We have to look at the FAR and all its supplements (DFARS and agency) to answer the question. The question references fixed price, but not FFP; FFP is one of the types under the fixed price family of contract types IAW FAR part 16. FAR 7.103(e) says the written acquisition plan is only required for other high-risk contracts other than FFP. If the contract type will be other than FFP under the fixed price contrac types, then look at the level of risk for the buy. DFARS 207.103(d) has further requirements for written acquisition plans, so that has to be looked at to see if the DFARS requirement applies to your acquisition. Agency procedures may also mandate when written acquision plans are required, so the agency supplement to the FAR and any applicable instructions will have to be reviewed as well.