TDY is the acronym for a temporary duty assignment used in the airline industry.  It is not necessarily the same as a temporary assignment as defined by the IRS.  It is important for crewmembers to distinguish between the two.  A TDY as used in the airline industry has no hard rules about time while the IRS definition of a temporary assignment does.  If a crewmember on TDY wants to write off travel expenses associated with getting to their temporary base, they must ensure that the TDY assignment also meets the time requirements of a temporary assignment as defined in IRS Publication 463. 

To understand this, it is important to understand the concept of a tax home.  Generally a pilot or flight attendant's tax home is their domicile, but in the case of a temporary assignment, that might or might not be true.