I thought ENG 2 Starter/Generator was powered by DC MAIN BUS 2, so we got to set BUS TIE SW to ON to start No.2 ENG first
This is correct.
but it is described in RFM 3-51 that ENG 2 IGNITER & START is powered by DC ESS BUS 2
This is NOT correct. There is no circuit breaker between the starter/generator and the respective main bus (in this case, main bus 2), but a "contactor"; in essence, a relay. This relay is powered by the respective essential bus, not the starter itself.
Battery start is always quicker on #1 engine. Less diodes to go through, so less voltage drop.
Bingo. Only time I'll agree to start #2 off internal battery is with both good voltage and cold engine. We tend to park with left side upwind so this is "almost never".
how to recharge the batteries from GPU. Very simple, but not in the manual!
I like that too! That way it's the engineers who'll be responsible for choosing that short-cut and the resulting hot start or worse if some battery cells happen to be damaged rather than the battery depleted. The level of system knowledge displayed here supports not letting pilots make these calls.
It will close in Auto mode when external power is connected or when one gen is failed.
At the time of starting the first engine NEITHER generator is working so the bus tie will be open => no power on main bus 2 => no power to starter/generator 2 even if the starter/generator "contactor" (see above) is closed.
out of interest, what are peoples thoughts about this?
There have been concerns on other types that allowing a rapid charging with the GPU
- following a low batt voltage situation for example - could create conditions for thermal runaway in the battery.
If you have gpu available, wouldn’t the safest option be to use it to provide start power, then let the battery recharge in the manner intended?
Would you rather have a thermal runaway in an empty aircraft sitting on the ramp, or in an aircraft with both engines running, and you being strapped inside it? Also, the GPUs I've used had slightly lower voltage (and therefore, lower charging current) than the generators gave after engine start, therefore IMO a lower chance of thermal runaway.
Yes, but what happens if you go out to the aircraft and discover someone has left a direct service on and the battery is lower than min for start?
This is indeed by far the most prevalent cause of low battery voltage on start-up, far more common than damaged battery cells or wiring/circuit faults, and is why recharging from the GPU is so popular.
If you don't have the time to change the battery, do you start it on GPU and then let the aircraft or GPU push charge into it?
How important is it to take off on that flight right now? Life or death type important? The procedure you propose definitely comes with an increased risk of problems that will preclude the flight from taking place at all, but maybe taking that chance is warranted under the circumstances? I only fly simple O&G offshore revenue flights and would rightly be censured for taking this option even if I were to get away with it.
I was lucky that I only had several unfamiliar CAS messages, but didn't have DUs black out. I heard DUs black out happened in the situation.
Not only that, the EECs (during engine start) and GCUs are also powered off the essential buses; would you even be able to abort the ongoing start when your only source of electrical power flakes out?
Could you let me know the way called "semi-automatic procedure"
I think it's in the RFM under "MANUAL ON GROUND START PROCEDURE". I would NOT recommend this for low battery voltage - if the screens blank out, how can you tell if the start is going hot? If you then abort the start because the blank screens, how do you know if you have pooled fuel inside the engine case still burning? Unless you see flames coming out of the exhaust once you get out, in which case what'll you do now, with really, conclusively, no battery power to speak of?