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Rob Logan wrote:
THE INSTRUMENT DISPLAY FAILED on that flight about one-third of
the way into the burn, but Melvill continued the trajectory by
referencing the horizon through the windows and was able to reach the
target apogee. The problem is believed to be the potentiometer that
adjusts display brightness - the calculations proceeded correctly but
the acceleration pulled the wiper off the contact. Once the motor
stopped, the display came back on. The pot has since been hard-wired.
This why some of us scream when we hear someone post things like "It is just a pot. It works great in my car, so I am going to put it in my plane. It has proven itself in automotive use so it is a proven design." Aaaaarrrrrgggghhhhhh!
This ties in well with the safety discussion. Sure, you can make changes to the design (you are the manufacturer) but can you be sure that a "minor" change will not have major consequences in ways that you did not anticipate? A stupid little pot for something as unnecessary as controlling the backlight intensity in a display that is used in the daytime only, brings down the backlight, which brings down the display which could have aborted the test or even brought down the aircraft. How many people could have thought through the exact sequence of events when they were installing that pot? Apparently none. And the caliber of people on that project is most probably a bit higher than the caliber on this list.
Remember the axiom: For the loss of the nail, the horseshoe was lost. For the loss of the shoe the horse was lost. For the loss of the horse the rider was lost. For the loss of the rider the battle was lost. For the loss of the battle, the war was lost. For the loss of the war, the kingdom was lost. Yes, I do mean to tell you that the whole kingdom was lost because someone said "It is just a stupid little nail....."
Hamid
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