Very High Frequency Omni-Directional Range (VOR)
Distance Measuring Equipment (DME)
When it becomes necessary to divert to an alternate airport, select the most suitable alternate, estimate the magnetic course to the alternate, and turn to the approximate heading to establish this course immediately. After becoming established on the new course, apply the new wind correction and compute the actual distance, estimated time, and fuel required.
Aeronautical charts are drawn using the geographic north pole as the north reference. However, a magnetic compass points to the magnetic north pole, which is not located at the geographic pole. The compass error caused by the angular difference between true north and magnetic north is called magnetic variation. Variation changes with location, but it is the same on all headings within the location.
Aeronautical charts show the variation error with isogonic lines that run diagonally and irregularly across the charts. Each line is labeled with the variation error that runs along it, and the pilot must apply the correction for this error to relate the magnetic course to the true course. Westerly variation must be added to the true course or true heading to get the magnetic course or heading. Easterly variation must be subtracted from the true course or true heading to get magnetic course or magnetic heading.
To determine the true heading required to fly a desired true course, crab the airplane into the wind. If the wind is from the left, subtract the wind correction angle. If the wind is from the right, add the correction. See Figure 9-1.

Figure 9-1. Course/heading formula
The most widely used charts for VFR navigation of small, slow airplanes are Sectional Charts. These charts are drawn to a scale of 1:500,000 (1 inch = 6.89 nautical miles), and are revised every six months, except for some Alaskan charts which are revised every 12 months. Sectional Charts are drawn on a grid in which the meridians of longitude are non-parallel straight lines that encircle the earth from pole to pole and cross the equator at right angles. When planning a long distance flight, measure the true course from the mid-meridian to get an average true course.
True course is determined by measuring the course on an aeronautical chart. True airspeed is known by applying the appropriate correction to the indication of the airspeed indicator. The wind direction and velocity are known from reports or forecasts from the Flight Service Stations (FSS).
The true heading and the ground speed can be found by drawing a wind triangle of vectors. One side of the triangle is the wind direction and velocity, one side is the true heading and true airspeed, the final side is the track, or true course, and the ground speed. Each side of a wind triangle is the vector sum of the other two sides.
ASA’s CX-3 Flight Computer is an electronic flight computer and can be used in place of the E6-B. This aviation computer can solve all flight planning problems, as well as perform standard mathematical calculations.
Problem:
Calculate the distance and time upon reaching 8,500 feet MSL. Given:
Departure path: straight out
Takeoff time: 1030 DST
Winds during climb: 180° at 30 knots
True course during climb: 160°
Airport elevation: 1,500 feet
True airspeed: 125 knots
Rate of climb: 500 fpm
Solution using a CX-3:
Find the ground speed and heading (Wind Correction on the FLT menu):
True Airspeed (TAS): 125 KTS
True Course (TCrs): 160°
Wind Speed (WSpd): 30 KTS
Wind Direction (WDir): 180°
Ground Speed (GS): 96.39 KTS
Heading (THdg): 165°
Find the total distance required to climb to get to 8,500 feet:
8,500 (destination) – 1,500 (airport elevation) = 7,000 feet
Calculate the time to climb 7,000 feet:
7,000 (total climb) ÷ 500 (rate of climb) = 14 minutes
Find the distance flown (Ground Speed on FLT menu):
Duration (Dur): 00:14:00 HMS
Ground speed (GS): 96.39 KTS
Distance (Dist): 22.49 NM
Find the time arrived at 8,500 feet (add takeoff time to time flown to get ETA):
Takeoff time: 1030 DST
Time flown: 14 minutes
ETA: 1044 DST
The takeoff and climb to altitude will require approximately 22.5 NM, and you will reach altitude at 1044 DST.
Problem:
Find the ground speed and magnetic heading given the following conditions:
True course: 258°
Variation: 10° East
Indicated airspeed: 142 knots
Ambient temperature: +05°C
Pressure altitude: 6,500 feet
Forecast wind: 350° at 30 knots
Solution using a CX-3:
Find true airspeed (Airspeed on the FLT menu):
Temperature (OAT): 5°C
Calibrated Airspeed (CAS): 142 KTS (use indicated airspeed)
Pressure Altitude (PAlt): 6,500 FT
True Airspeed (TAS): 157.08 KTS
Find ground speed and heading (Wind Correction on the FLT menu):
True Airspeed (TAS): 157.08 KTS
True Course (TCrs): 258°
Wind Speed (WSpd): 30 KTS
Wind Direction (WDir): 350°
Ground Speed (GS): 155.24 KTS
Heading (THdg): 269°
Find magnetic heading:
269° (true heading) – 10° (easterly variation) = 259°
When the true heading, true airspeed, the ground track, and ground speed are known, the wind direction and velocity can be found.
Solution using a CX-3:
Find the wind direction and velocity (Wind Correction on FLT menu):
Ground Speed (GS): 140 KTS
True Airspeed (TAS): 135 KTS
True Course (TCrs): 130°
True Heading (THdg): 135°
Wind Speed (WSpd): 12.99 KTS
Wind Direction (WDir): 245°
When ground speed and lapsed time are known, distance traveled can be found (use Ground Speed on the CX-3 FLT menu):
Lapsed time (Time): 2-1/2 minutes
Ground speed (GS): 98 KTS
Distance flown (Dist): 4.1 NM
When fuel consumption and ground speed are known, the amount of fuel required to fly a given distance can be found.
Problem:
Find the fuel required given the following conditions:
Fuel consumption: 15.3 GPH
Ground speed: 167 knots
Distance to travel: 620 NM
Solution using a CX-3:
Find the leg time (use Ground Speed on FLT menu):
Distance (Dist): 620 NM
Ground Speed (GS): 167 knots
Time flown: 03:42:45 HMS
Find the fuel consumption (use Fuel on the FLT menu):
Duration (Dur): 03:42:45 HMS
Fuel burn (Rate): 15.3 GPH
Fuel burned (Vol): 56.8 US GAL
The range available in an aircraft can be found when the fuel on board, the fuel consumption rate, the ground speed, the flight time since takeoff, and the required reserve are known. 14 CFR §91.151 requires that for VFR flight an airplane start off with enough fuel to fly to the point of intended destination and after that for 30 minutes in the daytime and 45 minutes at night.
Problem:
How much farther can the airplane can fly, under day VFR conditions, according to 14 CFR Part 91? Given:
Usable fuel at takeoff: 36 gallons
Fuel consumption rate: 12.4 GPH
Constant ground speed: 140 knots
Flight time since takeoff: 48 minutes
Solution using a CX-3:
Find the total amount of flight time provided by the usable fuel at takeoff (use Fuel on FLT menu):
Usable Fuel (Vol): 36 US GAL
Fuel burn (Rate): 12.4 US GPH
Duration (Dur): 02:54:12 HMS
Find the total time available, accounting for regulations and time already flown:
02:54:12 – 00:30:00 – 00:48:00 = 01:36:12
Find the distance available to fly (use Ground Speed on FLT menu):
Duration (Dur): 01:36:12 HMS
Ground speed (GS): 140 KTS
Distance (Dist): 224.47 NM
The required airspeed can be found when a specific point needs to be reached at a certain time.
Problem:
Find the required airspeed given the following conditions:
Distance between points A and B: 70 NM
Time between points A and B: 30 minutes
Forecast wind: 310° at 15 knots
Pressure altitude: 8,000 feet
Ambient temperature: -10°C
True course: 270°
Solution using a CX-3:
Find the ground speed that must be made between points A and B (Ground Speed on FLT menu):
Distance (Dist): 70 NM
Duration (Dur): 30 MIN
Ground Speed (GS): 140 KTS
Find the true airspeed (Wind Correction on FLT menu):
Ground Speed (GS): 140 KTS
True Course (TCrs): 270°
Wind Speed (WSpd): 15 KTS
Wind Direction (WDir): 310°
True Airspeed (TAS): 151.8 KTS
True Heading (THdg): 274°
Find the required indicated airspeed (or calibrated airspeed) (Airspeed on FLT menu):
True Airspeed (TAS): 151.8 KTS
Temperature (OAT): -10°C
Pressure Altitude (PAlt): 8,000 FT
Airspeed (CAS): 137.15 KTS
After flying for a given distance, you find yourself a known distance off course. You can find the correction angle needed to return the airplane to the desired course in a specific distance by using this simple relationship:
Miles off course |
= | Degrees to parallel or converge |
Miles flown or to fly |
60 |
Problem:
After 141 miles are flown from the departure point, the aircraft’s position is located 11 miles off course. If 71 miles remain to be flown, what approximate total correction should be made to converge on the destination?
Solution:
Find the change in heading needed to parallel the original course with the formula:
Degrees to parallel = |
Miles off course × 60 |
Miles flown |
11 × 60 |
= 4.68° |
141 |
Find the change in heading needed to converge on the destination in 71 miles with the formula:
Degrees to converge = |
Miles off course × 60 |
Miles to fly |
11 × 60 |
= 9.29° |
71 |
Add these two corrections to find the total correction required to converge on the destination:
4.68 + 9.29 = 13.97°
Radio signals in the very high frequency (VHF) band are nominally in the frequency range of 30 to 300 MHz. They operate according to the line-of-sight principle, following the same rules as light, and do not bend to conform to the surface of the earth. VHF reception distance varies in proportion to the altitude of the receiving equipment. The higher the aircraft, the greater the reception distance.
VHF Omni-Directional Range stations (VORs) operate within the 108.0 to 117.95 MHz frequency band and have a power output necessary to provide coverage within their assigned operational service volume. They are subject to line-of-sight restrictions and their range varies with the altitude of the receiving equipment. There are three classes of VOR:
To orient where the aircraft is in relation to the VOR, first determine which radial is selected (look at the OBS setting). Next, determine whether the aircraft is flying to or away from the station (look at the TO/FROM indicator), to find which hemisphere the aircraft is in. Last, determine how far off course the aircraft is from the selected course (look at the CDI needle deflection) to find which quadrant the aircraft is in. Remember that aircraft heading does not affect orientation to the VOR. If the station is directly abeam of the aircraft, the TO/FROM Indicator will show a neutral flag.
The time to the station is found by the formula:
Time to station = |
60 × Minutes between bearing changes |
Degree of bearing change |
The distance to the station is found by the formula:
Distance to station = |
TAS × Minutes between bearing changes |
Degree of bearing change |
Problem:
While maintaining a magnetic heading of 060° and a true airspeed of 130 knots, the 150° radial of a VOR is crossed at 1137 and the 140° radial at 1145. What is the approximate time to the station?
Solution:
Time to station = |
60 × Minutes between bearing changes |
Degree of bearing change |
= |
60 × 8 |
10 |
|
= |
48 minutes |
Problem:
Given these conditions, find the distance to the station.
Solution:
Distance to station = |
TAS × Minutes between bearing changes |
Degree of bearing change |
= |
130 × 8 |
10 |
|
= |
104 NM |
Therefore, the station is 104 nautical miles away.
To use the VOT service, tune in the VOT frequency on your VOR receiver. With the course deviation indicator (CDI) centered, the omni-bearing selector (OBS) should read 0° (or 360°) with the TO/FROM indicator showing FROM, or the OBS should read 180° with the TO/FROM indicator showing TO. Should the VOR receiver operate an RMI, it will indicate 180° TO on any OBS setting.
Note: sport pilot instructors can disregard this section.
A VORTAC station is a VOR station collocated with a military TACAN station. Civilian airplanes use the VOR for direction to or from the station, and the DME portion of the TACAN for distance information to the station. When a VORTAC station is undergoing routine maintenance, the identification code is removed. The VORTAC provides three individual services:
The mileage readout of DME is the direct distance in nautical miles between the aircraft and the DME ground facility. This is commonly referred to as the slant-range or slant-line distance. The slant-range error is greatest when the aircraft is directly above the facility at a high altitude. An aircraft flying at 6,000 feet AGL directly above the facility would indicate a DME distance of 1 NM.
DME furnishes distance information with a high degree of accuracy. Reliable signals may be received at distances of up to 199 NM at line-of-sight altitude, with an accuracy of better than 1/2 mile or 3% of the distance, whichever is greater.
The DME or TACAN coded identification is transmitted one time for each three or four times that the VOR or localizer coded identification is transmitted. When either the VOR or DME is inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded identification with a repetition interval of approximately 30 seconds indicates that the DME is operative.
When in the VOR mode, lateral deflection of the needle in the CDI represents degrees the aircraft is off course. A full needle deflection indicates that the airplane is 10° or more off course.
[10-2024]