There are many rules of thumb in the aviation industry. Things that we take for granted that our flight instructor told us to use but never explained why those rules of thumb really work. Why do we descend at 450 ft/min when on a glideslope in small airplanes? Where did that come from. How about this rule of thumb: For every dot deflection on the VOR it is equal to 200 FT per NM.
I want to explore these rules of thumb and if there is one you would like me to research please leave a comment below.
For every dot deflection of the VOR CDI needle we are 200 FT / NM off course. On the right is a circle representing the VOR signal emanating from the station. We have a 2° slice from the circle representing one dot deflection and we want to know how long the yellow segment is.
Notice that this is not a right triangle so we cannot use standard right triangle formulas of Sin(Angle) = Opposite / Hypotenuse. Time to pull out the trigonometry. We know the lengths of the white segment and they are equal: 1NM or 6076.1 feet and we know the angle of 2° for one dot deflection. The Side-Angle-Side formula will get us our result.
As it turns out, every dot deflection of the CDI on the VOR is exactly 212.09 FT if you were 2° or one dot deflection on the VOR CDI. This isn’t the whole story. If we were 180° from our intended course we would be at least 2NM off course. The deflection must change depending on the number of degrees we are off course.
It turns out that 212 FT works really well if you are 2° off your course. If you were 90° off your course then the 212 feet per dot per NM miles turns into 190.95 feet per dot per NM. Does this really matter? Let’s explore the difference we get from our rule of thumb versus the extremes of 2° and 90° off course. The next graph shows the dot deflection using 190 FT, 200 FT and 212 FT for each NM from the station if we are 5 NM off course.
We can see that with such a distance off course and as we get closer to the station the lines diverge. We may only be 7 dots deflection off while we calculate 7.5 dots deflection. That is not that big of a deal. As pilots we like things really easy and 200 feet is much easier to calculate than 190 or 212 feet. In the end, that average is good enough to keep us where we want to go.
I always try to improve on the processes that I have. Whatever it is, I like to analyze the process to complete the job and then find ways to optimize it… make it better, more effective and cost effective. The same goes with the airlines, who above all else, want to cut costs. They know that staying at altitude as long as possible is more fuel efficient than coming down on multiple steps far away from the airfield. If the airlines had their way, they would glide from their cruising altitude all the way to the runway with engines at idle. It is possible, the technology to make this happen already exists and the FAA is coming along with RNP type approaches.
One of the steps the FAA made with the former in mind was to create terminal arrival areas for standard GPS approaches. The big thing here is that GA aircraft will take more advantage of this approach than any other aviation operation. The idea is the same, ATC will clear the aircraft, sometimes from the cruising altitude, for the GPS approach. This allows the aircraft to descend when they want to make the approach. It is quite a cool thing.
The GPS approach into Vero Beach above shows a terminal arrival area. Notice the “T” approach design. There are IAF fixes at each of the three points. Once the FAA clears you for the approach the “free flight” concept relies on the pilot to maneuver as necessary to enter the approach at the given IAF at an airspeed and intercept angle to correctly fly the approach.
How does it work, once cleared for the approach the pilot can elect to descend down to the TAA altitude within that section of the approach. For example, if you are west of HOCHI intersection, once you get within 30 NM of HOCHI you can descend down to 2000 FT MSL. Imagine cruising along at 8,000 MSL and ATC clears you for the approach. There are 6,000 FT to lose and at a standard 3° descent will take 18-20 NM.
Obstacle clearance is also taken into account. The TAA from the NE has a step down arc. Most likely, there are tall antennas to the NE of the airport. Not that big of a deal though, plan to stay above 2700 till you are within 25 NM of MRINO intersection. I have also seen notches in a TAA section with a radial and DME arc to define the notch. The arcs cannot be within 4 NM of the fix or within 4 NM of the TAA boundary.
For more information reference AIM 5-4-5(b) or FAA Order 8260.58
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