Taxable per diem is per diem that is included in Box 1 of a pilot or flight attendant's W-2 form.  When pilots or flight attendants fly a day trip (out-and-back) without a layover, the per diem paid for that trip should be taxable.  This does not have a big affect on a pilot or flight attendant's per diem deduction because the crewmember does not accrue M&IE expenses on trips without layovers.  In other words, the crewmember does not get to deduct meals and does not get reimbursed for the meals on day trips.  In that sense, taxable per diem is simply wages.

Non-taxable per diem is different.  Non-taxable per diem is a true reimbursement for a flight crewmember's M&IE expenses.  Non-taxable per diem is reported on the W-2 in Box 12, Code L.  It offsets a pilot or flight attendant's per diem calculation.  This occurs on IRS Form 2106.