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## Time Intervals Between points

• Posts: 6

### Bill17 created the topic: Time Intervals Between points

29. Given: ATD 2200 DEP TO DEST is a straight-line track
Planned GS 117kt
TI- DEP to first feature/ proportional marker 23 min
TI- first to second feature/ proportional marker 20min
TI- second feature/ proportional marker to DEST 18min
A pinpoint is obtained at 2223 at the first marker and another pinpoint is obtained at 2243, 4 miles short of the second marker. The ETA at DEST

a) Is within ± 2min and need not be revised
b) Is revised to 2303
c) Cannot be revised without knowing the distances between pinpoints
d) Is revised to 2305

Had a bit of trouble with this question>

• John.Heddles
• Offline
• ATPL/consulting aero engineer
• Posts: 500

### John.Heddles replied the topic: Time Intervals Between points

OK, you can do this with your calculator/prayer wheel, whatever, or you can just do some rough, back of a fag packet mental calculations. Always be on the lookout for nice easy peasy round number problems which just cry out for a quick and dirty mental calculation solution.

(a) we are looking at a single leg. First assumption is to see if we can run the calculations with a constant wind.

(b) G/S = 117 kts. That's just too close to 120 kts to be coincidence (2 nm/min).

(c) planned interval to the first fix is made good. G/S matches plan.

(d) time expected at the next fix comes up 4 nm short of the fix so the wind has changed and G/S has reduced. Therefore, the time to get to the second fix will be a bit more than 2 min, ie we will be at least 2 min late at the second fix.

(e) distance to the destination is a bit less that from the first to second fix so, presuming that the wind change holds for the last calculation, we should be about 2+ mins (for the lost time from the first to the second fix) + 2+ mins (for the second fix to the destination) which is going to be, say, around 5 minutes late at the destination (give or take a bit).

The answer is going to be (d).

Engineering specialist in aircraft performance and weight control.