An example of an ethical appeal in a complaint letter would be a retired elementary school teacher expressing a concern about instructional practices in his child's third-grade classroom. An ethical appeal requires convincing the audience through the author's character and trustworthiness.
A retired elementary school teacher would have the professional expertise to raise questions about the instruction taking place in his child's classroom in a meaningful way. While parents who are not teachers might have legitimate questions as well, the teaching experience that this person has increases the level of character even more.
The word ethical comes from the Greek word "ethos," which translates to character. The way in which the writer of the letter uses tone and style to express meaning would express that character as well as his professional expertise as a teacher. The recipient is more likely to listen to instructional feedback from a professional colleague than to complaints about instruction from someone outside the educational profession. The writer would undermine his credibility if he used personal attacks in a letter to his child's teacher, as that would work against the aura of responsibility. Referring to such educational concepts as the phases of child development and strategies for teaching to children with different learning styles would bolster the writer's credibility.