When to initiate turn when approaching VOR

[Guest cloudtops]

When directly approaching a VOR station flying by hand, it can often be difficult to hit your departing radial at the desired departing heading, especially when encountering large angles of deflection and/or when flying faster aircraft (the beancounters pore over those JetA receipts, you know). I've been tinkering with a formula that allows you to plug in your groundspeed and desired angle of deflection so that a standard rate turn needs to be started/underway x.x miles from the station. This ignores wind, of course, which is to be factored in as one would other navigational maneuvers.I'm not too keen on embarrassing myself by putting my formula out yet, so I'm curious how you guys/gals fly VOR station approaches, both in the sims and in real life. I appreciate your feedback.Kevin

[brucek 38]

Hi Kevin,From a real aviation position, when intercepting a VOR radial the angle usually used is 30 degrees. This is what ATC will usually use, I believe that's in their ops manual.As you say, ground-speed (the aggregate of true airspeed and wind) will have an effect on how fast that intercept occurs, and further what the lead time will be to comemnce turning to capture the radial.Another effect is proximity to the VOR. The CDI is marked in + or - 10 degrees. At 60nm out from the VOR, 1 degree will be 1nm. But at 15nm out from the VOR, 1 degree will be 1/4 mile, which you will travel through at 1/4 of the time that 1nm would take. The CDI gets very sensitive as you get closer to the station. I'd like to see your formula. Bruce.

[Guest]

Isn't this the one where (ever so roughly speaking) you use the 1% of speed rule?Something along the lines of 1% of your speed equals the approx distance before the VOR that you begin a normal rate turn.Or maybe I just got a bit rusty :-)http://www.blandreynolds.btinternet.co.uk/DF_PG_sm.gif

[Guest CYOW_pilot]

Yup, I agree with Paul. Don't spend a lot of time trying to re-invent the wheel. There are all kinds of formulas based on the 1 in 60 rule that you can apply to radial intercepts but simple is better (you know, fly, talk, think and chew gum at the same time). Real pilots like to keep the mental gymnastics to a minimum.The 1% rule is a good guestimate for when to turn (ie your turn radius is about equal to 1 percent of your speed). And you know what, if you overshoot the radial a bit, so what, you have miles of "protected airspace" so just put a good correction on back to the radial, proportional of course to your distance from the VOR and number of degrees off (1 in 60 rule coming into play again).For intercepts inbound the recommended cut is 30 degs and for intercepting a radial outbound use 45 deg.Kevin in CYOW

[Guest cloudtops]

Ok, here goes.I really should have realized that this only accurately applies to low-level flying, as the "cone of confusion" keeps high altitude DME readings from being accurate or even available (without GPS enroute readings, that is).The real problem I've run into is that ground distance from the VOR is not what is displayed by the DME. It instead measures the distance from the station directly to the aircraft. I try to account for this by the following "chart". When x feet above the station elevation, subtract the given nautical mileage from your DME to get your ground distance from the station. 1,000 - 0.0 5,000 - 0.1 8,000 - 0.312,000 - 0.616,000 - 1.020,000 - 1.524,000 - 2.028,000 - 2.632,000 - 3.136,000 - 3.740,000 - 4.3As I mentioned before, the cone above the station may prevent reading accurately at the higher or even medium alt's, but GPS may lend a hand.----------------------I'd like to make a disclaimer that this formula/guide isn't much good as an enroute calculator. Rather, it is more useful in the planning stages to give one an idea where roughly to begin a procedure turn to intercept an outbound radial. There are many factors that affect the resultant number, such as:-wind,-turn bank other than standard rate,-accurate ground speed readings close to the station (though ground speed usually is well-established before reaching the station)-the time taken to establish procedural turn rate in given aircraftAs I understand it, differences exist between pilots, airlines, and aircraft as to the rate at which a procedural turn is entered, and should be factored accordingly.---------------------On to the formula.(Groundspeed x angle of departure*) / 20,350 = nm from station to be in procedural turn* departure, not deflection (thanks, Bruce)Kind of a weird, overly precise number, I know, but there's a little too much thinking behind it (you're right Kevin in CYOW), so that's what you get. :-rollHere's my rough thinking.Groundspeed in a 2-minute turn divided by 30 gives you the distance traveled in the full turn (circumference of that circle). Furthermore, if you divide that by (2 x pi), you'll get the radius of that circle. This would give you the rough ground distance from the station at which you need to be starting/well-into a procedural turn in order to roll out at a 90 degree departure from your initial course to the VOR.To illustrate this, John is flying his Skylane in no-wind conditions tracking course 360 to his local VOR, 1,000 feet above the station's elevation, with a groundspeed of 120 knots. He wants to depart the VOR on radial 270.GS / (30*2*pi) = nm to begin/be in turn120 / (30*2*3.14) = 0.637 nmJump in the default Skylane (if you still have it), set up as I've shown here, and try it (with or without AP engaged). Watch your airspeed/groundspeed closely, and be careful not to wait until you're right at that distance to start your turn with adjustments to the heading bug if on AP (it takes a bit to bank procedurally), or you'll overshoot the radial for sure. It worked for me, or I sure wouldn't be posting any of this.However, one won't be often turning 90 degrees to depart a VOR, will one? Here's where it got sticky for me, because turning 45 degrees does not mean you'll simply turn at the directly correlated half your 90 degree distance, as is the case with anything else under 90 degrees.So we change the formula to account for the somewhat-patterned deviations from subtracting "pro-rated" angle-distances.The previous number GS/(30*2*pi) is multiplied by the fraction of your desired angle of departure from your current course (e.g., 360 to 330 yields 30 degrees) over 90 degrees multiplied by 1.2 (the rough "pro-rating" factor). Thus, a 30 degree departure applied to the previous example looks like this: 120 x 30------------------------ = 0.18 nm from station (30*2*3.14) x (90*1.2)or 3600------ = 0.18 nm20350In a higher speed aircraft, the formula still holds, though elevation usually accompanies higher speeds. Let's try a Baron cruising at 200 knots at the same elevation attempting the same turn: 200 x 30------------------------ = 0.29 nm from station (30*2*3.14) x (90*1.2)Again, I gave these a whirl in the sim, and they worked.Ok, everybody. Let me have it. I am well aware that we pilots have plenty enough on our minds without having to perform mental gymnastics or punching a pocket calculator enroute. I simply pursued this line of thought because of how grossly I was under/overshooting outbound radials. Since most airline pilots, again as I understand it, are usually typed for one model at a time(the one they're flying), and most GA pilots only fly a type or two regularly themselves, I imagined this might be useful for figuring a rough range or getting a good feel in your usual bird as to when to begin turning on more largely angled departures on outbound radials.There are, after all, a lot of VOR's to be crossed out there!Man, I'm a nerd,Kevin

[Vulcan 14]

Kevin,You could always use 'Pilot's Assistant' and let it do the calcs for you ;-)http://www.tooby.demon.co.uk/P_Assist_Home.html

[Guest aca_dia]

In the real world I use a quick estimate for leading all intercepts. I find this trick also works great with MSFS.Lead the turn by 1% of 1/2 your ground speed.180kt ground speed. 1/2 of which is 90kts.1% of 90 = .9Start the turn to the new radial .9nm from the navaid.Works great for intercepts up to 90 degrees (like starting a dme arc)

[woodreau 0]

Well maybe I'm a sloppy pilot, but if you're already tracking on the inbound radial to the VOR, and the outbound radial is different, I just wait until the flag switches from TO to FROM. Then I turn. (and other things: T-T-T-T-T Time (stop or start the clock if you're timing the time between VOR's or checkpoints) -Turn (the aircraft) -Twist (OBS to new outbound radial) -Throttle (not needed) -Talk (if it's a compulsory reporting point - not normally). ) But then this is just a slow single-engine piston GA aircraft.At least in the heavy metal in FS2002, the FMC always turns the aircraft early so that I don't fly past the VOR.On a different note regarding DME:DME has nothing to do with the "cone of confusion" over the VOR. The DME is an entirely separate piece of navigation gear. DME gives you the "slant range" to a VOR so when you cross over the VOR at 5,000ft, it should give you - if the VOR is at sea level - 0.8 DME That's normal. If you cross at 35,000ft, then you should get 5.8 DMEThe way DME works on a transponder principle. The DME in your aircraft figures out the time it takes for the VOR to respond to its interrogation and determines distance from that time. Because of this, when there are lots of aircraft, DME performance degrades because it can't respond to every aircraft interrogation if there are over 100 aircraft interrogating it - There just isn't enough time if there is a lot of aircraft. Kind of like if 10 people started talking to you at the same time, who do you talk to first.... I think there is an upwards limit of about 100 aircraft that can utilize DME at a given time. Your VOR receiver on your aircraft is totally passive, and so there is no limit as to how many people can use a VOR at a given time. But the VOR receiver in your aircraft is measuring the time it takes between receiving the omni-directional "pulse" and the directional "sweep" to determine what radial you're on. Very simplified explanations, but it's my understanding as to how the navaid works. More knowledgable people can correct my misimpressions.Cheers :)

[CraigC 0]

I would have to check to be 100% sure, but I think initials on VOR approaches are fly-over points. I think you have to fly over the VOR before you can turn. The only time when you can use turn anticipation is on a GPS approach, and the fly-over points are the final and missed approach point. The reason I posted this was because it sounded like the original poster was asking about anticipating his turn to be on course once the turn was completed. On anything other than a GPS approach, I dont think this is possible.CraigEDIT: from this "I dont think this is possible", to this "I dont think this is legal".

[Guest cloudtops]

Thanks a lot, guys. I appreciate your very informative responses and helpful suggestions. I kind of figured all along that I was being way too technical. One of the reasons I really enjoy this forum is that its participants are so widely varied in experience and knowledge that there is almost always a balance of opinion and thought-provoking information shared.Happy flying,Kevin

[Guest aca_dia]

You bet it is legal. The idea or leading a turn is to stay as close to the navaid or approach course as possible. If you wait until you are actually over a VOR or NDB you will over shoot to some degree. There is nothing in the FAR's or AIM that speaks either way to this practice, but I asked 3 other CFII's and called the local FSDO. Everyone I talked to has some trick to lead into course interceptions. Keep in mind we are not talking about cutting a huge corner and getting away from the Navaid. Just enough lead to get you on the new radial, bearing, etc without over shooting. You will still pass through the cone of confusion and get a flip pretty clean flip. A great example is intercepting the start of a DME arc. If its a 9nm arc and you wait until 9dme to start your turn you could be down to 8 DME before you finish the 90 degree turn to start the arc. You have to lead the turn to enter right at 9 dme.

[CraigC 0]

Well, thats why I said I didnt think it was, because I wasnt exactly sure. And you shouldnt be so completely sure either just because a couple of CFIIs and an inspector said so. I went through this whole deal about 6 months ago over something so basic as the anti-collision light, and literally almost got a different answer from each person I talked to. The inspector at the FSDO ended up being wrong in his interpretation. That is where the fault lies in the FAR/AIMs. The FAA has left such a huge grey area, that they could probably nail you on it or make you at fault in an accident if they wanted to do so. I didnt find anywhere that specifically covered this maneuver in the FARs. The only thing I could find was on course reversal to the intermediate or final approach course. The maneuver in question from the original post may or may not fall under this category, but the FAR does say it is at the pilots discretion when to start the turn. I take that bit to be describing the maneuver you do once you are already established outbound to reverse course to intercept the final approach course. Im not saying that you arent right, but when it comes to the FAR/AIM, I try to say "I think" or "Im pretty sure" because you can bet there will be a hundred different interpretations of the simplest regs in the book.I will keep looking to see if I find anything, and ask a few different guys I know....one who went to ATC school in Minnesota. He is usually my fall back if I get a dead end from everyone else. I would also recommend from my experiences that you not stop at a FSDO if you feel like you havent received the valid explanation you were looking for. I have also called the Des Moines tower a couple of times. Those guys are pretty darn good about giving you a clear cut answer, if there is one. I also feel like these are the guys watching over me when I cant see anything, under IMC they are responsible for everyone out there to a certain degree so if its good enough for them, its good enough for me.Craig

[CraigC 0]

I just had a thought as I was flipping through the FAR/AIM.....if in fact it is perfectly legal to anticipate your turn on a VOR initial approach fix, it would only work if you had functioning DME. Otherwise you would have no idea how far away from the facility you are until you have actually crossed over it. This day and age just about everybody has DME, but I can remember a time when DME was a luxury.The reason I am saying this is because of a conversation I just had the other day with my buddy I mentioned above that went to ATC school. He is in the process of finishing up his CFI right now, and part of his ground is understanding new techniques that apply to newer equipment. Alot of old techniques keep getting passed down from CFI to CFI and we are still using old philosophies that arent always applicable to todays aircraft and airspace. Using a GPS that is capable of showing the entire approach (the newer GPS units like we will see in FS2004), I can now say that I wouldnt see any problem in anticipating the turn a little if you are absolutely sure of obstacle clearance and have good situational awareness of the terrain around you.I really like for real life regulation discussions to come up in this forum because it really is a great opportunity for people who only sim to see that the FAA regs arent always black and white.Craig

[Guest aca_dia]

Again the general rule you can follow with the FARS..."Unless the FARS state you cant do it... you can do it" I agree several CFI's (including myself) and several FSDO reps do not make it a hard rule. However it is pretty much standard practice and as far as I know there is know rule against leading intercept turns (again only just enough to avoid over shooting). If you do not lead the turn you will go just as far off course in the other direction and you will have to work to get back on course. We are not talking about getting off an airway or an approach and cutting huge corners. You should still be on the route or airway (remember airways are 8 miles wide and you are established on an approach when you are within 10 degrees of the approach course) I agree that it is most acurate when done with a distance from GPS. You would certainkly want to be on the conservative side if you are only using DME.I can

[dnunes 1]

Thank you guys! This has been one of the most interesting and instructive threads I have followed in the forum.Regards,