Electrical System Flashcards
The STANDBY POWER switch has 3 positions: OFF, AUTO, and BAT. What is this switch used for and what does each position do?
Generally, the captain’s critical flight deck equipment is on the standby system (comprised of 2 standby busses- APU and MAIN). This switch controls both the APU standby bus, and MAIN standby bus. Note: One bus is called “APU standby bus”, but it has critical flight deck equipment on it as well as the MAIN standby bus.
OFF: Will disconnect APU and MAIN standby busses from all power
AUTO: Allows APU and MAIN standby busses to be powered from available sources
BAT: Forces both standby busses to their backup power source (batteries). This is a MX function only, or to test that the standby system works off of battery only.
What does the BATTERY switch do?
ON: Powers the main and APU battery busses (normally by DC bus 3 for both)
OFF: Disconnects main battery and APU battery from their related battery busses.
NOTE: Even with AC power available, turning off this BATTERY switch will kill a running APU, or not allow you to start an APU. The switch is required to be ON for the APU to work.
There are 2 UTILITY switches, L and R. What do they do?
Description of system: Generally, attached to each AC bus there is 1 additional bus called utility. (4 AC busses means 4 utility busses). These utility busses contain non-critical or “utility” stuff like recirc fans and certain fuel pumps. If the load becomes too great on an AC bus, the load management (ELCU) takes over and removes the utility bus off of the overloaded bus.
ON: AC bus 1 and 2 (L) or 3 and 4 (R) ELCU’s are powered and can provide load shedding of the utility bus
OFF: Turns off associated utility busses
If automatic load shedding of a utility bus by an ELCU occurs, does it illuminate the utility switch?
Normal system logic ELCU load shedding does not trigger the light
What does the APU GEN 1 (or 2) switch do?
When AVAIL light illuminated, pushing this switch ON connects the APU GEN 1 (or 2) to the AC system
What does the EXT PWR 1 (or 2) switch do?
When AVAIL light illuminated, pushing this switch ON connects external power 1 or 2 to the AC system
What powers the ground service bus? What are some items on this bus?
AC bus 1.
Power outlets, flood lights, battery chargers, fuel pumps for APU start, etc.
What powers the ground handling bus? What are some items on that bus?
APU GEN 1 or EXT 1
Lower cargo lights and handling equipment, fueling system, and auxiliary hydraulic pump 4
What powers the main deck cargo handling bus? What are some items on that bus?
APU GEN 2 or EXT 2
Nose and side cargo doors
It can be confusing to keep track of the different “ground busses.” You have Ground service bus, ground handling bus, and main deck cargo handling bus. What is an easy way to keep track of them all?
2 of the 3 are for GROUND ONLY (I.E. get their power from external power or APU)
The one that can be powered on ground or in flight is the “ground service bus” (Because is powered by AC bus 1, available in flight unlike EXT or APU)
Just remember that if the term “handling” is in the title, its for on the ground only.
The “ground service bus” really should just be called the “service bus”
For the 2 busses related to “handling” on the ground (the ground handling bus and the main deck cargo handling bus), do they require switch activation to come online?
No. They are designed to be operated so that if the pilots aren’t around, the cargo loaders don’t have to mess with the flight deck switches as long as they plug in both GPUs.
KEY INFO: These 2 busses are unique in that they don’t require you to select “ON” on the overhead. They will still be powered if EXT or APU says AVAIL. In fact, if it is using for example EXT 2 for the main deck cargo handling bus (says AVAIL), and you push the switch to ON, it actually will shut off power to the main deck cargo handling bus. ANY HANDLING BUS CANNOT BE POWERED SIMULTAINOUSLY WITH THE REST OF THE AC SYSTEM OFF OF THE SAME SOURCE. Easiest way to remember this is one dedicated source per “handling” bus.
EXT 2 for the main deck cargo handling bus (says AVAIL), and you push the switch to ON, it actually will shut off power to the main deck cargo handling bus. Why is this?
-Because the main cargo deck handling bus and the rest of the AC system cannot be powered from the same source simultaneously. An interlock prevents this.
-The same can be said for the other “handling bus” (the ground handling bus), just regarding EXT 1.
If both APU and EXT are available and neither are selected ON, what will a “handling” busses prioritize?
Always EXT if both are available and neither selected ON.
NOTE: If you then select EXT ON, the “handling” busses will then switch over to the APU.
On the ground, you want all the “handling” busses to be powered, but it’s confusing as hell as to what configuration of what switches to push. What should you always select as your source to turn ON for your main AC systems, ensuring you don’t mess up the cargo loading process by turning off a “handling” bus?
Just simply select something labeled “1.” So either EXT PWR 1 or APU GEN 1 will do the trick. (The goal is to have an AVAIL on both sides.)
What do the 4 “BUS TIE” switches do? 2 positions, AUTO and ISLN.
AUTO: System logic can isolate an AC bus while keeping the DC bus powered
ISLN: Manual isolation of both the AC bus and DC bus
NOTE: during autoland, these switches will technically isolate each AC bus, however the overhead will not show this, will still say AUTO.
What is the synchronous bus?
It ties all of the AC busses together, allowing for all 4 to be powered off of 1 single IDG.
NOTE: Only an IDG can utilize the SSB, not any other source of AC power. That’s why I said “IDG” above instead of any AC source.
What is an SSB? What does it do?
The SSB is the split system breaker. It divides the synchronous bus into 2 parts, left (AC bus 1 and 2) and right (AC bus 3 and 4).
When would the SSB be open? When would it be closed?
Open: Each side powered by its own APU gen or EXT power
—or an IDG is powering one side, but EXT or APU power is on the other side
Closed: Whenever “LIKE POWER” is the only power online. So 1 single GPU would be an example. Any combination of IDG (as long as no other power sources are being used.) IDG’s can be synced, no other sources can.
What do the GEN CONT (generator control) switches do?
They turn each IDG either ON or OFF. (opens and closes the generator control breaker (GCB).)
What do the DRIVE DISC switches do?
These guarded switches will physically disconnect the IDG from the engine. Action cannot be undone, which is why they are guarded switches.
On a DRIVE DISC switch, you notice an amber DRIVE light. What is this telling you?
1 of 3 things:
1.) IDG oil pressure low
2.) IDG oil temperature high
3.) GCB is open due to uncorrectable frequency fault
Its saying, disconnect me before I catch on fire!
How many main AC busses? What are they?
4- 1,2,3,4
AC system: How many transfer busses? What are they?
2- Captain transfer bus and FO transfer bus
AC system: How many ground service busses and handling busses? What are they?
3
Ground service bus (in flight or on ground) (powered by AC bus 1)
Main cargo deck handling bus (ground only) (Powered by EXT 2 or APU 2)
Ground handling bus (ground only) (Powered by EXT 1 or APU 1)
AC system: How many utility busses?
- 1 per main AC bus
AC system: How many AC standby busses? What are they?
APU standby bus and Main standby bus
What is the point of a transfer bus?
They provide power to critical flight-related equipment. 1 for the CA and 1 for the FO. They are designed to switch over to a common backup if there was a failure of the AC power supply.
What is the CA transfer bus normally powered by? What is the backup?
Normally: AC bus 3
Backup: AC bus 1
What is the FO transfer bus normally powered by? What is the backup?
Normally: AC bus 2
Backup: AC bus 1
What is the point of the standby busses?
The standby busses provide electrical power to critical flight deck related equipment. (an even further level of redundancy to the transfer busses)
What normally powers the main standby bus? What is the backup?
Normally: AC bus 3
Backup: Main standby inverter (charged via AC bus 1)
What normally powers the APU standby bus? What is the backup?
Normally: Captain transfer bus
Backup: APU standby inverter (Charged via AC bus 1)
How many TRU’s? What powers them? What do they power?
4 of them
Each TRU is powered by a main AC bus
Each TRU powers its own DC bus
How many DC main busses? How many “other” DC busses and what are they?
4 main DC busses
Other: 4
-Main battery bus
-Main hot battery bus
-APU battery bus
-APU hot battery bus
What is a DCIR? What do they do?
DC Isolation Relay
They allow a DC bus to stay powered, even if its related AC bus, or TRU fails.
Is there any way to toggle a DCIR without also toggling its related AC bus tie?
No. You must press the BUS TIE switch to toggle the DC, but it also does it to the AC.
What charges the batteries?
The ground service bus.
What type of equipment is on the main hot battery bus?
-Shutoff valves for all engines and APU
-APU and lower cargo fire extinguishers
-Engine fire extinguishers/fire unlock switches
What type of equipment is on the APU hot battery bus?
-All IRUs
-Outflow valves
-APU controller
If the ground service bus receives AC power, and that same bus charges the batteries, how does that work? Batteries are DC?
Because both the APU battery charger and main battery chargers act like mini TRU’s to switch AC to DC to charge the batteries
If the ground service bus charges the batteries, and the ground service bus is normally powered by AC bus 1, how do the cargo loaders get the ground service bus to work without going up to the flight deck and activating AC bus 1?
They do this via the GND service switch located at door 1L. Activation of this switch powers the ground service bus.
During autoland, what busses are all isolated?
AC and DC busses 1, 2, and 3.
This is 1 for each autopilot. The 4th busses will step in if any failures occur
