraft till B787 design, there are three prime sources of motive power.
1. Electric. 2. Pneumatic. 3. Hydraulic.
To understand Boeing's problem now one has to know how the motive powers are derived on all previous aircraft.
Electric generators are on the engines and APU. With emergency power from batteries and RAT, which has a small generator the size of a pineapple, 10 KVA capacity on B787. B787 has normal generating capacity of 1450 KW.
Pneumatic is sourced from engine bleed or (if running)APU bleed below a certain altitude about 25000 ft.
Bleed air from APU, when on ground, is used to start engines. After an aircraft lands from a flight, as engine bleed is still available, it is used to start APU as in B777.
Hydraulic pumps are driven by engine, or electrically driven, or in the case of Boeing 777 and 747 bleed air driven during high demand.
When Boeing designed B787, which they called is a game changing design, they took a decision to do away with bleed air system and depend on the electrics and hydraulics only for motive power source. They called it one of the features of their game changing design. But one still needs compressed air for air-conditioning and pressurizing the aircraft. So they have these two large electrically driven compressors that require 270V DC power.
Another decision was to have electric brakes instead of hydraulic brakes. A questionable choice even if saves a few kilos. This required a large battery power for emergency brakes if you have total power failure before landing. There are 8 brakes, and each brake with 4 motors driving worm gears. So you are talking about 32 motors and associated electronics to be driven by battery. (This condition is rare, but it has happened on this plane once during a test flight - Laredo, Texas incident).
But doing away with bleed air is at the heart of the current problems with this design where there is very little room left for maneuver. Engines and APU can only be started with electric starters for which they use the same 3 phase generators, that normally supply the power the aircraft. Theyare driven as starter supplying power from available sources.
Let me give an emergency situations where there is a total power failure on B777 and B787 at cruise altitude of 40,000 ft.
On a Boeing 777 the bleed air from the engines is still available, (Bleed valves stay open when power is removed), and this is used to automatically start the APU through a pneumatic starter. This source is unlimited unlike a battery with a fixed charge. Bleed air is also available for pressurization of aircraft, so the crew, if they are 40,000 ft, need not worry about pressurization, and when APU starts, power is also restored. Crew have to deal with only the electrical failure during this critical period.

On B787, when electrical power fails, they have the additional emergency of pressurization stopping as the compressors that supply air would shut down.
So they have to quickly get the APU started to restore the electric power, using of course the battery. This battery can give 2 start attempts. APUs are traditionally difficult to start at high altitude. So if there is a delay crew will face the additional emergency of diving to a lower altitude to avoid a low cabin pressure situation, while you have a pineapple size generator on RAT powering this all electric plane.
As Boeing has done away with DC starter for APU (unlike in all older airplanes), they require a power conversion circuit to convert the 30V DC power from battery to 3 phase 235 V AC power to start the APU. That could explain why you need all the power drawing capacity for the APU battery.
It may not be easy to find a Ni-Cad battery to supply 150 A. They will need 2 batteries in parallel to supply that current.
Another draw back of doing away with bleed air is engine starting. If APU is not available for any reason, they require ground power. This is only available as 3 phase 115V at all airports. This has to be converted to 235V AC 3 phase to start engines using again a power converter on airplane.
Then the air compressors require 270V DC power, which is obtained using another large power conversion. This 270 V DC is also required to drive the high demand electric hydraulic pumps.
These power conversions generate heat and these panels that are located in the middle of the aircraft (Approximately below the first row of economy seats on ANA), are cooled using 2 large refrigeration units. In the middle of this power center is the suspect APU battery. So if this goes up in smoke one can expect a total power failure on this aircraft.
After seeing these problems, I doubt if we will ever see an all electric design in future. After all, doing away with bleed air has saved only 2% fuel, the figure many in the industry are already questioning. Is it worth the trouble?