The key to this incident in Ireland, just published by the AAIU, is the wording of a departure clearance to a corporate jet departing EIKY/Kerry – “Climb Flight Level Two Hundred”.
The crew report states: “Our altitude climb instruction was “climb level Two Hundred”. We read back the clearance and began the departure. As we began to climb we had some confusion as to what the altitude clearance limit was as we were unsure what level Two Hundred meant. We levelled at Two Thousand feet to ensure we didn’t exceed any altitude limits”
US operators are used to Flight Levels being that airspace above 18,000 ft, and in Ireland, as in much of Europe, Flight Levels can be as low as FL060 (6,000ft) – depending on the transition level – this international difference appears to have been the root cause of the confusion.
Standard ICAO phraseology dictates that “FL200” is Flight-Level-Two-Zero-Zero, but the UK in CAP413 suggest using “Flight-Level-Two-Hundred” to avoid confusion with FL210; this edict has been adopted de-facto many European ATC agencies.
The concern in this case, raised by ATC in Shannon, was that the aircraft levelled out at 2,000ft in the direction of high terrain – with, it appears from the report, real potential for CFIT.
The full report is here.