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.

Finding the actual wind in flight is a good example of irrelevance. I've been flying for over 60 years and have never done it in flight, nor have I ever met anyone who has (perhaps if had met a full-time navigator from the 1930/40s I might have).

I certainly would not agree that failure to perform that calculation constitutes an aeronautical knowledge deficiency.

nor have I ever met anyone who has

There are various tales, no doubt some apocryphal, which address this topic.

My favourite harks back to three likely lads motoring across the Bight to Perth at 0-dark-30 on an AN 727 freighter (dear old ready-mix) ....

Flight Service disrupts the peace, quiet and gentle snoring, with a simple request "can you guys give me a spot wind, please ?".

The said three likely lads looked at each other in amazement (horror ?) and, with some incredulity, asked in unison, "what ?". After the inevitable initial confusion as to how they might go about this monumental task, each managed (miraculously ?) to locate his CR thing in the nav bag (or shirt pocket, as the case might be).

Amongst the (quite ancient) B727 technology, they managed to figure out some guesstimates for values such as TAS and HDG (they were fairly easy) and GS and TR (very approximate best guesses) and proceeded to work their devices. At the conclusion of that very tiring exercise, three answers (near enough with winds coming from all directions of the compass) led them to conclude that, quite clearly, the wind must be L&V

It was so reported back to Flight Service which, quite wisely, saw no point in requesting any more spot winds from that particular flight, leaving the three lads to return to their slumbers ....

(PS that story is fact)

Thanks John and Bob much appreciated mix of theory, reality, and humor here. I'm now just as confident of how to do it as I am I'll never need to.

For posterity's sake I'll clarify that on the TAS/ETAS topic the instructions for the CR also use ETAS (step 4 in the manual), though I'm not holding my breath for many more questions about the CR2 going forward.

I'll clarify that on the TAS/ETAS topic the instructions for the CR also use ETAS

So long as you keep in mind that the approximation where the manual says to use TAS if the drift is less than 10 degrees is just that and is, mathematically, wrong. As there is no benefit in the approximation, I suggest that you discard it and just use ETAS ALL the time, regardless of drift.

That will give you the correct answer, especially for the exams.

Good one John!