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OldnGrounded
2025-06-18T23:16:00 permalink Post: 11905612 |
They're guarded by the switch design/operation itself. To move one of them, the toggle handle has to first be pulled outward.
6 users liked this post. |
bbofh
2025-06-19T01:30:00 permalink Post: 11905658 |
The 787-8 landing gear retraction is primarily hydraulic, using the center hydraulic system for the main operation. However, the alternate gear
extension
system utilizes a dedicated electric pump to pressurize fluid from the center hydraulic system for gear extension. Obviously due its size and weight and staged retraction, the effort required to raise and stow the gear greatly exceeds that required for extension.
The main gear retraction/extension is controlled by the center hydraulic system. It is apparent that the hydraulics failed when the engines shut down after breaking the down-locks and leaving the Main Landing gear bogeys in the tilt position, ready for a next step internal stowage and door closure (that was now never to happen). It is therefore apparent that the dual engine failure and consequent automated RAT extension was precipitated by this gear selection or retraction cycle and thus likely to be either WoW micro-switch or 5G Radar altimeter-effect associated. Due to accumulator depletion, the electric pump load would have spiked to replenish it. This may have precipitated the dual engine shutdown due to an unfiltered electrical surge affecting the Ground/Air microswitches (or a local 5G transmission affecting the RADALT) and resetting the TCMA. The RADALT? Another plausibility? Because of the furore over a spasticated frequency allocation by the US FCC, the US FAA had finally “bought in” and declared that individual nations and their airline operators were responsible for their own 5G frequency spectrum allocations and for taking essential steps to ensure mitigation of the interference effects upon aircraft automated landings and other critical systems caused by their own national approved 5G spectrum decisions. It was admittedly a situation calling for extensive modifications to (and shielding for) the three radar altimeters fitted for redundancy considerations to all modern airliners... for Category 3 ILS approach and landing in zero/zero visibility conditions. The RADALT also features in many air-ground sensing applications. (eg the 747-8). This was an unusual FAA “passing of the buck” to manufacturers such as Honeywell etc. (to sort out with client operators). But then again, it was not the US FCC’s right to dictate the specific 5G frequencies internationally. These spectrum allocations now vary over the wide selection of 5G phones available (and also nationally). 5G Radar Altimeters constitute a part of the ground/Air sensing that changes the TCMA from ground mode (able to fuel-chop engines) to the air mode (inhibited from doing so)... Ground activation is acceptable ...where fuel chopping of uncommanded thrust can prevent runway sideways excursions or runway length overruns. The question now becomes: “Is it more (or less) safe having an automated fuel-chopping capability on BOTH your left and right, rather than leaving it to the pilot to react via his center console fuel cut-off switches... in the unlikely event of a runaway engine after landing (or during an abandoned take-off)? 5G Frequency Variations The frequencies of 5G phones vary nationally based on the frequency bands allocated and used by different carriers in each country. In the United States, for example, carriers such as AT&T, Verizon, T-Mobile, and others use a combination of low-band, mid-band, and high-band 5G frequencies. Low-band 5G frequencies typically range from 600 MHz to 1 GHz, mid-band 5G frequencies range from 1.7 GHz to 2.5 GHz, and high-band 5G (mmWave) frequencies start at 24 GHz and go up to 40 GHz . These frequencies are allocated by regulatory bodies such as the Federal Communications Commission (FCC) and can vary between countries based on spectrum availability and regulatory decisions. In other countries, the specific frequency bands used for 5G may differ, leading to variations in the frequencies supported by 5G phones. Additionally, the deployment of 5G networks can also influence the frequencies used, with some countries focusing more on sub-6 GHz bands while others prioritize mmWave technology. 5G interference? It may be an avenue worth exploring? 6 users liked this post. |
Capn Bloggs
2025-06-19T01:40:00 permalink Post: 11905665 |
Originally Posted by
Sycamore
As an ex-tp, I would consider those cut-off switches a danger, and they should have guards either side of each.
Shirtsleeves/watches etc. can get caught, lift switch. Very poor design. Would not pass `military-muster.` Ours: ![]() 787: ![]() 2 users liked this post. |
Seamless
2025-06-19T09:30:00 permalink Post: 11905864 |
https://ad.easa.europa.eu/blob/NM-18...SIB_NM-18-33_1
The FAA recommends that all owners and operators of the affected airplanes incorporate the following actions at the earliest opportunity: 1) Inspect the locking feature of the fuel control switch to ensure its engagement. While the airplane is on the ground, check whether the fuel control switch can be moved between the two positions without lifting up the switch. If the switch can be moved without lifting it up, the locking feature has been disengaged and the switch should be replaced at the earliest opportunity.
Last edited by Senior Pilot; 19th Jun 2025 at 11:13 . Reason: Image 3 users liked this post. |
Capn Bloggs
2025-06-19T10:22:00 permalink Post: 11905892 |
Originally Posted by
DTA
That failure can be the result of physical damage or wear so that the knob is stuck in the pulled position. It would not be obvious if you did not look closely.
![]() 7 users liked this post. |
Seamless
2025-06-19T12:27:00 permalink Post: 11905984 |
The spec sheet says 100,000 cycles.
Switches fail sometimes. I have changed lots of lots of malfunctioning toggle switches in my day. But both at the same time? It has to be a common thing happening at the same time. Someone slipping both switches into a worn middle detent is such a thing though, that is one habit that could be developed. A slight bump at takeoff and they both go to cutoff. The switches themselves are on-on 4 pole toggles, and doesn\x92t fail into one position over the other, and gravity would prefer the cutoff position in this design. 2 users liked this post. |
Capn Bloggs
2025-06-19T12:29:00 permalink Post: 11905985 |
Originally Posted by
syseng68k
Have a few of that type here
|
Lead Balloon
2025-06-19T12:45:00 permalink Post: 11905991 |
The discussion about the fuel control switch design is interesting so far as it goes, but I'm more interested in the physical arrangements under the bracket to which they're fitted and the associated connection and wiring arrangements. No doubt the arrangements are designed so as to reduce to 'vanishingly small' the probabilities of any common shorts to the wrong places, but who knows, for sure, how the switches and connections and wiring were installed in the accident aircraft, or whether there was a piece of swarf or other FOD that ended up somewhere that it should not have been (i.e. anywhere on the aircraft) during manufacture or maintenance.
3 users liked this post. |
Capn Bloggs
2025-06-19T12:55:00 permalink Post: 11906000 |
Originally Posted by
syseng68K
I guess it depends on the model
Not so the Boeing fuel switches: they can be relatively easily "sat" in the middle, on the centre raised bit and could be bumped either way. Hence our (non-787) FCOM saying make sure you jiggle them when you put it in the On position to confirm it's locked there. 5 users liked this post. |
Xeptu
2025-06-19T12:57:00 permalink Post: 11906003 |
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OldnGrounded
2025-06-19T13:06:00 permalink Post: 11906009 |
Mods, feel free to remove this if you think it's not contributing.
Folks, the exchange here about the cutoff switches ("fuel control switches") is exactly the kind of discussion that contributes meaningfully to our collective understanding of one possible causal or contributing factor in the accident. Smart, well-informed people politely considering and evaluating the suggestions of others. No snark or sniping. Love it. 16 users liked this post. |
OldnGrounded
2025-06-19T13:51:00 permalink Post: 11906035 |
And add the radio altimeter(s). I think, but don't know, that they provide inputs to the FADEC TCMA function also. Last edited by T28B; 19th Jun 2025 at 14:38 . Reason: brackets completed 1 user liked this post. |
Seamless
2025-06-19T14:08:00 permalink Post: 11906053 |
I have read most of the thread (old and new). As a lawyer working in forensic investigations, I am constantly involved in problem-solving. My field of work also includes complex investigations related to insolvencies, which almost always require an analysis of the causes behind a specific, established outcome. In doing so, I naturally also have to deal with probabilities. However, it often turns out that the most likely or plausible explanation does not reflect what actually happened.
Many of the considerations I’ve read fail because the simultaneous failure of both engines is extremely unlikely, leading to a constant search for higher-order causes. It was suggested that an incorrect altitude setting led to an early thrust reduction. However, this would not explain the deployment of the RAT (Ram Air Turbine), especially since the thrust could have been readjusted. FADEC and TCAM are highly redundant systems, and TCAM failure is unlikely due to WOW (Weight on Wheels) logic, making a simultaneous engine failure after VR equally improbable. With that said, and with regard to my question concerning the AD that relates to the fuel control switches (FCS), my thought—and it was nothing more than that—was that their activation becomes more probable if it can occur accidentally. That’s how I came across SAIB: NM-18-33. Another user then brought up an iPhone. That notion would, of course, be dramatic—but how unlikely is it really that after approximately 10,000 actuations between December 2013 and June 2025, the two FCS no longer lock perfectly? Considering all of this, I find it quite conceivable that the A/T slightly reduced thrust in the first seconds after VR (e.g., if an incorrect target altitude had been entered) and that an object lying between the thrust levers and the FCS could have pushed the FCS into the “Off” position. Due to the buttons on top of the switches, which provide some resistance, it’s even possible that the object both pulled and pushed them. But all of this is speculation. The investigation report will bring clarity. Even if my theory is not confirmed, I still believe that the positioning and mechanism of the FCS are suboptimal. Switches of such critical importance should be better protected, and movements in the area in front of the switches (like reducing thrust) should not follow the same direction as shutting off the fuel supply. A different switching direction alone would provide more safety—especially considering that the FCS are protected laterally by metal plates. 5 users liked this post. |
Aerospace101
2025-06-19T14:11:00 permalink Post: 11906054 |
It does not follow that MCAS malfunction is a software malfunction.
As far as I know, the software functioned exactly as it was specified/required to function. The problem did not lie in the quality of the software, as you suggest. It lay in the functional requirements for the function, and the hazard analysis of those requirements, and those are manufacturer tasks. In a total electrical failure, when the system switches to emergency battery power, how are input variables like rad alt and wow switches processed? (these were inputs someone mentioned on the 747-8, have the TCMA inputs been identified yet?) I speculate the gear truck forward tilt is a symptom of a C hydraulic failure caused by a total electrical failure around the time of VR. Once they got 10 deg nose up on the rotation, with a total electrical failure, could the FADEC receive erroneous rad alt or wow inputs, and how would TCMA handle these inputs in the transition from ground to air logic? What is baffling is the simultaneous nature of the suspected dual engine shutdown. There is no obvious asymmetry, with the flight path or rudder movements. If the engine fuel control switches had been manually cut one at a time, there should have been some visible flightpath change or flight control response. Something happened to both engines at exactly the same time. 2 users liked this post. |
DTA
2025-06-19T14:36:00 permalink Post: 11906073 |
I have read most of the thread (old and new). As a lawyer working in forensic investigations, I am constantly involved in problem-solving. My field of work also includes complex investigations related to insolvencies, which almost always require an analysis of the causes behind a specific, established outcome. In doing so, I naturally also have to deal with probabilities. However, it often turns out that the most likely or plausible explanation does not reflect what actually happened.
Many of the considerations I\x92ve read fail because the simultaneous failure of both engines is extremely unlikely, leading to a constant search for higher-order causes. It was suggested that an incorrect altitude setting led to an early thrust reduction. However, this would not explain the deployment of the RAT (Ram Air Turbine), especially since the thrust could have been readjusted. FADEC and TCAM are highly redundant systems, and TCAM failure is unlikely due to WOW (Weight on Wheels) logic, making a simultaneous engine failure after VR equally improbable. With that said, and with regard to my question concerning the AD that relates to the fuel control switches (FCS), my thought\x97and it was nothing more than that\x97was that their activation becomes more probable if it can occur accidentally. That\x92s how I came across SAIB: NM-18-33. Another user then brought up an iPhone. That notion would, of course, be dramatic\x97but how unlikely is it really that after approximately 10,000 actuations between December 2013 and June 2025, the two FCS no longer lock perfectly? Considering all of this, I find it quite conceivable that the A/T slightly reduced thrust in the first seconds after VR (e.g., if an incorrect target altitude had been entered) and that an object lying between the thrust levers and the FCS could have pushed the FCS into the \x93Off\x94 position. Due to the buttons on top of the switches, which provide some resistance, it\x92s even possible that the object both pulled and pushed them. But all of this is speculation. The investigation report will bring clarity. Even if my theory is not confirmed, I still believe that the positioning and mechanism of the FCS are suboptimal. Switches of such critical importance should be better protected, and movements in the area in front of the switches (like reducing thrust) should not follow the same direction as shutting off the fuel supply. A different switching direction alone would provide more safety\x97especially considering that the FCS are protected laterally by metal plates. 6 users liked this post. |
sabenaboy
2025-06-19T14:51:00 permalink Post: 11906087 |
OK, I promised some
informed speculation
when I got back, so here goes:
Disclaimer: never worked the 787, so my detailed knowledge is a bit lacking. First off, this is perplexing - especially if the RAT was deployed. There is no 'simple' explanation that I can come up with. GEnx-1B engines have been exceptionally reliable, and the GE carbon composite fan blades are very robust and resistant to bird strike damage (about 15 years after the GE90 entry into service, I remember a GE boast that no GE90 (carbon composite) fan blades had needed to be scrapped due to damage (birdstrike, FOD, etc. - now that was roughly another 15 years ago, so is probably no longer true, but it shows just how robust the carbon composite blades are - far better than the more conventional titanium fan blades). Not saying it wasn't somehow birdstrike related, just that is very unlikely (then again, all the other explanations I can come up with are also very unlikely ![]() Using improper temp when calculating TO performance - after some near misses, Boeing added logic that cross-compares multiple total temp probes - aircraft TAT (I think the 787 uses a single, dual element probe for aircraft TAT, but stand to be corrected) and the temp measured by the engine inlet probes - and puts up a message if they disagree by more than a few degree tolerance - so very, very unlikely. N1 power setting is somewhat less prone to measurement and power setting errors than EPR (N1 is a much simpler measurement than Rolls EPR) - although even with EPR, problems on both engines at the same time is almost unheard of. The Auto Thrust (autothrottle) function 'falls asleep' at 60 knots - and doesn't unlock until one of several things happens - 250 knots, a set altitude AGL is exceeded (I'm thinking 3,000 ft. but the memory is fuzzy), thrust levers are moved more than a couple of degrees, or the mode select is changed (memory says that last one is inhibited below 400 ft. AGL). So an Auto Thrust malfunction is also extremely unlikely. Further, a premature thrust lever retard would not explain a RAT deployment. TO does seem to be very late in the takeoff role - even with a big derate, you still must accelerate fast enough to reach V1 with enough runway to stop - so there is still considerable margin if both engines are operating normally. That makes me wonder if they had the correct TO power setting - but I'm at a loss to explain how they could have fouled that up with all the protections that the 787 puts on that. If one engine did fail after V1, it's conceivable that they shut down the wrong engine - but since this happened literally seconds after takeoff, it begs the question why they would be in a big hurry to shut down the engine. Short of an engine fire, there is nothing about an engine failure that requires quick action to shut it down - no evidence of an engine fire, and even with an engine fire, you normally have minutes to take action - not seconds. The one thing I keep thinking about is someone placing both fuel switches to cutoff immediately after TO. Yes, it's happened before (twice - 767s in the early 1980s), but the root causes of that mistake are understood and have been corrected. Hard to explain how it could happen ( unless, God forbid, it was intentional ). 3 users liked this post. |
StudentInDebt
2025-06-19T14:52:00 permalink Post: 11906088 |
I\x92ve had a look at the Honeywell spec sheet for TL type switches. They are a common type with many available configurations and are essentially a normal looking snap action switch. The locking feature is an add-on which can be specified. I have to say that from the diagrams, the locking mechanism doesn\x92t look that robust and I\x92d guess that it is subject to wear which was probably the basis of the SAIB. Given that this is a mechanical locking device seeing frequent use possibly with less than full mechanical sympathy it is possible to see how wear would occur making the lock less effective. This does beg the question of whether a check on the mechanism has made it into maintenance routines. ( Note - the locking mechanism appears to be the same for all TL switch types) . Those familiar with the switches - what do you think?
For the avoidance of doubt, the above is a technical observation on the switch type NOT a causation theory for this accident. ![]() 6 users liked this post. |
Seamless
2025-06-19T15:04:00 permalink Post: 11906096 |
this isn\x92t the type of switch fitted to the 787 as a fuel control switch, totally irrelevant but has generated yet more nonsense. The switches are spring loaded (or so it feels) in addition to having a massive block to prevent inadvertent operation in either direction. Anyone suggesting they could be accidentally \x93knocked off\x94 is so clueless about their operation it\x92s actually painful to rebut
4 users liked this post. |
Europa01
2025-06-19T16:05:00 permalink Post: 11906149 |
Cutoff Switches
this isn’t the type of switch fitted to the 787 as a fuel control switch, totally irrelevant but has generated yet more nonsense. The switches are spring loaded (or so it feels) in addition to having a massive block to prevent inadvertent operation in either direction. Anyone suggesting they could be accidentally “knocked off” is so clueless about their operation it’s actually painful to rebut
Not looking over my shoulder here but I should add that I know and respect Honeywell switches. Those I have worked with are well designed and well made plus Honeywell incorporate improvements based on field experience. As posted by another contributor upthread I am puzzled by the odd ‘down’ = engine off configuration though. Last edited by Europa01; 19th Jun 2025 at 16:36 . 4 users liked this post. |
skwdenyer
2025-06-19T16:12:00 permalink Post: 11906161 |
this isn\x92t the type of switch fitted to the 787 as a fuel control switch, totally irrelevant but has generated yet more nonsense. The switches are spring loaded (or so it feels) in addition to having a massive block to prevent inadvertent operation in either direction. Anyone suggesting they could be accidentally \x93knocked off\x94 is so clueless about their operation it\x92s actually painful to rebut
That's a TL series switch with 4 poles (the "4" in "4TL"), a "type D" lock (meaning locked out of centre position per the Honeywell data sheet - the "D" in "3D." This is a photo of one: ![]() You can find the manufacturer's datasheet here: https://octopart.com/datasheet/4tl83...ywell-25749542 Problems with critical switches aren't new on 787-8s. For instance, in addition to the SAIB above, there's this AD: https://www.federalregister.gov/docu...pany-airplanes Looking at the photo above, it isn't just wear that's potentially an issue, but foreign object impingement. There don't appear to be gaitors fitted to these switches in the 788, so the locking mechanisms are potentially susceptible to a build-up of material if not kept clean. There are a range of other failure modes possible, whilst the SAIB specifically describes found-in-the-field problems with these switches. Yes, they're chunky, and very positive when new. That doesn't mean they shouldn't be discussed. 7 users liked this post. |
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