Boeing is defending the integrity of the fuselages on two of its largest planes, which have come under criticism from a whistleblower who warns that panels on the outside of one of the planes could eventually break apart during flight.

Two Boeing engineering executives went into detail Monday to describe how panels are fitted together, particularly on the 787 Dreamliner. They suggested the 787’s carbon-composite skin is nearly impervious to metal fatigue that weakens conventional aluminum fuselages.

Their comments during a lengthy media briefing served as both a response to news reports last week about the whistleblower’s allegations and a preemptive strike before he testifies to a congressional panel on Wednesday.

The whistleblower, Boeing engineer Sam Salehpour, said excessive force was applied to fit panels together on the 787 assembly line, raising the risk of fatigue, or microscopic cracking in the material that could cause it break apart.