Multiple turns 2

28.10.2004

Submission:

I've a discussion point about initial approach segment - base turn (so called teardrop turn). Is it possible to prescribe a base

turn in which:

- for the outbound track we use a navaid

- for the inbound track we use a different navaid ?

In case of it could be possible, how is the protection area? , how can I calculate the different parameters, like outbound track for different IAS?


Raquel Monje de Abajo - Spain

Answer or Commentary:

(I.W.

I think it is quite possible to use one navigational aid to position an aircraft onto the ILS. However it does make the approach dependent on both navigational aids being serviceable so if the VOR fails aircraft can no longer use the approach even though the ILS is fully serviceable. That is unless there is another way to position on the ILS track (i.e. radar vectors).

To determine the outbound track for different categories of aircraft one has to start from where you wish to finish (i.e. the IF). Usually I divide the 5 aircraft categories into 3, A&B, C&D and forget about E (military jets). Then you take the average speed for each sub-category (i.e. 140 kts and 220 kts) and calculate the turning radius at the ILS intercept altitude with a 25 degree bank angle. Some aircraft will be slower or faster and bank differently but you should catch most of them. Then draw a line from the VOR to the tangent of the 2 turning circles. At this stage you have the nominal out bound radials.

Then one uses the VOR area splays, bank establishment time, pilot reaction time and wind spirals to determine the protection areas. However in this case to cover all eventualities one uses the highest speed for each sub-category (i.e. 180 kts and 250 kts) in all calculations to cover the worst case, a bank angle of 15 degrees and the highest altitude deemed possible).