When Process Deviations Become Dangerous: Lessons for Manufacturing and Product Development

By: Jon M Quigley

When Process Helps—and When It Can’t

We are a big fan of Aircraft Disasters on the Smithsonian Channel (at least that is where we watch it). This often appears in our written materials, including the occasional blog post. This post origins from the season 4 episode 7, “Catastrophe at O’Hare” as well as some quality consulting work we have done. We watch this show because it illustrates what it takes to identify the root cause of the problem, which is not as trivial as root cause analysis often is treated. We use the term “operator training,” for example. As with many of these events, it is not a single factor; rather, it is a combination of factors that led to the situation. These two things all but guaranteed we would write something about articulated processes, and our inability to routinely follow.

Not everything can be turned into a process. This is especially true when organizations are doing something genuinely new—developing novel products, exploring untested technologies, or entering unfamiliar markets. In these cases, flexibility and learning are essential. However, problems arise when organizations treat established, proven processes as optional. Process deviation risks emerge when people bypass known safeguards without understanding why those safeguards exist. In both manufacturing and product development, history shows that unexamined deviations can quietly accumulate until consequences become unavoidable.

Understanding Process Deviation Risks

Processes exist to reduce uncertainty, control variation, and protect against known failure modes. Deviations occur for many reasons—schedule pressure, cost reduction, overconfidence, or the belief that “this time is different.” We have manufacturing processes to achieve repeatability and detect when we deviate from the desired quality (for example, using control charts). Processes, when done well, increase first pass yield and reduce waste.

In manufacturing, processes are often mature and repeatable. In product development, methods may be more adaptive, but they still contain critical checkpoints designed to surface risk early. Process deviation risks increase when teams normalize shortcuts and fail to reassess the assumptions behind their actions.

The O’Hare Catastrophe — A Deadly Deviation from Process

One of the most cited examples of catastrophic process deviation risks occurred at Chicago O’Hare International Airport in 1979: the crash of American Airlines Flight 191.

What Happened

During maintenance of the DC-10 aircraft, American Airlines deviated from the manufacturer’s approved process for removing the engine and pylon assembly. This deviation from the prescribed process affected the engine mounting bracket, creating a stress point that would fatigue over time and result in the physical loss of an engine. This would not have been a problem, as the aircraft has 3 engines and can take off on the two remaining. However, the engine that powered the pilot’s information center (instrumentation) was from the engine that fell off. This meant the pilot could not readily discern what had happened and, therefore, how to respond adequately. This deviation damaged critical components. The damage went undetected. When Flight 191 took off, the compromised structure failed, resulting in the loss of the aircraft and all on board.

Input, supplier, process output, is a change even within the organization.

Why the Deviation Persisted

This deviation arose from a desire to shorten maintenance schedules (cost reduction via throughput improvement) without consulting the aircraft manufacturer, and it was not in compliance with the manufacturer’s specifications.

The shortcut “worked” previously.
Schedule pressure rewarded speed over compliance.
The risk introduced by the deviation was poorly understood.
Process ownership was unclear.

This tragedy illustrates how process deviation risks can remain hidden when organizations fail to challenge informal workarounds.

Manufacturing: When Deviations Undermine Quality and Safety

In our decades of product development and manufacturing experience, we have seen many times when the process is dumped to expedite. In practice, material substitution is a source of manufacturing deviation. In fact, this experience is recent; there was a material substitution for a specific product build sub-assembly. This assembly became an end product, but the sub-assembly did not fit into the housing as needed. When it came to assembling this interference was discovered.

We consult from time to time, and one of these, was a company that was moving a manufacturing line from one location to another. They wanted to know what they needed to do in advance of this movement to ensure a successful move and reduce risk. Besides logging all of the tools and equipment, including information on the maintenance, we noted spending time on the manufacturing line as it now runs, and capture that juxtaposed against the present work instructions. From experience, there is good probability that as documented and as executed do not match. This can look like process drift only if we are measuring specific associated attributes.

In manufacturing, deviations from process often begin as minor exceptions:

Skipping inspections
Using non-approved materials
Adjusting tolerances without authorization

These actions may appear harmless in isolation. Over time, however, they erode process integrity. The result is increased defect rates, safety incidents, recalls, and regulatory exposure. Manufacturing systems depend on disciplined execution; unmanaged deviations introduce silent failure paths.

Processes, procedures, and work instructions are connected.

Product Development: Balancing Innovation and Discipline

Product development requires experimentation, but that does not mean abandoning structure. In my career, I worked at a vehicle OEM, where we mapped the product development processes of the engineering group, from marketing idea input to product delivery through the manufacturing line. I recognized, from experience, that the prescribed process may not work

Effective development processes include:

Defined decision gates
Risk reviews
Design verification and validation
Change control

Problems arise when teams bypass these mechanisms in the name of speed or creativity. In product development, process deviation risks often surface late—during integration, launch, or customer use—when correction is most expensive.

The key distinction is intentional adaptation versus accidental erosion. Adaptive processes evolve consciously; degraded processes drift without oversight.

Deviation Management in Manufacturing and Product Development

Deviation management is the disciplined practice of identifying, evaluating, approving, and learning from departures from defined processes. Without formal deviation management, organizations rely on informal judgment, where process deviation risks are most likely to grow unnoticed. It is essential to acknowledge that there are times when we cannot use the documented process. We were the process manager at a large OEM for a time, and for the individual, specific process there are times when the inputs are not perfect. In this case, we will need to either wait to get the inputs perfect or try to move while missing these prerequisites.

Deviation Management in Manufacturing Processes

Manufacturing environments depend on stability, repeatability, and control. Effective deviation management ensures that necessary exceptions do not compromise safety, quality, or compliance. Ideally, this system is a searchable record of the events that have transpired regarding any given deviation.

Key elements include:

Formal deviation requests with documented rationale
Impact assessment on safety, quality, regulatory, and customer requirements
Time-bound approvals with clear ownership
Root cause analysis to prevent recurring deviations
Trend monitoring to identify systemic process weaknesses

In mature manufacturing systems, unmanaged deviations often indicate deeper issues such as unrealistic cycle times, inadequate training, or poorly designed processes. Treating deviations as learning opportunities—rather than administrative burdens—reduces long-term process deviation risks.

Deviation Management in Product Development Processes

Product development requires flexibility, but flexibility without structure leads to late-stage surprises. Deviation management in development focuses less on compliance and more on risk visibility.

Effective practices include:

Explicit deviation criteria distinguishing experimentation from uncontrolled change
Risk-based reviews at design and decision gates
Documentation of assumptions introduced by deviations
Revalidation triggers when deviations affect performance, safety, or reliability
Feedback loops to update future development processes

In product development, deviations are often justified as “temporary.” Without disciplined follow-up, these temporary decisions become permanent sources of technical debt. Proactive deviation management keeps innovation intentional while controlling process deviation risks.

A Shared Principle Across Both Domains

Whether in manufacturing or product development, the goal of deviation management is not to eliminate deviation—but to make it visible, deliberate, and reversible. Organizations that manage deviations well understand that every exception carries risk, and every unmanaged deviation is a decision made by default.

Managing Process Deviation Risks Without Killing Innovation

Organizations do not need more rigid processes—they need better awareness of why processes exist.

Practical Guidance

Identify which processes are safety-critical versus exploratory
Require justification and review for deviations
Capture and analyze deviations as leading risk indicators
Reinforce process purpose, not just compliance

Process discipline and innovation are not opposites. The real risk lies in failing to recognize when deviation crosses from learning into danger.

Deviations Are Signals, Not Shortcuts

The lesson from O’Hare and countless manufacturing and product failures is clear: deviation from process is never neutral. It is a signal that assumptions have changed—or been ignored. Organizations that understand and manage process deviation risks improve not only safety and quality, but also long-term performance and trust.