June 12, 2025, 20:42:00 GMT
permalink Post: 11899713
ALT HOLD ON TAKEOFF
Look at FAA SAIB AIR 22-09.
Likely what happened here. ALT wiNdow set to 300.
Altitude hold and throttles rolled back, plane stopped climbing and descended into the ground.
Likely what happened here. ALT wiNdow set to 300.
Altitude hold and throttles rolled back, plane stopped climbing and descended into the ground.
June 12, 2025, 20:56:00 GMT
permalink Post: 11899728
June 19, 2025, 11:01:00 GMT
permalink Post: 11905926
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 could occur. This does beg the question of how 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?
June 19, 2025, 14:08:00 GMT
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.
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.
June 19, 2025, 14:36:00 GMT
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.
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.
June 19, 2025, 14:52:00 GMT
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.
For the avoidance of doubt, the above is a technical observation on the switch type NOT a causation theory for this accident.
June 19, 2025, 16:05:00 GMT
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 June 2025 at 16:36 .
June 19, 2025, 16:12:00 GMT
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.
June 19, 2025, 19:01:00 GMT
permalink Post: 11906272
Happy to accept any challenge to my post. The SAIB refers to 4TL837-3D switches which are explicitly covered in the Honeywell data sheet I looked at (p16). Given that the SAIB was based on some shortfall in the locking mechanism it maybe that all these switches have been replaced by something with an improved specification since the SAIB was issued and I\x92d be very pleased to acknowledge that. Those who know facts what do you think?
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 \x91down\x92 = engine off configuration though.
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 \x91down\x92 = engine off configuration though.
June 19, 2025, 20:52:00 GMT
permalink Post: 11906364
The type of switch being discussed is the specific type reported as being liable to problems. The SAIB is here
https://ad.easa.europa.eu/blob/NM-18...SIB_NM-18-33_1
and specifies a part number for the B788 as 4TL837-3D
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.
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.
June 19, 2025, 21:09:00 GMT
permalink Post: 11906376
But the SAIB is clear - they can be assembled / installed incorrectly, resulting in no lockout protection, with actual failures / anomalies apparently found in the field.
None of which means they were the cause of this crash.
July 11, 2025, 20:47:00 GMT
permalink Post: 11919775
FAA issued an SAIB in 2018 about these fuel control switches being installed without the locking feature engaged\x97Air India didn\x92t act on it (not mandatory).
No evidence of birdstrike, fire, or mechanical failure. Both engines had good health data. The only obvious explanation at this stage is an inadvertent dual shutdown by crew,
No evidence of birdstrike, fire, or mechanical failure. Both engines had good health data. The only obvious explanation at this stage is an inadvertent dual shutdown by crew,
July 11, 2025, 20:53:00 GMT
permalink Post: 11919779
FAA issued an SAIB in 2018 about these fuel control switches being installed without the locking feature engaged\x97Air India didn\x92t act on it (not mandatory).
No evidence of birdstrike, fire, or mechanical failure. Both engines had good health data. The only obvious explanation at this stage is an inadvertent dual shutdown by crew,
No evidence of birdstrike, fire, or mechanical failure. Both engines had good health data. The only obvious explanation at this stage is an inadvertent dual shutdown by crew,
then again this isn\x92t all pointing at the pilots which is interesting
July 11, 2025, 21:00:00 GMT
permalink Post: 11919790
FAA issued an SAIB in 2018 about these fuel control switches being installed without the locking feature engaged\x97Air India didn\x92t act on it (not mandatory).
No evidence of birdstrike, fire, or mechanical failure. Both engines had good health data. The only obvious explanation at this stage is an inadvertent dual shutdown by crew,
No evidence of birdstrike, fire, or mechanical failure. Both engines had good health data. The only obvious explanation at this stage is an inadvertent dual shutdown by crew,
Last edited by Diff Tail Shim; 11th July 2025 at 21:18 .
July 11, 2025, 21:17:00 GMT
permalink Post: 11919817
I remember the SAIB. I assessed it as “recommended” to all our customers and all agreed. I then raised work orders to check all fuel cut-off switches for the missing locking bar conditions. None were found defective. The preliminary report says the switches were found in the Run position. There is no mention of whether the locking mechanism is correctly in place. We need to wait for the full report for that, as this matter is still under investigation.
July 11, 2025, 22:04:00 GMT
permalink Post: 11919871
Does anyone have a link to Special Airworthiness Information Bulletin (SAIB) No. NM-18-33. I did not find it in a quick search.
Last edited by Saab Dastard; 11th July 2025 at 22:26 . Reason: Removed errors
July 11, 2025, 22:12:00 GMT
permalink Post: 11919889
Spoiler
I wouldn't put too much significance in the "01 second" since this still is close enough for an unintentional cause.
No matter what: The design and position of the fuel cut off switches is potentially prone for mishaps.
July 11, 2025, 22:17:00 GMT
permalink Post: 11919895
Background
The Boeing Company (Boeing) received reports from operators of Model 737 airplanes that the fuel control switches were installed with the locking feature disengaged. The fuel control switches (or engine start switches) are installed on the control stand in the flight deck and used by the pilot to supply or cutoff fuel to the engines. The fuel control switch has a locking feature to prevent inadvertent operation that could result in unintended switch movement between the fuel supply and fuel cutoff positions. In order to move the switch from one position to the other under the condition where the locking feature is engaged, it is necessary for the pilot to lift the switch up while transitioning the switch position. If the locking feature is disengaged, the switch can be moved between the two positions without lifting the switch during transition, and the switch would be exposed to the potential of inadvertent operation. Inadvertent operation of the switch could result in an unintended consequence, such as an in-flight engine shutdown.
...The table below identifies the affected airplane models and related part numbers (P/Ns) of the fuel control switch, which is manufactured by Honeywell.
...787-8, -9, and -10
The Boeing Company (Boeing) received reports from operators of Model 737 airplanes that the fuel control switches were installed with the locking feature disengaged. The fuel control switches (or engine start switches) are installed on the control stand in the flight deck and used by the pilot to supply or cutoff fuel to the engines. The fuel control switch has a locking feature to prevent inadvertent operation that could result in unintended switch movement between the fuel supply and fuel cutoff positions. In order to move the switch from one position to the other under the condition where the locking feature is engaged, it is necessary for the pilot to lift the switch up while transitioning the switch position. If the locking feature is disengaged, the switch can be moved between the two positions without lifting the switch during transition, and the switch would be exposed to the potential of inadvertent operation. Inadvertent operation of the switch could result in an unintended consequence, such as an in-flight engine shutdown.
...The table below identifies the affected airplane models and related part numbers (P/Ns) of the fuel control switch, which is manufactured by Honeywell.
...787-8, -9, and -10
Last edited by 13 others; 12th July 2025 at 01:40 . Reason: Bold emphasis mine, fixed link
July 11, 2025, 22:18:00 GMT
permalink Post: 11919901
Agreed, it's most unfortunate that that the preliminary report hasn\x92t closed off speculation, it's simply created opportunity for more.
It confirms that both fuel cutoff switches were moved to OFF at Vr, within a one second interval which is as extreme and inexplicable as it gets. Then it gives us a CVR quote that\x92s so neutered, "Why did you cut off?\x94 / \x93I didn\x92t\x94, that it raises more questions than it answers. If the goal was to reassure or clarify, it\x92s had the opposite effect.
It confirms that both fuel cutoff switches were moved to OFF at Vr, within a one second interval which is as extreme and inexplicable as it gets. Then it gives us a CVR quote that\x92s so neutered, "Why did you cut off?\x94 / \x93I didn\x92t\x94, that it raises more questions than it answers. If the goal was to reassure or clarify, it\x92s had the opposite effect.
Action slip
SAIB NM-18-33
Intentional.
July 11, 2025, 22:28:00 GMT
permalink Post: 11919916