Ladder expert Dr. Morse has investigated over 475 ladder and climbing equipment accidents involving stepladders, extension ladders, articulated ladders, fixed ladders, scaffold ladders, telescoping, combination, orchard, manhole and window washing ladders, stepstools, folding attic stairways, rolling ladders, fixed stairways and scaffolding. He has testified many times as a ladder expert witness, both in deposition and in court. Dr. Morse has investigated ladder accidents involving ladders made by Werner, Louisville, Keller, Davidson, Cuprum, Krause, TriCam, Versaladder, Telesteps, Cosco (Dorel), Michigan, Lynn, Century and Memphis Folding Stairs, among others.
He has performed testing and evaluation of ladders and ladder accessories.
Dr. Morse has conducted research into ladder safety, including extension ladder slide-out, false lock and human factors, including warnings.
Dr. Morse has co-authored three papers on ladder safety; one on extension ladder slide-out, one on tiplock false lock of extension ladders and one on flipperlock false lock. This places him among a select group of ladder experts who have published papers on ladder safety. 
-Morse, J. S. and D. M. Paul, "Mathematical Analysis of Extension Ladder Flipperlock", Paper number IMECE2008-66064, Proceedings of IMECE2008: International Mechanical Engineering Conference and RD&D Exposition, October 31 - November 6, 2008, Boston, MA (paper presented by J. S. Morse).
-Morse, J. S. and C. R. Rasnic, “Analysis of Tiplock Phenomenon in Extension Ladders”, American Society of Mechanical Engineers, Paper number IMECE2004-59631, Proceedings of IMECE2004: International Mechanical Engineering Conference and RD&D Exposition, November 14 – 19, 2004, Anaheim, CA.
-Morse, J. S., L D Ryan, and E. Henrickson, “Minimizing Extension Ladder Slide-out Accidents”, 1999 ASAE/CSAE Annual International Meeting, Toronto, Ontario, Canada, July 1999, paper 997042 (paper presented by J. S. Morse).
The Flylock Mechanism
A rung lock or flylock is used to hold an extension ladder in the extended position. The flylocks are usually attached to the fly or upper ladder section and are engaged by a particular combination of raising and lowering this section. When extending the ladder, the flylocks will slide past each rung on the stationary base section. When the appropriate height is reached, the fly section is raised so the flylock tip is slightly above the desired base section rung. The flylock pivots forward over the base rung. By then lowering the fly section, the flylock engages the base rung and locks the fly in place (Figure 1). 
To disengage the flylock, the fly is raised high enough for the tongue or flipper to clear the base rung. The flipper closes off the slot when pushed up by a base rung. With the tongue covering the slot and preventing rung engagement, the fly can telescope all the way down.

The False Lock Phenomenon
False lock can occur if the flylocks do not fully engage the base rung when the fly is supposedly locked. There are several types of false lock. The two most common are tiplock and flipperlock.

Tiplock occurs when the tip of the flylock hangs up on the top surface of the base rung. As the fly section is being raised, the flylock tip catches the top of the rung, holding the fly in position (Figure 2). 

Flipperlock is when the tongue hangs up on the leading edge of the base rung. This can happen if the fly is raised and then lowered enough to rotate the flipper over the slot. The tongue slides on the base rung, and can generate enough friction force to hold the fly in place (Figure 3). 

Camlock occurs when the base rung catches the flylock below the flipper. Typically this will occur at the junction of the flipper and flylock. 

Other False Lock Modes
Other false lock modes exist, but do not have names in common use. One type arises when a flipper becomes broken or has the end chipped. A form of tiplock occurs, with the flylock tip ending up further back on the rung top than in normal tiplock. Another type of false lock involves the flipper becoming trapped between a pair of base and fly rungs. Still another form involves the flipper caught on top of a rung and the flylock prevented from rotating due to its design. 

John S. Morse, PE  18 Pullman Street. Brocton, NY 14716  
479-233-9785  email: jmorse-pe at
Angle-Rite ladder level
Extension Ladder Failure - False Lock
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Figure 1.  Flylock in Fully Locked Position.
fully locked flylock
tiplock false lock
Figure 2.  Tiplock False Lock.
flipperlock false lock
Figure 3.  Flipperlock False Lock.