When to Instruct an Expert Witness: A Practical Guide for Solicitors and In-House Counsel

Expert evidence is often the hinge upon which technical disputes turn. 

In matters involving machinery, plant and engineering systems, the tribunal rarely has the specialist knowledge needed to interpret the complicated data of telemetry, fracture surfaces, lubrication regimes or finite‑element outputs.

It is here that an expert can provide invaluable support, translating those specialist facts into a clear, impartial opinion that the court or arbitrator can use. 

In practice, the quality of that translation depends as much on when and how counsel instructs the expert as on the expert’s technical skill. 

Instruct too late, or with an imprecise remit, and you pay for avoidable work, leave crucial evidence to degrade, or create needless points of attack in cross‑examination, weakening the case in the process. 

Instruct early, and with discipline, and the expert can narrow issues, preserve evidence and produce a tightly reasoned opinion that materially improves settlement and trial strategy.

This article will act as a guide for those seeking to employ the services of an expert witness, covering everything from the initial consideration through to what to expect in the arbitration itself.

 

When should you consider instructing an expert?

Technical disputes do not all require an expert. Many commercial disagreements can be resolved on contractual interpretation or simple factual proof. 

But ask yourself, is the contested question essentially one of how or why a machine behaved as it did? If the answer is yes, it’s time for specialist input.

There are several scenarios in which solicitors and in‑house counsel should consider an expert. One is contested causation, when parties advance mutually incompatible explanations for the same event. 

Another is when the dispute turns on interpretation or application of specialised standards or codes, the kind of detailed normative material that sits beyond a lawyer’s routine knowledge. Be it fatigue life calculations, tribological tolerances, non‑destructive testing reports, or the interpretation of telemetry. 

A third, and urgent, situation is human harm. Where injury or fatality has occurred, early expert involvement helps to preserve perishable evidence, communicates immediate safety steps, and coordinates interactions with regulators.

Commercial exposure is also a critical factor. If losses could include large repair or replacement bills, extended business interruption, product recall costs, or reputational fallout, the value of a technically sound opinion rises sharply. 

Patent and design disputes are another common ground for technical experts. Detailed side‑by‑side comparisons of functioning components and claimed features require specialist assessment.

Finally, any time a party’s contemporaneous statement clashes with simple engineering plausibility, for example, where a claimed sequence of events contradicts measured telemetry, an expert can quickly test credibility.

Beyond the myriad scenarios that may require an expert witness, timing also matters, as early technical triage often repays its cost many times over. 

A short, focused inspection and a preliminary opinion can identify the most persuasive hypotheses, save unnecessary disclosure, and steer testing toward what will genuinely affect outcome. 

There are also practical red flags that should prompt immediate action. Perishable residues, components scheduled for scrapping, discrepancies in telemetry records or maintenance logs, and regulator involvement such as HSE notification all demand swift preservation and an expert’s eye. 

In short, if key items are at risk, or the technical issue is central to outcome, act early.

 

How to choose the right expert witness

Choosing an expert begins with matching the question to the specialist, not the job title. 

Mechanical failure, fatigue, and fracture mechanics call for different skills from corrosion, tribology, or control‑software debugging. A vibration specialist is no substitute for a materials' scientist in a case where microstructure and grain‑boundary segregation are at issue. 

Part of counsel’s early work is to diagnose the likely failure modes and then short‑list experts whose day‑to‑day work addresses those mechanisms.

Equally important is experience with the legal process. Producing a methodical, well‑documented report that will survive adversarial scrutiny requires more than technical excellence, it needs familiarity with the conventions of court or arbitration. 

Independence must be explicit with any potential expert witness. A prima facie competence argument will be undermined if an expert has undisclosed ties to one party, or a publication record that demonstrates firmly established positions on a contested technical debate.

Finally, practical capability should not be overlooked. Does the expert have access to accredited labs, NDT resources and trusted subcontractors? For many failures, a range of tests (such as metallography, chemical analysis, hardness testing, fractography) will be necessary. 

This is where an expert operating within a consultancy with in-house facilities can prove vital. Not only has their expertise already been rigorously tested, but if the issue spans multiple disciplines, they have the capability to coordinate tasks and maintain chain-of-custody records with their team.

 

What to consider when briefing an expert witness

When briefing an expert witness, a clear written instruction is the single most effective tool for controlling cost, focus, and quality. 

Start by outlining the chronology and litigation posture, while not burying the practical context. The heart of the instruction is the questions, which should be precise and limited in scope. 

Instructions such as “give an opinion on all matters relevant to the failure” are a recipe for billable scope creep and indefensible generality. Instead, frame specific tasks so the expert can assist identifying what forensic inspections should be made, what physical evidence must be retained and needs testing. 

If counsel wishes the expert to work on the basis of certain factual propositions, list them clearly, but require the expert to identify and test any which are material and unsupported. 

Supply, wherever possible, the documents likely to bear on the issue, be it maintenance and inspection logs, calibration records, telemetry, photographs, lab reports, and relevant correspondence. Additionally, a list of missing or disputed documents helps the expert prioritise investigatory steps.

It is advisable to adopt a staged approach. Begin with a rapid technical triage that sets priorities. The triage memo should combine annotated photos with a plausibility ranking of likely mechanisms, a list of urgent preservation steps and a targeted plan for testing. 

If the triage indicates that the issue is central and that further work is proportionate, proceed to a formal expert report. That report must be methodical, including a clear statement of scope, a description of methods, full disclosure of data and raw results, reasoned analysis, discussion of alternative hypotheses, and a conclusion stating the degree of certainty and limitations.

Drafting instructions well also means avoiding everyday drafting errors. Keep questions precise, avoid advocacy in remit, and insist that the expert list additional information that could materially alter their opinion. 

Requesting a short, plain‑English executive summary is beneficial, as it can support settlement discussions, alongside technical appendices for exchange.

 

What to expect from an expert witness report

The Crown Prosecution Service describes the duty of an expert witness as “to help the court to achieve the overriding objective by giving opinion which is objective and unbiased, in relation to matters within their expertise”.

Oftentimes this is achieved through the receipt of an expert report which is submitted for arbitration.

A credible expert report follows a predictable pattern because predictability is essential to scrutiny. It should open with scope and instructions, and list documents relied upon. 

The report should describe inspections and methods used, present raw data or appendices containing it, explain the analysis, and set out alternative explanations before reaching a conclusion. 

Language matters. An expert should frame conclusions probabilistically where appropriate, rather than using imprecise rhetoric. Terms such as “probable”, “possible” and “unlikely” are far preferable to unqualified assertions. Counsel tend to focus almost obsessively on the language used by a witness (factual or expert) in an effort to squeeze the maximum meaning from the precise word(s) employed. 

Expressions such as “extremely important”, “jump”, “surprising” or even “relatively” can be latched onto and subjected to microscopic cross-examination, when what may have been meant is “significant”, “increase”, “unexpected” or “reasonably”.  It is essential, therefore, to exercise caution in the selection of the language used to express a view, particularly where adverbs and adjectives are employed.

Where numerical estimates or modelling results are relied upon, statements of uncertainty or margin of error are necessary. Reports that obscure uncertainty invite cross‑examination and court scepticism.

A good report should counter the points the opposing counsel will attack with. Frequently it is the assumptions baked into models, the selection of test methods, the integrity of the chain of custody, or the decision to rely selectively on particular data. 

 

How experts deal with missing data

In incidents involving machinery, plant and engineering systems, missing or incomplete data is commonplace. 

Good experts will not pretend certainty where records are absent, instead they present conditional opinions and explain what additional information could alter their conclusions. 

Where data is missing, and that absence is materially prejudicial, consider preservation orders or urgent applications to secure evidence. Experts should state clearly when their opinions are provisional and what else would be necessary to firm them up.

 

Using expert evidence outside the trial

Expert evidence is not only for trial. 

A focused preliminary opinion can be an instrument of settlement, giving parties a realistic view of technical strength, and often concentrates the negotiation on quantum rather than on technical plausibility. 

Single joint experts and joint statements in mediation can save cost and defuse its adversarial tone, but they require careful thinking. A single joint expert may limit a party’s ability to test adverse hypotheses vigorously, while joint statements can clarify agreed facts and narrow contested issues. 

Choose the approach that best fits case dynamics and the appetite for adversarial testing.

 

Preparing an expert for a hearing

The trial is the accumulation of months of work.

Demonstratives must be trial‑ready. This includes annotated photographs, exploded diagrams and simplified telemetry plots to help judges and tribunals follow complex points. 

Have the expert produce written summaries that can be read into the record and rehearse testimony until the expert can explain technical points simply without losing necessary nuance. 

Ensure the expert can produce model files, raw data and calibration certificates on short notice; absence of these materials can be fatal in cross‑examination.

In cross‑examination, experts face pressure to concede alternative interpretations or to show that modelling choices materially affect conclusions. 

Prepare your expert by walking through the most damaging lines of attack, rehearsing plain‑English explanations for technical choices and ensuring familiarity with demonstratives such as cross‑sections, exploded views and annotated telemetry plots.

There are some red flags in reports that should prompt further investigation. Missing raw data, absent calibration certificates, vague modelling descriptions, reliance on unsupported simulations, and failure to consider obvious alternatives, are all grounds to seek clarification or challenge admissibility.

 

Neale Consulting Engineers: experts in mechanical engineering

Expert evidence in engineering and plant disputes is as much a procedural and strategic discipline as it is a technical one. The best results come from aligning counsel’s litigation strategy with the expert’s technical method. This is something that Neale Consulting Engineers excels on.

As part of Brookes Bell, Neale Consulting Engineers has a large, multidisciplinary team of industry experts to rely upon to provide clear, accurate, and technically-sound reporting.  Our consultants are market leaders in expert witness work with experience of giving testimony on an international basis.  Regardless of jurisdiction, as a matter of best practice, we adhere to the Civil Procedure Rules, Part 35, Guidance to Experts. 

If you have a matter where causation, testing, or preservation is critical, a short preliminary technical triage is usually the most cost‑effective next step. Contact our team today to learn more about how we can support you.

Contact Neale Consulting Engineers today

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