The Investigation also notes that the Flight Crew’s transmissions featured several instances of single digits being read back in group form e.g. “Two Zero” read back as “Twenty”,“Three Zero” as “Thirty”, “Four Zero” as “Forty”, “Five Zero” as “Fifty”, “One Two Four decimal Seven” as “Twenty Four decimal Seven”. It therefore appears that, although it is not ICAO standard phraseology, the Flight Crew were comfortable working with grouped digits and should not have had a difficulty interpreting “Flight Level Two Hundred".
This is a classic example of why it is not a good idea to invent one's own RT shorthand, and why it is important to use the correct phraseology - otherwise it could lead to dangerous situations. Another one I hear sometimes is "Runway three" for example. Well, is that three zero, or zero three? It's kind of important.
Correct RT phraseology can be a real ball ache, especially in busy airspace, and it can be tedious to recite every digit. However, it is done for very good reasons - to avoid confusion.
We were once flying to Cancun, I think it was, and were cleared down to an altitude with a QNH of "nine nine two". We queried this several times, but the Mexican ATC guy kept saying "nine nine two". On hand over to the next sector, we queried again and were were told "
two nine nine two" (i.e. mmHg, not millibars). The first guy had been using his own RT shorthand, but it would have put us at the wrong altitude.