Posts by user "ignorantAndroid" [Posts: 18 Total up-votes: 72 Pages: 1]

ignorantAndroid
2025-06-17T04:20:00
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Post: 11903956
Originally Posted by RickNRoll
Electrical engineer here. Delete if you want.

Electrical failures are often not passive and clean. A short of high voltage to low voltage lines or circuits can be highly destructive and disruptive. The "magic smoke" comes out and what happens next can't be predicted.
That's why the regulations require separation between high-voltage and low-voltage wiring.

Subjects: None

ignorantAndroid
2025-06-17T04:46:00
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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
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Post: 11906415
Originally Posted by Aerospace101
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.
None of the flight controls can be powered by the battery. Both the stabilizer motors and the electrically-actuated spoilers require high-voltage DC which is only available with at least one engine-driven generator or APU generator working. They cannot be powered by the RAT generator either.

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)

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ignorantAndroid
2025-06-20T01:22:00
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Post: 11906524
Originally Posted by skwdenyer

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.
TCMA is simply a bit of software in the FADECs, so it has the same separation as everything else. There's no inter-engine interaction when it comes to TCMA.

Originally Posted by skwdenyer

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.
This is technically possible, but of course the FADECs would have the ability to shut down the engines anyway, even if TCMA didn't exist. If there's a software bug, it could involve TCMA but it could easily be somewhere else.

Originally Posted by skwdenyer

Or even just a "dirty power supply" (cf all the reports of dodgy passenger-side electrics on this a/c) leading to spurious inputs and unexpected consequences.
Like I mentioned above, there's no communication between engines wrt TCMA. Therefore a software bug is plausible, but any kind of transient hardware malfunction can be essentially ruled out.

Originally Posted by skwdenyer

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.
TCMA can't be disabled electrically. It's just software, and all of the hardware involved serves other functions which are still needed while in the air. For example, the FADECs would command the HPSOV closed in case of N2 overspeed. That would have the exact same effect as TCMA.

Subjects: Dual Engine Failure  Engine Failure (All)  Engine Shutdown  High Pressure Shutoff Valve  TCMA (All)

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ignorantAndroid
2025-06-20T01:26:00
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Post: 11906527
Originally Posted by Someone Somewhere
Loss of all electric power to flight instruments both sides: total AC electric loss, and I think battery/static inverter too?
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

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ignorantAndroid
2025-06-20T04:28:00
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Post: 11906580
Originally Posted by skiingman
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.
Boeing added such a comparator to the NG circa 2005. The 737 in the Turkish accident was manufactured prior to that.

Subjects: None

ignorantAndroid
2025-06-20T04:57:00
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Post: 11906593
Originally Posted by Lead Balloon
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.
Originally Posted by Someone Somewhere
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.
I don't have any direct knowledge, but yes, that's my understanding based primarily on tdracer's comments. It also just makes sense. I'm pretty confident that all the necessary hardware already existed because of the need for N2 overspeed protection. A failure in one FADEC channel could drive the FMV fully open, leading to an overspeed and uncontained engine failure. For regulatory purposes, it would be unacceptable to have a single point of failure with catastrophic consequences, so it would be necessary to make the inactive FADEC channel capable of cutting off fuel in that case.

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)

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ignorantAndroid
2025-06-20T08:53:00
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Post: 11906736
Originally Posted by skwdenyer
So saying "TCMA would only trigger if WoW and RA have failed somehow" is incorrect? Those are simply inputs to software, which might itself fail badly for other reasons.
In general, we can classify computer errors into 3 categories:
  1. Errors in system design, specifications, or algorithms. These are cases where the computer does exactly what it was designed to do, but the design itself was flawed or inadequate, or had unforeseen consequences. This would include things like MCAS on the 737 MAX.
  2. Software errors. These are cases where the computer does exactly what the code tells it to do, but not what the designers wanted it to do. This results from mistakes in writing the actual code, and this is usually what we'd refer to as a "bug." This includes things like race conditions, loops that fail to terminate, incorrect type conversion, etc.
  3. Hardware malfunctions. These are cases where the computer does something different from what the code instructs. It can involve memory corruption or data bus corruption. It can result in a system that appears to work, but returns incorrect outputs, e.g. a calculator saying 2+2=5. It can also cause the computer to execute valid instructions, but at an inappropriate time. It can result from manufacturing defects in components, cosmic rays (SEU or SEE), radiofrequency interference (HIRF), moisture ingress, failed solder joints, and numerous other things.
As I said previously, I think we can rule out category 3 in this case. Hardware malfunctions are essentially random events. They're inherently unpredictable since there's little to no relationship between what the system was supposed to do and what it actually does. It would be astronomically improbable for the same failure to occur on both engines at the same moment.

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:
  • Airplane on ground
  • Thrust higher than commanded by thrust lever angle (TLA) for some period of time
It's reasonable to assume that the first condition is common to both engines. But that still leaves the second. To my knowledge, there's no plausible scenario that would cause that condition to be met on both engines simultaneously. Each thrust lever has 2 resolvers which are wired directly to the corresponding FADECs. That means the signals don't pass through any common component. An incorrect reading from one resolver could conceivably trigger TCMA, but I don't see any reason why that would happen to both engines simultaneously.

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
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Post: 11907444
Originally Posted by MJA Chaser
The term battery may be misleading us. Think supercapacitor. Minimal to zero maintenance required.
It is a battery, not a supercapacitor. Most commonly nickel-cadmium, because that chemistry has been used in aircraft for decades. They're safe and readily accepted by regulators. Li-ion can be used, but a "special condition" from the regulators is needed, plus the weight savings would be negligible.

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
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Post: 11907903
Originally Posted by Recidivist
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
When it stops receiving power from the aircraft. The RIPS sits between aircraft power and the CVR. It pretty much works just like an uninterruptible power supply that you'd use with your PC.

Subjects: CVR  RIPS

ignorantAndroid
2025-06-21T18:02:00
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Post: 11907949
Originally Posted by jdaley
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.
Boeing specifications say that the RAT will provide hydraulic power within 6 seconds and electrical power within 10 seconds. That would be the worst-case scenario, so it should usually be a bit less than that. Almost everything that gets electrical power from the RAT can also be powered by the main battery. So you don't have to wait for the RAT to spin up before you have instruments.

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
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Post: 11908002
Originally Posted by Lead Balloon
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.
I don't think ‘worst case scenario will be preferred’ is the philosophy they use. The way tdracer explained it, there can't be any single failure that leads to uncommanded high thrust on the ground. Presumably, each FADEC channel is treated as a single 'fault isolation area.' That's why the inactive channel has to be able to effect a shutdown in case the active channel causes a runaway.

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

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ignorantAndroid
2025-06-21T20:40:00
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Post: 11908039
Originally Posted by Aerospace101
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?
There's actually no discrepancy.

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

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ignorantAndroid
2025-06-22T01:41:00
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Post: 11908217
Originally Posted by TryingToLearn
If just the BTBs on the 230V grid got some humidity (hot, no AC, water cooling...) and went up in one big arc (I think they made them semiconductor relays, too).
The BTBs are traditional electromechanical contactors, and they're not water-cooled. Solid-state switches are used for some of the smaller loads, but not BTBs.

Originally Posted by TryingToLearn
Or if the engines were in a condition not far from compressor stall, one was stalling and 400kW load jumped from one engines generators to the other...
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
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Post: 11908225
Originally Posted by EDML
Strange environment to manufacture complex carbon fiber components. A wooden rig? Seriously?
It appears that photo is showing one of the very first 787 fuselage sections ever built, back when the manufacturing process was still being developed.

Subjects: EDML

1 user liked this post.

ignorantAndroid
2025-06-22T17:39:00
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Post: 11908706
Originally Posted by za9ra22
India has become one of the foremost global centres of IT hardware and software development, so I'd agree that it seems quite probable that it would have the capability to work with data extraction at component level.
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

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ignorantAndroid
2025-06-22T18:39:00
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Post: 11908749
Originally Posted by Musician
"AOA disagree" indicators are considered a luxury item for Boeing
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.

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ignorantAndroid
2025-07-01T07:10:00
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Post: 11914056
Originally Posted by megan
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.
They stay off until engine start because of load shedding. Once both engines are running, the pumps should be running too.

Subjects: None