---
title: "Mechanical Engineering Expert Was Not Allowed to Opine on the Defective Condition of the Lawnmower's Design"
meta:
  "og:description": "The mechanical engineering expert was not allowed to testify about the defects in lawnmower design, manufacture, or warnings"
  "og:title": "Mechanical Engineering Expert Was Not Allowed to Opine on the Defective Condition of the Lawnmower's Design"
  author: "Shuva Guha Thakurta"
  description: "The mechanical engineering expert was not allowed to testify about the defects in lawnmower design, manufacture, or warnings"
---

# Mechanical Engineering Expert Was Not Allowed to Opine on the Defective Condition of the Lawnmower's Design

Posted on August 20, 2026 by Shuva Guha Thakurta

This case arises out of damage to Plaintiffs' real and personal property after a riding lawnmower in their garage ignited in flames. The subject product at issue is a Cub Cadet LTX1040 riding lawnmower, manufactured by Defendant MTD Products Inc. ("MTD") some time between November 2009 and April 2011.

Plaintiffs sought to present the testimony of [Michael Zazula](https://expertwitnessprofiler.com/expert-witness/Michael-Zazula/1521378) as evidence about the fire's causation and defects in design, manufacture, and warnings. Defendants asked the Court to deem his testimony inadmissible because he is not qualified as an expert and his opinions are unreliable.

![Mechanical Engineering Expert Was Not Allowed to Opine on the Defective Condition of the Lawnmower's Design](https://cdn.jurimatic.com/images/blog_pic_640X480_2026_08_20T170010_310.webp)

## **Mechanical Engineering Expert Witness**

[Michael J. Zazula](https://expertwitnessprofiler.com/expert-witness/Michael-Zazula/1521378) specializes in applicable codes and standards for mechanical system components and the installation and maintenance of fire suppression systems, gas storage and delivery systems, boilers, HVAC equipment, and other fuel fired appliances.

With over 20 years of forensic experience, he has conducted over 2,000 investigations.

[**Want to know more about the challenges Michael Zazula has faced? Get the full details with our Challenge Study report**](https://expertwitnessprofiler.com/order/add?eId=1521378&pId=3).

## **Discussion by the Court**

### **i. Summary of Zazula's Opinion**

Plaintiffs relied on Zazula's expert testimony to show that "a gap that developed over time between the muffler and the exhaust deflector on the lawn tractor caused hot gases to be misdirected, toward the inside of the machine instead of outside, causing combustion and the subject fire."

Zazula's report stated that following the incident with Yuan's lawnmower, he "was requested to participate in a scene and evidence inspection/examination of the subject tractor to determine the cause of the tractor fire" and "to determine if a malfunction or defect in the tractor caused or contributed to this fire."

In detailing the subject lawnmower that he examined following the fire, he noted that its muffler was missing the deflector extender, and without the deflector extender, "the exhaust gases do not properly exit away from the muffler/muffler guard." He also found an "outward deformation/bulging of the muffler," submitting that "the deformation of the deflector from the muffler and the lack of the deflector extender . . . impacts the proper exhaust flow through the deflector and allows hot exhaust gases to escape and impinge on components/debris within the engine area."

Zazula performed testing on an exemplar Cub Cadet lawnmower of a different model, with the difference between the models being the horsepower rating and cutting decks. He ran the engine on the lawnmower over a ten-minute period, recording data in ten-second intervals from five probes attached to the tractor engine and exhaust stream. He recorded temperatures that he concluded were "high enough/in excess of-to ignite leaking oil from the valve cover gasket, which was observed on the exemplar tractor muffler and grass/debris in/around the exhaust componentry."

He noted that he had "inspected or been involved with inspecting in excess of 50 MTD LTX/LT tractor fires throughout 20 years of forensic investigations" and that "a common and consistent occurrence/observation is the . . . muffler outward deformation and separation or gap between the muffler and deflector."

Zazula ultimately concluded that the subject lawnmower's muffler was deformed due to backfiring, which caused the deflector shield to deform away from the muffler, that the gap formed between the muffler and the deflector allowed hot exhaust gases to impinge upon combustible material including plastic, grass, oil, and other materials, that temperatures of the exhaust gases and exhaust componentry exceeded the ignition temperature of debris, grass, and oil, and that hot exhaust gases and excessive heat impinged upon the plastic engine shroud, valve cover, valve cover gasket, and any accumulated oil, grass, and debris within the area of the muffler, causing the fire. He opined that the fire would not have occurred if the deflector had not deformed away from the front of the muffler or if the deflector extender were present, as the hot exhaust gases would have been directed out and away from the engine and ancillary components.

At his deposition, Zazula testified that he had never designed an engine that was mass produced. When confronted with the question of whether he had an opinion about an alternative design for the engine of the subject lawnmower, he responded that "the design would be for the muffler not to deform," although he did not design such an alternative engine. He testified that the subject lawnmower's muffler was not deformed at the time it was manufactured.

He did not subject a lawnmower with clogged cooling fins to testing, nor did he consider potentially clogged cooling fins in his opinion. He testified that he had tested a machine where there was no gap between the muffler and the deflector approximately ten years ago, although he did not discuss the results of that test.

### **ii. Zazula's Opinion on Fire Causation**

Defendants argued that Zazula's opinion on fire causation must be excluded because it is unreliable, first on the grounds that he "selectively ignored evidence and eschewed a first fuel analysis," and second, because he "failed to follow the scientific method and test his hypothesis."

As to the "first fuel analysis," Defendants submit that Zazula "undertook no analysis of first fuels that may have started the ignition chain" and failed to consider that combustible materials in front of or below the lawnmower may have been the first fuel.

Zazula's report as to ignition source relies on Yuan's deposition testimony that when he saw the fire, it was in the area of the lawnmower's engine, and the fire origin expert's report, which stated in relevant part that "the only competent ignition source found in the origin area was the Cub Cadet riding mower." He proceeds to conclude that "as the deflector/muffler was found with the gaps, hot exhaust gases and excessive heat impinged upon the plastic engine shroud, valve cover, valve cover gasket and any accumulated oil, grass and debris within the area of the muffler, resulting/causing the fire."

It cannot be said that Zazula's conclusion as to the source of ignition was anything beyond mere speculation. Here, Zazula does not specifically address other potential first fuels in his report, such as potential combustibles in front of or below the lawnmower. Although an expert is not required to definitively rule out all other possible causes of a fire, they should still "provide some explanation for ruling out plausible alternative causes as the sole cause of the incident

In this Court's opinion, Zazula has not excluded other first fuels reliably, as he only refers to another individual's opinion that the lawnmower was not parked up against something combustible. This does not address potential combustibles underneath the mower, nor did Zazula use his own methodology to reach such a conclusion, making his opinion as to the ignition source unreliable.

However, even if Zazula's first fuel analysis was considered reliable, the Court would still exclude his testimony as to fire causation based on Defendants' second raised grounds for exclusion: failure to test his hypothesis. Defendants argued that Zazula's methodology failed to follow the scientific method as outlined in NFPA 921 because he did not test his hypothesis.

The Court agreed with Defendants that Zazula's testing cannot be said to prove his theory that the gap in the cannister and/or the missing port extension was the cause of the increased temperatures which ignited any combustible materials.

Defendants emphasized that Zazula's method does "consist of a testable hypothesis," which he failed to actually test, and that NFPA 921 provides a set of "standards and controls for fire investigations," which Zazula failed to adhere to. But Plaintiffs in no way refute Defendants' arguments that Zazula's opinion as to fire causation is unreliable, rather just nakedly asserting that Zazula's test was "consistent with the scientific method outlined in NFPA 921." Plaintiffs in no way show that Zazula's conclusions are anything beyond mere speculation, so his opinion as to fire causation is inadmissible.

### **iii. Zazula's Opinion on Defective Condition**

Defendants next argued that Zazula's opinion on the defective condition of the subject lawnmower must be excluded because he is not qualified and his opinion is not reliable.

Defendants contended that Zazula is not qualified to opine as to any defect in the lawnmower's design, manufacture, or warnings because he "lacks the specialized expertise that would be needed to aid the jury in understanding evidence on the separate issues of lawnmower design, engine design, materials science, production processes, product quality control, human factors, ergonomics, and product warning psychology."

Zazula specifically testified that he was not a warnings expert, so the Court will not further consider his qualifications in that field. He additionally testified that he had never designed a mass-produced engine, he had not designed a muffler for the type of lawnmower at issue in this case, and he had never designed a lawnmower or engine exhaust system for a lawnmower.

Even if Zazula were qualified to opine on design or manufacturing defect, his opinion is not reliable. As to any defect in design, Zazula testified that a feasible alternative design would be one in which the muffler did not deform. He did not provide any details as to what such a design would entail, nor did he provide any other evidence of a design defect beyond the fact that the muffler was apparently capable of deforming. And as to defect in manufacture, Zazula was unable to identify anything in the lawnmower's assembly that caused the muffler to deform. As Defendants point out, Zazula merely offers the "observation of the altered operating condition of a 12-year old lawnmower that, in his view, might hasten the occurrence of a fire in the engine area," which "is not a design defect," but merely "the symptom of a hypothetical design defect." Without any supporting methodology or scientific procedure to support his opinion on defect, his opinion cannot be considered anything beyond subjective belief. Therefore, Zazula's opinion as to the defective condition of the lawnmower's design, manufacture, or warnings is not admissible.

## **Held**

The Court granted****Defendants' motion to exclude the testimony of Michael Zazula.

## **Key Takeaway**

The basis of specialized knowledge can be practical experience as well as academic training and credentials, and at a minimum, a proffered expert witness must possess skill or knowledge greater than the average layman. But Plaintiffs provide no specifics as to why Zazula is qualified to testify as to defects in lawnmower design, manufacture, or warnings.

## **Case Details:**

| Case Caption: | Xiang Yuan V. Stanley Black & Decker, Inc. |
| --- | --- |
| Docket Number: | 2:24cv2363 |
| Court Name: | United States District Court for the Eastern District of Pennsylvania |
| Order Date: | August 17, 2026 |

---

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