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.
