Speed Dist Time question

[flyingpom]

Thanks All.

Makes perfect sense now. The reason this came up relates to some rules of thumb I'm using namely:

Note: My aircraft has a 120kts TAS

• For every 1 min early/late in 20nm I have 10kts of HW/TW. This then becomes 2 mins early/late in 40 and 3 in 60 etc
• So for say 20kts of HW / Tail wind I will be 2 mins late in 20, 4 in 40 etc etc
• I use 10% of the HW/TW component as the mins early or late in 20

Now from your mathematical obervations, the Speed / Time relationship is not linear which I've just realised doesn't matter for how I am applying this technique:

Lets say we are flying a 60nm Leg and have a 30kts tail wind i.e 3 mins early in 20 = 150kts GS

Initial calc is:

• At 20miles = 7mins ( Math calculated ETI = 8 mins )
• At 40miles = 14mins ( Math calculated ETI = 16 mins )
• At 16miles = 21mins ( Math calculated ETI = 24 mins )

So our ETI's are slightly out from the actual mathematical calculation T=D/S etc

-Assuming we are actually doing 150kts GS we would arrive at our first check point @ 8 mins which means we are 1 min late, which means I just update my ETI's for each of my next check points

• 2 mins late @ 40
• 3 mins late @ 60

-Updating out ETI’s we now get

• At 20miles = 7mins
• At 40miles = 16mins
• At 60miles = 24mins

And we get an accurate ETI for our destination

I am using this a technique in diversions so I work purely in time i.e no need to calc ground speed. Plus i use some small % factor for < or > 20mn intervals