Hello there
I’m currently studying for my ATPL theory exam in Aircraft General Knowledge and have for some time been quite confused as to the function of a synchronising bus in a parallel electrical system.
According to both my textbook (PadPilot: Aircraft General Knowledge – Electrics) and my instructor, a synchronising bus cannot carry a load. The textbook reads, “The purpose of a synchronising bus is purely to allow for a parallel connection between the four AC buses, thus no loads are connected to either of the synchronising buses”.
I cannot see that this could be correct; even by the logic of
their schematic (attached below). It illustrates the APU’s generator circuit breakers as being connected to the synchronising buses; were it the case that the synchronising bus could not carry a load, the APU generator would be rendered useless.
My other cause for cynicism is that I believe that the electrical system illustrated, despite being drawn as an overlay of a BAe 146, is in fact that of a 747. It certainly is not that of the Bae 146, which has generators only on the numbers 1 and 4 engines. For example, the term chosen for the circuit breaker which divides the synchronising buses, “Split System Bus”, is one I’ve seen used only by Boeing for the 747’s electrical system. Beneath is a schematic of the 747-400’s electrical system.
Furthermore, in describing system logic, as an example of a situation which requires bus isolation, PadPilot states that “Generally the BTB will be closed unless particular conditions exist, the requirement for redundancy and individual power sources during an autoland is one”. I know this to be identical logic to that of the 747-400. Its FCOM states (Chapter 6.20.8) “During automatic ILS approach, AC and DC busses 1, 2, and 3 are isolated from the synchronous bus to provide independent power sources for the three autopilots. AC bus 4 continues to power the synchronous bus”.
If this is in fact a 747’s electrical system, I must say that I’m particularly confused as to why the textbook states that the synchronising bus cannot supply a load. In the case of the 747-400, this is most definitely not true. The FCOM states (Chapter 6.20.4) “If the IDG is not able to maintain acceptable power quality, the GCB opens and the BTB closes to provide power from the synchronous bus”.
The reason as to why I’m going to such lengths in asking about this is that between my two databases of exam questions, I have two different answers to the same question. Please see below.
Whilst I am quite happy to learn logic which is to my mind is flawed and accept that I’ve probably just misunderstood something along the way, in this case it would seem that is not an option. My question is, what is the basis of the notion that a parallel, constant frequency AC electrical system cannot supply loads from its synchronising bus? Was this historically the case or is the 747 simply something of an exception to the rule? I certainly doubt that it is the latter, given that the MD-11’s parallel system follows fairly similar logic to that which I’ve stated above.
Additionally, is anyone aware of the CAA’s current thinking? They appear surprisingly fickle in spite of their legal omniscience, and that applies to more questions than just this one.
Finally, as point of personal interest, I’ll ask about no-break power transfer. PadPilot states that “On some aeroplanes [synchronising voltage, phase and frequency] allows for no-break power transfer between various sources, unlike for the split busbar system…” I fully understand the reason for stating that the feature is unavailable to aircraft with split bus electrical systems, although it is not the case. Several split bus systems integrate the feature including that of the A330, whose FCOM states (Chapter 1.24.10) “No break power transfer… This function avoids busbar power interruption during supply source transfer on ground in normal configuration.”
In the case of the parallel system, this feature is clearly necessitated to allow for sources to be paralleled, however I’m unsure as to its benefit in a split bus system. Whilst it could in some circumstances eliminate the need for hold-up voltage fed to computers heavily reliant on volatile memory, such as Display Electronic Units during power transfer, it would not do so at all times. The FCOM states that “If synchronisation is not achieved within allowed time, transfer is performed anyway…” Therefore, a continuous DC power source and the wiring which goes with it is still required to such components. Given the complexity of the logic required of the Electrical Contactor Management Units which allow for this, it seems a significant engineering challenge for the sole benefit of avoiding flickering of the cabin lights during start-up. I'd love to know what the real world benefits are.
Kind regards
Charlie Reed
