DC Electrical System
Understanding the ATR 72 DC Electrical System
Reading time: 10 mins
Firstly, welcome to what is ATR Theory’s first in what I hope to be a long list of technical blogs. Aimed at simplifying the ATR 42/72’s systems, helping you sharpen your knowledge and understanding on the aircraft. It won’t just be copy & paste of the FCOM, I will break the systems into more simplified terms. Any questions throughout please drop them in the comments box below and I’ll respond to each of them :).
What qualifies me? I am a current ATR 72-600 pilot, operating between the UK and Europe, I have been operating the type for the last 2 years, and thought in my spare time I would start a blog and share my technical knowledge with you all.
ELECTRICS
Well, kicking of with the ATR’s electrical systems, comparable to the nervous system of a human, the ATR’s electrical system is key to its functionality, and is used in all systems from starting the engines, to making that first morning cruise coffee.
The basics of the ATR’s electrical system can be broken down into 2 key parts; DC (not batman) and AC, Direct & Alternating Current. Each type of current is separate to the other and does not mix, although power can be transferred from one to the other, but we’ll come back to that later.
DC ELECTRICS
Let’s start with the DC systems, unlike other aircraft types, the ATR is a predominantly DC powered aeroplane. The DC systems looks a little something like this:
Key DC components and their functions are:
2 x Batteries: Emergency Battery (17 Ah) & Main Battery (44 Ah) both 24V – The rest of the DC system is 28V, which is important, because when the generators (28V) are online the higher voltage of the system will recharge the lower voltage batteries. Ah or Amp-Hours, refers to the capacity of the battery, how much charge they hold.
Lots of Electrical BUS’s: A ‘BUS’ or Busbar is essentially a big 4 socket extension cable to find at home. However, it has many ‘sockets’ and is crucial to distributing electrical power across the aircraft’s systems. I will add a list of each bus and what it powers at the bottom of the page (if you’re struggling to sleep give it a read).
2 x Generators: Or to give them their full name DC Starter/Generators. One is connected to the engine accessory gearbox on each engine. Unlike other aircraft that use compressed air to start its engines, the ATR does this electrically via the DC starter/generator, more on that later.
1 x TRU: The Transformer Rectifier Unit (TRU) acts like a one way bridge between AC & DC. Whereby the AC system can power the DC side, but not the other way round. It’s used to maintain the batteries when DC power is under emergency supply conditions (Dual DC Generator fail).
2 x Inverters: The two inverters do the opposite job to the TRU as above. They act as a one bridge allowing the DC side to supply the AC side of the electrical system. By converting DC power to AC.
CONTROLS
Top half of electrical panel:
Here is the control panel of the DC electrical system of the ATR 72. The white hashed lines indicate the AC system and is separate to the DC side.
– ATR 72-600 FCOM DSC.24.2
Key:
1. DC BUS LGHTS; indicate when the DC BUSES are powered, normal state lights are off.
2. EMER & MAIN BAT CHG; Controls the associated Battery Charge Contactor (BCC), the contactor that recharges the batteries when EXT PWR ON, or engines running. When ON, the BCC is controlled by the MFCs and opens if:
- Battery thermal runaway.
- Under voltage of DC MAIN BUS (<25V).
- Start sequence initiated, closed when START selector leaves START or CRANK position.
- OVRD signal on BAT switch.
Fault light illuminates if overheat is detected or a after a failure of the charge contactor, the contactor will open.
3. TRU Pushbutton; “ON” illuminates when button pushed in. The green arrow illuminates when power is being converted from AC to DC (officially when supply of DC EMER BUS, DC STBY BUS, INV 1, DC ESS BUS from TRU is effective).
4. Emergency supply indicator; Indicates when the batteries are discharging! Right indicates Main battery is discharging, Left for the emergency battery.
5. OVRD pushbutton; When on battery supply, when pushed this button transfers supply of the DC STBY BUS & INV 1 from the HOT MAIN BAT BUS to HOT EMER BAT BUS.
That’s very technical but when would you push the OVRD button?
UND/V Caption illuminates when the DC STBY BUS voltage is less than 19.5V. Normally OVRD push button would be selected in this situation.
- After an electrical failure, This would occur when operating in basic mode with BATT toggle in OVRD pos (a separate switch! More than that later). Once the main battery runs down to 19.5V, the UND/V light comes on, alerting the crew to select the override push button, thereby directing the DC STBY BUS to operate from the DC EMER BUS, and therefore the EMER BATT. Ideally, you wouldn’t reach this point until you’re ready for the approach since you would be down to the emergency battery only.
- When the OVRD button is pressed in DC STBY BUS and Inverter 1 will be powered by the source that supplying the DC EMER BUS and OVRD caption illuminates.
6. Battery Toggle Switch; Controls the supply of DC power by the batteries.
- ON: Essential bus, DC STBY BUS, INVERTER 1 are powered by the Main Hot Battery Bus, if the engines are off and no EXT PWR. In addition DC EMR BUS is powered by HOT EMER BATT BUS.
- OFF: all buses except the HOT MAIN BAT BUS are disconnected from BATT. Same with EMER BATT/BUS
- OVRD: Bypasses system protections, allows buses to be powered by respective BATT. Reverts to basic mode – BPCU keeps DC STBY BUS connected to the DC ESS BUS and does not permit it to toggle over to the DC EMER BUS. The OVRD pos is used for a dual generator fail, when managing battery power is critical. A guard prevents it from being accidentally selected.
7. DC AMP Indicator; Indicates the charge or discharge current of the selected battery.
8. BAT SEL Switch; enables to select the battery checked by the DC AMP indicator.
Bottom half of electrical control panel:
1. DC GEN Pushbutton: Controls the energisation of associated generator and the resetting of the protection system after failure.
2. BTC (Bus Tie Contactor) pushbutton: when BTC is closed it connects both main DC BUSSES. The BPCU (Bus Power Control Unit) automatically controls the BTC, when both engines running BTC is open isolating each side of the DC system. When EXT PWR is connected BTC is closed and the flow bar is illuminated.
3. DC EXT PWR pushbutton: Illuminates green when conditions of DC external power connection are met.
4. DC SVCE ULTY BUS pushbutton: Controls connection of DC SVCE BUS and both UTLY BUSSES to associated main DC BUSSES. SHED illuminates when a load condition controlled by BPCU is present and at least one UTLY BUS is disconnected from associated main DC BUS. Since the “SHED” caption wont indicate which ULTY bus has been shed, you can use the recirculation fans to make the determination. Fan 1 is on UTLY BUS 1, Fan 2 is on ULTY BUS 2.
– ATR 72-600 FCOM DSC.24.2
Absolutely brilliant!
It’s helped so much with my training and learning of the ATR!!