Posts about: "Switch Guards" [Posts: 87 Pages: 5]

One thing that I remember from when I was a simulator TRI/TRE on a Boeing was that as an instructor you get very used to operating critical
switches rapidly without following any procedure, in order to set the sim up for a single engine landing etc. When I was then line flying next I had to guard against doing the same thing in the real aircraft.

Muscle memory is a strange and (usually) wonderous thing. It allows us as humans to perform amazing things without actually thinking about what we are doing. Professional Athletes have perfected this to a high art, but the rest of us do things using muscle memory on a regular basis. Back when I was still racing, I happened to look down at my hands on the steering wheel in fast, bumpy corner, and I was simply amazed at the large, rapid steering inputs that I was making to compensate for the bumps - with absolutely zero conscious thought. Muscle memory at its best.

However, it can also bite us. The Delta dual engine shutdown during takeoff from LA (referenced way back when in the 1st accident thread) was caused by muscle memory - the pilot reached down to set the EEC switches (located near the fuel On-Off switches) but muscle memory caused him to do something else - set both fuel switches to OFF. Fortunately, he quickly recognized his error, placing the switches back to RUN and the engines recovered in time to prevent a water landing (barely).

It is conceivable that a pilot - reaching down to the center console to adjust something unrelated - could have muscle memory cause him to turn the fuel off to both engines. While all new engines are tested for "Quick Windmill Relight" - i.e. the fuel switch is set to CUTOFF with the engine at high power - and the engine must recover and produce thrust withing a specified time (memory says 60 or 90 seconds) - it takes a finite amount of time for the engines to recover (spool down after a power cut at high power is incredibly fast - plus moving the switch to CUTOFF causes a FADEC reset, which means it won't do anything for ~ 1 second). Doing that at a couple hundred feet and the chance that an engine will recover and start producing thrust before ground impact is pretty much zero

Is option 2 possible? It was my idea too but does the switch "connect" when it is not properly in its guarded possition?

We are talking about the Engine 3 position stop/run/start switches here? SB stopping engine shut down if thrust lever are beyond flight idle? Blimey, seen a photo of the switches in question. Assume they are detented? Actually why are they not guarded?

If you look at the mechanism, and imagine the physical obstacle that makes the lift necessary not being there, it could conceivably be connecting, yes. It would then be like any other switch, really.

The switch state could be either on or off in that position. However, the CVR makes it pretty clear that the switches were moved deliberately to off. Just not why. Also, remember the report says one switch was moved then the other. Not together.

Actually, why are switches that are only really required on the ground or in an engine shut down event where they are? Embraer's have the switches out of the way on an overhead panel and as well as detent locked, have a protective guard, one has to physically lift. If the PF had to physically carry out the engine shut down on an Embraer, he would have to raise his arm. Would be obvious to the PnF.

They are guarded. To refresh everyone's memory here's a photo from the previous thread.

No there is not. Further investigation will likely see if the locking detents were in a decent state or not. TBQH, such a switch should be properly guarded. Like a Embraer is, like other types that one sees with switches one has to lift a guard to operate and is locked on with the guard in position. Boeing designers found out again in not making processes fool proof?

I raised this on one of the other threads, they are supposed to be isolated systems, and I am sure they are according to electromechanical tradition, but for both human factors and electromechanical reasons, should\x92nt they be off to each side of the cockpit to be considered isolated?

Like this? Actually, the middle switches should look like the port side ones IMHO. Properly guarded.

Might there have been something resting in front of the switches that moved backwards on rotation? A phone, small iPad, book, wallet?

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.

The Boeing 717 had a near identical Fuel Switch setup.

An Australian crew once had a Tech Log folder fall off top of the Instrument Shroud and land on a fuel switch which initiated a shutdown. This was at altitude so unlike this incident there was time to troubleshoot and recover the engine.

Australian registered B717s were then required to install a guard above the fuel switches to help prevent anything that falls on them from bumping them to the Off position.

Interestingly there was also a requirement to \x91jiggle\x92 the fuel switch after selecting it to ON after start to ensure it was seated correctly and wouldn\x92t accidentally flick to OFF due to it not being fully in the ON Position.

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?

I operated such guarded pull-move switches on several plane types uncounted times and not a single time in my entire pilot career i had a situation that such a switch "sits" on top of the locking gate and suddenly snaps by itself .
It is either positivly on or off .

In theory i could with much fantasy imagine dirt around the locking gate that keeps it pulled where you believe its latched but imaging that both switches were sitting on top of the gate and snapped back at the very moment of rotation is too much fantasy for me .

Further , even with a non working locking gate such a switch would never move by accleration forces of the plane alone , it needs positive manual input .

their position roughly behind the thrust levers does not offer any space to store there lets say your phone which slips down and moves them .

one more point : during the very moment of rotation there is normally not a single reason to change anything in the vicinity of the center console at all to explain why you even had your hands there .

Thank you! We are getting deep into fantasy land when people who've never touched one of these switches in their lives are convincing themselves of the nearly impossible probability that someone on the flight deck did not physically move the switches from RUN to CUTOFF one at a time. Who and why are the real questions.

Boeing has put the fuel control switches in more or less that same position starting with the 707 (it may go back even further). Over a billion flight cycles on 707, 727, 737, 747, 757, 767, 777, and 787 (not to mention various military installations such as the KC-135.

Toggle refers to the over-center mechanism inside the switch. It is independent of the method used to guard against inadvertent operation. You could have a switch with simple sliding contacts that also had a pull to operate guard but that would not be a toggle switch.

The whole point is that toggle switches do not fall between one position and the other whether they are guarded, not guarded, or have a defective guard mechanism. It takes force on the lever to swing the internal over-center toggling mechanism to the other position.

This is a preliminary report. It is quite detailed for a preliminary report.

Examining the before-takeoff checklists seems like it would be akin to examining the re-arrangement of the deckchairs before the titanic even hit the iceberg.

The engines were switched off. Unlike Embraer, B & A have no protections stopping you switching an engine off inadvertently. From everything in the report, everything operated exactly as designed. I am not certain of how long the relight window is without windmilling speed, but +- 10 seconds seems entirely reasonable.

The outstanding question that presumably requires much more in-depth investigation of the wreckage items and CVR audio is whether:

• the cutoff switches were operated deliberately (and by who)
• the cutoff switches were operated inadvertently (and by who)
• the cutoff switches were bumped (by what) and the guards failed or weren't installed
• some electrical failure perfectly mimicked both many-pole switches being operated, then being operated again (seems unlikely)