ye olde whizzwheels
I still carry not 1 but 2 in my flightbag, a Jeppesen CR3 and a Pooleys CRP5 ( i've never used them in 8 yrs of professional flying but they are there...) I seem to recall that for any given set of input conditions that they would often yield markedly different answers for things like TAS, Mach, drift etc. (thats why we had to use the CRP5 in the ATPL's) So, not wishing to start a bun fight, which one is correct? Why do they give different results? Surely the mathematical formulae these devices work to must be the same that ADC's use and they are always spot on. I did jot down the numbers in the cruise one day and ran them thru the whizzwheels and the Jeppesen was almost spot on compared to the ADC generated numbers. The Pooleys was way out (20 kts). However this was hardly a definitive survey as i only did it the once! I'm curious to know why something as straightforward as this results in little consistency between these devices.