Divided Loyalties - S7.E4

A pilot-friend of Johnno's, Keith Lorrigan, has a plane crash and dies. His wife Vikki finds some heart pills, and Johnno doesn't want anybody to know, they'd only think it was Keith's fault. Johnno tries to work out what happened and discovers Keith thought he was flying barrels of destilled water, when in fact it was sulphuric acid in them. The barrels leeked and caused the crash.