VOR Transitions

[motocheme]

I just finished the I-3 and I-4 ratings over the last couple of days and was hoping some of you might have some advice. I am having trouble making smooth transitions when passing over a station. For instance, after proceeding direct to Seal Beach VOR from KSNA and trying to join the V3 airway I blow the turn. Or when transitioning from V23 to Santa Monica at POPPR I made a horrible turn as well. Does anyone have any advice for making those turns and transitions more smoothly? I am sure this will be a handy skill when I actually start my instrument training. Thanks!

[Keith Smith]

Practice....practice....practice A lot of it depends on the equipment you have on board. If don't have GPS or DME, it's going to be hard to know when you pass over the station until it's literally happening, or if you're hand flying (rather than NAV mode on the AP), you'll noticed the CDI moves with more and more gusto as you get closer to the station. That's your clue that it's getting close (yet another reason why I'm not a giant AP fan for anything other than long, boring enroute segments).

You can either lead the turn (ie, turn to the new heading prior to station passage) if your equipment allows you detect that you're getting close, or you can PROMPTLY start the turn after station passage. If it's the latter, know that you're going to have to execute the course change and then some to reintercept the desired course. Also know that it's going to come up very quickly (if you started the turn promptly) because you're still relatively close to the station, hence the radials are not very spread out at that point (ie, the lateral distance between each degree radial is much less than at a point 20-30 miles out from the station).

Bearing all those things in mind, you can start honing your skills. You're very smart to be practicing this in advanced. This is one of those cases where the sim is 100% as good as the plane. So, practice...practice....practice now so you can nail it in the plane later.

Course changes along airways are an excellent way to keep the mind engaged. That's why I greatly prefer flying 10-15 30-40nm segments on a long IFR flight than a single 400nm direct leg, even if the direct leg is shorter by a few miles.

[Mark Hargrove]

Based upon my own early experiences with VOR navigation I think I might have a guess at what you're struggling with.

If you're tracking a radial into and then away from a VOR remember that the CDI needle gets very sensitive when you are close to the transmitter. Trying to chase the CDI when you're within a couple of miles of the VOR facility is an exercise in frustration -- so don't do it. If you've been tracking inbound to the VOR with a centered needle, nothing magical is going to happen in that last couple of miles that takes you wildly off course -- so just keep the wings level and fly the heading rather than the needle when the CDI starts to get twitchy. If you're using your autopilot to track a radial it will usually do a pretty good job of staying on the inbound radial but will probably suck at tracking outbound if you have to make a turn at the VOR (for the same reason -- it will try to chase the CDI and make a bunch of lovely S-turns back and forth across the outbound radial until you get a few miles away, after which it will settle down) -- so don't use the autopilot coupled to the VOR either within the first couple of miles outbound.

If I'm using VORs alone for navigation and have to make a turn at a VOR to follow a specific radial outbound (to track a Victor airway, for example), I make my turn to the appropriate heading for the airway (either by hand or by using the autopilot coupled to the heading bug) and then continue to fly that heading while I turn the OBS to the appropriate setting, and then wait until I'm at least a mile or so away from the VOR before I worry about centering the needle.

Also consider using DME, if you have it equipped, to help anticipate and plan for the turn. If you're waiting for the TO/FROM indicator to switch to FROM, it's already too late to make a turn that will smoothly intercept your new course! Say you're flying direct to SLI on the 140 radial (so your heading will be 320). Your turn to intercept V23 (which is the 272 radial) will be about a 50 degree left turn. If you wait for the FROM indicator to indicate you've crossed SLI before you begin your turn, you will be north of V23 when you roll out on the 272 heading. How far north you'll be will depend upon your airspeed and your rate of turn -- but there is no way to roll out directly onto V23 if you don't anticipate the turn. Without DME you're just guessing (and that's not a great idea), but with DME you can anticipate the turn and cut the corner a bit, allowing a much smoother intercept of the outbound radial you need to track.

Try flying the route with autopilot coupled to a GPS sometime and watch what happens. The flight management system (on the Garmin family, anyway) uses your current airspeed and a pre-programmed turn rate (of about 25 degrees of bank) to perform "turn prediction" -- and it always begins the turn before it reaches the particular fix its flying towards to allow for smooth intercept of the new leg.

-M.

[Peter Grey]

There is a lot of good stuff here on course changes, but nothing concrete so far on actual turn lead numbers. Here are some good numerical rules of thumb to help.

For a 90 degree turn, lead your turn by .5% of your ground speed. For example if your ground speed is 100 knots, you should lead a 90 degree turn by .5 NM, note this is the DME where you should be established in the turn, not starting it, start it a tenth or 2 early. If you do not have ground speed you can estimate it (as long as its within 20 knots it won't affect the formula too much).

For a 60 degree turn, lead your turn by 90% of the value for a 90 degree turn (unless your going fast this isn't worth the extra calculation, just use the 90 degree number)

For a 45 degree turn, lead your turn by 70% of the value for a 90 degree turn.

For a 30 degree turn, lead your turn by 50% of the value for a 90 degree turn.

For turns beyond 90 degrees you can do the lead as 2 separate leads added together (1 for 90 and 1 for the remainder). Note the further away you get from 90 degrees the less this works.

[Keith Smith]

Quick addendum, be sure to start CALCULATING the lead distance 3-5 hours in advance of the flight.

Kidding aside, I never even knew there were rules of thumb for lead calc, those are pretty neat. I've never been a rule of thumb guy. After reading that, I am reminded why that is

I'll be honest, I calculate most of leads by visualizing how long it's going to take me to complete the turn, and how quickly I'm approaching the turning point (via DME or GPS). If I'm paying attention, it goes well. Peter, have you actually memorized those figures and used them in an airplane? Have you found them to be practical?

Note: I'm not trying to state with certainty that they're not practical, I'm just checking to see if you've found them to be practical.

[Peter Grey]

I use the 90 degree one a lot as its a easy calculation, divide by 2 and move the decimal 2 places over, or the rule of 100 knots = .5 DME lead works as well.

The other ones aren't normally needed until you are going fast, but they do work as published, we use them quite a lot at my flight school.

As a note, there are numbers and rules of thumb for everything.

One of my favorites (this is one to not try and figure out in flight).

VSI change per degree of pitch change = 100 * TAS/60.

[arb65912]

I would separate the VOR navigation into 3 cases.

- No DME - this one is the hardest of course, one HAS to wait until the needle starts moving and then it is going to be a bit of overturn when getting back on radial. In this case, I would have to watch the needle and when it starts moving fast ( passing the station), I would make my turn to new heading, wait about minute or two and re intercept the radial with 30 deg rule.

- VOR receiver with DME - that one is much easier, I would start the turn a bit ahead of the passing VOR ( one might use rules as Peter described above) , how far of course would depend on the speed.

- GPS and moving map equipment without using FMS of course, that one is the easiest, you see the plane moving on the map, you start the turn by judging what is going to take to make a turn and you can can compensate while turning ( you can see the course line which is a big help in this situation)

- GPS , moving map and FMS flying the plane - well, nothing to do here..

I am sorry if I described something wrong, this is the way I am navigating using VOR and I just wanted to share.

Cheers, Andrzej

[motocheme]

Thanks everyone for all the great information! I knew I could count on the PE gang for help

[Calvin Waterbury]

I'll have to find out the "official" word in my FAR/AIM (I loaned it to a friend), but how much leeway (how many miles) will FAR/AIM and ATC allow in using the "cutting corners" method of anticipating a VOR turn?

Assuming a ~90-degree course change and VOR/DME on-board, I'm thinking a turn 3 miles before the VOR would allow more time for the workload and 5 miles would allow even more time.

[Ryan Geckler]

The internet is your friend The AIM is published online and updated on a regular basis.

http://www.faa.gov/air_traffic/publicat ... 503.html.7