We have always known that different versions of the nav computer produce different results.
There is also considerable scope for errors when using the nav computer. If we use 2 degrees per 1000 feet for the lapse rate we get -5 at 10000 feet. If we use 1.98 we get -4.8. Distinguishing between –5 degrees Celsius and –4.8 degrees Celsius in the airspeed window for example is clearly subject to errors. These types of error are what I meant when I said that the differences were probably due to the approximations used.
There is also the question of differences between individual examples of any given type of nav computer. At EPTA we had one of those large classroom versions of the CRP5 which rarely if ever produced the same results as any normal sized version.
When I do the 230 knots CAS at 10000 feet question on my rather old and tatty CRP5, I get 268 TAS and Mach 0.42. These figures are pretty close to those produced by the two Apps. But the first couple of times I misread 0.42 as 0.44, which I suspect is what you did. (Damned treacherous thing this age related brain fade!)
As you say, hopefully the “incorrect “ options are far enough apart to enable exam candidates to pick the “correct” option. I do think that going for answers to three decimal places is a bit over-optimistic on the part of the author of the question.
On the up side, researching this question did lead me to find those two rather entertaining Apps, which i hadn't seen before.
Last edited by keith williams; 8th Dec 2014 at 13:30.