Please can somebody explain to me how you put 8 into a transponder and how it is recognised by the ground station. I thought all transponding equipment worked using the octal code, therefore nothing higher than a 7 could be entered and recognised. Also the only way of knowing that you had a transponder failure would be when you are told so by a controller. Then you would be told to squawk 0000 or switch it off. If it only the mode c which is innacurate and independant switching of mode a and c is available then just the mode c can be switched off. I do not think that 7800 is recognised in any books as transponder failure.