"Push-button switch in a receptacle behind a locked cover on the side of the escalator. Here. Circuit runs to the servo motor here. It retracts the pin, releasing the access panel."
"So," Rhyme mused. "What could have caused it to pop? Ideas? Come on, think."
Archer: "The latching bracket broke off."
But an examination of Sachs's pictures seemed to show that it was still attached to the bottom of the access panel.
"Maybe the pin snapped," Rhyme said. "The de Havilland Comet. Nineteen fifties."
Both Archer and Cooper looked his way.
He explained: "First commercial jetliner. Three of them exploded in midair because of metal fatigue--a window failed at high altitude. Fatigue is one of the main mechanical failure modes. Other modes are buckling, corrosion, fouling, fracture, impact, stress, thermal shock, a few more. Fatigue occurs when a material--could be metal or anything else--is subject to cyclical loading."
"The jetliner," Archer offered. "Pressurization over and over."
"That's what happened. Right. In that case, there were square windows and doors; the stress was concentrated in the corners. The redesigned planes had round ports and windows. Less stress and fatigue. So the question here is, did the opening and closing of the access panel on the escalator lead to fatigue on the part of the latching pin?"
Cooper highlighted the latch. "No signs of wear on this one but it's new. I wonder how old the original was, how many times the door had been opened and closed."
Rhyme felt once more the frustration at not having the actual evidence before him.
He heard a sound of a jostled table as Juliette Archer maneuvered closer to him, clumsily manipulating the chair's controller with her right finger; handling a two-hundred-pound wheelchair deftly took considerable practice.
New to the game...
"The one that failed, in the mall, was six years old," she said.
"How did you find that out?"
"Press releases from Midwest Conveyance, announcing they'd been awarded the contract for the escalators at the mall. Seven years ago. Construction occurred the next year. According to maintenance recommendations, the unit should be inspected and lubricated five times a year. Allowing for breakdowns and unplanned repairs, I'd say the door was opened and closed fifty times."
Rhyme looked at Cooper's picture of the pin holding the triangular bracket that kept the panel shut. It was only about an inch long but thick. It seemed unlikely that the pin would fatigue with that limited number of openings.
Archer added, "And one of the maintenance steps is to examine the pin for excessive wear. Presumably for fatigue too."
"What's it made of? Steel?"
Archer said, "That's right. All the escalator parts are steel, except for a few housings that had nothing to do with the accidents. And the exterior pieces. They're aluminum and carbon fiber."
She certainly had attacked the manual and specification sheet quickly.
Rhyme said, "Even if it was in good shape, the latch might have come loose and the pin might not have fully reseated itself. Vibrations could have worked it loose."
Maybe... Lot of speculation in this case.
"Who made the locking mecha
nism?"
Without looking at the documents she'd loaded onto her screen she said, "The manufacturer. Midwest Conveyance. Wasn't a separate company."
Rhyme said, "Possibly metal fatigue, possibly maintenance issues. What else might've caused it to open?"
"Could somebody," Archer asked, "have hit the switch accidentally or as a prank?"
Cooper called up some pictures. "Here's the switch. It's on the outside of the unit, on the bottom, near the emergency cutoff." He pointed. "But it's behind a small locked door."
Rhyme said, "Amelia checked on that. She looked over the CCTV at the mall. She said nobody was near the access switch when the panel opened."