Navigation

Navigation is the procedure by which the pilot flies from one point to another. A single method of navigation is rarely used by the pilot operating under Visual Flight Rules (VFR). There are several methods of navigation in use today by the VFR pilot.

Methods of Navigation

The principal methods of navigation used today by light aircraft are:
  • Dead Reckoning
  • Pilotage
  • VOR
  • ADF
  • LORAN
  • GPS

Dead Reckoning

This is the primary navigational method used in the early days of flying before adequate aeronautical charts and electronic navigation were available. It is the method on which Lindberg relied during his first trans-Atlantic flight. It is based on Time, Distance, and Direction only. The pilot must know the distance from one point to the next, the magnetic heading to be flown, and have some idea of the effects of the winds expected to be encountered during the flight. It is the most fundamental aspect of VFR flight.

Even in todays environment, the pilot should prepare a basic planning log of check points along the route of flight. This planning log should include such data as True Course (TC), Distance, anticipated wind drift (or wind correction angle), estimated ground speed and magnetic heading by which to steer. This data should be measured, or in some cases estimated, for each leg of the flight. The purpose for this log is to allow the pilot to estimate the time and heading for each leg, and to make minor corrections to the plan for the next leg based on the experience of the previous leg.

A sample flight planning log will be demonstrated later in this section.

Pilotage

Pilotage is the art of following an aeronautical chart to fly from one point to another. True pilotage may not always follow straight lines for long distances, but rather may follow terrain features such as rivers, coastlines, mountain ridges, roads, railroads, etc. The pilot is relying on the recognition of major features shown on the chart, and correlating them to what is seen below..

The pilot may keep a primitave log of checkpoints, or may even write the time directly on the chart as prominant features are passed over.

During training, instructors will usually train student pilots to navigate over a course of 50 to 100 miles, using a combination of dead reckoning and pilotage. This type training is important for later use, since electrical and electronic navigation equipment may occasionally malfunction. The experienced pilot should be able to fly relatively long distances using a combination of these two basic methods.

Even when using electronic means of navigation, it is a good procedure to also utilize dead reckoning and pilotage procedures in addition to the electronic instruments.

VOR Navigation

The principal electronic navigational system in use today is the VHF Omni-Range (VOR). This navigational method relies on a system of ground-based transmitters which emit signals that a VOR receiver can interpret. The VOR receiver can use the signal emitted by a selected ground station to arrive at an azimuth reading from the station. This azimuth FROM the station is called a RADIAL of the VOR. Another way to envision the VOR radial is to think of a wagon wheel with 360 spokes. One is called 360 (representing Magnetic North). The others are numbered 1 through 359. If a fly lands on spoke 37, the fly is on the 037° RADIAL of the wheel. It makes no difference which way the fly is headed. He can turn adound in a complete circle; but as long as he stays on the same spoke, he is on the 037° radial.

The diagram below demonstrates an aircraft (#1) flying on a Victor Airway (V54) whose outbound radial from VOR (A) is 095°. The VOR instrument shown on the bottom of the diagram is called a Omni Bearing Selector (OBS) (the ADJ knob and numbers on top and bottom) and then Omni Bearing Indicator (OBI) comprised of the needle, white dots and Yellow/Blue arcs and the TO/FROM flags.. When the needle of the instrument is centered, and the FROM FLAG is showing (is WHITE), the radial is indicated by the marks and numbers at the top of the instrument

To fly an outbound radial of 095°, rotate the selector knob labled ADJ until the needle centers and the FROM (white) flag is showing. When this is achieved, you are on the 095° radial of VOR (A). Note that the heading of the aircraft has no effect on the presentation of the instrument, or the radial you are on.. The radial and the instrument presentation of aircraft #2 and #3 are the same as aircraft #1.

When you are about half way to VOR B, you can tune the same ( or a second) VOR receiver to the frequency of VOR B. Again, you will rotate the ADJ knob (if necessary) in order to center the needle: but this time you want the TO FLAG to be showing (WHITE) since you are flying TO station B.

As you pass over station B, you will encounter a zone of indecision. The needle may swing wildly from one side to the other, and both the TO and FROM flag will be off (dark). Note in the diagram above, you will want to start tracking the 110° outbound from station B. When the FROM flag shows white, turn the aircraft to the new heading of 110° and rotate the ADJ knob until the 110° mark is under the selector arrow. You may need to turn a few degrees more until the needle centers, then track the 110° outbound radial.

The VOR station makes a good checkpoint. Not only do you get the "station passage" indication on the OBI, but you can visually see the VOR ground station in good visibility conditions. It is a low building with a truncated cone on the top, which houses the antenna. The truncated cone is fiberglass and white in color.

Whenever you tune the VOR receiver to a new station frequency, you should always turn the VOR receiver volume up to hear the 3 letter morse code identifier. The dots and dashes are shown in the VOR identification box near the VOR compass rose. Some stations may have voice identification in lieu of the Morse code. In this case, the full name of the VOR is spoken. See the VOR OmniRange description in the previous section titled Chart Symbols to review the how the station frequency, name, ID and morse code is indicated on the charts.

Correcting for Wind Drift

Seldom when flying a course will the pilot encounter a "no wind" condition. Wind will always add or subtract from Indicated Airspeed to create a different speed across the ground called Ground Speed (GS). Wind will also drift you off course.

The OBS Selector (your radial) indicates the Magnetic Course (say 095°) that you want to fly over the ground. Seldom will the wind allow you to track your radial without making some correction to the left or right. The wind, will drift you off course, either to the left or right of your radial. The OBI needle will start drifting to the left or right rather than staying centered.

If you are flying the 095° radial from VOR A for example, and the needle drifts right, it means the wind is blowing from your right ( South ), thus drifting you off course to the left. The needle always points in the direction toward which you need to make correction in order to get back on the selected radial. Take the 095° radial for example. After flying for a while holding a steady 095° heading on the compass (Magnetic Heading MH) you see the needle has drifted to first dot on the right. This first dot represents a 5° deviation from the radial. What do you do to correct for the wind drift, and get back on course? If you said, change the OBS to 100°, WRONG!. Do not change the OBS, because that is the Magnetic Course (MC) of 095° is what you want to track over the ground.

Rather , change your magnetic heading to the right, say 10°, to a MH of 105°. Hold that for a while. If the needle drift ceases (it steadies), you are now flying parallel to your course, but to the left of course. You now need to get back on course, so take another 10° correction to the right (now a 20° total correction) to a MH of 115°. When the needle comes back to center, you are back on the desired radial. Now remove that last 10° correction (i.e. come back to a Magnetic Heading (MH) of 105°). That should now hold you approximately on the 095° radial by holding a 10° right Wind Correction Angle (WCA) (i.e. holding a 105° MH on the compass).

However, if the initial 10° R WCA fails to stop the needle from drifting right, increase the WCA to 20° R (MH=115°). If the needle steadies, increase the MH to 120-130° to get back on course, then hold a 20° WCA (MH=115°). Maintain this for a while to see if that holds the needle centered.

Continue to make this type of correction, left or right until you have the correct WCA pinned down to hold you "on course" with needle remaining centered. Usually when flying cross country, you do not make large radial changes; so the WCA you held on the last leg should be close to appropriate for the next leg. After some practice, you will be able to estimate the WCA fairly quickly by watching how fast the needle drifts from center.

Testing the VOR Indicator for Accuracy

Generally, the accuracy of the VOR is within one degree. However, due to age or other factors, the error may increase. The accuracy of a VOR receiver can be checked several ways.

  • FAA VOR Test Facilities (VOT)
  • Airborne Check Points
  • Ground Check Points

The Airport/Facility Directory is a document describing all public airports and navigational facilities. It can be purchased at most airports and pilot supply stores for a nominal fee. This document lists certified checkpoints that may be used to check VOR Receiver accuracy. These are selected ground or in-flight checkpoints which have a known radial bearing from the specified VOR.

The FAA VOR Test facility is called a VOT. These facilities are usually found at larger airports. It is a special test facility which can be tuned while on the ground at the airport. When you tune the VOT frequency, you will hear a series of dots, or a steady tone as the VOT identifier. Turn the OBS until the CDI centers. The course indicator should read either 0° or 180°. If 180°, the flag should be TO. If the OBS reads 0°, the flag should be FROM.

The OBS reading must be accurate to +/- 4 degrees for ground based checkpoints and +/- 6° for airborne checkpoints..

Using VOR Intersections

You can determine your position by using a radial reading from two VOR stations. To locate your position using 2 VORs, tune one receiver to one of the stations. Rotate the OBS knob until the CDI is centered and a FROM flag shows. Do the same with the second station. Draw a line from the center of the the VOR compass rose on the chart through radial (the degree number on the pheripery of the compass rose). Now do the same for the radial reading of the second VOR. The intersection of the 2 radial lines is your position at the time the readings were made. For best accuracy, the two radials should be at right angles as much as possible.

In the example, you read a radial of 150° from the top VOR, and 060° from the lower VOR. By drawing lines on the chart representing these radials, you are at the intersection of the lines. Often, where Victor Airways cross, the intersection will be indicated on the chart by crossed arrows and an intersection name.

Distance Measuring Equipment (DME)

VOR/DME and VORTAC stations provide distance information for aircraft equipped with Distance Measuring Equipment (DME). DME operates in the UHF radio band on frequencies from 962 MHz to 1213 MHz. Whenever you tune the VOR station frequency, the DME receiver automatically selects the correct UHF frequency. This is called “paired frequency selection”. The pilot need not be concerned with the UHF frequency.

Some DME systems can also be “slaved” from a normal VOR receiver, so that the DME automatically operates on the VOR station selected by the VOR receiver. Tuning a DME receiver is similar to tuning the VOR receiver; i.e. the frequency of the VOR is selected on the DME tuner dials.

The DME unit uses a “shark fin” appearing antenna, normally mounted underneath the aircraft. The aircraft DME unit sends an interrogation signal to the VOR/DME or VORTAC station. The ground station then transmits a responding signal. The DME unit measures the time elapsed between sending the interrogating signal and receipt of the response signal. From this information, the DME unit calculates the distance to the station.

Some units also calculate groundspeed and “time to the station”. However, GS and time to the station are only valid if you are flying directly to or from the station. If you are flying in any other direction, these values will be incorrect. However, the distance to the station will be correct.

The indicated distance is known as “slant” distance to the VOR station. If you are some distance from the station and not at a high altitude, the DME slant distance will be very close to the ground distance. As you become closer to the VOR station, the slant range error becomes greater. !
For example, if you are flying at 6000 feet, the DME will register 1 NM to the station when you are directly over the station.

Automatic Direction Finder (ADF)

Some aircraft are equipped with an ADF receiver. They receive radio signals in the medium frequency band of 190 Khz to 1750 Khz. The ADF receiver can “Home” on both AM radio stations and Non-Directional Beacons. Commercial AM radio stations broadcast on 540 to 1620 Khz. Non-Directional Beacons (NDBs) operate in the frequency band of 190 to 535 Khz.

The aircraft equipment consists of two antennas, the ADF Receiver, and the ADF Instrument. The two antennas are called the (1) LOOP antenna and the (2) SENSE antenna.

The loop antenna can sense the direction of the signal from the station, but cannot discriminate whether the station is in front or behind the aircraft. The sense antenna can discriminate direction, and solves the ambiguity of the loop antenna.

The receiver unit has tuning dials to select the station frequency A volume control allows the audible volume to be controlled for identifying the station. The volume can be reduced to prevent interference with other communications. You should, however, continuously monitor the identifier while using the NDB for navigation.

The navigational display contains a compass rose dial graduated in 5 degree increments from 0° to 355°, a pointer with an arrow on one end, and a square form on the other end. We will call the arrow end the “Pointer”, and the square end the “Tail” for the sake of identification.

There are 2 types of compass rose dials that can exist in the navigational unit. One is a fixed compass rose, called a “Fixed Card” ADF. Zero degrees is always shown on top of the card. The “Rotateable Card” ADF allows the compass rose card to be rotated. Interpretation of these displays will be more fully described in later paragraphs.

Non-Directional Beacon (NDB)

Non-Directional Beacons are depicted on aeronautical charts as a circular band of magenta colored dots. A rectangular magenta box near the NDB symbol shows the name of the station, the 2 or 3 letter identifier, and the Morse code transmitted by the station.

NDBs may be located on the surface of airports, or may be within a few miles from an airport. Sometimes they are co-located with the Outer Marker in ILS approaches. The NDB provides two principal functions; (1) homing for VFR operations, and (2) ADF instrument approach capability for IFR operations.

Because the frequency is below and within the commercial AM band, reception is subject to the same atmospheric disturbances as AM radio, in particular, noise generated by lightening.

ADF Orientation

The pointer end of the ADF navigation unit ALWAYS POINTS TO THE STATION. The degree reading on the display is dependent on the aircraft heading. In the diagram if the heading of the aircraft changes, the arrow will always point to the station and the degree reading on the instrument which the pointer indicates also changes..

Fixed Card ADF

The relationship of the aircraft to the station is referred to as “bearing to the station” (MB). There are 2 elements to determining MB. One is the Relative Bearing (RB) which is the reading of the ADF DIAL: the other the Magnetic Heading (MH) of the aircraft. This relationship is given by the equation

MB (to the station) = RB + MH

In the example, the MH of the aircraft is 270° as read on the compass. The RB read from the ADF dial is 45° . Therefore the MB to the station = 270° + 45° = 315°. This equation applies to any problem on the FAA Written Exam relating to the Fixed Card ADF. If any two values are known, the third can be computed.

Moveable Card ADF

Some aircraft are equipped with an ADF instrument in which the dial face of the instrument can be rotated by a knob. This is called a Moveable Card ADF. By rotating the card such that the Magnetic Heading (MH) of the aircraft is adjusted to be under the pointer at the top of the card, the Bearing to the Station (MB) can be read directly from the compass card. More sophisticated instruments of later design automatically rotate the compass card of the instrument to agree with the magnetic heading of the aircraft. Thus MB to the station can be read at any time without manually rotating the compass card on the ADF face.

Area Navigation

Three types of navigation receivers can be called Area Navigation. They are:

  • RNAV
  • LORAN
  • GPS

RNAV

This type of navigation allows a pilot to fly a selected course to a predetermined point without the need to overfly ground-based navigation stations. Flight can be from waypoint to waypoint. A waypoint is a position determined either by Latitude/Longitude or Radial and distance from a VORTAC or VOR/DME station.

These navigation receivers use VOR/DME or VORTAC stations as a base point. A waypoint is defined as a distance along a radial of the VOR. The RNAV equipment electronically translates (moves) the VOR station to the waypoint position. You then fly the VOR as though it were located at the waypoint’s geographical location.

Through triangulation, the navigation unit measures the radial and distance of leg A. By knowing the entered data for leg B, the azimuth and distance to the waypoint along path C is repeatedly calculated calculated. It is as though the VOR were located at the waypoint position.

Long Range Navigation (LORAN)

LORAN operates on the principle of time measurement. A Master station and up to to 4 secondary stations transmit a synchronized pulse. The time differential between the master station pulse and the secondary pulses is measured. From this data, the communication receiver can calculate the position of the aircraft within 0.25 Nm or better. The North East LORAN chain is shown. A database of airports and navigational facilities can be loaded into the memory of the LORAN unit.

The LORAN unit can indicate:

  • Present Position - in Latitude/Longitude and/or relative to a destination, waypoint or checkpoint.
  • Bearing and distance to your destination.
  • Groundspeed and estimated time enroute.
  • Course Deviation Indicator.
  • Storage in memory of all US airports, pilot selected fixes, minimum enroute and obstruction clearance altitudes, and Class B and C airspace warnings.
  • Continuous computation of bearings and distances to the nearest airports. Computation of wind direction and velocity.
  • Add ons, such as fuel flow analyzers to estimate fuel needed to reach destination and alternates; ELT’s to transmit exact location of ELT.

Add-on programmable and updatable databases.

Since LORAN operates on a low-frequency signal, it is subject to the same disturbances that AM radio sustains. It is possible to loose signal when operating near thunderstorm and in heavy rain areas.

The LORAN receivers know the frequency of the Master and secondary stations; no tuning by the pilot is necessary.

Global Positioning System (GPS)

The GPS system is the latest in technology that can be used by aircraft. It has many of the attributes of LORAN. The complete system will contain up to 21 satellites in earth orbit. The "clocks" and "positional data" is updated periodically to insure accuracy of the data from the satellites. It sense 4 or more satellites in orbit. The system is maintained by the US Department of Defense.

Like LORAN, it operates on a time-based methodology. Each satellite transmits coded pulses indicating it’s position, and the precise time the pulses are sent. The GPS unit listens to the satellite’s signal, and measures the time between the satellites transmission and receipt of the signal. By the process of triangulation among the several satellites being received, the unit computes the location of the GPS receiver. Not only can Latitude and Longitude be calculated, but altitude as well.

Like LORAN, the GPS unit contains data about all the commercial airports in the US, including runway lengths, directions, and location. There are numerous forms of display among the various manufacturer. The units can range from “hand held” to “panel mount” with altitude information input from an encoding altimeter. They can warn of Class B, C, and Prohibited and Restricted airspace. They can calculate direction and time to nearest suitable alternate airports in event of emergency.

The database in most units can be updated via a connection to a Personal Computer. The maximum error is within 100 meters (0.05 Nm). Work is in progress to give the GPS system adequate precision for instrument approaches.

No frequency tuning is required, as the frequency of the satellite transmissions are already known by the receiver.

Work is currently underway to provide sufficient accuracy for use of GPS for instrument approaches.