The two most fundamental methods of finding your way in an airplane are pilotage—the identification of present position and direction of flight by seeing features on the ground, and Dead Reckoning. Dead reckoning is the navigation procedure to plot and fly a course based solely on mathematical calculations. Dead Reckoning and Pilotage are so interdependent, though, that they are essentially one method. Ask a pilot how he intends to navigate to his destination, and if he says "by Dead Reckoning," one knows that he also intends to look out the window at the ground features to check his progress. No assistance from electronic aids to navigation. If the weather is good, and it's daytime, and you can see the ground at all times, DR can be very satisfactory. After all, both Charles Lindbergh and Amelia Earhart soloed across the Atlantic navigating in this manner.

Three properly-performed actions are necessary for dead reckoning navigation to get you to your destination, at the estimated time of arrival:

1. The pilot must properly calculate the course and fuel consumption.
2. The pilot must accurately fly the aircraft.
3. The weather service must correctly predict the winds aloft.

Omitted is "Put enough fuel in the tanks for the journey, and then some." That slight oversight has ruined more than one perfectly-navigated flight. Few will argue that if the aircraft is running low on fuel, the fuel indicator becomes the most fascinating gauge on the panel. The first two items in the list are under the pilot's control and most pilots have the good sense to do them right. The third criteria is the one that notches up the interest level. Winds-aloft predictions are seldom accurate, and may never be. Sometimes the miss is huge. No big deal, though, if you can see the ground, have chart in hand, and it is properly oriented to follow your flight. You simply adjust for the wind based on landmarks spotted below. Assume that your estimated course was to take you directly over the power company with its 100-ft. tall smoke stacks. But there it is a mile or so to your left, its plume of white smoke laying down horizontally blowing towards you. Obviously, you must turn left some few degrees to get back on course plus make a heading correction once back on course to accommodate those strong winds. After returning to course you check the next landmark to see how good your heading correction was, and so on for each landmark, until you reach your destination. Very simple in theory. There is nothing inherently inaccurate about dead reckoning; it is limited only by the information provided. Dead reckoning is, in fact, the basis of all navigation.

The equator is an imaginary circle equidistant from the poles of the earth. Circles parallel to the equator (lines running east and west) are parallels of latitude. They are used to measure degrees of latitude north or south of the equator. The angular distance from the equator to the pole is one-fourth of a circle, or 90°. Thus latitude would run from 90° North to 90° South of the equator. Meridians of longitude are drawn from the North Pole to the South Pole and are at right angles to the equator. The "Prime Meridian" which passes through Greenwich, England, is used as the zero line from which measurements are made in degrees east and west to 180°. Any specific geographical point can thus be located by reference to its longitude and latitude. From the earliest days, determining latitude was relatively simple; measure the height of the sun with a sextant. Longitude was a different matter. Greenwich Observatory was set up by King Charles II in 1675 to study means of fixing longitude, and the observatory became the acknowledged world authority on the subject. The telescopes and other instruments there determined the exact position of the meridian, and in 1884 an international conference in Washington agreed that Greenwich should be sited at zero longitude By using meridians, direction from one point to another can be measured in degrees, in a clockwise direction from true north. Draw a course line on a chart and measure the angle which this line forms with a meridian. When moving north or south by one degree of latitude, the distance remains approximately the same whether the observer is at the equator or in London, New York, Tokyo, or Sydney. That number would be unchanged vs. latitude if the earth were a perfect sphere, which it isn't. The earth is somewhat flattened at the poles. The distance situation is quite different when moving east or west.

The circumference of the earth is divided into 360°. Each degree is further divided into 60 minutes. If you move one minute east or west on the equator, you have gone one nautical mile. Thus a nautical mile is the circumference of the earth divided by 360, giving the distance in one degree, and that is further divided by sixty for the distance in one minute of arc. Earliest estimates of the earth's diameter was 18,000 statute miles. As methods of measurement improved over the years, the earth's circumference "increased" to 24,901.55 statute miles. Hence the distance of a nautical mile similarly increased with time, too. What a nightmare that must have been for cartographers and navigators. Regardless of calculations of the earth's circumference, the nautical mile has been standardized at 6076.113 feet, plus another gazillion decimals. One nm = 1.15 statute miles for the purposes of estimating. Again, there are a gazillion decimals in the actual conversion. Using 1.15 will not fly you into a mountain. There is a lot of lore behind the knot ... tying knots into lines and clocking the time for each to pass a sailing ship, etc. Suffice it to say that a knot is one nautical mile per hour and hence is 1.15 statute miles per hour.

The first is VFR weather vs. IFR weather. If the weather is "good," which the FAA defines as a visibility of at least three statute miles and that you can maintain at least 500 ft. clearance from the clouds above you, then you may fly by Visual Flight Rules. Flying at VFR minimums is not very comfortable and you better know where those 1000 ft. TV towers are, because you'll never see them in time to avoid them. They are marked on your charts. In practical terms, VFR is 3 miles visibility and 1000 ft. ceiling, because the minimum altitude one can fly in an uncongested area is 500 ft. above the surface. But that minimum altitude increases to 1000 ft. above obstacles near cities. So, if you depart VFR from Podunk Hollow Airport, with reported ceilings of 1200 ft., you're not going to be legal with that ceiling flying above New York City with all of its skyscrapers reaching for your landing gear.

Pilots must fly by Instrument Flight Rules, or IFR, if the weather is below VFR minimums, or if they are in Class A airspace, which is anything above 18,000 ft. MSL. A pilot must be instrument rated to fly IFR, which requires passing a written test plus demonstrating the ability to safely fly and land the aircraft solely by reference to the instruments. Before an IFR flight, an IFR flight plan must be filed with Air Traffic Control (ATC) and it must be approved. Normally ATC personnel read back the approved flight-plan shortly before departure. It can be brief and pleasant, such as "Approved as Filed," or, still brief, but less pleasant such as "Approved as Filed with the following changes ..." and then they completely modify your routing. Pilots may file an IFR flight plan no matter what the weather. There are times when it is prudent to file IFR even in VFR weather. For instance, it might be wise to file IFR if one were to fly a small, single-engine plane over a large body of water after dark when the horizon is difficult to discern. Not supposed to file IFR though if you're not IFR rated. All U.S. commercial airliners file IFR for every flight.

The second subject worth mentioning is flight altitude. A pilot doesn't hob-nob about in airspace at whatever altitude they want. The FAA has a set of rules for flight altitude. And, wouldn't you know, there is one set of rules for VFR flight and a slightly different set of rules for IFR flight. So here goes. In a moment you'll learn an easy way to remember these rules.

If your VFR flight is above 3000 ft. AGL (Above Ground Level) when flying a magnetic course of 0° to 179° fly at ODD thousands of feet plus 500 feet. In non-gov't. language, fly at 3500 ft. or 5500 ft., or 7500 ft. and so forth if your magnetic course is from 0° to 179°. And of course, from 180° to 359° magnetic course, your altitude should be EVEN thousands plus 500 ft.—like 4500 ft., 6500 ft., 8500 ft., etc. How does one remember that rule? It's easy. "Easterners are Odd" will do it. If your VFR course is easterly, i.e., from 0° to 179° fly at odd thousands plus 500 ft. If one leg of your flight is easterly and you are at 5500 ft., and the next leg is to the northwest, you need to change your altitude to adhere to the rule. Notice that the term "magnetic course" has been carefully used throughout this altitude discussion. With a crosswind, the aircraft's magnetic heading will differ from the planned magnetic course.

How does IFR differ? Forget the "plus 500 ft" adder. So from 180° to 359° on your IFR flight, fly at 4000 ft., 6000 ft., 8000 ft., etc. All of this is a moot point on IFR, though, because ATC assigns your altitude, at which you are obligated to fly even if it violates this altitude rule. Not to worry, though, ATC won't assign your Cessna 172 to 17,000 feet. The IFR flight plan that you filed includes your aircraft type, and since computers never make mistakes ... The pilot must set the aircraft's altimeter to the local barometric pressure or else it will not show the correct altitude, which is vital to know when near the ground. At 18,000 feet and above, pilots no longer need worry about local barometric pressures. They uniformly set their altimeters to 29.92 in. Hg. and adhere to the altimeter reading regardless of the actual barometric pressure. Also, from 18,000 feet and above, the term Flight Level is used, rather than actual altitude. Drop the last two zeros of the altitude for the Flight Level. Thus, 24,000 feet becomes Flight Level 240, written FL240, and spoken Flight Level Two-Four-Zero.

IFR Flight Altitudes

The Non Directional Beacons [NDBs] are the oldest established, and though technically obsolescent, still the most common radio navigation aid. The beacons are usually located at or near an airfield although a very few are still sited to mark waypoints along air routes. The reason they are called 'non directional' is that the aural radio ranges they originally replaced had directional antennas. The NDBs transmit an omni-directional carrier signal in the low frequency band between 190 and 535 kHz. Their effective range is primarily dependent on the operating power. Most inland NDBs have a transmitter power between 100 and 500 watts providing a range, during daylight hours, usually between 40 - 100 nm but tending toward the lesser figure. The rated coverage of each NDB is shown in the ERSA entry for the airfield or waypoint. Low power NDBs, known as 'locators' with a range of 30 nm or less, are sited around major airports and are associated with their Instrument Landing Systems [ILS]. There are also high power [2 - 3 kilowatt] NDBs sited near the coast to provide guidance for over water routes, their over water range being much greater than their inland range.

Identification
The carrier wave is transmitted on a specific frequency but a two or three letter Morse code signal is continually superimposed on the carrier for NDB identification. The frequency and ident for each beacon is given in ERSA and shown on VNCs, ERC(L)s and VTCs. Some NDBs may provide an intermittent 'voice-over' facility for airfield information. If you want to practice, the Morse code dots and dashes are rendered phonetically as dits and dahs. Broadcast stations must be used with caution because of identification problems. There are long intervals between station identification calls and even then the transmitter to which you are tuned may be relaying programs from another station. The information contained in ERSA may not be up to date.

The ADF, or radio compass, equipment consists of an antenna system, a receiver/control box system and a panel mounted indicator instrument. The antenna system comprises a loop antenna and a sense antenna which, depending on the age of any particular unit, may be completely separate or combined into one unit. The ADF receiver includes the frequency selector [probably 190 to 1799 kHz] and usually some test capability. The loop antenna nowadays may be a fixed square ferrite core with two perpendicular windings and may be coupled with a goniometer – [a device for measuring angles, with a great number of scientific applications] – in the receiver. Such a system automatically ascertains the direction of the transmitter relative to the longitudinal axis of the aircraft. Hence the reason for the term "Automatic" DF because in earlier days the loop antenna was a physical loop [mounted on top of, or beneath, the fuselage and often enclosed in an egg shaped fairing] which, simply put, had to be manually rotated by the operator to find the direction of the transmitter, which was read off a scale. At that time, and later, the sense antenna was a wire from the top of the tail fin to a fuselage connection. The output from the receiver is fed to the panel-mounted instrument, which is a needle indicating the direction to the NDB, or broadcast station, as an angle relative to the aircraft's longitudinal axis. Behind the needle is a circular card marked off in 5 degree azimuth divisions from 0° to 355° with a mark at the top dead centre [TDC] indicating the aircraft's nose. Depending on the age of the instrument that card may be fixed, in which case 0° is always at TDC, or, more commonly, manually rotatable by turning a heading knob on the instrument. If the card is rotated so that the aircraft's current magnetic heading is situated at TDC then the head of the needle indicates the magnetic track to the transmitter and the tail of the needle indicates the reciprocal bearing – the aircraft's magnetic bearing from the station. When using the ADF indicator it should be normal practice to adjust the card whenever the aircraft's heading is changed. The illustration shows the ADF instrument with the heading knob [HDG] rotated so that the aircraft's heading of 350° magnetic is at TDC, the needle head indicates the track to the NDB is 155° magnetic while the bearing from the transmitter is 335°. However, whether the card is fixed or rotatable, the head of the needle should always point directly to the transmitter and the angle [the number of degrees] between TDC and the head of the needle is always the angle between the fore and aft axis and the direction of the transmitter. In the illustration that angle is 10 + 155 = 165°. Heavier aircraft are usually fitted with a more complex, and very expensive, form of ADF called a Radio Magnetic Indicator [RMI] which incorporates, or is slaved to, a directional gyro. It may also have a two needle display, the second needle being tuned to another navigation aid which of course makes position fixing remarkably easy.

There are several applications for the ADF in light aircraft cross country VMC navigation – remembering the Visual Flight Rules require that the pilot must be able to navigate by reference to the ground and position fixes must be taken at least every 30 minutes. The applications briefly described below will be detailed in the 'Using the ADF' module. Position fixes. If two [or better - three] transmitters are in range then the bearing from each can be ascertained, the lines of position roughly plotted on the chart [after converting to true bearings] and the aircraft position will be close to the intersection point. In most of Australia to have two NDBs in range at the same time is not so common and three would be most unlikely, so the most likely position fixing use is to combine a surface line feature with an NDB bearing. Running fix / distance from NDB. The 1 in 60 rule can be applied when the aircraft is within range of a transmitter by turning the aircraft so that the station is abeam and then measuring the degrees traversed against time. This is a form of running fix in that two bearings are taken, at an interval, from one source and the aircraft's position is the distance along the second LOP from the NDB. For example:- Distance [nm] to NDB = elapsed time [minutes] × ground speed [knots] / degrees traversed.

Homing & tracking to or from an NDB. If there is no crosswind component then tracking toward an NDB is quite simple, just keep the head of the ADF needle at TDC and you will arrive overhead; the track over the ground will be straight and the magnetic heading consistent. However if there is a crosswind component and you just endeavor to keep the head of the ADF needle at TDC, you will eventually arrive but, due to the drift, the track followed will be curved and the magnetic heading will need to be consistently changing. This is called homing, and you will arrive at the NDB on an into-wind heading. Thus tracking, or flying directly towards, or from, an NDB is exactly the same as tracking from A to B – you have to calculate a wind correction angle. Passage overhead an NDB is signified by a "cone of silence" [if the ident volume has been turned up beforehand] and the needle then swinging to the reciprocal bearing. Using the ADF probably appears to be fairly simple, which it is, but there will be difficulties, for the uninitiated in perceiving, from the position of the needle, the headings to fly when attempting to intercept and then track along a particular magnetic bearing to or from the ground station. As in all navigation you should always maintain an awareness of the aircraft's position in terms of being north, south, east or west of the NDB and, when initiating a turn, think in the same terms e.g. a left turn will take you further east.

NDB/ADF Errors

• Electrical interference. Radio waves are emitted by the aircraft alternator in the frequency band of the ADF. An alternator suppressor is fitted to contain those emissions but this component does not have a long life and it is wise to test the ADF for correct operation during pre-flight checks. The test is made by selecting a transmitter – which must be a reasonable distance away, say 30 nm – then watch the ADF needle during the engine run up. If the needle moves as rpm increase there is electrical interference and probably the alternator suppressor should be replaced. Magnetos may also interfere with the ADF. Thunderstorms emit electrical energy in the NDB band and will deflect the ADF needle towards the storm.
• Twilight/night effect. Radio waves arriving at a receiver come both directly from the transmitter – the ground wave – and indirectly as a wave reflected from the ionosphere – the sky wave. The sky wave is affected by the daily changes in the ionosphere, read the ionisation layers section in the Aviation Meteorology Guide. Twilight effect is minimal on transmissions at frequencies below 350 kHz.
• Terrain and coastal effects. In mountainous areas NDB signals may be reflected by the terrain which can cause the bearing indications to fluctuate. Some NDBs located in conditions where mountain effect is troublesome transmit at the higher frequency of 1655 kHz. Ground waves are refracted when passing across coast lines at low angles and this will affect the indicated bearing for an aircraft tracking to seaward and following the shore line.
• Attitude effects. The indicated bearing will not be accurate whilst the aircraft is banked.

A useful ADF application in visual navigation is to locate a particular NDB and then track - or home - directly to it. The ADF receiver is tuned to the NDB frequency, and the audio volume turned up, so that the NDB can be identified as soon as the aircraft comes within range. The ADF needle indicates the bearing to the NDB and the wind correction angle necessary to maintain that track is then ascertained by bracketing, a technique which bears some similarity to the double track error method. [The term 'bracketing' is derived from the artillery technique for ranging the target by deliberately placing initial rounds behind and in front of it.] Note: this sequence is best performed if the heading being flown is positioned on the ADF card at TDC, the diagrams in the left column below indicate the readings with those settings.

Needle Position	Compass Heading	Event Sequence
	060°	Position A. When receiving the NDB signal turn the aircraft so that the head of the ADF needle is pointing to TDC, then check the heading from the compass. That heading is the track required to home directly to the NDB, for our example 060° magnetic. Rotate the ADF compass card to set 060° at TDC and the needle head will also indicate 060°. Remember that all heading changes should be logged.
	060°	Position B. As the flight progresses, holding the 060° heading, the crosswind causes the aircraft to drift to the south of the required track and the ADF needle has moved left about 5° to 055°.
	030°	Position C. We now have to make a first rough cut at the track error - it is best to initially overestimate so let's choose 15° and, applying the double track error technique, we turn left 30° on to an intercept heading of 030° magnetic. Positioning the 030° heading at TDC, the head of the needle will still initially indicate 055° but will move towards 060° as we close with the required track.
	045°	Position D. When the needle reaches 060° the 060° track to the NDB has been regained. Now halve the intercept angle [i.e. subtract the track error] and turn right onto an initial wind correction heading of 045° magnetic, i.e. the estimated track error was 15°, we turned left 30° onto the intercept heading of 030° and now, having regained the required track, we turn right 15° onto a wind correction heading of 045°. Now rotate the card to the 045° heading and the needle remains at the 060° bearing.
	045°	Position E. If the 15° WCA is correct then the ADF needle will remain at the 015° position whilst the 045° heading is maintained. However it is most likely that we have overcorrected, the aircraft will drift north of track, shown by the needle moving clockwise a few degrees from the 015° position so we now have to refine the wind correction angle.
	055°	Position F. We might guess that we have overestimated the WCA by about 5° so, applying the double track error technique, we turn right 10° on to an intercept heading of 055° magnetic. Positioning the 055° heading at TDC, the head of the needle will still initially indicate something greater than 060°, say 063°, but will move towards 060° as we close with the required track.
	050°	Position G. When the needle reaches 060° the 060° track to the NDB has been regained. Now halve the intercept angle and turn 5° left onto a wind correction heading of 050° magnetic, rotate the card to the 050° heading and, if we've estimated correctly, the needle will remain at the 060° bearing, maintaining a 10° WCA, while we continue along the required 060° track to the NDB.

Another useful application for the ADF in visual navigation is in determining track error when departing from an airfield equipped with an NDB or when overflying an NDB. For example: the flight plan calls for a departure – from overhead an NDB – on a track of 240° magnetic with any necessary wind correction to be assessed after departure – using the ADF – with the track recovery and heading correction to be made by a slightly modified double track error method. [The modification is that rather than timing the intercept leg to estimate track recovery we will use the ADF needle to indicate when we are back over the required track.] In this ADF application the ADF card may be used with the 0° position set at TDC or your personal preference may be to set the 240° heading at TDC. The diagrams and the text below indicate the procedure and the readings with 0° positioned at TDC but the additional text in italics is the procedure when rotating the card to the new heading for every change. Hopefully you will be able to see that the latter method is easier to handle. Note that when tracking away from an NDB we use the tail of the ADF needle, rather than the head, as the indicator.

Needle Position	Compass Heading	Event Sequence
	240°	Departing from overhead the NDB to track 240° magnetic. The magnetic compass heading is 240° [ i.e. no wind correction provision] and the tail of the ADF needle swings to the 0° position. [ With the 240° magnetic heading set at TDC the position of the needle relative to TDC is exactly the same as in the diagram but, on the background card, the needle tail indicates the 240° heading.]
	240°	Position B. As the flight progresses holding the 240° heading the crosswind causes the aircraft to drift to the south of the required track. The tail of the ADF needle has moved about 15° to 345° and is in the left half of the card. Thus the opening angle, or track error, is 15° and the tail of the needle represents the track made good, which is 15° to the left of the required track. [ With the 240° magnetic heading set at TDC the tail of the needle will indicate the track made good, 225° or an opening angle, or track error, of 15°.]
	270°	Position C. Use the double track error method to intercept the required track. The aircraft is turned 30° [2 × 15] onto a heading of 270° magnetic. The ADF needle tail initially moves 30° to 315° then commences to reverse direction as the 270° heading is maintained and the aircraft is closing the 240° track out. [ The aircraft is turned 30° [2 × 15] onto a heading of 270° magnetic and 270° magnetic is now set at TDC, the tail of the needle will then initially still indicate 225° but will move towards 240° as you close with the required track.]
	270°	Position D. When the needle has moved through a 15° arc and is back to the 30° left position [330°], on a heading of 270°, the 240° track out from the NDB has been regained. [ With the 270° magnetic heading set at TDC the 240° track out from the NDB has been regained when the tail of the needle reaches 240°.]
	255°	Position E. Subtract the track error [15°] and turn left onto the new heading of 255° which will then maintain the necessary 15° wind correction angle. The ADF needle moves 15° clockwise and the aircraft should hold the required track – if the heading is maintained and the needle kept at the 345° position. [ Subtract the track error [15°] and turn left onto the new heading of 255° which will then maintain the necessary 15° wind correction angle. Set the 255° magnetic heading at TDC, the tail of the needle now indicates 255°.] After flying this heading for a while you may find that you still have some drift - indicated by movement of the needle. In this case a small heading correction is usually enough compensation.

Whenever your track will pass abeam an NDB it is quite easy to obtain a running fix, using the 1 in 60 rule and a little mental arithmetic, providing you have a reasonable idea of your groundspeed. The technique is illustrated in the diagram:

Procedure

1. Tune and identify the NDB, set your heading at TDC and watch the ADF needle, the NDB is directly abeam when it moves to 90° either side of TDC, position A in the diagram.
2. Note the time and continue flying your heading, for example 040° magnetic.
3. When the needle has moved a sufficient amount to get a good reading [position B on the diagram], note the time and the bearing from the NDB, indicated by the tail of the needle. Let's say the needle has moved 10°, the elapsed time is 8 minutes, the bearing from the NDB is 110° magnetic and you reckon your groundspeed at 70 knots.
4. Now we calculate the distance we are along that bearing using the 1 in 60 rule: i.e the distance [nm] from the NDB = elapsed time [minutes] × ground speed [knots] / degrees traversed = 8 × 70/10 = 56 nm. The aircraft's position at time B is then 110°/56 nm from the NDB. The real difficulty now is to measure and plot that position on the navigation chart whilst flying the aircraft [not forgetting to convert the bearing to °true] - so best to get the passenger to do it.

If you are wondering what happened to the '60' in the 1 in 60 application the answer is it is negated by the usage of minutes in one factor and nautical miles per hour in another. In the diagram the dashed red line outlines the right angle triangle on which the calculation is based - the distance from the NDB to position B forms the hypotenuse.

The VHF Omni-directional Radio Ranges [VORs] operate in the Very High Frequency aviation navigation [NAV] band between 112.1 and 117.9 MHz. As VHF transmissions are line-of-sight the ground to air range depends on the elevation of the beacon site, the height of the aircraft and the power output. The VOR beacons are usually located at airfields but as they serve to define designated air routes [ airways] some are located away from airfields, often on high ground. A simplified concept of the ground beacon is that it simultaneously transmits two signals, a constant omni-directional signal called the reference phase and a directional signal which rotates through 360°, during a 0.03 second system cycle, and consistently varies in phase through each rotation. The two signals are only exactly in phase once during each rotation – when the directional signal is aligned to magnetic north.

Imagine a wheel with 360 spokes, at one degree azimuth spacing, with the VOR beacon being the hub. The spokes are numbered clockwise from one to 360 and each spoke or radial represents a magnetic bearing from the VOR beacon. The airborne navigation circuitry measures the phase angle difference between the directional signal phase received and the reference signal phase and interprets that as the angular, or 'radial', indication currently being received. Radials are identified by magnetic bearing – e.g. the 30° radial – and thus form the basis for VOR, and designated air route, navigation. Essentially the system indicates a line of position, from the selected VOR, on which the aircraft is located at any time. The beacon also transmits a Morse code aural identification signal at about 10 second intervals. The airborne system utilizing the VOR beacon transmissions usually consists of an antenna [a V - type dipole mounted horizontally on the fin or fuselage but could be the more expensive 'blade' or 'towel rail' types], a conventional VHF receiver [if combined with the VHF communications transceiver it is then called a NAV / COMM unit], navigation circuitry and the separate panel mounted navigation indicator or 'Omni Bearing Indicator' [OBI]. Some hand held aviation COMMS transceivers can also receive the NAV band VOR transmissions and appear to have some navigation circuitry but, from all reports, their VOR navigation capability, if it exists at all, is limited.

A basic Omni Bearing Indicator, like this Bendix-King model, has a manually operated radial or 'omni bearing' selector [OBS] which rotates an azimuth ring marked from 0° to 355°. The OBS selected radial – is indicated by the arrow at top dead centre and the reciprocal bearing is indicated by the bottom arrow. The other features of a basic OBI are the TO-FROM indicators, a deviation bar, a deviation indicator needle and a NAV / OFF alarm flag.

The TO-FROM Logic
The TO - FROM indications on the OBI are dependent on the aircraft's position relative to a notional ground baseline, formed perpendicular to the selected radial and passing through the beacon site. Unlike the NDB the indication is completely independent of the aircraft's heading. The navigation circuitry compares the difference between the radial being received and the radial selected. If the aircraft is located anywhere within range on the radial side of the baseline the 'FROM' indication will be displayed on the OBI and, if located within range on the reciprocal side, the 'TO' indication will be displayed. For example if the 030° radial is selected on the OBI, the ground baseline is established between 300° and 120°. If the radial received indicates the aircraft is anywhere in the blue shaded area of the diagram and no matter whether it is headed towards or away from the VOR, or in any direction whatsoever, the OBI will display 'FROM'. Similarly if it is in the yellow area the OBI will display 'TO' no matter which direction the aircraft is headed. There are two areas of ambiguity – near bearings at right angles to the radial [e.g. shown at 120° and 300°] – where the OBI will give fluctuating indications, or display the 'OFF' flag.

The Course Deviation Indicator

The deviation bar and the deviation indicator needle together form the Course Deviation Indicator or CDI. If the needle is over the centre point the aircraft is then located at some position along the selected radial – or its reciprocal. The five division marks or dots either side of the centre point are spaced at two degree intervals, thus if the needle is over the third mark, left or right of centre, the aircraft is positioned at a radial six degrees in azimuth from the selected radial, or its reciprocal. [Actually the aircraft is at the centre mark and the needle indicates the position of the selected radial]. Full travel of the needle from the centre to either side represents 10° – or more – of azimuth. The ambiguity of whether the OBS selection is the radial or the reciprocal is determined by the TO / FROM indication; in the diagram at left 030 must be the radial as the aircraft is in the FROM area. When the aircraft passes overhead the beacon the needle will swing from side to side, the alarm flag may temporarily indicate that navigation is 'OFF' and the TO / FROM indication will reverse.

Difficulty for a non IFR trained pilot using the VOR is a lack of perception of which way to turn the aircraft to fly to a selected radial, using the CDI indications. However, for VFR purposes, this is easily ascertained if the pilot follows two simple rules:

1. To track FROM a VOR select the radial required and ensure FROM is indicated.
2. To track TO a VOR rotate the OBS until the CDI is centered and TO is indicated.

In both cases as wind effect drifts the aircraft off track the deviation indicator needle will move to one side and that movement indicates the direction to turn to regain track. i.e. turn towards the needle.

VOR Applications

Like the NDB / ADF there are several applications for the VOR in light aircraft cross country VMC navigation. Homing & tracking to a VOR. Even with a crosswind component tracking toward a VOR is quite simple, rotate the OBS until the CDI is centered and TO is indicated, turn onto that magnetic heading and then just keep the CDI centered and you will track more or less direct to the VOR. Tracking from a VOR. Rotate the OBS to the required track [radial], ensure FROM is indicated, turn onto that magnetic heading and just keep the CDI centered and you will maintain the track. Position fixes. If two VORs are in range then the bearing from each can be ascertained, roughly plotted on the chart [after converting to true bearings] and the aircraft position will be close to the intersection point of the LOPs. Alternatively a VOR bearing and a NDB bearing can be used or a VOR bearing and a line feature on the chart, the latter technique being the most frequently used. Running fix / distance from VOR. The 1 in 60 rule can be applied when the aircraft is within range of a transmitter by turning the aircraft so that the station is abeam and then measuring the degrees traversed against time, as in the NDB running fix application above. The advantage with the VOR is that the CDI needle indicates the degrees traversed. As in the NDB application the position fix is the distance along the second radial from the beacon.

A useful VOR application in visual navigation is to locate a particular VOR and then track - or home - directly to it. The receiver is tuned to the VOR frequency, and the audio volume turned up, so that the VOR can be identified as soon as the aircraft comes within range and the 'OFF' flag disappears. Rotate the OBS until the CDI is centered and 'TO' is displayed by the TO/FROM indicator, and turn onto that indicated magnetic heading. and then just keep the CDI centered and you will track more or less direct to the VOR.

Instrument landing system (ILS) facilities are a highly accurate and dependable means of navigating to the runway in IFR conditions. When using the ILS, the pilot determines aircraft position primarily by reference to instruments. The ILS consists of:

1. the localizer transmitter;
2. the glide path transmitter;>/li>
3. the outer marker (can be replaced by an NDB or other fix);
4. the approach lighting system.

ILS is classified by category in accordance with the capabilities of the ground equipment. Category I ILS provides guidance information down to a decision height (DH) of not less than 200 ft. Improved equipment (airborne and ground) provide for Category II ILS approaches. A DH of not less than 100 ft. on the radar altimeter is authorized for Category II ILS approaches. The ILS provides the lateral and vertical guidance necessary to fly a precision approach, where glide slope information is provided. A precision approach is an approved descent procedure using a navigation facility aligned with a runway where glide slope information is given. When all components of the ILS system are available, including the approved approach procedure, the pilot may execute a precision approach.

Ground Equipment

The primary component of the ILS is the localizer, which provides lateral guidance. The localizer is a VHF radio transmitter and antenna system using the same general range as VOR transmitters (between 108.10 MHz and 111.95 MHz). Localizer frequencies, however, are only on odd-tenths, with 50 kHz spacing between each frequency. The transmitter and antenna are on the centerline at the opposite end of the runway from the approach threshold.

The localizer back course is used on some, but not all ILS systems. Where the back course is approved for landing purposes, it is generally provided with a 75 MHz back marker facility or NDB located 3 to 5 NM from touchdown. The course is checked periodically to ensure that it is positioned within specified tolerances.

Signal Transmission

The signal transmitted by the localizer consists of two vertical fan-shaped patterns that overlap, at the center (see ILS Localizer Signal Pattern figure, below). They are aligned with the extended centerline of the runway. The right side of this pattern, as seen by an approaching aircraft, is modulated at 150 Hz and is called the "blue" area. The left side of the pattern is modulated at 90 Hz and is called the "yellow" area. The overlap between the two areas provides the on-track signal.

The width of the navigational beam may be varied from approximately 3º to 6º, with 5º being normal. It is adjusted to provide a track signal approximately 700 ft wide at the runway threshold. The width of the beam increases so that at 10 NM from the transmitter, the beam is approximately one mile wide.

The localizer is identified by an audio signal superimposed on the navigational signal. The audio signal is a two-letter identification preceded by the letter "I", e.g., " I-OW ". The reception range of the localizer is at least 18 NM within 10º degrees of the on-track signal. In the area from l0º to 35º of the on-track signal, the reception range is at least 10 NM. This is because the primary strength of the signal is aligned with the runway centerline.

Localizer Receiver

The localizer signal is received in the aircraft by a localizer receiver. The localizer receiver is combined with the VOR receiver in a single unit. The two receivers share some electronic circuits and also the same frequency selector, volume control, and ON-OFF control. The localizer signal activates the vertical needle called the track bar (TB). Assuming a final approach track aligned north and south (see ILS Localizer Signal Pattern figure, above), an aircraft east of the extended centerline of the runway (position 1) is in the area modulated at 150 Hz. The TB is deflected to the left. Conversely, if the aircraft is in the area west of the runway centerline, the 90 Hz signal causes the TB to deflect to the right (position 2). In the overlap area, both signals apply a force to the needle, causing a partial deflection in the direction of the strongest signal. Thus, if an aircraft is approximately on the approach track bur slightly to the right, the TB is deflected slightly to the left. This indicates that a correction to the left is necessary to place the aircraft in precise alignment. At the point where the 90 Hz and 150 Hz signals are of equal intensity, the TB is centered, indicating that the aircraft is located precisely on the approach track (position 3). When the TB is used in conjunction with the VOR, full scale needle deflection occurs 10º either side of the track shown on the track selector. When this same needle is used as an ILS localizer indicator, full-scale needle deflection occurs at approximately 2.5º from the center of the localizer beam. Thus the sensitivity of the TB is approximately four times greater when used as a localizer indicator as opposed to VOR navigation. In the localizer function, the TB does not depend on a correct track selector setting in Most cases; however, the pilot should set the track selector for the approach track as a reminder of the final approach. When an OFF flag appears in front of the vertical needle, it indicates that the signal is too weak, and, therefore, the needle indications arc unreliable. A momentary OFF flag, or brief TB needle deflections, or both, may occur when obstructions or other aircraft pass between the transmitting antenna and the receiving aircraft.

Transmitter

The glide slope provides vertical guidance to the pilot during the approach. The ILS glide slope is produced by a ground-based UHF radio transmitter and antenna system, operating at a range of 329.30 MHz to 335.00 MHz, with a 50 kHz spacing between each channel. The transmitter is located 750 to 1,250 feet (ft) down the runway from the threshold, offset 400 to 600 ft from the runway centerline. Monitored to a tolerance of ± 1/2 degree, the UHF glide path is "paired" with (and usually automatically tuned by selecting) a corresponding VHF localizer frequency.

Like the localizer, the glide slope signal consists of two overlapping beams modulated at 90 Hz and 150 Hz (see Glide Slope Signal Pattern figure, below). Unlike the localizer, however, these signals are aligned above each other and are radiated primarily along the approach track. The thickness of the overlap area is 1.4º or .7º above and .7º below the optimum glide slope.

This glide slope signal may be adjusted between 2º and 4.5º above a horizontal plane. A typical. adjustment is 2.5º to 3º, depending upon such factors as obstructions along the approach path and the runway slope. False signals may be generated along the glide slope in multiples of the glide path angle, the first being approximately 6º degrees above horizontal. This false signal will be a reciprocal signal (i.e. the fly up and fly down commands will be reversed). The false signal at 9º will be oriented in the same manner as the true glide slope. There are no false signals below the actual slope. An aircraft flying according to the published approach procedure on a front course ILS should not encounter these false signals.

Signal Receiver

The glide slope signal is received by a UHF receiver in the aircraft. In modern avionics installations, the controls for this radio are integrated with the VOR controls so that the proper glide slope frequency is tuned automatically when the localizer frequency is selected. The glide slope signal activates the glide slope needle, located in conjunction with the TB (see Glide Slope Signal Pattern figure, above). There is a separate OFF flag in the navigation indicator for the glide slope needle. This flag appears when the glide slope signal is too weak. As happens with the localizer, the glide slope needle shows full deflection until the aircraft reaches the point of signal overlap. At this time, the needle shows a partial deflection in the direction of the strongest signal. When both signals are equal, the needle centers horizontally, indicating that the aircraft is precisely on the glide path. The pilot may determine precise location with respect to the approach path by referring to a single instrument because the navigation indicator provides both vertical and lateral guidance. In the Glide Slope Signal Pattern figure, above, position 1, shows both needles centered, indicating that the aircraft is located in the center of the approach path. The indication at position 2 tells the pilot to fly down and left to correct the approach path. Position 3 shows the requirements to fly up and right to reach the proper path. With 1.4º of beam overlap, the area is approximately 1,500 ft thick at 10 nautical miles (NM), 150 ft at l NM, and less than one foot at touchdown. The apparent sensitivity of the instrument increases as the aircraft nears the runway. The pilot must monitor it carefully to keep the needle centered. As said before, a full deflection of the needle indicates that the aircraft is either high or low but there is no indication of how high or low.

Instrument landing system marker beacons provide information on distance from the runway by identifying predetermined points along the approach track. These beacons are low-power transmitters; that operate at a frequency of 75 MHz with 3 W or less rated power output. They radiate an elliptical beam upward from the ground. At an altitude of 1,000 ft, the beam dimensions are 2,400 ft long and 4,200 ft wide. At higher altitudes, the dimensions increase significantly.

Outer Marker (OM)

The outer marker (if installed) is located 3 1/2 to 6 NM from the threshold within 250 ft of the extended runway centerline. It intersects the glide slope vertically at approximately 1,400 ft above runway elevation. It also marks the approximate point at which aircraft normally intercept the glide slope, and designates the beginning of the final approach segment. The signal is modulated at 400 Hz, which is an audible low tone with continuous Morse code dashes at a rate of two dashes per second. The signal is received in the aircraft by a 75 MHz marker beacon receiver. The pilot bears a tone over the speaker or headset and sees a blue light that flashes in synchronization with the aural tone (see the Marker Beacon Lights figure, above right). Where geographic conditions prevent the positioning of an outer marker, a DME unit may be included as part of the ILS system to provide the pilot with the ability to make a positive position fix on the localizer. In some ILS installations, the OM is replaced by an NDB.

Middle Marker (MM)

Middle markers have been removed from all ILS facilities in Canada bur are still used in the United States. The middle marker is located. approximately .5 to .8 NM from the threshold on the extended runway centerline. The middle marker crosses the glide slope at approximately 200 to 250 ft above the runway elevation and. is near the missed approach point for the ILS Category l approach.

Back Marker (BM)

The back course marker (BM), if installed, is normally located on the localizer back course approximately four to six miles from the runway threshold. The BM low pitched tone (400 Hz) is beard as a series of dots. It illuminates the aircraft's white marker beacon light. An NDB or DME fix can also be used and in most locations replace the BM.

Various runway environment lighting systems serve as integral parts of the ILS system to aid the pilot in landing. Any or all of the following lighting systems may be provided at a given facility: approach light system (ALS), sequenced flashing light (SFL), touchdown zone lights (TDZ) and centerline lights (CLL-required for Category II [Cat II] operations.)

Runway Visibility Measurement

In order to land, the pilot must be able to see appropriate visual aids not later than the arrival at the decision height (DH) or the missed approach point (MAP). Until fairly recently, the weather observer simply "peered into the murk", trying to identify landmarks at known distances from the observation point. This method is rather inaccurate; therefore, instrumentation was developed to improve the observer's capability. The instrument designed to provide visibility information is called a transmissometer. It is normally located adjacent to a runway. The light source (see the Transmissometer figure, on the right) is separated from the photo-electric cell receiver by 500 to, 700 ft. The receiver, connected to the instrument readout in the airport tower, senses the reduction in the light level between it and the light source caused by increasing amounts of particulate matter in the air. In this way the receiver measures the relative transparency or opacity of the air. The readout is calibrated in feet of visibility and is called runway visual range (RVR).

Runway Visible Range (RVR)

The RVR is the maximum distance in the direction of take-off or landing at which the runway or the specified light or markers delineating it can be seen from a height corresponding to the average eye-level of pilots at touchdown. Runway visual range readings usually are expressed in hundreds of feet. For example, "RVR 24" means that the visual range along the runway is 2,400 ft. In weather reports, RVR is reported in a code: R36/4000 FT/D; meaning RVR for Runway 36 is 4000 ft and decreasing. Because visibility may differ from one runway to another, the RVR value is always given for the runway where the equipment is located. At times, visibility may even vary at different points along the same runway due to a local condition such as a fog bank, smoke, or a line of precipitation. For this reason, additional equipment may be installed for the departure end and mid-point of a runway. Runway visual range reports are intended to indicate bow far the pilot can see along the runway in the touchdown zone; however, the actual visibility at other points along the runway may differ due to the siting of the transmissometer. The pilot should take this into, account when making decisions based on reported RVR. Runway visual range is not reported unless the prevailing visibility is less than two miles or the RVR is 6,000 ft or less. This is so because the equipment cannot measure RVR above 6,000 ft. When it is reported, RVR can be used as an aid to pilots in determining what to expect during the final stages of an instrument approach. Instrument approach charts state the advisory values of visibility and RVR.  Runway visual range information is provided to the ATC arrival control. sector, the PAR position, and the control tower or FSS. It is passed routinely to the pilot when conditions warrant. RVR information may be included in aviation weather reports. Ground visibility will continue to be reported and used in the application of take-off and landing minima. At runways with a transmissometer and digital readout equipment or other suitable means, RVR is used in lieu of prevailing visibility in determining the visibility minima unless affected by a local weather phenomenon of short duration. The normal RVR reading is based on a runway light setting of strength 3. If the light settings are increased to strength 4 or 5, it causes a relative increase in the RVR reading. No decrease in the RVR reading is evident for light settings of less than setting 3. Pilots shall be advised when the runway light setting is adjusted to 4 or 5. If the RVR for a runway is measured at two locations, the controller identifies the touchdown location as "ALFA and the mid runway location as "Bravo". In all cases, the pilot can request a light setting suitable for his or her requirements. When more than one aircraft is conducting an approach, the pilot of the second aircraft may request a change in the light setting after the first aircraft has completed its landing. Because of the complex equipment requirements, RVR usually is only available at more active airports and not necessarily for all runways. If RVR equipment is not available or temporarily out of service for a given runway, the pilot uses the observer method to provide visibility information. In this case, the visibility is expressed as miles or fractions of a mile.