A private pilot recently asked me about whether he could be reimbursed by his employer for fuel he uses in flying his personal aircraft in pursuit of his employer’s business. (Apparently the pilot’s territory included a number of states in the upper midwest). Unfortunately, he would not be able to receive reimbursement for his fuel expenses since the FAA would view that as compensation paid to him in violation of FAR 61.113. However, as long as his flying is only incidental to his business, and he does not carry passengers or property, an exception to the regulation would allow him to be paid his regular salary while flying his personal aircraft on company business.
Figuring out what is “incidental” can be difficult and will always depend upon the particular facts of each case. If you are in doubt, contact an aviation attorney for help!