There are quite a few aircraft flying around, without Area Navigation/ FMS /FMGS.. .There are many reasons why there may be a problem with FMS/FMGS/IRS/GPS etc.. .The point is this, what ever navigation equipment you have, if you are in any doubt as to it's accuracy, ASK ATC. They have a multi-million pound radar system for your use. I am sure that they would wish you to ask for help at an early stage, it is simple Airmanship.. .As to the F50 crew, WELL DONE, it may have been flying the other way with 400 passengers in the back.