Looking for Aircraft Electrical Power Generation System Design Books 1

[kontiki99]

Hi All,

I primarily work in line operation Engineering support. I want to better understand the design concepts used in the design of power generation and management on large transport category aircraft.

The OEMs gives us plenty of repair into, it's the troubleshooting when fault messages don't take the technicians to the fault, that's the struggle. I keep thinking that if I could recognize the design approach being used, I could make sense of things better. It's really worse on on older aircraft. we're always seeing instances where the original designer are long gone.
Thanks,

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

 

[IRstuff]

That sort of information is typically found in technical manuals. While I laud your proactiveness, I doubt that will help much, given that your specific concern about fault messages and the fact that the fault messages were likely written by, or in conjunction with, the original designers.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

yes, trouble shooting is a learned experience. The manuals only go so far. The OEMs can only imagine how the fault may have come about in a limited number of ways and show you how to fix those. Maybe the best you can do is the write down the lessons learned and append to the OEM's MM.

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

Some mechanics take it upon themselves to write down some troubleshooting notes on every problem they encounter, perhaps also collecting from a team of engineers until they have quite the trove of insightful knowledge. This stuff may start to circulate, but not on "official" channels.

It may not be kosher to say this on Eng-Tips, but you would want to explore PPRuNe and Airliners for stuff like that. Ask the "old guys" on the shop floor.

Sadly the only example I have was done for a poorly-documented helicopter, mass-produced in Europe starting in the 1990's... I tried to read the manuals for those helicopters and it was a nightmare. The words looked like english but the message wasn't in english... Discovering the "Dunstun files" was like finding the legend of a secret code.

http://www.sparweb.ca

 

[WKTaylor]

K99... Might find something useful here... 'spaghetti-on the wall'... OTH... maybe a bit too much... it’s late.

FAA-H-8083-6 ADVANCED AVIONICS HANDBOOK
FAA AIRCRAFT EWIS PRACTICES JOB AID 2.0 AIRCRAFT ELECTRICAL WIRING INTERCONNECT SYSTEM (EWIS) BEST PRACTICES - JOB AID
FAA FSAW 2-10 ELECTRICAL WIRING INTERCONNECTION SYSTEM (EWIS) PROTECTIONS AND CAUTIONS DURING MAINTENANCE AND ALTERATION
MIL-PRF-21480 GENERATOR SYSTEM, ELECTRIC POWER, 400 HERTZ, ALTERNATING CURRENT, AIRCRAFT; GENERAL SPECIFICATION FOR
MIL-DTL-23982 PANELS, ELECTRICAL, POWER DISTRIBUTION AND MANUAL TRANSFER, CIRCUIT BREAKER TYPE
MIL-E-81910 ELECTRICAL POWER GENERATING AND CONTROL EQUIPMENT, AIRCRAFT, GENERAL SPECIFICATIONS FOR
MIL-E-85583 ELECTRIC POWER GENERATING CHANNEL, VARIABLE INPUT SPEED, ALTERNATING CURRENT, 400 Hz, AIRCRAFT; GENERAL SPECIFICATION FOR
MIL-HDBK-525 ELECTRICAL WIRING INTERCONNECT SYSTEM (EWIS) INTEGRITY PROGRAM
MIL-HDBK-704-* GUIDANCE FOR TEST PROCEDURES FOR DEMONSTRATION OF UTILIZATION EQUIPMENT COMPLIANCE TO AIRCRAFT ELECTRICAL POWER CHARACTERISTICS
MIL-STD-704 AIRCRAFT ELECTRIC POWER CHARACTERISTICS
ARMY ADS-68-IS AERONAUTICAL DESIGN STANDARD - INTERFACE STANDARD AIRCRAFT ELECTRICAL POWER CHARACTERISTICS [ARMY]
AFGS-87219 ELECTRICAL POWER SYSTEMS, AIRCRAFT
ARINC 413* GUIDANCE FOR AIRCRAFT ELECTRICAL POWER UTILIZATION AND TRANSIENT PROTECTION
ARINC 431 NO FAULT FOUND - A CASE STUDY
ARINC 609* DESIGN GUIDANCE FOR AIRCRAFT ELECTRICAL POWER SYSTEMS
JSSG-2009 APPNDX H AIR VEHICLE ELECTRICAL POWER SUBSYSTEM REQUIREMENTS AND GUIDANCE [CONT DIST]
AFWAL-TR-81-2117 PROTECTION OF ADVANCED ELECTRICAL POWER SYSTEMS FROM ATMOSPHERIC ELECTROMAGNETIC HAZARDS
NATO STANAG 3456 AIRCRAFT ELECTRICAL POWER SYSTEM CHARACTERISTICS
NATO STANAG 3457 GROUND ELECTRICAL POWER SUPPLIES FOR AIRCRAFT
NATO STANAG 7039 TEST PROCEDURES TO ENSURE COMPATIBILITY OF EQUIPMENT WITH AIRCRAFT ELECTRICAL POWER SYSTEMS
NRL R-7298 VOLTAGE LEVEL AND WIRING WEIGHT FOR AIRCRAFT ELECTRICAL POWER SYSTEMS
AIR4174 A GUIDE TO AIRCRAFT POWER TRAIN MONITORING
AIR4365 115/200 VOLT, 400 HZ AIRCRAFT EXTERNAL ELECTRICAL POWER CONNECTOR CONTACT MAINTENANCE PROCEDURES
AIR6139 WAYS OF DEALING WITH POWER REGENERATION ONTO AN AIRCRAFT ELECTRICAL POWER SYSTEM BUS
AIR6326 AIRCRAFT ELECTRICAL POWER SYSTEMS MODELING AND SIMULATION DEFINITIONS
ARP5584 DOCUMENT FOR ELECTRIC POWER MANAGEMENT
AS1212 [CX] ELECTRIC POWER, AIRCRAFT, CHARACTERISTICS AND UTILIZATION OF
AS1831 ELECTRICAL POWER, 270 V DC, AIRCRAFT, CHARACTERISTICS AND UTILIZATION OF
AS7974/* CABLE ASSEMBLIES AND ATTACHABLE PLUGS, EXTERNAL ELECTRICAL POWER, AIRCRAFT,... ...
SAE/TP 2008-01-2922 ADVANCED PROGNOSTICS FOR AIRCRAFT ELECTRICAL POWER SYSTEMS
SAE/TP 2008-01-2923 ADVANCED DIAGNOSTICS OF AIRCRAFT ELECTRICAL GENERATORS
SAE/TP 2009-01-3190 INTEGRATING ELECTRICAL PROGNOSTICS AND MONITORING INTO AN ELECTRONIC POWER DISTRIBUTION SYSTEM
SAE/TP 2010-01-1804 INTEGRATED AIRCRAFT ELECTRICAL POWER SYSTEM MODELING AND SIMULATION ANALYSIS
SAE/TP 2011-01-2623 ADVANCED TECHNIQUES FOR ACCELERATED SIMULATION STUDIES OF COMPLEX AIRCRAFT ELECTRICAL POWER SYSTEMS
SAE R-441 NO FAULT FOUND: THE SEARCH FOR THE ROOT CAUSE
CIVIL AIRCRAFT ELECTRICAL POWER SYSTEM SAFETY ASSESSMENT: ISSUES AND PRACTICES ISBN 9780081007211
SIMULATION-BASED DESIGN OF AIRCRAFT ELECTRICAL POWER SYSTEMS

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.385.2809&rep=rep1&type=pdf

RESEARCHGATE PAPER INTERMITTENT WIRING FAULT DETECTION AND DIAGNOSIS FOR SSPC BASED AIRCRAFT POWER DISTRIBUTION SYSTEM
AVIONICS TECHNICIAN HANDBOOK- VOLUME ONE: AVIONIC SYSTEM FAMILIARIZATION AND TROUBLESHOOTING
AVIONICS TRAINING: SYSTEMS, INSTALLATION, AND TROUBLESHOOTING, ISBN 1885544219
EMI TROUBLESHOOTING TECHNIQUES, ISBN 0071344187
‘BASIC ELECTRICAL/ELECTRONICS REFERENCE BOOKS’ [VARIOUS]

http://www.actechbooks.com/subcategories/47/

I, AVIONICS ENGINEER, MARIJAN JOZIC, 2007
YOU, AVIONICS ENGINEER, MARIJAN JOZIC, 2012
CAL TECH THESIS - DESIGN, SPECIFICATION, AND SYNTHESIS OF AIRCRAFT ELECTRIC POWER SYSTEMS CONTROL LOGIC

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[jaceb]

Will,

Here is my vote to promote you to the field marshall of knowledge rank - endless admiration Sir.....

 

[WKTaylor]

jaceb... wow...

Kinda weird, huh?! This has been decades in the making.

REALY long path and long story... it is all 'very boring to 99.9% of real people' [quoting my wife of 39+years].

I do proudly claim to be an Aero Enginerd... and I've finally realized I seem to 'have the knack' [YouTube, Dilbert].

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[kontiki99]

Thanks all, I appreciate the info. I've been a licensed A&P for over 30 years, and have had an EE degree for over 20. I likely have all the Technician material. What I'm looking for are insights into design concepts.

What triggered this particular search was troubleshooting an APU generator that will not come online. There is Description and Operation material, and fault codes that take you to the fault a lot of the time. Sometimes it all just leaves everyone scratching their head.

In this case, the fault message was related to "OVERLAP." Hence, the question, just what is power generation system overlap? Everyone involved understands Differential Fault Current Transformers and fault protection. The the concept of overlapping zones of differential fault protections in a power generation system is not something I've seen in any aircraft system repair manuals.

I did find an explanation in a downloadable paper on power substation design. And it made sense at least partially. Unfortunately, in this specific case, the APU generator does not have an overlap zone like the main engine generator systems do. My best guess is that we see this message because the same generator control unit is used on the main engine generators and APU generators.

Regards,
bt


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

 

[Sparweb]

It sounds like the manufacturer of the APU is not available for tech support or answers to these questions.

<cringes> ...which APU is it? <instantly regrets asking>

http://www.sparweb.ca

 

[jaceb]

is it airplane with long service history? if so my advice is to check PN of APU's control unit, could be this one you have installed should be similar not necessary identical, next question is how frequent problem is - permanent or appear/dissapear nature?

 

[WKTaylor]

K99...

I am assuming You did the obvious and validated the UPU install in another Acft?

From my field service experience, when an electrical discrepancy occurs similar to this... and the main components seem to 'check-out OK'... then the problem is hidden in component-to-component connections... wiring, connectors/pins, grounds/bonding/jumpers, EMI/lightning shielding, damaged wire harnesses [which include clamps, jumpers and shielding], and damaged 'electro-mechanical devices' [circuit breakers, switches, relays, etc]... and possibly interactions within a defective wire harness... in the aircraft or within the component-mounting system.

In one particular incident on an F-15C a system was intermittently-faulty. Troubleshooting pointed-to the 'black box'... however on the test bench the black-box was clearly serviceable. This remove-examine-CND [cannot duplicate] cycle went on for weeks. Finally I was called-over and we 'went thru the process' on the jet. When the box was removed we examined it in great detail with the electrical tech. Perfect. Then dumb-old-me asked about the condition of mating connector/pins on the Acft wire-harness 'both-ends'... something the techs rarely looked-at in detail. The male connector/pins in the black box were straight/smooth/clean... but the female connector on the wire harness was discolored and visibly dirty... and the bores of the pin-receptacles looked scored/rough/dark. Changing the connector/female pins solved the problem. NOTE: this became troubleshooting standard field-practice, since inspection of the wire-harnesses/connectors was not part of the depot maintenance process... and was always supposed-to-be a 'field-level task'.

Another example...
In the book I, AVIONICS ENGINEER, MARIJAN JOZIC, 2007 ... a hair pulling No Fault Found [Cannot Duplicate] situation existed. Airborne the discrepancy was 'real but intermittent'... and all ground TESTING was fruitless. This was driving troubleshooters and engineers crazy. One engineer had the test equipment [TE] set-up and was running test-after-test in various conditions/states. In absolute frustration, the engineer began to scream and took his frustration out by hitting on the equipment mounting rack with his fist... and low-and-behold... he saw the TE detect a FAULT... that went stable when he wasn't hitting the rack... TA DA! The engineer began 'troubleshooting' the rack with his hands... hitting/shaking/pulling/squeezing [I think] the rack and wiring... and he was able to isolate the fault.

Another example...
A ‘large Boeing military aircraft’ was having problems keeping generators synced and on-line... which was mission-critical fault. The jet was being flown back for depot maintenance in CONUS... when one-by-one all-but ONE of the AC generators went-off-line and could not be recovered. Pretty hairy situation ensued over the north Pacific. When it FINALLY arrived at Depot, the electrical generation system was ‘torn-down end-to-end’... and corrosion/contamination was evident on MOST airframe bonding/grounding points and within a few wiring connectors. The aircraft was stationed on a Tropical island.

One final example...
A light tactical jet I was lead engineer on experienced two inflight cockpit fires that were catastrophic... first one was a foreign air force [FMS] jet... the second was a USAF jet. In the first instance the fire on the RH side of the cockpit went from a low/long-smoldering concentrated fire to a massive catastrophic-flare-up in seconds... the aircraft crashed with a long-trail of fire behind. The pilot ejected and lived with massive burns and a damaged parachute [not much of a useful witness]. The copilot was directly in the path of the flaring fire and never left the jet. Lots of speculation but NO clear evidence-based scenario.

I was NOT permitted to go on the investigation of the FMS jet... but did investigate the USAF jet... same scenario except the fire was on the left side of the cockpit and the pilot felt in imminent danger of not surviving... so he ejected... and the jet crashed into the ocean! Based on the pilots witness statement and burned-wiring and a burned-up fiberglass[?] box [recovered from the wreckage], the preliminary evidence strongly suggested an electrical fire. We focused in on the space directly behind the pilot over the center section fuel-tank. I presented the tangled mess of wire to the jet mechanics who identified the tangled mess as the 300-Amp starter-generator-relay-to-battery wires/box located directly behind the pilot’s ejection seat. We started inspecting jets in maintenance and found one aircraft with [1] electrical-cable [0.625-dia] of [2] parallel cables with the plastic insulation jacketing ‘oozing’ thru the protective wire braiding. Weird. The mechanics pulled the cable-pair out of the jet. On the cable that was in ‘good condition’, the upper-terminal pin was clean and fresh; while the lower-terminal pin had a severely corroded silver-plating finish. On the cable that was oozing plastic insulation thru the wire braid, both the upper and lower terminal pin had excellent/clean silver-plating finishes... and the copper conductor wires had obvious signs of severe current overload/overheating. It was immediately evident that current was by-passing the cable with the corroded pin into the cable with clean/conductive pins. Further-more, the terminal on the fiberglass starter-generator relay box [SGRB, upper-end] was showing signs of distress due to over-heating. Scary... a few flying hours away from losing another jet!!!

NOTE.
By coincidence, the damaged power cable we found [on the jet in maintenance] was on the right side of the space behind the copilot... about where the fire MAY have begun on the FMS jet. So I looked harder and noted a 3/8-OD aluminum tube running Fwd/Aft ~5-inches above the top SGRB... that tubing was NOT present on the same area on the LH side of the cockpit. The mechanics informed me that the 3/8-OD aluminum tubing was CREW OXYGEN system tubing. Dammmmmm... the copilot never had a chance... the electrical fire suddenly intensified by the rush of pure oxygen [from the fire-damaged-O2-tube] over his nylon parachute.

Too much useless information?

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

K99... FYI...

Also... Suggest You take a CLOSER-peek at...

SAE R-441 NO FAULT FOUND: THE SEARCH FOR THE ROOT CAUSE
ARINC 431 NO FAULT FOUND - A CASE STUDY
ARINC 672 GUIDELINES FOR THE REDUCTION OF NO FAULT FOUND (NFF)
SAE AIR1336 Ground Support Equipment Electrical Systems
SAE AIR4204 Commercial Aircraft Auxiliary Power Unit Installations
SAE AIR5317 A Guide to APU Health Management
SAE AIR5992 Descriptions of Systems Integration Test Rigs (Iron Birds) For Aerospace Applications
SAE ARP5015 Ground Equipment - 400 Hertz Ground Power Performance Requirements

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

Also... forgot about this spec...

NATO AECTP-500 ELECTROMAGNETIC ENVIRONMENTAL EFFECTS TESTS AND VERIFICATION

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[kontiki99]

Mr Taylor,

Thanks for all the feedback. I believe our group exists at the pleasure of an FAA interpretation that does not allow Airline FAR Part 121 Maintenance people any ability to let an airplane go with ANY visible damage or the smallest alteration without Engineering Department approval. Our internal authority to approve minor issues prior to deferring a system per the FAA approved MEL and dispatching the airplane saves our company significant recovery costs.

Because we are here, we also field configuration questions and are asked to assist with chronic troubleshooting questions. We are not located on an airport any more. Sometimes the airplane is literally on the other side of the planet.

One of the things about wiring diagrams and schematics is that they show only the connections that are supposed to be there. The fault or damage is always on the airplane. We also never know for sure exactly what's going on or what's been done on the airplane. We do a lot of fast one-off reverse engineering analysis. Then may not have an issue on the same model system for years. That being said they are ALL eventually corrected sooner or later.

We live through all manner of situations similar to ones you described above, and help the best we can. We have OEM support for every model fleet, but that does not mean any of the original designers are still around. When the footprint of an old aircraft model shrinks to some point, support for the LRUs may be divested to 3rd parties with no connection to the original parent company.

I did meet MARIJAN JOZIC at an ARINC Avionics Maintenance Conference probably 10 years ago. Nice guy, I have been unable to buy his books, they are out of print. Mx did fix this particular issue. I can't recall what it was. We've had a dozen other issues since.

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

 

[WKTaylor]

K99...

MARIJAN JOZIC books are 'self-published/available at Lulu.com, thus...

https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=marijan+jozic

NOTE.
In late 2000's I attended an SAE Aerospace GA conference/meeting in Wichita KS. A discussion panel with various avionics engineers/vendors presented their latest-greatest systems and capabilities... then 'took questions from the audience'. So dumb-ole Wil Taylor asked "how are you all addressing the issue of avionics obsolescence"? There was an uncomfortable silence for about 10-seconds... finally the panel leader spoke up... 'not well'. Things are changing so fast that warranties/parts/test-equipment/etc... are only guaranteed available for 8-to-10 years after date of purchase. DoD typically purchased current generation AE equipment intended for mod/installation a couple of years in advance... and did a poor job of 'provisioning' for maintenance and repairs. This appears to be a universal problem [GA, Commercial, DoD, space]... and doesn't just include the 'black boxes'... includes minor and major associated parts/components/connectors, wire/cables, etc.

DOT/FAA/TC-15/33 OBSOLESCENCE AND LIFE CYCLE MANAGEMENT FOR AVIONICS
ARINC 662* OBSOLESCENCE MANAGEMENT STRATEGIES FOR COMMERCIAL AIRCRAFT
MSFC-STD-3620 MSFC ELECTRICAL, ELECTRONIC, AND ELECTROMECHANICAL (EEE) PARTS OBSOLESCENCE MANAGEMENT AND CONTROL REQUIREMENTS
DSTO-TR-2437 A SURVEY OF ELECTRONICS OBSOLESCENCE AND RELIABILITY
ICOAF THESIS DIMINISHING MANUFACTURING SOURCES AND MATERIAL SHORTAGES: SOLUTIONS TO OBSOLESCENCE IN MICROCIRCUITS

http://www.dtic.mil/dtic/tr/fulltext/u2/a331473.pdf

IDA DOC NS D-5483 OBSOLESCENCE MANAGEMENT FOR STRUCTURAL, MECHANICAL, AND ELECTRICAL ITEMS IN CONJUNCTION WITH ELECTRONICS

http://www.dtic.mil/dtic/tr/fulltext/u2/a627527.pdf

ISU THESIS [2015] SELECTION OF OBSOLESCENCE RESOLUTION STRATEGY BASED ON A MULTI CRITERIA DECISION MODEL
NPGS THESIS MICROELECTRONIC OBSOLESCENCE MANAGEMENT
UM THESIS ELECTRONIC PART OBSOLESCENCE (LIFE CYCLE MISMATCH)

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[kontiki99]

Mr. Taylor

I've been through 1 or 2 obsolescence goat ropes. Not in my current role, but with previous employers. I have felt like something of an avionics archeologist with fleet standardization projects. Build a fleet out of lease returns and there is no telling how they are equipped when you get them.

I was the cognizant engineer for TCAS and Transponders for about 10 years at 2 airlines. TCAS was mandated in the mid 90s so it was a retrofit on most of the airplanes standardized in that decade. It was a retrofit on new airplanes. The company had an TCAS installation STC and Boeing didn't like it. They would install some provisions, but not all of them and no activation. We'd complete the installation and activation at a pre-service maintenance visit.

The retrofit TCAS indicator was usually a dual use VSI/TCAS color LCD display. At one point the vendor told us they were going to make one last lifetime buy of glass for the displays. They wanted usage data from us. I guess the lifetime buy was a 2 day production run and then the manufacturer would never make that old glass again. They would have no way to repair our displays. Not a big deal, we complied and moved on.

A year or two later they told us, they could not repair our displays and we we had to make some hard choices going forward. I asked them what happened with the lifetime buy, they told us the US Navy had failed to submit their data by the deadline. They eventually did. The entire inventory was going to support them. Then without explanation, all that went away and we had no more issues. I never got an explanation.

I did some bench support back then too. There were always electrical component issues.

The folks I'm with now have some very old airplanes. We get the word, were going to park them. There is an uptick in business. They come back out of the desert. I've seen airplanes built in the 70's reach 100,000 hours before they retired.

Thanks for the info.




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

 

[WKTaylor]

K99 there are more obsolescence resources available... but mostly DoD related.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]