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ignorantAndroid
2025-06-17T04:20:00 permalink Post: 11903956 |
That's why the regulations require separation between high-voltage and low-voltage wiring.
Subjects: None |
ignorantAndroid
2025-06-17T04:46:00 permalink Post: 11903963 |
I'm honestly mystified by the obsession with TCMA. The FADECs control almost every aspect of the engines, so there must be numerous ways they could cause a failure or uncommanded shutdown. So, even if we assume that the engines failed due to faults in the FADECs, why assume that TCMA would be involved? Surely it's more logical to simply posit that some unspecified bug in the FADEC software caused the failure. That bug
could
be related to TCMA, but it could just as easily involve any one of the dozens of other subroutines that likely exist.
Various posters seem to assume that all it takes is an incorrect air/ground signal, and the engines would shut down. But in fact it would also require the FADECs to read the thrust levers as being at or near idle... AND the engines failing to respond to closure of the fuel metering valve. I've read the entirety of both threads, and I haven't seen anyone even attempt to explain how a malfunction within the airframe could cause both of those things to occur on both engines (or even one engine!). Subjects: Engine Failure (All) Engine Shutdown FADEC TCMA (All) 9 users liked this post. |
ignorantAndroid
2025-06-19T21:51:00 permalink Post: 11906415 |
No, very unlikely the pilots lost all control. In a total power failure (hydraulics and electrics) the pilots can control roll (by a specific spoiler pair) and pitch through the stabiliser. All electrically powered through wiring direct to control column.
It's possible the RAT was not providing correct power in the last moments, remember RAT provides Hydraulic (C system flight controls only) and Electrics. The evidence is the spoiler pair deflection behind the engines prior to impact. That specific spoiler pairing is only controlled by electrical power (directly connected to pilot control column). We should see other flight control deflections if the RAT was powering hydraulics. In the final moments I would speculate the flight controls had only emergency electric power from the battery. The direct wiring you're referring to is intended to provide minimal control in case of a complete failure of all ACEs. It allows for control signals but does not provide power. Subjects: APU Generators/Alternators RAT (All) 1 user liked this post. |
ignorantAndroid
2025-06-20T01:22:00 permalink Post: 11906524 |
In the interests of completeness, we should perhaps also consider the possibility of some other previously-unknown software issue capable of creating an uncommanded dual engine shutdown. TCMS is the most likely candidate due to the deliberate separation of other systems from being able to achieve this outcome. The question then isn't whether there's some odd combination of input faults that would confuse TCMS into believing it were on the ground, but rather whether there's any way in which the software side could crash in such a way as to create an anomalous state within the system leading to engine failure. For instance, another overlooked software counter with an unwelcome failure mode. Whatever is the cause will likely turn out to be have been a very low-probability event. But unless we have a TCMS expert who can state canonically that (say) the WoW sensor electrically disables TCMS when airborne (as opposed to merely being an input to the TCMS logic) then we cannot say with certainty that multiple inputs would have to have failed / been corrupted in order to reach the end state of this flight. Subjects: Dual Engine Failure Engine Failure (All) Engine Shutdown High Pressure Shutoff Valve TCMA (All) 3 users liked this post. |
ignorantAndroid
2025-06-20T01:26:00 permalink Post: 11906527 |
There is no static inverter on the 787. All essential flight instruments use 28 VDC, so they can be powered directly from the batteries.
Subjects: None 4 users liked this post. |
ignorantAndroid
2025-06-20T04:28:00 permalink Post: 11906580 |
In that era of federated avionics, aircraft equipped with dual RAs usually also had a separate comparator box that would compare the outputs and alert the crew if the values diverged - even if the RA LRU hadn't "flagged" the indicator. I am surprised that comparator output wasn't checked by the autothrottle system.
Subjects: None |
ignorantAndroid
2025-06-20T04:57:00 permalink Post: 11906593 |
Just so I have this clear, are you saying that the implementation of the TCMA functionality involved
no
new components being added to the pre-existing FADEC? Are you saying, in effect, that the two switch relays described in the TCMA patent application, which relays and their configuration achieves the described two channel redundancy, were already there as components or are mere depictions of what the software does itself?
I am not suggesting you are wrong and, as I've said before, the descriptions and schematic in the patent application are just 'big hands / small maps' concepts. However, if TCMA functionality "is simply a bit of software in the FADECs", merely sending a 1 or 0 or other signal into a point in the pre-existing FADEC that already had control over fuel cutoff (with the TCMA software merely monitoring data busses, rather than direct sensor outputs, to work out thrust lever position and whether or not the aircraft is 'on the ground' for TCMA purposes) I for one would really like to know that for sure and get my head around the implications.
That is the implication I have heard all along, particularly from tdracer's posts.
It uses existing thrust-lever-angle inputs, existing N1 inputs, and (presumably) existing WoW inputs, does software stuff inside the ECU, and if necessary uses the existing overspeed cutout outputs to stop the engine. The air/ground signal would've already been present as well. It would be needed for switching between ground idle, flight idle, and approach idle. Tdracer has discussed that as well, in past threads. Subjects: Engine Failure (All) FADEC Fuel (All) Fuel Cutoff TCMA (All) TCMA (Shutdown) 4 users liked this post. |
ignorantAndroid
2025-06-20T08:53:00 permalink Post: 11906736 |
Categories 1 and 2 would be common to both engines, so they both remain plausible. For category 2, it would be impossible to identify the issue without analyzing the complete source code. Since we don't have access to that code, this is a dead end. It could be the cause, but we won't be able to figure it out. Looking at how the FADECs are designed to work isn't going to be very useful here, since by definition, they'd be doing something they weren't supposed to. Category 1 is a bit different. There are 2 functions we know of that can close the fuel shutoff valve: TCMA and N2 overspeed protection. We don't have the complete specifications, but the basic logic of both functions has been described. If we assume that one of these was the cause, then the conditions for one of those functions must have been met. The conditions for TCMA, at least as it's been described in this thread, are:
As for the overspeed protection, as far as I know, there's only one condition: N2 greater than a certain value. That reading comes from sensors that are inside each engine and wired directly to the FADECs. I don't see any way this could affect both engines simultaneously either, but it still seems a bit more likely than something involving TCMA since it only requires 2 separate, simultaneous failures rather than 3 or more. For the sake of accuracy, I should also note that not everything fits neatly into one of my 3 categories. For example, let's say we have a machine that's programmed to shut down if any one of 3 parameters goes above a certain value. If one of those values gets corrupted by a faulty memory chip, the machine could shut down unnecessarily. If we add more parameters to the list, the probability of an inadvertent shutdown increases since there are more critical areas in memory. As another example, consider a case where corruption of the CPU's program counter causes it to inadvertently jump to a particular subroutine. If we add more subroutines that can trigger a shutdown, we make the machine more vulnerable, albeit to a very small degree. Changes like these are sometimes referred to as "increasing the surface area." Due to those types of scenario, I will admit that the existence of something like TCMA probably makes an engine ever-so-slightly more likely to fail. Whether the benefit is worth the cost could be debated. In any case, I still find it pretty unlikely that any of this will turn out to have been a factor in this accident. Last edited by ignorantAndroid; 20th Jun 2025 at 09:11 . Subjects: Dual Engine Failure Engine Failure (All) Parameters TCMA (All) TCMA (Shutdown) 9 users liked this post. |
ignorantAndroid
2025-06-21T02:03:00 permalink Post: 11907444 |
The 10-minute limit has nothing to do with the battery capacity. It's to prevent the recorder from continuing to operate after an accident and thus overwriting the audio of interest. There's a timer in the RIPS module. It will stop providing power after exactly 10 minutes, regardless of state of charge. Last edited by ignorantAndroid; 21st Jun 2025 at 02:58 . Reason: spelling Subjects: RIPS 20 users liked this post. |
ignorantAndroid
2025-06-21T17:00:00 permalink Post: 11907903 |
SLF here, but former electronics tech in coms/navaids in civil aviation.
ignorantAndroid yours is a good and helpful explanation, thank you. Can you please advise what is the trigger that starts the 10 minute period? I guess I mean, what signal tells the battery to stop supplying power 10 minutes from "now". Thanks |
ignorantAndroid
2025-06-21T18:02:00 permalink Post: 11907949 |
Another post referenced the RAT only supplying electrical power after 10s - I find that hard to believe, not instant obviously because there has to be some stabilisation time and startup/boot time but it would imply the LH flight instruments would only be active very late. Hopefully the RAT hydraulics would be effective quicker than that.
The engine-driven hydraulic pumps should still work for at least a few seconds after flameout. There's also a small amount of stored energy in the hydraulic systems even after the pumps stop. So even with that 6-second delay for the RAT, there shouldn't be any significant interruption in hydraulic power for the primary flight controls. Subjects: Hydraulic Failure (All) Hydraulic Pumps RAT (All) 6 users liked this post. |
ignorantAndroid
2025-06-21T19:33:00 permalink Post: 11908002 |
If we now consider the ‘worst case scenario will be preferred’ concept that apparently applies to the TCMA design so as to achieve redundancy, the number of sensor inputs it’s monitoring to decide whether, and can change its decision whether, the aircraft is on the ground, becomes a very important matter. The TCMA is only supposed to save the day on the ground, if the pilots select idle thrust on a rejected take off but one or both of the engines fail to respond. In the ‘worst case’ (in my view) scenario, both TCMA channels on both engines will be monitoring/affected by
every
WoW sensor output and
every
RADALT output data and, if
any one of them
says ‘on ground’, that will result in
both
engines’ TCMAs being enabled to command fuel shut off, even though the aircraft may, in fact, be in the air.
For the sake of argument, imagine if every air/ground sensor had to say 'ground' to enable TCMA. That should still meet the 'no single failure' requirement since you'd need at least 2 failures to get a runaway engine: the original thrust control problem, and a faulty air/ground sensor. IIRC, he said that the 747-8 looks at weight on wheels, gear truck tilt, and radio altimeters. At least one of each has to say 'ground' for TCMA to be enabled. Subjects: FADEC Fuel (All) Fuel Cutoff MLG Tilt TCMA (All) Weight on Wheels 1 user liked this post. |
ignorantAndroid
2025-06-21T20:40:00 permalink Post: 11908039 |
Without going round the hamsterwheel again does anyone have an actual reference for this? Because I've gone back through each of tdracer's very informative posts about this
see here
and there is a discrepancy in the two points he makes below in adjacent posts. Is tdracer talking about the same HPSOV valves? Can anyone confirm that with both AC power loss and and a temporary DC power loss there are no critical engine related shutoff valves that will fail safe (unpowered) in a closed position?
The HPSOV is made up of 2 parts which I'll call the main valve and the pilot valve. The pilot valve is actuated by a solenoid and supplied with fuel from the high-pressure side. The main valve is held shut by a spring. As long as the pilot valve is open and the high-pressure fuel pump is operating, fuel flows through the pilot valve, then pushes and holds the main valve open. The pilot valve and solenoid are 'latching,' i.e. they maintain their position until electrical power is applied. However, a certain pressure still has to be provided by the pump in order to hold the main valve open. Note that when I say 'high-pressure fuel pump,' I'm referring to the one that's mechanically driven by the engine's high-pressure shaft, not any of the electric pumps. ![]() Note: The HPSOV is mistakenly labeled as 'PSOV' in this diagram. Subjects: Fuel (All) Fuel Pumps High Pressure Shutoff Valve 7 users liked this post. |
ignorantAndroid
2025-06-22T01:41:00 permalink Post: 11908217 |
That wouldn't happen. Loads are preemptively shed before the busses are paralleled. Subjects: Generators/Alternators 5 users liked this post. |
ignorantAndroid
2025-06-22T02:02:00 permalink Post: 11908225 |
Subjects: EDML 1 user liked this post. |
ignorantAndroid
2025-06-22T17:39:00 permalink Post: 11908706 |
Those are two drastically different things. I've encountered software engineers who astound me with their knowledge of programming, but most of them are dumbfounded as soon as they see a soldering iron or oscilloscope.
Subjects: None 2 users liked this post. |
ignorantAndroid
2025-06-22T18:39:00 permalink Post: 11908749 |
That's not quite right. First of all, the AOA DISAGREE warning was supposed to be basic (standard). It was linked with the AOA indicator by mistake, and that bug was already being fixed before the first crash.
The AOA indicators themselves weren't considered 'luxury items' either. The only reason they exist is that they're mandatory in Russia. When Russian airlines started buying western-built aircraft in the 90s, both Boeing and Airbus had to add them as options. I've never been able to confirm whether the AOA indicator was even an added-cost option or not. There are several similar options that can be configured by maintenance at no cost. Subjects: None |
ignorantAndroid
2025-07-01T07:10:00 permalink Post: 11914056 |
Hold your horses there
Bloggs
, I didn't say they did, I said centre tanks were typically turned on at that altitude (using a certain 737 operator as a guide). As the check list that you posted shows the centre pumps will automatically turn off because of load shedding once an engine is started.
Subjects: None |
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