No point putting this in a pilot thread

[RMC]

Only a knowledgeable 787 engineer could answer to this.

The first time this happened I wasn’t sure if it was because lowering the gear had caused the problem. It has now happened to me twice ,both times as soon as the gear is dropped.

An Electric Brake Actuator fault appears as soon as the gear is lowered. If power were removed from the gear once it is retracted then I assumed we wouldn’t get tyre pressure indications throughout the flight.

Could it be that electrical power is only removed from the braking system once the wheels are up (but I thought the brakes were used to stop rotation during retraction). Confused.com

[Uplinker]

I know you want an Engs input - quite rightly - but I would say that the brakes are probably applied as the gear is retracted, so when they are fully stowed, no braking is required, as the wheels will have stopped by then. Therefore the main brake actuator power can be automatically removed.

Pressure transducers do not need high current, only low current at 5 or 12V, so the main power supply to the undercarriage can be removed, leaving only the pressure and presumably the brake temperature transducers working.

When you drop the gear, the main power will be reapplied, and presumably a BITE test is performed on all the (fully electrical) 787 brake actuators, at which point the fault is detected and flagged up to the cockpit. My guess would be a bad electrical connection somewhere in the brake actuator brought on by the low air temperature and cold soak at altitude.

[Tom Sawyer]

Essentially as the above post says - From the 787 AMM SDS;

"The BSCU (Brake System Control Unit)s remove a power enable to the EBAC (Electric Brake Actuator Controller)s 60 seconds after the landing gear lever moves to the UP position. This causes a shutdown of the EBAC (Electric Brake Actuator Controller)s during flight. The BSCU (Brake System Control Unit)s set the power enable to the EBAC (Electric Brake Actuator Controller)s when the landing gear lever moves to the DOWN position. If the LRRA (Low Range Radio Altimeter) data show 500 ft (152 m) or above, the EBAC (Electric Brake Actuator Controller)s do a brake operational test that sets the EBA (Electric Brake Actuator)s to the running clearance before landing."

So, if one of the EBA did not actuate to running clearance on gear down selection , a fault message would be generated.
Elsewhere it states that 7.4% braking pressure is applied for 20s during gear retraction.
Brake temp and wheel pressure are sent over the CDN direct to the BSCUs which are still powered in flight, hence why you still get that data and any required warnings (i.e. it is only the actuation system that is not powered in flight).

[RMC]

Brilliant guys. We just don’t have access to this kind of detailed information. Thanks.

[TURIN]

Re tyre pressures, I thought the data was sent over Bluetooth from the wheel px transducer to the wheel hub. To power the transducer the wheel has to be spinning creating a current through the flux link. The nose wheels are small enough not to require a flux link as the transducer rotates within the hub magnetic field.
Does the px transducer retain enough battery power during flight?

[Tom Sawyer]

The SDS doesn't mention the flux ring having to rotate to provide any power. It just talks about the transmitter getting power via wifi inductive coupling from the wheel remote data concentrator. The flux ring improves the reading of the wifi signal.

[TURIN]

I'm going to have to re read my notes. 😁