Posts about: "Special Airworthiness Information Bulletin" [Posts: 85 Pages: 5]

The SIAB relates to defective throttle units at the point of installation (on a different but related aircraft). I might be onboard with that theory if maintenance happened to have replaced the unit a day or 2 before the incident. But it's a tough sell that a defective switch that's used literally on every flight had flown for 2 years without someone noticing and snagging that the locking mechanism wasn't working

it would have to be small enough to fit between the idle throttle position and guards on each side of the switches and weigh enough to push the switches over the detent, unless the SAIB cones into play.

Yes, which is why I said a small such range of objects. I can't see how both switches could unintentionally be moved individually one after the other unless there was something wrong with the detent mechanisms. I suppose a careless hand is also a possibility, removed from the thrust levers on rotation?

Timeline of known events with source attribution from the preliminary report:

08:07:33 ATC: Takeoff clearance
08:07:37 A-SMGCS: Aircraft starts rolling
08:08:33 EAFR: V1 153kts
08:08:35 EAFR: Vr 155kts
08:08:39 EAFR: Gnd-Air mode transition
08:08:42 EAFR: Max IAS 180kts, Eng 1/2 Cutoff switches activate within 1 second of each other
08:08:42 CVR: "Why did you cut off", "I did not" (exact time not specified)
08:08:42 A-SMGCS: RAT deployed (exact time not specified)
08:08:47 EAFR: Both engine N2 below min idle. RAT hyd pwr commences
08:08:52 EAFR: Eng 1 cutoff to RUN
08:08:54 EAFR: APU inlet door opens (auto start logic)
08:08:56 EAFR: Eng 2 cutoff to RUN
08:09:05 ATC: Mayday call
08:09:11 EAFR recording stops

Fuel cutoff switches operated within 1 second of each other suggests to me that the locking mechanism wasn't working as per (SAIB) No. NM-18-33. Any loose item could have accidentally (or not) operated the switches (including hands).

Interesting that the report mentions SAIB No. NM-18-33. Can you see a detent on the left switch base in the mishap photo on page 10., Fig. 13? Yes kind of blurry when blown up but maybe we have some photo gurus.

I think they know. Page 10 Fig. 13. It's telling that they referenced the SAIB in the report.

There would already be an emergency airworthiness directive, for lots of different planes, IMO. But there isn't.

Unless defective. I only encountered one in a 30 plus year career where my Captain shutdown at block in and did not like the feel and checked that the detent was worn. Wrote it up. But Boeing installed some that did not have a detent and the FAA issued a SAIB, referenced in the Air India mishap report, to check for these switches because they could inadvertently be positioned to off. Whether they meant human or other inadvertent action was not clear.

Really? It suggests to me and I would imagine the vast majority of us who have flown modern Boeings that they were physically moved, by one of the crew, one at a time, the question is why.

Well, speaking of fuel-cut switches, I read NM-18-33 SAIB with attention, and as a not-that-fluent english speaker, I stumbled on this sentence (my bold):

I could hardly figure what the "disengaged" word meant in this context, so I did a Google search for the switch part numbers (especially "766AT614-3D") to figure the difference between them, and a page from this chinese web site was part of the results.

And while I was painfully crawling the thread, I noticed the following picture about an "undesirable condition":

Incorrect lock tab position on fuel cut-off switch

If this incorrect mounting is actually possible, it would possibly remain unnoticed from the pilots (normal "pull-up then move" action is unaffected), but it would cancel the protective function of the so-called "locking tab", and even limit the travel of the switch handle in both directions, making it more vulnerable to an undesired change of state.

The photos above seem convincing enough, but I'd be very grateful for an informed opinion on this assembly mistake. Even if this is possible, the probability of both switches being unexpectedly snapped seems very remote to say the least, but not as remote as previously estimated.

NM-18-33 SAIB left me wondering the same thing. It give a procedure for detecting the defect but omits to explain the cause. I was thinking that the wrong switches had been supplied/fitted. Assuming the image from the Chinese web site is correct, it is disappointing that the actuator can get into that state. I did not see anything that said whether that was how the switch arrived from Honeywell or if there was a defect that allowed the actuator to turn.

One other useful thing from that web site is a partial schematic which shows the connection of the 4 poles in the switch. I believe this is from a 737NG but it should be the same idea.

Why are you wasting your own time?

If any of that were true why would it happen after Vr and why would one pilot ask: \x93why did you go to cutoff\x94. That is an observed deliberate action. Cmon folks enough with the nonsense!

Why are people shying around the truth!?

I have rebuilt the site that organises these three threads by subject here: https://paulross.github.io/pprune-th...171/index.html

Changes:

• Build threads up to July 12, 2025, 11:59:00 (built at July 12, 2025, 13:03:09).
• Add thread about the published report: Preliminary Air India crash report published
• Add subjects: "Guards", "Timeline (Preliminary Report)", "EICAS", "RUN/CUTOFF", "Special Airworthiness Information Bulletin", "SAIB NM-18-33", "Gear Lever", "Startle Effect" and "Cockpit Area Audio".
• Minor improvements to the index.html page.

Project is here: https://github.com/paulross/pprune-threads
Raise issues here https://github.com/paulross/pprune-threads/issues or PM me.

That account, which is posted as being authoritative, appears to disregard SAIB NM-18-33 which states, in part:

"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.'

Since the preliminary report does not specify that this SAIB was actioned on this aircraft we do not know if it was fitted with defective switches.

In my personal opinion a defective locking feature does not explain the reported event sequence. That does not mean the switches were not defective.

Too many people not wanting to acknowledge what is staring them in the face.

The switches were physically moved to cut off. The report says so, they will have heard them on the CVR.

The switches don\x92t move unless you intend to move them. You can give me all the worn mechanism, SAIB, phone/ipad theories you like. You\x92re clutching at straws.

If you had a massive brain fart and moved one to cutoff by mistake, you would realise instantly. These switches are distinctive in sound and feel. You would know what you had done before you had even released it in the cut off position.

Almost instantly you would get aural warnings and pages of EICAS Warnings and advisory\x92s. You\x92d get a massive clunk and momentary blanking of screens as power transferred.

if it was a mistake, you would instantly move it back to run\x85..you sure as hell wouldn\x92t double down and do the second engine.

it\x92s very sad, but I fear it is staring us in the face.

Both of them? 1-2 seconds apart? That\x92s extremely unlikely.

They don't have to be positively lifted if the detent rounded out due to wear or if the switch installed was one that the SAIB referenced with the locking feature disengaged. I can't discern a raised boss on the body of the left switch in Fig.13, page 10 of the AAIB report. I assume those switches have some internal over center locking mechanism also besides the spring on the lever that would not be needed if there was no detent. I do remember writing up a 767 switch a long time ago when the Captain noticed the detent was worn and he could shut down without lifting.

See here

It’s very easy to tell if the switch is wearing or defective. It’s also the norm for everyone operating the switches to give them a tug to insure they are in the detents. It’s simply inconceivable that both switches failed in exactly the same way at almost exactly the same time and no pilot who flew the aircraft in the last year or so noticed the issues. Add to that the CVR statement and it’s beyond inconceivable.
On the other hand pilots deciding to end their lives in a spectacular fashion is not inconceivable. In fact over the last 25 years it may be the single most numerous reason for a catastrophic loss of a transport category aircraft operated at a major airline.

Upthread there was quite some discussion about the preliminary report's use of the word transition in regard to the operation of the CUTOFF switches.

Several posters have quoted the SAIB recommendation to check these switches. I find it interesting that this Bulletin, an official document, uses the same word.

Perhaps the investigation team, who reference that Bulletin In their report, were influenced by it to use the same word to describe its physical action.

Quote:
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.