Michael Birbeck wrote
Question is how much alcohol is just enough before it all goes pear shaped . Very little.
Actually, and I speak with some authority here as a former QA tester and medical subject for breath screening devices (retd.), you'd be surprised.
In the units I am familiar with - % blood alcohol - The figures are typically, 0.05% for a basic infringement and 0.08% for a serious infringement.
For most people, 0.05% seems 'O.K.' and you think you are still able to drive, while with 0.08% you know you are definitely not safe to drive.
I guess here, but perhaps 2-3 pints will get you to 0.05% BAC. A lot depends on your body mass, fat composition, strength of beer, and time to ingest.
Studies have shown that for BAC of 0.05% you are as likely to have an accident as if you have drunk nothing at all. More surprising a BAC somewhere between 0.00% and 0.05% has
less probability of an accident than driving tee-total. After 0.05% it all goes pear shaped.
Now these figures are based on accidents using motor vehicles, and probably measure motor skills and reflexes. They do not address cogitative abilities for higher functions - route planning etc.
Whether you can extrapolate motor vehicle BAC to aircraft control, flight planning, decision making is totally another matter.