Originally Posted by
EDLB
O.k. So it is possible to at least stop the motion. However since the seat movement control seem to go over software, why did they not put in some safety mechanism for a stuck switch. Stuck switch is a typical scenario for any functional safety analysis. Continuous forward movement until the forward mechanical stop of the seat should be looked as a dangerous situation. Now with 20/20 hindsight.
And what if someone wanted to move the seat to the forward stop?
The clear problem isn't the simply the switch, but that there apparently was an added obstruction, the meal tray, that was trapped. Can't tilt down - pilots thighs are in the way. Can't go right or left because of surrounding structure. And can't go up if it's under the wheel and maybe a bit of belly fat or seat-belt buckle (no photos, but perhaps a pilot might clarify) . The seats don't move lightening fast, and it may be the pilot even started out having to suck in a bit while tucking under the wheel, which would have been fine but doesn't leave an exit strategy if a flight attendant then directly or indirectly pushes the go forward button.
Look at the typical analysis - the switch jams and the seat moves. If it moves forward it cannot reach the control column. The pilots legs are supposed to be on either side of the control column, so they cannot be forced against it. The edge of the seat should be ahead of the pilot's abdomen, unless the pilot is morbidly obese; I haven't see all pilots but I think I would have noticed, but that is covered in the human factors books for 5th and 95th percentile design guidance, so any body outside those limits would be unlikely to be considered. Sure, it's uncomfortable getting pushed up with the knees folded, but without cockpit dimensions and the human factors limits, I can't be sure it would be "unsafe."
One could suppose a second switch in series with the first that applies power and the rocker switch controls direction. Seems good, but then there are two mechanical places for the seat to fail to operate. Likewise is one switch can override the other and one fails then the seat is also unable to move.
Frankly, I'm not a fan of power seats, but then I've read on a number of cases where a mechanical lever gets stuck and when the pilot get to Vr, pulls back, nose comes up, and the seat comes loose and takes the pilot away from the controls,.
If it's the meal tray - the fix is really simple. Get new trays so that this cannot happen.
I also really, really, really want to see a photo of the switch that is being blamed. I worked with military equipment for 30 years and the rule was, if they could break it they would break it. We put equipment into HMMWVs, Bradleys, M113s, and Striker vehicles - people with big feet, big boots, swinging guns around. Did not have a problem with broken switches. Mostly because they had switch guards, similar to ones I see in cockpit photos. Does this rocker not have a switch guard?