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APU issues
Quote from TURIN:
"Chris Scott. Temp/Time limits on the APU are due to a rotor bow problem caused by excess heat soak after shutdown.. The quick fix is to leave the APU inlet door open to allow sufficient ventilation of the APU powerplant. Long term fix will be hardware/software changes." So let's get this right. We have an APU that has to be treated with kid gloves, including 30-minute rest breaks, and currently no on-board battery available to start it. I remember flying a long-haul airplane about forty years ago that was still dependent on a GPU for meaningful electrics on a turnround. It was called a Seven-Oh-Seven. Right now, its great-great grandchild is looking to be a bit of a dog. Shame. End of rant... :) |
Quote from TURIN: "Chris Scott. Temp/Time limits on the APU are due to a rotor bow problem caused by excess heat soak after shutdown.. The quick fix is to leave the APU inlet door open to allow sufficient ventilation of the APU powerplant. Long term fix will be hardware/software changes." So let's get this right. We have an APU that has to be treated with kid gloves, including 30-minute rest breaks, and currently no on-board battery available to start it. I remember flying a long-haul airplane about forty years ago that was still dependent on a GPU for meaningful electrics on a turnround. It was called a Seven-Oh-Seven. Right now, its great-great grandchild is looking to be a bit of a dog. Shame. End of rant... Kid gloves is correct--among the systems on an aircraft that should report fit for duty when called upon the APU is certainly in that august group. Having to wet-nurse this thing as Turin describes is not instilling much confidence at the moment... |
Now my information may be about 2 years out of date, but what I have seen says that after APU shutdown "there is no cooldown period", several times. To me that indicates it can be started again immediately after shutdown.
The only cooling period during which no restart should be attempted is after two consecutive failed start attempts from the APU battery alone. And that is presumably a battery limitation, and not an APU limitation. Also, [After shutdown] (t)he APU continues running in a two minute cooldown cycle. [...] At any time during the cooldown period, the APU may be returned to its running condition by placing the APU selector back to ON. |
Bernd,
Yes. The limitations you describe from two-years-old information are in line with normal practice for conventional APUs back to the 1960s. Unfortunately, as I've commented before, I was never clear whether the delay before a third start attempt was to avoid overheating the starter motor, or the battery, or to give unbernd fuel a chance to drain into the drains tank (from which it is subsequently ejected!). However, this APU, unlike others, is not required to provide the vast amounts of bleed air needed for air conditioning and (pneumatic) engine starting. So it is a very different beast, I guess. Presumably, the "rotor-bow" problem described by TURIN only came to light on service entry. |
Correct. As I said, this quick fix is to ensure continued operations of the APU, albeit with awkward ramifications for the operator.
All a/c have these type of issues throughout their service life. It just so happens that the spotlight is shining ever so brightly on this one and we are seeing them. |
It might not provide massive bleed air for AC and engine start, but it is going to have to provide massive amounts of electricity to accomplish the same things.
The starter for the engines on the 78 is electric, not pneumatic? Lots of new stuff. |
The starter for the engines on the 78 is electric, not pneumatic? "Another fundamental architectural change on the 787 is the use of variable frequency electrical power and the integration of the engine generator and starter functions into a single unit. This change enables elimination of the constant speed drive (also known as the integrated drive generator, IDG), greatly reducing the complexity of the generator. In addition, by using the engine generator as the starter motor (an approach used with great success on the Next-G eneration 737 APU), the 787 has been able to eliminate the pneumatic starter from the engine." AERO - Boeing 787 from the Ground Up |
@ TURIN, saptzae;
Thanks for your responses to my query at #363. The blocking "diode" will most likely consist of parallel high power MosFets to reduce voltage drop and hence power dissipation. My overall impression is that a Li-Ion battery consisting of multiple cells in series must have stringent manufacturing requirements for matching the impedance of each cell over the max/min operating cell temperatures. Only by selecting matching cells will charge/discharge cell balancing have a reasonable chance of keeping the battery as a whole in a stable state. |
Questions
Hi,
1) Why Boeing 787 canīt be equipped with another battery? We can list the reasons? And quantify the implications? Weight, Volume, etc. (for 787 required currents and Amperes x hour) 2) An alternate battery (or a set of alternate batteries) could be used to shorten the "grounding time"? 3) What about the implications (of a change) WRT "integration" to 787 circuitry, chargers, circuitry (Diode modules, etc.) 4) There is (are) other(s) reason(s) for the grounding? :confused::confused::confused::confused: |
Are any of the alternatives certified for use though?
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@mm43
Either schottky diodes or power mosfets with reverse current cutoff circuitry. Yes, cells must match closely. Charging includes cell voltage balancing by current bypass. Discharge is not balanced. Questions on my mind (Picture: CT scan of JAL APU bat at BOS) http://i337.photobucket.com/albums/n...SYuasa7872.jpg Cells top left #1, top right #2 to bottom left #7, bottom right #8. #3 was identified by NTSB as shorted (#6 in their rotated picture).
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I would say #1 and #3 are the closest to normal, and the streaks are normal. When the rest of the cells were overcharged they all bulged and the spaces between the individual "plates" (not sure that is the right term), and the batteries, disappeared.
It looks like 7 of the cells were damaged. Nice picture. Which battery is this? Different aircraft? |
@USMCProbe
Picture is of CT scan of JAL APU bat at BOS Thank you for your interpretation. |
#3 looks the least damaged. To me that means the BMS did a good job balancing the charging, and the whole thing was overcharged. IMHO that means there was not a single defective cell. 7 defective cells? Possible but I think #3 just got lucky.
7 bad cells is possible. If they are, at least Yuasa is consistent. I think Boeing has their work cut out for them. I think the problem is systemic, and the only way to recreate the problem is to..........fly them and instrument the heck out the charging/BMS/batteries. |
NTSB stated Feb 1:
The auxiliary power unit battery, manufactured by GS Yuasa, was the original battery delivered with the airplane on December 20, 2012. It is comprised of eight individual cells. All eight cells came from the same manufacturing lot in July 2012. The battery was assembled in September 2012 and installed on the aircraft on October 15, 2012. It was first charged on October 19, 2012. Examination and testing of an exemplar battery got underway earlier this week at the Carderock Division of the Naval Surface Warfare Center laboratories in West Bethesda, MD. The tests consisted of electrical measurements, mass measurements, and infrared thermal imaging of each cell, with no anomalies noted. The cells are currently undergoing CT scanning to examine their internal condition. In addition, on Thursday, a battery expert from the Department of Energy joined the investigative team to lend his expertise to the ongoing testing and validation work. An investigative group continued to interpret data from the two digital flight data recorders on the aircraft, and is examining recorded signals to determine if they might yield additional information about the performance of the battery and the operation of the charging system. Next week, the NTSB battery testing team will initiate a non-invasive "soft short" test of all cells of the exemplar battery. This test will reveal the presence of any high resistance, small or "soft" shorts within a cell. Also, an NTSB investigator will travel to France with the battery contactor from the JAL event battery, for examination at the manufacturer. The battery contactor connects a wiring bundle from the airplane to the battery. |
B787 battery
Looking at the CT scans it appears the metal cases are touching & the internals are pushed out of shape which results in shorts everywhere!! Nothing would protect the pak if that occured & the charge circuit continued to provide a nice stable 30 V into this mess. It would not be a good thing to continue with this type of cell if could short out to the case & the metal box containing the cells !!. There is some idea not to float charge Li Ion but to charge then disconnect until needed. Could this get the aircraft back flying if the battery was isolated for flight? After all this is not reliant on the battery for systems in flight. It is still available for the energency case if required. I would be pleased to help Boeing further all they have to do is call me.
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Saptzae;
Can you post a CT scan of a perfect, undamaged battery? |
@USMCProbe
If I could, I would have. Scan was published by the NTSB. |
I can't see any pattern. If it was a single bad cell going off, I would expect to see a "damage gradient", no matter how the heat was generated.
I would also guess this was a single event. Had it happened numerous times I would think all the cells would have bulged. Each time it would have happened, heat would be generated, and then the whole pack would get heat soaked, and slowly cool down over time. Single event, stopped by the fire crew, and cooled down. They have lots of smart folks working on this. I am sure they are going down a hundred different paths at the same time. |
Question for the BMS/electrical gurus:
For some reason they put the main battery terminals extremely close together on the outside of the case. 30V is not a lot, but the higher the voltage, the more it can jump. Lets say a little dirt/condensation/grease is allowing a very very small amount of current to flow on the outside of the battery case between terminals. How would this affect the sensing function of the BMS? I would think it would be a slow draw, always pulling the battery voltage down slightly, requiring - more charging? I am not an aircraft mechanic. Do normal 28V aircraft batteries put their main terminals an inch or two apart? |
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