Off-route Crit Points

[Dunhovrin]

I'm sure we're all happy with calculating the Crit Point on a journey from A to B. But what if there is an off-route airfield 'C' abeam the route? Does anyone know how to calculate a crit point along route AB for AC and then CB? Assuming one has all the required winds. At the moment I just use an iterative primary digital method (ie thumb along the map every 10 minutes until C slips past) but there must an elegant solution.

[Check-in]

This is a crude method, assuming that you do not have FMS. Draw equal circles centred on A, B and C, of suitable radii eg each circle could equal half an hour or one hour or whatever was appropriate to your speed (be it all-engines or engine-out). Where they intersect your track should enable you to eyeball equi-time points. Of course, if you want to really finesse it, the radii will have to represent ground speed achievable TO either A, B or C and of course that is a problem with an off track situation. Better to use still air radii and apply common sense with the wind component.
Crude, but accurate enough, given that anytime you have an emergency, you can quickly look at where you are and then point the aeroplane at the nearest place while you deal with the problem. Much simpler with FMS, just hit the Direct to key twice (some installations) and read off which is closest. Don't execute it until you've thought about effects of wind and destination weather etc .

[Tinstaafl]

The old (ie about 10 yrs ago) Australian Senior Commmercial/ATPL navigation exam used to require the calculation of off track ETPs.

This may have changed since the new syllabus was introduced.

As I recall, one method involved plotting + a degree of interpolation once in the approximately correct place. The wind's effect was included in the calculations.

Can't remember anymore. http://www.pprune.org/ubb/NonCGI/redface.gif

Anyone with an old SCPL/ATPL textbook?

[invalid entry]

Hi,
Here's a crude method I use which works well and can prevent a gross error.
1. On a chart draw your route.
2. Mark the airfields in question with a large dark coloured blob of ink.
3. Hold the chart up to the light, fold it in 2, so that the two blobs are superimposed.
4. Make a crease in the chart in this posn.
5. Unfold the chart.
In still air, the crit point is where the crease intersects your route.
You can allow for the wind by shifting the crease upwind.
Sounds complicated but after a little practice it is quick and easy.
Regards

[This message has been edited by invalid entry (edited 29 November 2000).]

[Jim lovell]

STEPS TO FIND THE OFF-TRACK ETP

1. Find the halfway point on a line that connects the two stated alternates. This point is known as position X
2.Draw a line from position X found in step 1, up to your flight planned track. This line must be at 90 degrees to the line that connects the two alternates. This position is the Equi-distant pint between the two alternates- and is known as position A
3.From position A found in step 2, measure the scale distance to either of the two alternates
4. To find the length of the wind vector line divide the distance found in step 3 by the TAS and multiply by wind speed given in the question
5.With your protractor centred on position X, mark the direction from which the wind is blowing
6. Now draw a pencil line from position X up to the mark found in step five. This is the wind vector line
7. Measure up the wind vector line the scale distance found in step four. This is position Y
8. Finally, draw a line from position Y up to your flightplanned trackline. This line must pass through the line connecting the two alternates at 90 degrees. You have found the ETP. position B

[Dunhovrin]

Thanks guys for all your gen.

Jim - your's seems the most scienterrific but it makes my brain hurt.

I think I'll try Check-Ins with groundspeed adjusted circles,

or...

Plan c: Get onto an FMS aircraft..

Thanks again guys - now back to the monkey spanking