Equal Time Points

[Chairman 8]

Here's a screen capture of my ND partway through Craig Read's tutorial.Table 3 of the tutorial and the surrounding text list the en-route alternates and the distances between them, and the radii of the circles you need to draw to work out the ETPs so you know which is closer at any given point. Fair enough, and it works when you're flying from one alt airport to the next, but what about when you're not ?The 190nm circle around CYZS is for the CYFB-CYZS ETP, the 120nm circles are for the CYZS-CYRT ETP, all exactly according to Table 3.Why would I want to know the ETP of a line between CYZS and CYRT, which is nowhere near where I'm flying. Wouldn't it be more useful to draw bigger circles so I could see where on my route is the ETP ? Like this, using 205nm circles ? How is it done in the real world ?ThanksGary

[downscc 1,327]

There's a saying I learned in the military, "Plan, Plan and Plan then improvise like **** when it happens." Aviation can be like that. When selecting a place to get on the ground, the most important consideration is why you need to in the first place. Is it crucial that you land as soon as possible or do you have time to select a location that would best serve your needs (medical help, equipment available to unload the pax, etc.).The point is, the nearest might not be the best. It depends entirely on the situation.

[Guest jumbojock]

Hi GaryYou're absolutely right. In the "real world" most guys I fly with who bother to draw ETP rings (I do) do it the way you've described, where the 2 equidistant rings cross the flight plan. Some do it so the range rings touch as in your first example,some just put in a blanket 250 miles (don't know why), some don't bother.Dan's comments about suitability are most valid, but for a giggle and as a purely academic exercise, consider the following:If you want to make it even more accurate (stay with me here) alter the range rings by a wind factor equating to (1/2 distance between diversions x Wind Component)/TAS. In your example, the TAS is 498, half the range is 120 nm and the wind component is about 24 kts between the two diversions. So you would alter your 120 nm range rings by 6 nm, making the CYZS ring larger (126 nm) and the CYRT ring smaller (114 nm). That way, you've accounted for the wind component between the 2 diversions. However, please bear in mind that this is only valid if the winds are constant between the diversions and for all flight levels, as you'll most likely be descending. However, worth noting when your nearest diversions are ZLLL and ZMUB.Apparently you can draw a sheep with the lines and rings but I haven't seen it done!David "jumbojock" Robertson

[Chairman 8]

I gotta give that a try LOLForgot about the wind, thanks for that :-)Gary