hey,

Just wondering if you can guide me in the right direction with this situation...i have the CR2 computer and not the one shown on page 4.46...im hoping this makes sense..

TAS = 180mph
T/C = 175degs
T/H = 160degs
G/S = 144mph

when finding the actual wind and velocity on track, my CR2 computer states that if the crab angle fairly high which occurs in the situation above for which it caused a e.g 47kt left x-wind. It states to use the effective true airspeed(ETAS) in this case it reduced the TAS to 174Mph
the difference is 30mph between the ETAS and GS....if you didnt use the ETAS, the difference is 36mph

The wind when calculated gives very different answers due to the H/W component experienced depending on the ETAS of 30 or the TAS of 36..

Is there a rule of thumb that you use to differentiate the two, for example greater than 30kts xwind convert the tas to etas??

its all good, ive figured it out now thanks anyway

Missed your post Madox, must have slipped in under the radar. I'm glad you got it sorted out

If you happen to get this, could you describe what you figured out? Got the exact same problem but I can't even make sense of the manual enough to work out a method 14 years later :/

Never mind I have also worked it out; www.manualslib.com/manual/3928215/Jeppes....html?page=22#manual manual page 41 & 42 (web page 22 & 23). Only thing I was uncertain about is which track, TMG or FPT was considered "true course", but our textbook explains this specifically at the bottom of page 119 (TMG).

If anyone else finds themselves in this very specific predicament let me know on this thread.

Is there a rule of thumb that you use to differentiate the two, for example greater than 30kts xwind convert the tas to etas??

It is VERY simple. If you choose to use the ETAS/TAS approximation (why you would do so is way beyond me) then you inject an absolutely avoidable error into your solution. The mathematically/geometrically correct approach is ALWAYS calculate ETAS and use it in determining GS. Considering that, when you set up the sine scale to figure X/W and drift angle, you AUTOMATICALLY set up the cosine scale to figure ETAS, why wouldn't you spend all of, say, 1-2 seconds to switch your gaze over to the cosine scale and read off ETAS ?

So, the best rule of thumb is ALWAYS calculate ETAS and use it to determine GS, regardless of drift angle.