Elevators are (usually) very safe but complicated devices, so this post is geekier than usual; if you're not into mechanical stuff, feel free to surf elsewhere.
NYC's Department of Investigation (DOI) released a ten-page report on Monday with some pretty firm conclusions, based on inspections and simulations by the Dept. of Buildings and a private forensics firm.
Both the report and transmittal letter are available at the NYTimes City Room. The DOI says evidence supports a conclusion that a mechanic left a jumper wire in place on the controller panel of Elevator No. 9 during maintenance work earlier that morning.
The purpose of that service call, ironically, was to bring the elevators into code compliance by adjusting the hoist-motor governors so the elevators in this bank would move about 16% slower.
Supporting evidence included a pair of linked wires that investigators found under the grating of the elevator machine room, near No. 9's panel. It was about sixteen inches long and had the telltale signs of being used as a jumper.
The machine room was on the fourteenth floor; Elevator No. 9 was one of a low-rise bank of elevators that served floors one through twelve. The shafts for the low-rise bank stopped at the twelfth floor.
As I said in my post last month, a jumper wire -- a short cut between two terminals -- is a standard widget in the toolboxes of people who service machines controlled by circuit boards. It's legitimate to cut out safety interlocks and alarms ... but only while the unit is out of service.
Transel mechanics started arriving at 5:09 AM that day, with several jobs to do: replacing EPROM circuit cards in the fourteenth-floor machine room, and recalibrating speed governors mounted at the top of the shafts. The work required mechanics to step from the twelfth-floor lobby onto the roof of each car to recalibrate the speed governor. With one exception -- Elevator No. 9 -- they used a local control on the car to move the car a few feet so they could reach the governor.
Why would mechanics want to climb onto the cars' steel roofs, which can be slippery with oil and offer a tripping hazard? The reason is that the governor for each elevator was located on a headframe at the top of the elevator shaft; the easiest way for mechanics to reach the governor for a given elevator was to stand on the elevator car's roof, using the car like a work platform.
From Buildipedia, here's a photo showing elevator shafts in a tower under construction.
To get to a car's roof following standard procedure, the mechanics had to open two sets of doors, and to open the doors they had to bypass a safety device that normally prevents the doors from opening whenever the car is out of position.
The outer set of doors, visible from the lobby, are attached to the building, so they don't travel with the elevator. The second set of doors is the inner one -- the set you see from inside the elevator car when it's moving. This set of doors (obviously!) travels with the elevator. Special techniques are needed to open the inner door when the car is a few feet out of position, because the door interlocks try to prevent doors opening in this situation. Otherwise we citizens would be tumbling down shafts every other day.
(Why didn't they bring a ladder and climb up through an emergency hatch in the car roof, which every action-movie screenwriter knows about? That possibility isn't spelled out in the report, but my guess is that the emergency-egress openings could only be opened from the exterior. That's standard, to prevent misbehavior.)
Now to Elevator No. 9, which posed a frustrating problem the others hadn't. The mechanics on the twelfth floor couldn't get Elevator No. 9's inner door to open, and that meant they couldn't climb on the roof and use the local control to maneuver it manually into the desired position. According to the DOI report, they had expected to reach in the space between the door by hand, or failing that, had expected to use a "tomahawk tool." (A tomahawk tool, when slipped into the space between the inner and outer doors, can release a clutch that keeps the inner door from opening in such circumstances.)
For whatever reason, the mechanics couldn't get either arms or tomahawk tool through the narrow space available, so they radioed another Transel worker in the fourteenth-floor machine room to bypass the safety interlock from there, and to shift the car remotely.
According to the DOI report, the Transel employee in the machine room used a jumper wire to bypass the safety interlock, which was okay, but failed to remove it before the two mechanics on the twelfth floor, the ones working on Elevator No. 9, returned it to service. (And Transel did so without notifying the Department of Buildings, which had the authority to inspect the work before the car went back into service.)
Here's the reason such a jumper wire is so dangerous when a car goes back into service: the jumper wire in the machine room closed the safety circuit, so the car would respond automatically to summons by button-push, heedless of whether No. 9's doors were open or closed. Normally the car wouldn't have budged an inch until both sets of doors are secure, which closes the safety circuit.
While the mechanic denied leaving a jumper wire on Elevator No. 9's controller panel in the machine room, and investigators did not find one in place when they inspected the panel, the report said a jumper in place at the time of the fatality is the best explanation of why No. 9 acted with fatal results the first time anyone tried to ride it from the ground floor, minutes after the mechanics left the twelfth floor for a break.
Responding to a button push from the ground floor, Elevator No. 9 left the twelfth floor and arrived at the ground floor. The doors opened. Two people entered the car and pressed call buttons; nothing happened for a few seconds. Then as Suzanne Hart tried to enter, the car lurched upward, catching her in the opening, and didn't stop for twenty feet.
To wind up: Jumper wires left in place too long, cutting out alarms and safeties, have been a problem in many trades. The good news is that simple steps can greatly reduce the likelihood of leaving a jumper in place accidentally. Here's one: start the job with a count of jumper wires in the toolbox. Don't return the machine to service until all jumpers are accounted for.
That's like the precaution that well-run operating rooms take: the staff tallies sponges and small instruments before and after a procedure. That avoids rolling patients off to the recovery room with foreign objects inside.
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