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ETI TO THE ETP

  • jukzizy
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jukzizy created the topic: ETI TO THE ETP

Small piston day VFR charter operation:

Find the ETI to the ETP from alpha, Given the following

Distance from alpha to Bravo- 320 nm
Track 295*M
Tas 140 kts
W/V 250*M/30kts
fuel flow 120 L/hr
Taxi 20L
Etas- 138kts
320×159/280=181.7nm

Time= D/s
= 181÷117kts
= 93 minutes
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Carello replied the topic: ETI TO THE ETP

When doing this type of problem always start with a diagram.



In nil wind conditions, the ETP (CP) will be half-way between A and B. If a wind is blowing the ETP will be displaced upwind from the track mid-point position. Use this to check if you get a sensible answer after the calculations are done.

Track Home = 115 M
TAS = 140 kts
W/V = 250 M/30
GShome = 160 kts

DETP = (Total Dist x GShome)/(2xTAS) = (320 x 160)/(2 x 140) = 182.8 = 183 nm

Note: 183 nm is more than half-way to B - this is to be expected as we are punching into a headwind on route to B.

Track Out = 295 M
TAS = 140 kt
W/V = 250 M/30
GSout = 117 kts

TETP = DETP/GS out = 183/117 = 1.56 hr = 94 min
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bobtait replied the topic: ETI TO THE ETP

If that is one of my questions, could you please let me know where you found it.
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  • jukzizy
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jukzizy replied the topic: ETI TO THE ETP

it is a question from casa exam one of my friend got last time he sat his exam
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  • John.Heddles
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John.Heddles replied the topic: ETI TO THE ETP

A minor comment, if I may.

Some of the processes we use when working the wind triangle depend, for their validity, on the wind speed's being a sensibly small fraction of the TAS. As that fraction increases, things can require a little more care and precision and, in the extreme, the solution doesn't always function well at all.

For this particular case, the wind is 30 kt, a decent proportion of the TAS. The 2 x TAS approximation for (G/S on + G/S home) is starting to unravel a little.

Suggest you rework the calculations to check out the difference. Just for interest ....

Engineering specialist in aircraft performance and weight control.
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Carello replied the topic: ETI TO THE ETP

Just out of curiosity, I reworked the ETP using the actual GSout and GShome

Using TAS
DETP = (Total Dist x GShome)/(2xTAS)
= (320 x 160)/(2 x 140) = 183 nm

Using the actual GS's
DETP = (Total Dist x GShome)/(GSout + GShome)
= (320 x 160)/(117 + 160) = 185 nm

Absolute Error = 185-183 = 2 nm
%Error = 2/185 = 1%

An interesting exercise!
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  • jukzizy
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jukzizy replied the topic: ETI TO THE ETP

Thank you very much..planning to sit the exam this thursday.
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John.Heddles replied the topic: ETI TO THE ETP

%Error = 2/185 = 1%

Hence my comment that the approximation was starting to unravel at that sort of drift angle.

Now run the sums for larger drift angles and you can find yourself rapidly up to errors of 10% or more and the implications on flight management become significant. Generally, this is not too much of a concern for high speed aircraft - the fast jet folks commonly tend not to worry much at all about wind and just point and go - but, at the other end of the spectrum (low speed GA) the effects are much more obvious.

Hence my comment that folks need to be a bit circumspect about blindly following any approximations unless one understands and is appropriate about the application of any limitations inherent in such approximations ...

Engineering specialist in aircraft performance and weight control.
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