Seeking Aircraft Electrical Power Generation and Load Management Info 1

[kontiki99]

All,
Anybody know of a good design reference for subject topic. I'm trying to bring myself up to speed on the principles after an incident on an older airplane. I'm not seeing what I need in the aircraft maintenence guidance.

Thanks

In theory, there is no difference between theory and practice however, experience suggests that in practice, there is!

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm affiliated with.

 

[VEBill]

Try the keywords: electrical load analysis
Or: aircraft electrical load analysis

 

[Sparweb]

Operators tend to tabulate an electrical load analysis (ELA) for their aircraft not just for safety's sake, but as a reference when considering equipment upgrades, mission flexibility, etc. It's not strictly necessary on small privately-operated aircraft, but it's pretty much the norm for commercial operations, even when it's not mandatory under the operating rules the company follows. There has probably been an ELA done for every type of aircraft by now, even if they didn't do it "back then" when the type was certified. Whatever aircraft you are concerned about, there should be an ELA out there. Even if it's not for the same serial number of aircraft, the same basic equipment will be on board the others of the same type.
You could look for an ELA for a similar type of aircraft, particularly one in the same weight class, or having the same engines, etc. Reading that ELA you will find many items of equipment common to both aircraft, and even if the specific type of (for example) landing light is different on the two aircraft, you may find that the electrical load is roughly the same anyway.

STF

 

[IRstuff]

Part of the loads analysis is also to determine which parts of the aircraft are essential and which are non-essential. The aircraft I've been involved with have separate busses for essential and non-essential equipment. That's to ensure that failures of non-essential equipment which take down the buss don't take down the aircraft itself. That applies for both DC and AC powered equipment.

TTFN
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[VEBill]

There are also "Conditions" to consider.

For example, the analysis for a 2-engine aircraft would have to include analysis of the condition when one engine has failed (meaning only one generator). Obviously the design has to account for such eventualities.

 

[dgapilot]

Kontiki99,
You reference "older airplane", I've seen lots of times when alterations are done that add electrical equipment, even if the 337 indicates an electrical load analysis was performed, there is no analysis to be found. AC 43.13-1b Chapter 11 paragraph 11-35 and 11-36 cover the topic od electrical loads and controlling them. There is also an Australian AC 21-38 that goes into detail on what an ELA is and how to develop it. While it is for Australian certification, it provides a lot of good info that would be applicable to US certified aircraft. MIL-E-7016F is another good reference document for the development of an ELA.

 

[kontiki99]

Thanks for the replies, I'm really not looking at performing an ELA. Describing exactly what I'm looking for has been part of my problem.

I'm trying to better understand load management from an operating perspective after an in flight incident.

I'm wondering if various commercial aircraft wouldn't have individual load management philosophies based on a failure effects analysis of their electrical power generation system. Certainly the ELA would play a big part too I suppose.

I'm not in the hot seat for this, just trying to understand what I watched unfold.

I've got a copy of IEEE STD 128-1976 from an internet search, but it's very dated.


In theory, there is no difference between theory and practice however, experience suggests that in practice, there is!

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm affiliated with.

 

[IRstuff]

You've mentioned "incident" twice now, but failed in both cases to explain what the incident was. Most aircraft don't really have much in the way of loads control, other than pulling breakers on the breaker panel.

TTFN
I can do absolutely anything. I'm an expert!
homework forum: //

http://www.engineering.com/AskForum/aff/32.aspx

faq731-376 forum1529

 

[verymadmac]

Sounds almost like what you are after is xx.1309 - System Safety Analysis and Assessment
such as the following guidance.

https://www.faa.gov/regulations_pol...fm/go/document.information/documentID/1019681

Aircraft specific load shedding information is most likely held in the Flight manual (if you are lucky).

Yes can we get more information about the "incident" please

 

[dgapilot]

It all depends on the aircraft and the design philosophy used. The ELA is the start of what you are looking for. Simple single engine aircraft will have a single bus system, and perhaps an avionics bus. As the complexity of the aircraft increases, the bus arrangement increases. The arrangement of the bus system is based on the importance of the items needing power. Basic flight instruments, at least one comm, and minimal lighting along with control systems for gear and flaps would be considered Essential services, and most likely powered by a battery bus or emergency bus. Some systems will have automatic load shedding busses for non essential services and cabin power. Rad the applicable regulations for the type aircraft you are concerned with and the power distribution system will be designed to meet the requirements of the regulation. As stated above, a 2X.1309 analysis will help in determining what services are essential. Part of that analysis is a Functional Hazard Analysis, that helps in the determination of what systems are critical to the operation and which ones are not. You can also look to SAE ARP 4754 and 4761 for guidance in compliance with 2X.1309.

In your original post, you referenced "older" aircraft. Depending on age, your answers may be in CAR 3, CAR 4a, Car 4b, or if old enough, Aero Bulletin 7a. You need to look at the TCDS to see what the Cert Basis of the given aircraft is to understand what the regulatory requirements were at the time of certification. Most "older" aircraft have a lot more electronic equipment installed that what was originally envisioned. If these alterations weren't well thought out, that could be the source of your issue. Keep in mind that with a few exceptions, based on 21.101 any alterations made after 2003 need to comply with the regulations in effect at the time of application for approval. So even if you have a CAR 4b airplane, the changes being incorporated after 2003 will most likely need to comply with the requirements of 14 CFR Part 25.

 

[kontiki99]

Apologies for sounding cryptic.

I think what I may really be looking for is an explanation on how the MEL for a large multi engine commercial jet is developed.

That is, what methods are used to determine what equipment in an electrical power generation system can be inoperative or deactivated at departure?

In theory, there is no difference between theory and practice however, experience suggests that in practice, there is!

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm affiliated with.

 

[WKTaylor]

Dang...

Had a long post for this question... but somehow managed to wipe it out [+1-hour work] with a few off-cadence key strokes or some other spastic keyboard actions. This is about the 5th time I have wiped-out long post replies by some action I cannot fathom.

That will teach me to type/edit to a word document first... then post to Eng-Tips. I'll try again tomorrow.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[Sparweb]

Wil,
It happens to all of us, if we post often enough.
I have a habit of pressing "Ctrl-A" then "Ctrl-C" to get everything on the the clipboard before clicking either Preview or Submit. My problem is having my internet connection interrupted by a lousy ISP, but it's a good habit for any reason.

STF

 

[dgapilot]

OK, that's something totally different. First you need to understand the difference between the MMEL and an MEL. The MMEL is a document assembled by a working group that contains the AEG, the ACO, the FDSO, and selected operators. Using the base certification basis, Part 25 for transport airplanes, and the aircraft system design along with an extensive safety analysis, the MMEL is generated. The MMEL is the base document that all MELS are derived from. An MEL can never be more lenient than the MMEL. It is a scrubbed version of the MMEL, editing it for the actual equipment installed in the given aircraft. In addition to the editing for actual installed equipment, the MEL is also edited for all the "M" and "O" items. The operator has to develop all the "M" items as procedures based on their maintenance and inspection program, and how they think they can safely operate the aircraft. The "O" items are procedures that must be written to account for a specific piece of equipment being inop. So for every "M" item, there will be a Maintenance Procedure that must be performed prior to departure if that item is inop. For every "O" item there is an operating procedure that must be adhered to after departure and until the item is repaired. The MEL is submitted to the FSDO having geographic control over the operator for approval. Once approved, the MEL, and the approval letter constitute an STC for the aircraft, allowing it to operate with certain items inop based on the original safety analysis.

So for an electrical system, lets say you have a 2 engine aircraft with an APU. Lets say the APU has a limitation that it cant be started if the OAT is below -20. Lets continue and say this aircraft is certified for extended range, and normal cruising altitude FL320 and above. The OAT at FL320 is below the starting temp limit of the APU. Based on a safety analysis lets say that if a given bus relay fails, the only way to get power to a required system is with the APU. The MMEL may say that with some combination that may cause the relay to fail, the APU must be operating prior to feet wet or no extended range (Operating Procedures). There may be associated maintenance procedures like disconnect, cap and stow a connector, pull and collar a CB, placard the panel. Lots of other possibilities. All based on the basic safety analysis of the system using things like Fault Trees, FAMECA Analysis and all kinds of other things to show compliance with 25.1309.

Hope that helps.