You can be anywhere in the world and want to be something (PPRuNe's PPT section is a good start). A distinction under 14 CFR 61 exists between instructors and
authorized instructors - to the best of my knowledge there is no rule under Part 61 requiring a person to be in possession of a US flight instructor certificate to give instruction in flying to any person flying for any purpose other than becoming qualified for the issuance or variation of a US pilot certificate, rating or qualification. Viz, instruction given cannot be credited (save perhaps by some special arrangement with the authority) towards requirements for training made under the respective sections of Part 61.
Subject to my interpretation of
14 CFR 61.3(d) the giving of instruction in flying by JAR-FCL FI holders (who hold no US flight instructor certificate) on board civil aircraft of US-registry within US airspace, for the purpose of the instructed person meeting training requirements under JAR-FCL, is not absolutely prohibited under 14 CFR 61.
Special requirements are established under Appendix 1a to JAR-FCL 1.055 (or 2.055 if applicable) in respect of such arrangements. In the case of JAR-FCL licences granted by the UK CAA refer to
Standards Document 39.
A JAR-FCL professional licence holder can, without prejudice to national security and other related rules, undertake
training as required (i.e. voluntary or mandatory training as the case may be) to meet with the aeronautical experience requirements established under Part 61 for a US commercial pilot certificate and a US instrument rating. Possession of both a US CPC and a US unrestricted IR form part of the eligibility requirements under
Sec. 61.183 for a flight instructor certificate.
Be careful with the
Instrument Foreign Pilot test as a means of including in a Sec.
61.75 certificate (foreign-based FAA private) privileges to operate under IFR / in IMC. This process does not replace the eligibility criterion requiring candidates to hold an
unrestricted IR for an FI certificate. A
restricted IR included in a Sec. 61.75 certificate cannot be transferred to a US commercial.
The optimum solution, from a time and expenditure standpoint, to obtaining a Part 61 US CPC/IR requires judicious/attentive inspection of the governing regulations. As mentioned many times on this forum the usual 'gotya' is in the definition of
cross-country flying experience, among others. Other subtle points work in your favour - Sec.
61.39(c) for example.
If a US flight instructor certificate is obtained with the express intention of carrying on flight instruction outside of the United States (employment ineligibility, etc) then careful consideration must be paid to regulations in force in the territory in whose airspace the training is given. In the UK for example Article 36 of the ANO 2005 (as amdt) [
c.f. CAP393] is relevant (but would not prohibit flights for Sec. 61.56 purposes, etc).