Flying in the high seas and the transition altitude
I was flying the other day VFR on a light airplane across the Gulf of Mexico (from Louisiana to SW Florida) started thinking about the issue of flight levels vs altitudes, and, in particular, where is the transition altitude located in the high seas. I have tried to research the answer but with no luck, so here is my question (and some thoughts):
- When in international waters (or "high seas", meaning >12 NM from shore), where is the transition altitude located? And the transition level?
My thoughts:
- Transition altitude is defined by the country (for example, 18,000 ft in the continental US), or may be defined by a regional agreement. It applies to both IFR and VFR traffic, and should not be discretionary (i.e. ATC assigned), as some VFR traffic may not have radio communications.
- I have found the NA and EUR regional supplements to say nothing about it, and no particular country controls the high seas for the purpose of aviation (as per ICAO Chicago Convention art 12), so it must be ICAO.
- An interesting question could arise here: if in the US and flying outside the 12 nm mark but inside the outer limit of the ADIZ do you follow ICAO or FAA rules - but I think the FAR (91.703) and the charts, as well as ICAO art 12, answer that favoring ICAO.
- Another interesting question is what happens if the controller says, while 100 nm from shore, "Tampa altimeter 30.11. Maintain 11,500", assuming ICAO transition altitude would be lower. Would it make a difference flying VFR and IFR?
- A partial answer to the question (applying to IFR traffic in uncontrolled airspace) may be found in ICAO doc 8168:
"4.1.5.1 For IFR flights outside controlled airspace, including flights operating below the lower limit of controlled airspace, the determination of the lowest usable flight level is the responsibility of the pilot-in-command. Current or forecast QNH and temperature values should be taken into account."
"4.1.5.2 It is possible that altimeter corrections below controlled airspace may accumulate to the point where the aircraft's position may impinge on a flight level or assigned altitude in controlled airspace. The pilot-in-command must then obtain clearance from the appropriate control agency."
Any comments and thoughts will be greatly appreciated.