Jessica Simpson and Ex Nick Lachey Had a Run-In on Flight to Hawaii

Jessica Simpson and Nick Lachey recently came face to face in the sky ... and TMZ has learned it was as awkward as you'd expect.

Sources with direct knowledge tell TMZ ... the former couple had an unexpected run-in on the same flight from Los Angeles to Kauai, Hawaii on March 29, where both were seated in first class. Despite being in close quarters, we're told the exes went out of their way to avoid each other the entire time.

One source on board the plane tells us Nick turned bright red when he realized Jessica was on board ... and from that moment on, there was zero interaction.

We're told Jessica made her intentions crystal clear ... even switching seats with someone in her group to make sure she wasn't anywhere near Nick.

Our sources say Jessica's estranged husband, Eric Johnson, was also on the flight, but sitting in coach while she stayed up front. We're also told Nick's wife, Vanessa Lachey, was on board as well.

Jessica and Nick famously started dating in 1999 and were married in 2002, with their relationship playing out on their hit reality show, "Newlyweds: Nick and Jessica." The two split after four years of marriage, finalizing their divorce in 2006.

Jessica later moved on with Johnson -- the two got engaged in 2010 and tied the knot in 2014. They share three children together.