AI171 B787 - YT on lithum battery

[andrej]

Came across this interesting video, explaining lithium battery diode module failure. Not sure of it is accurate, this is for our in-house experts to confirm.

https://www.youtube.com/watch?v=OLxLN3osS1Y

Enjoy!

[John H Watson]

I see a few "issues" with his theory.

1) He was under the assumption that the Main DC Busses (e.g. the Captain's Instrument Bus) were only isolated from the battery by a diode.



In his scenario, if the diode shorted out, then you would have both the battery charger and Captain's Instrument Bus feeding the main battery. However, the circuit to me looks no different from the 747-400's. The Main Battery Relay (MBR) is only closed when the aircraft is on Standby Power (or the Standby Power switch is put to BAT).
Normally, with the MBR closed, the battery charger is not active and the battery is actually feeding the Captain's Instrument Bus, not vice versa.

If you look at normal configurations, the MBR (Main Battery Relay) is open as it is on the 747-400.



(See battery in bottom left with MBR symbol to the right of it. It shows open -||- )

The diode only seems to be an additional safety measure... perhaps if the MBR failed to open with the application of power on the main busses (Ext power, APU power, engine power)

2) He assumes there are no protections against bus tie closures with shorts on the busses being tied.

3) He assumes that the fuel relays and valves are properly represented by the simplified colour schematics in online training notes. i.e. that the valves will close with lack of power on them. Typical fuel control relays are not only dual coil (run coil/cutoff coil), the last powered position is latched with permanent magnets. You need power to get them to change state. Also, the SOSV  (Shutoff Servo Valve) which controls the HPSOV (High Pressure Shutoff Valve) in the Fuel Metering Unit is most likely magnetically latched, too, like it is on the CF6 and RB211. Also, the Fuel Spar Valves in the wings are powered by DC motors, which are not going to move without power. Although I don't have proper schematics for the 787, on the 777, there are two latched relays in series and a motor controlling each spar valve. You could have a nuclear meltdown and that valve wouldn't close.

Any valve which takes up to 15 seconds to close, is definitely not spring loaded to close.

(Google  Boeing Service Bulletin B787-81205-SB280015-00 FUEL - Fuel Distribution - Fuel)

For some reason, this is far slower than the 747-400 spar valves which take 3~5 seconds to close.

Some Air India pilots had a 787 sim session and shut off all electrical power with the engines running. The engines continued to run. PSX does the same. Because of this, the FAA mandated that all new aircraft be fitted with an emergency power supply to be able to provide power to close the fuel valves if the aircraft lost power.

Here's a quick video of one such power supply being changed on a 737NG

https://www.instagram.com/reel/CrRT3DXAQID/

Just noticed that the box has an "FAA Approved" sticker on it.

Captain Steeeve (777 pilot) on YouTube attemped to explain the operation of the valves, but he got a lot of things muddled up. e.g. He still believes the valves are held open by the battery until the battery goes flat.

People seem to think that the 787 is some exotic beast, but I see it only as an evolution. The 777 was an evolution from the 747-400 and the 787 was an evolution from that. An (almost) bleedless aircraft with electric brakes and A380 style "frequency wild" generators. Meh.

I still wish I had some proper wiring schematics for the 787. That would solve a lot of arguments, including those on the Geoffrey Thomas YT channel. There are always some conspiracy theorists who refuse to believe the preliminary reports.. and will continue to do so even after the final report has been released.