Airliners.net

Hi,

First of all, great forum. I learned a lot reading the past posts

I am trying to understand how electrical systems work on aircraft.
My understanding is that it is split between Generation. Distribution and Conversion. The components of conversion is pretty straight forward but I find generation and distribution a bit more confusing.
- How does the generation actually start, There is an APU start and an engine start? Do all aircraft have both? Is it always an electric start or is mechanical sometimes?
- Do all generators have a control unit?
- Do all aircraft have primary and secondary distribution? Is it dependent on the size of the aircraft?
- Where could I find information on the suppliers of generation, distribution equipment? in particular APu & engine electric start

Thanks for your help!

On the 777, the APU has 2 starters, electrical and pneumatic. The APU and both engines each have their own individual generator With their own switches in the flight deck. The APU Generator can be turned on or off but the IDG requires the engine to be running for it to run. The IDG ( Intergrated Drive Generator) can also be disconnected from the engine with switches if the generator fails to prevent further damage. The APU is required to be on to provide bleed air for the engine penunatic starters, if the APU is inop, then an external air source provides air for engine start. The 777 also has a BUG (Back Up Generator) on each engine that also has its own individual switches in the flight deck.

The MD11 has 3 AC Busses. Each one is connected to one IDG via a Generator Relay (GR). During normal operation phase angle and frequency of all three IDGs are synchronyzed to allow parallel operation. This means that all three AC Busses are connected to the AC Tie Bus via Bus Tie Relays (BTR). The APU generator is also connected to all three AC busses via Auxiliary Power Relays (APR). External Power is connected to the Ground Service Bus via the Ground Service Relay (GSR). But when external power is switched on in the cockpit, the GSR opens and the Bus Tie Relay - External Power (BTR-EP) connects external power to the AC Tie Bus. The Ground Service Bus is then powered by the AC Bus 2 via the AC Ground Service Tie Relay (AGSTR).

Priority on the AC Busses is 1. related IDG, 2. APU generator, 3. AC Tie Bus. Meaning when parallel operation is not possible, the BTR opens and the AC Bus is powered only by its own IDG. When generator power is not available the APR closes (provided APU power is available) or the BTR closes to get power from the AC Tie Bus (which is powered by external Power or the other IDGs).
On The AC Tie Bus external power has the highest priority, meaning when external power is available the BTR of all AC Busses open until IDG power on the respective bus is no longer available and APU power is not available.

The Left Emergency AC Bus is normally powered by the AC Bus 1 via the Left Emergency Control Relay (LECR). When emergency power is switched on, the LECR connects the Left AC Emergency Bus via the ADG/BAT Control Relay (ABCR) to a static inverter, which is connected to the Main Batteries. Then the Air Driven Generator (ADG) is deployed and operates in ELEC Mode, the ABCR connects the Left Emergency AC Bus to the ADG.

The Right Emergency AC Bus is normally powered by the AC Bus 3 via the Right Emergency Transfer Relay (RETR). When emergency power is switched on and the ADG is deployed and operates in ELEC Mode, the RETR connects the Right AC Emergency Bus to the ADG.

When the ADG is operated in HYD Mode, the RETR and LECR dieconnect the Amergency Busses from the ADG (L EMER AC Bus is powered by the Batteries) and connects the AUX Hydraulic Pump 1 via the AUX Hydraulic Pump Transfer Relay (AHXR) to the ADG.

There are transformer/rectifiers on AC Busses 1 (TR-1), 2 (TR-2A), GND Service (TR-2B) & R EMER (TR-3), which power DC Busses 1 (TR-1), 2 (TR-2A), GND Service (TR-2B) & R EMER and 3 (TR-3). All 4 TRs (the the DC Busses) are connected to a Tie Bus similarly to the AC Busses. DC Bus 3 is disconnected from TR-3 by the Right Emergency Isolation Relay (REIR) in case Emergency Power is switched on.

Each generator (3 IDG, 1 APU generator) is controlled by a Generator Control Unit (GCU). The Electrical Power Control Unit (EPCU) controls most of the relays and gives frequency references to the IDG GCU.

This has probably been more specific than you asked, but I hope it helps to get some insight on how complex electrical systems on airliners are... and the MD11 is not the newest plane out there.

Thanks! Wow it is even more complex than I thought but really interesting as well.

Is there any documents that summarized which Engine&APU starter each aircraft program has (type and manufacturer)?
Do you know who are the main manufacturers of APU/Engine electrical starter?

ldn17 wrote:

Thanks! Wow it is even more complex than I thought but really interesting as well.

Is there any documents that summarized which Engine&APU starter each aircraft program has (type and manufacturer)?
Do you know who are the main manufacturers of APU/Engine electrical starter?

Pretty much all engines are started with a pneumatic starter. They use air from the apu or a ground cart to run the starter. Apu and turboprops are usually electric starters. I think many electric starters carry the Honeywell name now. In the case of turboprops the starter is a combo starter/generator.

Dalmd88 wrote:

ldn17 wrote:

Thanks! Wow it is even more complex than I thought but really interesting as well.

Is there any documents that summarized which Engine&APU starter each aircraft program has (type and manufacturer)?
Do you know who are the main manufacturers of APU/Engine electrical starter?

Pretty much all engines are started with a pneumatic starter. They use air from the apu or a ground cart to run the starter. Apu and turboprops are usually electric starters. I think many electric starters carry the Honeywell name now. In the case of turboprops the starter is a combo starter/generator.

Exception for large airplanes is the 787 which uses an electric starter for the engines on the ground and inflight (as necessary).

7BOEING7 wrote:

Dalmd88 wrote:

ldn17 wrote:

Thanks! Wow it is even more complex than I thought but really interesting as well.

Is there any documents that summarized which Engine&APU starter each aircraft program has (type and manufacturer)?
Do you know who are the main manufacturers of APU/Engine electrical starter?

Pretty much all engines are started with a pneumatic starter. They use air from the apu or a ground cart to run the starter. Apu and turboprops are usually electric starters. I think many electric starters carry the Honeywell name now. In the case of turboprops the starter is a combo starter/generator.

Exception for large airplanes is the 787 which uses an electric starter for the engines on the ground and inflight (as necessary).

Thanks is it different for business jets or military jets? do they have electric start?
Regarding distribution I guess only large planes need primary and secondary distribution or do you find the two also in smaller planes such as business jets ?

Business jets can use either air start or electric start..depends on the size of the engine. Generally speaking, the smaller turbo jet engines such as the TFE 731 which power small-midsize biz jets such as Learjet 35, HS 125, Astra, etc use an electric starter...most of these types don't have APU nor do they need one.
Large biz jets such as GV, Global Express, using BRR 710 engines employ APU bleed air for start.

I would surmise the difference between electric vs. air start has something to do with the mass of the engine. An electric starter for the BRR 710 would be robust, massive and quite heavy. Bleed air is not heavy but requires an APU under normal ops.

There are exceptions to the above as someone will soon shout,..they always do...but this is the general way of things with biz jet types.

ldn17 wrote:

Hi,

First of all, great forum. I learned a lot reading the past posts

I am trying to understand how electrical systems work on aircraft.
My understanding is that it is split between Generation. Distribution and Conversion. The components of conversion is pretty straight forward but I find generation and distribution a bit more confusing.
- How does the generation actually start, There is an APU start and an engine start? Do all aircraft have both? Is it always an electric start or is mechanical sometimes?
- Do all generators have a control unit?
- Do all aircraft have primary and secondary distribution? Is it dependent on the size of the aircraft?
- Where could I find information on the suppliers of generation, distribution equipment? in particular APu & engine electric start

Thanks for your help!

A330:
- There are three main generators. One in each engine and one in the APU. These are identical and produce 115KVA.
- There is also a hydraulically driven emergency generator powered by the green hydraulic system, producing 8KVA. If the engine driven hydraulic pumps are offline the green system can be (weakly) powered by the RAT, leaving you with a whopping 3.5KVA.
- There are two external electric power connections, giving 90KVA each.
- Gennys and external power are three phase 115/200 V 400 Hz.
- If the airplane is completely "cold", you power on the three batteries. You can then start the APU with the APU battery. Alternatively you can connect external electric power and start the APU with that. Once the APU is on you have have on board generation of both pneumatic pressure and electric power.
- The engines are started with pneumatic pressure from the bleed air manifold. The manifold is pressurized by the APU, by the "other" engine for a crossbleed start or by external high pressure air.
- All gennys have some kind of electronic control.
- The electrical system is rather complex. Some buses are always powered. There's DC, AC, etc...

ldn17 wrote:

[quote="7BOEING7"

Thanks is it different for business jets or military jets? do they have electric start?
Regarding distribution I guess only large planes need primary and secondary distribution or do you find the two also in smaller planes such as business jets ?

Yes the smaller business jet can use a electric starter. Someone mentioned the TF731. That engine shares the same core as the apu found on the the 767/757. It is also the same core as the Tubo Prop TPE331 found on Metros and MU2. it all comes down to weight. Yes you could start a larger engine with an electric motor. You just would not want to fly around with a huge heavy starter motor. Military jets say from F-16 and up would use airstart. even if they don't have an APU. They require a ground start cart. Many also have a provision for a charge start. The crewchief can insert a big shotgun like shell into a starter on the motor and fire it off. The blast gives enough air to get the motor turning for start.

As far as primary and secondary for business jets, I would say yes. Secondary is things like galley power and entertainment systems, lavs. Business jets tend to have those.

Starlionblue wrote:

Do you have a higher resolution image of that schematic?

But throw out much of the above info when it comes to the B 787 -- totally different beast.

Electric start, no IDG's, 230 VAC primary distribution on ring-tie bus, +-270 VDC motor controllers to replace all the pneumatics, bi-directional 230-115 Auto-transformer units...

787 Power Distribution:

Horstroad wrote:

Starlionblue wrote:

Do you have a higher resolution image of that schematic?

I have the FCOM images, but I can't put them on a public forum.

This preso has good schematics in high res:. https://issuu.com/skyhawkss/docs/airbus ... nd_systems

You're all over the spectrum with your questions, but I'll give it my best shot.

Electrical power generation on aircraft is similar to the power you get at your house.

A turbine, of some sorts, powers a generator which outputs three phase A/C power. 60Hz for the house, 400Hz for aircraft (with either CSD's or IDG's). Non-regulated (varying frequency) can be regulated to 400Hz by converters if needed. These converters are seen on GIII's and GIV's as two examples.

Smaller corporate aircraft usually have 28VDC generators. They use inverters to change the DC to 400Hz AC for the equipment that needs AC power.

Larger aircraft, go the opposite way. Alternating Current is changed to DC by transformer rectifiers.

All generators need a ground plane (earth for home use and the aircraft frame in aviation). The power output is all dependent on exactly how the generator is wired.

Power distribution is really just a matter of who needs what. All loads are balanced as evenly as possible via the distribution buss's. Contactors (big ass relays) are used to connect generator out puts to specific buss's or connect buss's if there is a failure of a generation source.

AC generators can be paralleled to help balance the load. This is just a matter of matching the phases sine wave outputs. And yes.... almost all generators (AC or DC) have controllers to control and monitor the generator's output.

http://rotate.aero

Dalmd88 wrote:

Many also have a provision for a charge start. The crewchief can insert a big shotgun like shell into a starter on the motor and fire it off. The blast gives enough air to get the motor turning for start.

Ah, the ominous "Coffman Starter" from Flight of the Phoenix.
https://en.wikipedia.org/wiki/Coffman_engine_starter
do they still exist?