novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

Basic Parameters Of An Aircrafts Electrical System

Tue Apr 30, 2013 4:04 pm

From my notes it states that the basic parameters of an aircraft's electrical system are

1. No paralleling of the ac sources of power.

2. All generator bus sources have to be manually connected through the movement of a switch that also will disconnect any previously existing source.

Can anyone explain are elaborate on these two points further to help my understanding of them?

Thanks
 
fr8mech
Posts: 6580
Joined: Mon Sep 26, 2005 9:00 am

RE: Basic Parameters Of An Aircrafts Electrical System

Tue Apr 30, 2013 9:20 pm

Quoting novice (Thread starter):
1. No paralleling of the ac sources of power.

Incorrect. Most of your older generation aircraft (B727, DC8, B747, etc) allowed their generators to parallel as normal operation. I know the MD11 flies around with its generators paralleled. The B757 and B767 run isolated. Not sure about the other more modern aircraft.

Now, it is true that you can not parallel an APU generator to the engine generators, nor can you parallel external power to any of the ship's generators.

Quoting novice (Thread starter):
2. All generator bus sources have to be manually connected through the movement of a switch that also will disconnect any previously existing source.

Again, incorrect. I know that the B757/B767/B744/A300 and MD11 all automatically bring their generators onto their respective busses (or the sync buss) when the generator comes online and the power is "good".

I'm not sure where you got your information.

The very basic requirements are spelled out in FAR 25.1351, 25.1353 and 25.1355

http://www.ecfr.gov/cgi-bin/text-idx...browse/Title14/14cfr25_main_02.tpl

[Edited 2013-04-30 14:27:00]

[Edited 2013-04-30 14:31:11]
When seconds count...the police are minutes away.
 
airbuster
Posts: 306
Joined: Fri Mar 30, 2007 12:43 am

RE: Basic Parameters Of An Aircrafts Electrical System

Wed May 01, 2013 2:13 pm

Novice,

The text you wrote is an exact copy of what is written in the 737 FCOM. And is correct for that type. As stated above, some do parallel and on the MD11 this gives no power transient and it runs very smoothly.
FLY FOKKER JET LINE!
 
User avatar
Starlionblue
Posts: 17053
Joined: Fri Feb 27, 2004 9:54 pm

RE: Basic Parameters Of An Aircrafts Electrical System

Wed May 01, 2013 2:31 pm

Quoting airbuster (Reply 2):
The text you wrote is an exact copy of what is written in the 737 FCOM.

The 737 is used as the "standard" jet airliner in several JAA/EASA exams. Are those the notes that you have novice?

[Edited 2013-05-01 07:41:19]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

RE: Basic Parameters Of An Aircrafts Electrical System

Thu May 02, 2013 1:34 pm

'Airbuster' what is the type that the information is correct for?

Quoting fr8mech (Reply 1):
Now, it is true that you can not parallel an APU generator to the engine generators, nor can you parallel external power to any of the ship's generators.

Is this because of different voltage requirements?

Source: 'ACE THE TECHNICAL PILOT INTERVIEW' by GARY V. BRISTOW - Page 181
 
fr8mech
Posts: 6580
Joined: Mon Sep 26, 2005 9:00 am

RE: Basic Parameters Of An Aircrafts Electrical System

Thu May 02, 2013 7:34 pm

Quoting novice (Reply 4):
Is this because of different voltage requirements?

No, it's because external power units can not be controlled by the aircraft and APU generator speed, and therefore frequency, is regulated by the APU speed itself. The APU is not nearly as troublesome as an external power unit.

In order to parallel power sources, the frequencies must match. Ground power units are notorious for being a little squirrely in frequency. Yes, newer units have much better controls, but the bottom line is that the aircraft would be unable to tweak the frequency of the external power unit.

The aircraft can control the speed of the APU, therefore controlling the frequency of the generator output, but the APU also supplies air to the aircraft and APU speed will tend to vary a little as demand shifts on the pneumatic side, before settling back down to the speed that provides 115V/400hz power. Maybe not enough to kick the generator offline, but why risk it? The APU generator is sufficient, in of itself, to power the whole aircraft (as are each of the engine generators).
When seconds count...the police are minutes away.
 
novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

RE: Basic Parameters Of An Aircrafts Electrical System

Sat May 04, 2013 8:43 pm

I don't understand why can't the external power units can not be controlled by the aircraft if the aircraft can control the speed of the APU which therefore controls the frequency of the generator output?



Quoting fr8mech (Reply 5):
but why risk it?

Risk using the APU to supply air to the aircraft?
 
fr8mech
Posts: 6580
Joined: Mon Sep 26, 2005 9:00 am

RE: Basic Parameters Of An Aircrafts Electrical System

Sat May 04, 2013 9:14 pm

Quoting novice (Reply 6):
I don't understand why can't the external power units can not be controlled by the aircraft


Because the external power unit is external to the aircraft. The units vary in design and how they generate the power. They can be diesel engine powered, they can be powered through the terminal network. Heck, I've seen GPU/Airstart units that are powered by a small centrifugal jet engine. The only thing they have in common is that they produce 115V/400hz AC and have a standard plug.

Bottom line is that there is no provision on the aircraft to control the quality of power being supplied to the aircraft. It can only kick it off if it doesn't like the power, by opening the external power breaker.

Quoting novice (Reply 6):
Risk using the APU to supply air to the aircraft?


No. The risk, and it's minimal, of an APU generator's output fluctuating while it's paralleled to an aircraft generator, during pneumatic operation.

As far as I know, the only time an engine generator is paralleled to the APU is just before the engine generator takes the buss in order to execute what is called a "no break power transfer". Immediately after the generator is synched to the buss, the APU is forced off. This makes for a smooth transition that isn't accompanied by flickering screens and lights.
When seconds count...the police are minutes away.
 
FlyHossD
Posts: 1243
Joined: Mon Nov 02, 2009 3:45 pm

RE: Basic Parameters Of An Aircrafts Electrical System

Sun May 05, 2013 2:57 am

Quoting novice (Reply 6):

I don't understand why can't the external power units can not be controlled by the aircraft if the aircraft can control the speed of the APU which therefore controls the frequency of the generator output?
Quoting fr8mech (Reply 7):
Because the external power unit is external to the aircraft. The units vary in design and how they generate the power. They can be diesel engine powered, they can be powered through the terminal network. Heck, I've seen GPU/Airstart units that are powered by a small centrifugal jet engine. The only thing they have in common is that they produce 115V/400hz AC and have a standard plug.

Bottom line is that there is no provision on the aircraft to control the quality of power being supplied to the aircraft. It can only kick it off if it doesn't like the power, by opening the external power breaker.

   Well stated.

Further, in my experience, the quality of maintenance of the ground power units also varies widely. The older they get, the more variable the output becomes.
My statements do not represent my former employer or my current employer and are my opinions only.
 
novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

RE: Basic Parameters Of An Aircrafts Electrical System

Sun May 05, 2013 4:48 pm

Thanks for clarifying that fr8mech    makes sense now.
 
celestar345
Posts: 23
Joined: Wed May 08, 2013 5:35 pm

RE: Basic Parameters Of An Aircrafts Electrical System

Thu May 09, 2013 3:23 pm

Quoting fr8mech (Reply 7):
Bottom line is that there is no provision on the aircraft to control the quality of power being supplied to the aircraft. It can only kick it off if it doesn't like the power, by opening the external power breaker.

Second to that. At where I work there are a few diesel units that the output quality is so bad that the aircraft will just rejects and shuts down... Many occasions when the electrical hydraulic pump is being selected on and the sudden load caused the power cut - quite funny (with hindsight) watching his/her stunned expression and the ADIRU warning sound at the background....

Who is online

Users browsing this forum: bdlord and 20 guests