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I remember part of it had the inside of an airplane and you had to look around at all of the people on the airplane for clues. I think another part had a big mansion you were looking for clues in as well. Throughout the book you were searching for clues to solve the main mystery.
I'm pretty sure I got it at a Scholastic book fair around 1998.
This question may be too broad ... there are a lot of illustrated "solve the mystery" books for kids, and neither planes nor mansions are particularly uncommon settings. However, one very possible answer is the first three stories in the Usborne Puzzle Adventure series, published in the late 1980s. I owned a single-volume copy of all three of these stories, and I've found some images from them online which might jog your memory. It's worth noting that the mansion part and the plane part are two separate stories in the same volume, not part of one overarching mystery. But that's the sort of detail which is easy to misremember.
"Escape from Blood Castle" is set in a large mansion/castle, where the main character (a boy of perhaps 10-12) is visiting/rescuing his relative (uncle? cousin?) Boris, who owns the place, from an impostor who's imprisoned the real Boris in a cage. There's a lot of illustrations of detailed plans and booby traps which the main character has to pass through to find Boris.
"The Curse of the Lost Idol" is about an ancient Egyptian relic. I'm only mentioning it because it was in the same volume as the other two.
"Murder on the Midnight Plane" is set on an overnight flight to the Tsetse islands. Two siblings(twins?) Sam and Spike, are travelling with about a dozen other people, one of whom dies on the flight. The brother and sister solve the mystery together and identify three of the other passengers as responsible for his death.
