Torque at intercell connections
 

Hi,

Rocchi:

Low or high torque are not good. Low IMO is worse. Could overheat the connection.

The model i imagine is another: Heat, short to case and LOC. :)

Your point could be one factor to damage (cell # 3).

Welcome!

I think at least in ANA, the stainless case contained the event. The "top" is deformed, having been subjected to sufficient pressure to significantly bulge the structure with the fasteners preventing separation....

I'm not sure this is a totally valid comparison. The fuel is inert on its own and the bleed system can be shut off as soon as abnormal temperatures are sensed. If the wings carried a mixture of LOX and kerosene, then yes...

Errrrm, not really? Uncontrolled fire in one aircraft on the ground, meltdown and discharge of battery internals in another, leading to an emergency landing and evacuation? What would a non-working system look like? :confused:

There is more energy in half a pound of butter than in an average grenade but which would you rather hold in your hand? Now pull the pin.

It just appears to me that the combination of cobalt chemistry and a lot of high-capacity cells jammed together isn't the right way to go for anything that flies. Or drives. Or floats...

@grebllaw123d

Rating is 5 * C (A). C is 65Ah. 5 * 65 = 325A

The inverter powered starter motor soft-starts and draws well below the the battery limit.

@hetfield
No, lead acid can't either and also consists of sub-cells.

Failure mechanism, example.....

Quote......
"The use of copper as the current collector for the negative electrode has particular reliability and safety implications. At very low cell voltages (usually approximately 1 V for the cell), the potential at the copper current collector increases to the point where copper will begin to oxidize and dissolve as copper ions into the electrolyte. On subsequent recharge, the dissolved copper ions plate as copper metal onto negative electrode surfaces, reducing their permeability and making the cell susceptible to lithium plating and capacity loss. Usually, once a severe over- discharge event has occurred, cell degradation accelerates: once the negative electrode has become damaged by copper plating it will no longer be able to uptake lithium under “normal” charge rates. In such an instance, “normal” charge cycles cause lithium plating, which result in a greater loss of permeability of the surfaces. Ultimately, over-discharge of cells can lead to cell thermal runaway." ....Unquote

http://www.nfpa.org/assets/files/pdf...rieshazard.pdf


So it would seem that "internal protections" would be limited practically to a strict control of voltage within a frame of charge/discharge. protection from mechanical damage, and environental stressors are straightforward, nothing below -18 or above 60 (C).

Since the listed resrictions are "mitigating" rather than "prohibitive" (eliminating) as to failure, they can certainly be construed as "permissive of" smoke, runaway and even fire "events". A "mitigation" is by definition "may" not "may not"....

B787 battery
 

On the question of containing a problem within a cell. How many prismatic shaped pressurised aircraft are flying? The question on the backup internal protection is where the fault will be found. I cannot find the same sort of protection built into the prismatic cells as can be found within the cylinder type which has at least 2 methods to prevent this sort of trouble. What I would like to find out is if there was power being feed back to the batteries in both cases while this fault condition existed or if the process of disassembly was all within the battery case. i.e the one on the ground- was ground power or APU supplying DC to the batteries & I assume the A/C flying had the main battery powered from its charging source. By design, if the battery is fully charged it should be disconnected to protect the aircraft (imo). Li-Ion has no loss of capacity over any flight time of any aircraft. All the Aux. battery does is to start the APU when Ext, AC power is not available after which the AC alternators provide the systems power for normal ops. (maybe needed to "clean up" the DC but that could be addressed) There are several issues not just one cell failure that must be addressed before this aircraft is returned to service on just how safe the interface is to other components within the DC system. I hope all are identified.

Hopefully the fix is close.

Bill, the battery does not start the engines.
The APU battery can be used to start the APU if no other AC available.
Main engine start is from high energy AC sources only.

Short as consequence of heat inside box
 

Hi,

FullWings @ # 523

"It just appears to me that the
combination of cobalt chemistry and a lot of high-capacity cells jammed together isn't the right way to go for anything that flies. Or drives. Or floats..."

And for anything in ground too. So the question is: Why the cell(s) started to overheat?

The "equivalent circuit" derived from a visual analysis of the ANA battery shows clearly the temperature inside the case was so high that caused a dramatic short circuit that "fused" an intercell strap and a thick ground wire. I tend to think as the increasingly hot cells inside the case like you put created the short circuit of cell # 3 to ground. (~ 12 V applied in resistance of miliOhms. Surge current of hundred Amps. (probably thousands Amps).

I don´t put as probable a short circuit as the trigger of the battery destruction. FDR iirc data is compatible with the model i presented.

@RR_NDB

The ground strap shows little thermal damage and its cross section is too small to have carried any significant current in destroying #3.

The ground strap was likely mechanical damaged and partially cut either before the event or by intervention.

Severe damage seen is tertiary. I would not spend time on it.

Important is Primary failure, cause of deterioration leading to shorting cell and Secondary failure, if/why the battery was not protected from charging.

Edit: If ground strap was cut pre-event and could have touched BMU PCB, it possibly could have induced the battery failure.

Edit: Arcing at the pole of #3 due to loose nut, I think it would have welded shut, not burned the pole off.

B787 battery
 

Ths Turin, I was focused on the local problem at the battery installation level!!
I hope it reads correct now.

Anyone able to give min voltage alowable before dispatch for this A/C?

APU battery usage in BOS
 

Hi,

TURIN,

JAL APU at BOS was started by AC or from battery?

A model i am imagining is:

1) Batteries discharged (for many possible reasons) more than expected
2) Thermal aspects (cell heating, improper thermal cell monitoring, inadequate battery case, inadequate cell spacing) created conditions for thermal runaway (a positive feedback that in the end destroy the device)

So, my question is because ANA 787 main battery very probably was being recharged (hi curr.)when event started.

BOS JAL APU battery probably was also being recharged? (high curr.)

My feeling, was NOT.

I'd like to see some of the other 99 batteries they replaced, and some images of what the battery box looked like when the fault messages showed up...

Hi,

Saptzae @ # 529

First i need to clarify some points to prepare my comment.

1) Destruction of # 3 was by what? Internally generated heat (due subcell inbalance, dendrites, voltage), ohmic losses due thousand Amps flowing to case (and ultimately to ground), or what?

2) On ground THICK wire you has two details: One seeming mechanical and another VERY PROBABLY caused by excessive current. Please refer to the equivalent circuit where i put the (highly probable due evidences) short circuit path.

3) Severe damage shows a PATTERN. It´s up to us to establish a model rhat if is robust enough could explain most. I am just taken into account everything. Could we discard sabotage? Answer is NO. I prefer look to all details and balance them in the best proportions.

4) Shorting cell? Which one? Due over voltage, improper charging? Temperature of individual cell(s) not being measured?

5) to be continued...

RR,

I would suspect the way the cells are wired as well...not with bus bars, or ground straps, but simple braided wire...

and I certainly would not consider this 'thick' wiring by any definition...

@RR_NDB
I continue to rule out thermal runaway as cause of Primary failure. My scenario is same as at BOS, the same pattern. Primary failure was in #3 (#5 at BOS) due to deterioration of unknown origin. Secondary failure overcharge and thermal runaway of several other cells. Tertiary failure is total destruction of #3, how the pole can burn away, leaving the nut unscathed, beats me. The cause of "deterioration" leading to Primary failure remains as elusive as ever. By now, NTSB could have said something, perhaps them looking for micro-shorts by testing last week gives a hint toward the short of a cell as the cause of Primary failure.

Yes, it could be that the frayed, not severed wire burned up the way you suggest. It could also be that the severed wire caused shorts on a BMU PCB. Now, that would be really awkward and destroy the pattern of both batteries and feed sabotage conspiracies.

I think it is tertiary damage pattern, like structural breakup, it can go any way.

#3, (#5 at BOS), deterioration of cells by mishandling after assembly** / by maintenance or mismanagement by BMS during flight operations.

** I don't know how battery could be safely assembled when cells carry a charge. Any mistake like a short would lead to a firework or damage. Perhaps, battery is assembled while cells not carry a charge, and are charged after assembly.

I am not confident about cause of deterioration leading to Primary failure.

Mishandling like for example resetting BMU cutout of deep discharged battery, thereby charging damaged cells would be a convenient and fixable explanation. Mismanagement by BMU would be harder to fix.

If I would want to figure evtl mismanagement out, I would connect transient processing by way of a 8 channel differential 12bit transient recorder at 1megasamples/sec to the battery, one channel per cell and perform real-time transient analysis to find unique voltage transients on cells.

I feel confident toward the Secondary failure pattern, it is a very tough job for BMS to detect cell short in a timely manner. That will have to be fixed. It may well take something like the above mentioned transient processing to be responsive enough. Another way could be an infrared camera in the case. BTW, Temperature sensors would be not much use as the response is way too slow.

Tertiary failures - breakup - I don't really think about.

It all may well boil down to lack of understanding in a highly decentralized operation. Examples are that nothing seems to have been done about deep-discharged and failed batteries over many months.

Edit: BMU -> PCBs in battery box. BMS -> BMU + charger + all other related functionality.

@FlightPathOBN
The cells are connected using at least 350A rated bus bar and the wires are only for monitoring and balancing at currents below 5A.

Edit: I also would like to see CT scans of in-service as well as of failed batteries.

RR,
Agreed. Reading through this thread there are many cogent theories as to why the overheat began and I have to say most of them seem at least possible.

The main question for me is: why did batteries whose technology was known to be vulnerable to catastrophic thermal runaway (more than virtually any other kind of cell) get put into a configuration that sent the whole lot up in flames if there was a *single* failure? Who in their right minds would install that in an aircraft that could be 3hrs from the nearest airfield?

If it had been designed to contain an individual cell going AWOL, then you'd get something like a status message "ELEC BAT MAIN SYS" which would show reduced redundancy in flight and maybe no dispatch at the other end until the problem had been sorted. You don't expect (or want) a large percentage of the chemical energy in the whole battery being released over a short period of time inside the aircraft... :ugh:

Looking at it from a statistical POV, if all it takes is a microscopic rupture in a nanometre-scale film in one cell to trigger a destructive event, when there are 800 787s flying 18hrs a day carrying (insert number) of cells, how likely does this become?

Regarding the number of batterie that have been changed-out.-

First, one has to go along with the officoal viewpoint at Certification....."safe, well-proven, long service-life......."

So, What we're looking at, is an overgrown cellphone or Laptop battery.

they cost a fraction of (16,0000 USD?) the price of the Thales "box of energy" They're charged by non-technical and sometimes low-intelligence owners ,using cheap and nasty power-supplies.

IT ALL WORKS MILLIONS OF TIMES BETTER THAN THE "SCREAMLINER" SETUP

The number of documented failures have been a minute percentage of the Thales failures.

All these "Domestic" batteries are properly fed/controlled by their interface circuitry.

When your phone needs charging, it switches itself off....you connect a supply and it automatically "sees" it and commences recharging....does this for hundreds of cycles.

So, WHY is the "screamliner's" system so pi55-poor that it won't stop an over-discharge but instead converts it to a disposable?

The lack of sub-cell monitoring and the whole lack of fitness for purpose makes one think of short term profiteering.

For whatever reason, Boeing plumped for the most sensitive and unstable of the Lithium technologies.

You would have thought that the person(s) making that decision, would have used every resource available to ensure that it worked properly.

they didn't ......it doesn't. someone has a lot to answer for. that many "dead" batteries in that short an operating-life points to a major problem......so, who was brushing it under the carpet?


If the volume of components ordered is twice the number that the customer can use (new-builds) the balance is going somewhere.....in this instance, a massive pile of replacements well before their anticipated service-life.

I feel there's a lot we don't know, as yet.

I think this was covered in either this or one of the other (five?) threads.

In an emergency you don't want your battery to shut down just to save itself from the scrap heap.

Imagine this scenario.

Full inflight electrical supply failure (doesn't matter why-lets call it fuel starvation so no APU either. The Airtransat A330 gliding into the Azores springs to mind.)


The main bat kicks in while the RAT drops and spins up.

The a/c lands safely due to excellent pilot skills:E but on the roll out as the RAT winds down due to lack of airspeed, the brakes are being powered by the main battery. The battery monitoring system at this point senses a low charge state and shuts the battery down. Pilots get a black screen, microsoft windows icon, the words "hibernating" flash on the PFD and a little yellow box pops up and says "please connect external power or change to a new battery".


Meanwhile, the poor chaps at the front end are smashing their feet through the brake pedals as the end of the runway looms at about 140kts.

OK I'm being facetious, and you could argue that a simple logic based on air/ground, airspeed, wheel speed etc should be incorporated to stop that.

Maybe it was suggested and dismissed as unnecessary expense/weight/design complication. Who knows?

No info on that sorry.

In normal ops the APU would have been started on taxi-in off the normal AC supply. However, in my experience it is not uncommon for the ground electrical supply to drop off line (after engine and APU have been shutdown) requiring the APU to be started off the battery.

Happens a lot where I work as the airport infrastructure is diabolical in regard to suitable multiple 90KvA supplies.

This is where the problem is... If Boeing admits it is this simple, they will not lose some face, their face will fall completely off....

An organic corollary.....

Imagine the human brain suffering random amyloid deposition (sclera) that block neural pathways, and cause the brain to lose efficiency...

Yuasa's batteries have rapid onset Alzheimers....

Leave them alone, save them for emergency supply needs only.

Unless the charge range and heat cycle can be rigidly, completely controlled...

Fragile, these little boxes....

Interesting update mentioned in r&n:

Boeing Plans Battery Redesign to Get Dreamliners Airbone Again | Frequent Business Traveler

A couple of weeks or so ago, I was trying to determine if each cell
had it's own associated temperature sensor, rather than one to cover
the whole enclosure. The following from the above article suggests that
there may *not* be one per cell:

"Boeing may <snipped> possibly add better heat sensors".


I seem to be the only one out of step here, as everyone else
still seems to be obsessed with the battery and it's construction,
but there are other possibilities to consider.

Still think that the battery would only fail in such a way if it
were abused, which suggests either operation outside cell voltage
limits, or inadequate cell temperature sensing, during fast charge
or high current load.

Might need just a software mod to fix, though after all the hysteria
only an ejectable, fully armoured enclosure would
be good enough for some :*...

I just went out and had a 6 mm thick SS enclosure fabricated for my Galaxy S2 so I don't self immolate with it in my pocket. I also put a 100mm spacer between the battery and the phone so it doesn't short out. I have 3 temperature and pressure sensors installed, hooked up with Bluetooth to my headset, to warn me about impending doom. All of this so I can hook up my cheap 2 dollar knock off charger to my 7 dollar knock off Chinese battery. I believe everybody should have this setup and am going to demand all government safety bodies implement these measures immediately.

Does this sound silly? Of course it does.

This is extremely mature technology, that has a minor flaw that will be fixed.

All the short circuits were effects, not causal. You can put all the space between cells you desire. You can put 100 meters between each cell, but the cells will still short circuit internally if they heat and swell up. This is thermal runaway.

Extra sensors, in an aircraft? Maybe. I would vote yes.

From Boeing Land...
The National Transportation Safety Board (NTSB) has pinpointed the start of the 787 Dreamliner battery fire on a parked Japan Airlines jet a month ago today as a short circuit inside a single cell.

The agency still hasn’t identified the cause of the initial short circuit but has narrowed down the suspects.

Details provided by the NTSB make clear that Boeing will have to redesign the battery for a long-term fix.

In addition, the NTSB pointed to failures in the airplane certification process conducted by Boeing and the Federal Aviation Administration (FAA), which failed to identify the hazards revealed by this incident.

“The assumptions used to certify the battery must be reconsidered,” said NTSB chief Deborah Hersman in a detailed press briefing. “Our task now is to see if appropriate layers of defense and checks were built into the design, certification and manufacturing process.”

NTSB questions Boeing&rsquo;s 787 battery design and certification after short circuit | Business & Technology | The Seattle Times

USMCProbe:
So long as you realise that the phone would no longer function as a
phone, due to the sceening ?, but hey, it's still got the rest of it's
functionality, games, camera at both sides etc etc. Seriously though,
such irony may be lost on many, who would have this drag on for as
long as possible.

Of course it is, but there are deficiencies in this particular design,
otherwise the batteries would not have ended up in the state that they did.


Most likely, though I suspect that improved battery management will
be the key to solving the problem. Can I say that again ?. At least
it appears as though they are (finally) looking seriously at that
area (charger, BMS) now...

USMC

Technically, I agree. Cause? Shutdown syndrome. Each Separator fault at the "Capillary" level occludes a penetration, preventing Lithium ions from transiting the separator. This creates heat, and a degradation of battery CAPACITY, which "ages" the cell. The range of permitted charge level narrows, the battery degrades. Eventually the cell produces a thermal runaway, and we see JAL, ANA.

The best way to remedy the problem is to wind the cell radially, not "folded".

The prismatic shape and the folds produced, cannot dissipate heat quickly enough, or efficiently enough, to ennable a long life and efficient profile for the "Eight battery group" as built by YUASA.

It's the BATTERY....

And BOEING'S immature pride, and stubbornness in trying to push a bad position.

:ok:

Lyman:

So, what about all the other applications for these cells, for which we
have no news of any problems ?. As I said in an earlier post, they
have been approved for use by NASA, the ESA and others only after very
extensive testing, so are they all wrong ?.

The whole idea is for the cells not to overheat in the first place, for whatever
reason and that's down to good design for the bms..

Battery topology really doesn't matter, so long as they are operated
within data sheet limits. All li chargers utilise temperature sensing and
should backoff charge current to avoid overtemperature. Sensing is
critical, hence the earlier disussion about individual cell temperature
sensing, as opposed to overall enclosure sensing.

Laptop batteries may get away with a single sensor, but it's just not
enough for an enclosure of that area. If there's adequate sensing, the
cells won't overheat to start with.

Of course, this doesn't cover the high rate of discharge case, such as
apu starting, but once again, the battery can be safely disconnected from the
load if excessive temperatures are seen by the bms...

Chris.

with great respect, you are absolutely right. So too, am I. In your recent post your questions all have answers. My interpretations of the problems and the solution are based on prior art, prior knowledge, and I create an interpretation.

Architecture, partially, Control/management, parenthetically. Application, absolutely.

In post #525 I quoted what I perceive to be the basic and fundamental problem.

For YUASA. BOEING did not feverishly replace the BATTERIES because they thought the chargers and controls needed replacement.

OCCAM.

One more time. BOEING knew. All along. There is no mystery.

The current charade is a produced drama.

Too bad BOEING is too sick to admit the problem, fix it, and start flying.

PRIDE can be way expensive.

Chris. I think the separator will be upgraded, thickened, and a better method of perforating the film will be found. Also, I believe quality of paste, and applicating method will be upgraded. Of course I think cylindrical shape will be utilized, if it can be demonstrated that it will peform in a Battery of this capacity and size.

I wanted to say ceramcs will play a part, but I think that is a way off, plus the possibility that in an explosion, ceramics will fracture and become shrapnel.

Hersman claims the method of certifying the technology will be examined.

YA THINK?

Lyman:

Sorry, but that's just guesswork without any of the required science. Perhaps
ok in r&n, but tech log ?. Just because one man who has experience of the
technology for a different application says it's wrong doesn't mean that
this design is wrong. If the cells are managed properly, then there should
never be a problem. If the cell management fails, then anything might happen,
so perhaps the enclosure and cell mounting should be revised to better
account for worst case conditions. I'm not qualified to comment on that
though.

Kevlar or carbon fibre ?. Perhaps they use that for intercell spacing
already ?. Internally, such cells can use an interplate material that's designed to
be self healing.

New technology and new application of a sensitive technology may need a
different set of design rules to standardise usage for aviation.

For those who are not engineering bods, will try and get an image together
that describes the overall charger - bms - battery system. While the battery
enclosure layout might have been better, you can't consider such a failure
by looking at any single item in the system. You have to look at the whole
system and understand how each part interacts with another.

While we only have limited information, it should be possible to work out
how all the bits fit together, their interactions and possible failure
modes...

Any method of winding will disguise almost all the surface area of the separator.
Since each failure begins at a perforation in material that is twenty five microns thick, the capability to monitor all the area is impossible, and sampling the entire cell is not sufficient.

I am sorry, it is not guesswork. It is based on knowledge I have researched, and based my intrpretive skills upon.

Kevlar and Carbon fiber (sic), are not stable in these temperatures, and off gas.

Ceramics is a natural, even as a suitable separator, in and of itself. Porous, heat resistant, and non conductive.

As to certification, the FAA relied almost entirely on the applicant, BOEING, to write its own rules.

I am grateful for the opportunity to have this discussion, Chris.

For a better understanding (and hopefully avoid endless conjecture) of the prior art vs the new art of packet cell construction, here's a glimpse into patent 7629077 awarded to some smart lads at Qinetiq Ltd:



Prior

http://i337.photobucket.com/albums/n...ps86dec984.jpg


New

http://i337.photobucket.com/albums/n...psc029a0df.jpg

http://i337.photobucket.com/albums/n...psb00235be.jpg

http://i337.photobucket.com/albums/n...ps2b55c8ba.jpg

http://i337.photobucket.com/albums/n...psb2fe630b.jpg

Fold at separator 3 is the mechanical and heat management weak link, imo. And line "D" where the stack "returns" on itself. (10).

Thank you Machaca.

Edit.....The "POUCH" concept is N/A on GSYUASA, as the container is likely ("rigid") polypropylene. Note in the CT scan of the failure, the more deformed batteries tend to mimic what should be, imho, the correct "shape", a cylinder.

The weaker "sides" deflect the most, the "corners" very little, and the "ends" very little, as expected.

Ly[ing]man:


Absolutely beyond belief. Aramid and CF don't burn!

You seem hell bent on emulating your namesake, Lyman Gilmore.

I did not say they burned.

I cannot imagine why you are so incensed at my presence. If you ask, i will leave.

APU operation when (if) it´s battery fails
 

Hi,

1) From NTSB briefiing: APU stops if it´s battery fails. :confused::confused::confused:

I don´t understand why APU stops if it´s yet running (and generating power)

I watched the video and would like a confirmation. (not so sure) :confused:

2) Any mention on cell # 6 voltage when shorts initiated BOS event? :confused:

This info was lost? (shorts could resulted from improper cell voltage)

:confused:

PS

One month after BOS event i would expect more info.

Charred to a crisp: New pictures show how full extent of damage to burnt Dreamliner batteries which prompted emergency landing | Mail Online

I found this just now. If the link works, and you scroll down, you will see a CT scan of the ANA battery, which shows different damage, at least to my eyes, than the JAL battery.

There is a CT scan of the battery. It clearly shows a damage gradient, starting at one end, and the other in is far less damaged.

To me this makes the investigation even more difficult. On the JAL battery, I did not see a single pattern of damage. i saw at least 6 cells, all being damaged in their own separate events, and finally one shorted and entered thermal runaway causing the fire.

The ANA damage shows heat damage radiating from one end of the box to the other. The last two cells at the end show little damage.

The investigators and Boeing really have their work cut out of themselves. To me it looks like two different failures.

My opinion.

ANA battery:

One battery entered thermal runaway, causing heat damage to the others.
causes:

A single defective/mishandled/damaged cell.
Bad battery charger/BMS


JAL battery:

Most of the battery was overcharged. One cell finally let go and shorted. Firemen put out the fire before too much heat damage to adjacent cells.
causes:

Bad battery charger/BMS
Aircraft electrical system noise/interferance/bonding caused overcharging.

The only common denominator is the battery charger/BMS. The two batteries, to me, look like they had two completely separate failure modes.

Lyman,
While playing armchair engineer is great fun, I am sure Yuasa has a lot of very bright boys and girls who do this for a living and have a level of experience and expertise in the subject you and I can never aspire to. I am quite confident that the cell design and testing is top notch - dare I say on this forum, from my personal experience even if in another area, more so than they would be from any US manufacturer - and that the cells will not cook off if operated within their specified envelope.

Likewise, designing electronic circuits to meet certain current and voltage targets is almost a trivial matter these days and I would not look for any design problems in either the balancing units or the charger - with a caveat on the charger in that it apparently implements a novel charging algorithm. I'm confident that the electronics faithfully applies that algorithm, but how good is the algorithm? "Novel" does not exactly square with "well known".

What it boils down to then is the system integration aspects. Despite the complexity of the individual items, it really is no different from ensuring that a bolt or rivet isn't stressed beyond its specified limits - either that or specify a better rivet. The key factor here is whether the aircraft system as a whole - the one great unknown before the aircraft started flying, since it too is "novel" - is respecting the specifications provided to the manufacturers of the various components.

Of course the investigative bodies have to take things step by step and rule out the more obvious culprits first. My reading of the reports to date is that this has been more or less done with the batteries and electronics. Leaving....?

Fizz57

I could not agree more about Yuasa and would add the same comment about ANA and JAL as operators. I have friends that used to work for both. There may be some other airlines in the world that might equal their fanatical attention to detail concerning maintenance. Maybe. There are none better. This fact also reduces a lot of the uncertainty of how maintenance procedures were followed. i would expect they were followed nearly perfectly, and to the letter.