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luckyb52

Question About Turns > 90 Deg

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Hello Fellow Simmers,

 

I'm approaching a fix. At the fix I am supposed to turn more than 90 deg onto a VOR track.

 

1) Do I have to slow down, and if so by how much?

 

2) Should I start turning early to a heading less than 90 deg from my current heading, then make another turn onto the VOR track? If so, how do I decide what the heading for the first turn is and when I should make the first turn?
 

I have researched the wazoo out of the Internet for the answer to no avail.

 

Please help me out with wise counsel?

 

Thanks a ton in advance!

 

Best,

 

Lucky

 

/////////

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If you fly with aircraft equipped with HSI/Flight Director - this instrument could guide you to make a precise turn, yes, it would ask you to start turning before you actually overflew the VOR and it would guide you precisely through the turn.

 

And then there is a difference between VFR and IFR flights.

 

When you do it in VFR - it actually doesn't matter what you would do, you can actually overfly the VOR and then start your turn or make anticipatory early turn - it is up to you.

In IFR flying it is usually obvious from the chart if you are supposed to overfly the VOR (so called fly-over) or you can make an early turn (flyby). Modern RNAV equipment is able to distinguish between both cases.

 

If you fly some basic Cessna and want to practice such a manoeuvre I would overfly the VOR first and then make the required turn, yes, you will overshoot but it is considered a safer practice because overflying VOR gives you a solid positional  feedback. If I were in the clouds in a non-RNAV basic aircraft with a VOR receiver and had to make such a turn - this is what I would do, I wouldn't try to make any shortcuts, it would probably be even illegal. Those who design approaches try to avoid charting such sharp turns unless the waypoint is specifically designed as fly-by and you fly an RNAV capable aircraft.

 

You can read more about it in those FAA handbooks available in pdf format for free.

 

Sorry for this long answer because the answer really is "it depends". If you had a specific example which is on a map - it is always better to bring such map to discussion.

 

And no, no slow down is necessary unless the phase of the flight calls for such a slow-down.

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If you go for a fly by then you can calculate when (time, not where/distance) you must start the turn.

 

TimeBeforeFix = Tan( AngleOfTurn / 2 ) * 19 seconds

 

This assumes a standard rate turn and does not take into account the time needed to roll into the turn.

 

As a table:

 30º -  6s

 60º - 11s

 90º - 19s

120º - 33s

150º - 71s

 

On charts fly-over waypoints have a circle around them.

 

Wether a fly-by with a shortcut on the inside or a fly-over with an overshoot on the outside is btter would depend on wether there is anything inside or outside of the turn that you do not want to run into.

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It depends on what you are using to navigate. 

 

In the real world (IFR) you have to overfly a VOR/NDB and positively identify you have over flown the beacon. This means you need to pass the cone of confusion and get a from flag, and then turn, or on an ADF, the needs tail needs to be in the top half of you instrument.

 

If you are flying using RNAV, it depends on the equipment you are using. Most IFR approved GA GPS units, like the garmin 430 ie, will tell you "next track" at the bottom in a white box, which means you can start your turn. If you are in approach mode (which is where you will most likely find a 90 degree turn, on a GPS approach) it even announces "turn now" at which point you can start a rate 1 turn to end up perfectly on track. (Unfortunately in FSX the 430 doesn't simulate this)

 

Lastly, the above table is incorrect unfortunately. When doing rate 1 turns, a 360 degree turn would take precisely 2 mins. so 180 1 min. 90 degrees would take 30 secs etc. 

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Lastly, the above table is incorrect unfortunately. When doing rate 1 turns, a 360 degree turn would take precisely 2 mins. so 180 1 min. 90 degrees would take 30 secs etc. 

 

 

I believe that the table was stating how many seconds early the turn needs to be started (for a flyby) - not the length of time needed in the turn?

 

Eugene

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