The role of the appraiser and appraisal in Legal matters
The question of value tends to come about after a damage event or during repossession of the aircraft. Regardless of the reason, any client needs reliable information that can be supported by years of research and data and not "rules of thumb", "rough percentages" or something someone told the evaluator at some point in the past.
An ethical and professional aircraft appraiser is not a champion for any client's cause or position but will act more as an advocate for the aircraft itself. In some cases, this may support your client's position and in other cases it may not. Regardless, a professional attorney needs to understand what they will be up against before negotiations with the other party begin.
What should an attorney look for?
The aircraft appraisal industry is unregulated. Anyone with a published guide can call themselves an aircraft appraiser and offer their services to the general public. It is also possible for one or more people to form an "appraisal association" that is really little more than a name. It may be helpful to review the article - What You Need to Know Before Hiring an Aircraft Appraiser to better understand what attributes you should be looking for.