What is Air Navigation

The process of planning and directing the progress of an aircraft between selected geographic points or over a selected route. The primary tasks involved are planning the flight, guiding it safely along the desired route, conforming with the rules of flight and with special procedures such as noise abatement, and maximizing fuel efficiency.

The simplest form of air navigation is pilotage, in which the pilot directs the aircraft by visual reference to objects on the ground. More complex methods rely upon navigational aids external to the aircraft or upon self-contained, independent systems.

Dead reckoning is the process of estimating one's current position from measurements of the change in position since the last accurate position fix. In an aircraft, dead reckoning requires an airspeed indicator, outside-air-temperature gage, altimeter, clock, compass, and chart (map). The high accuracy and the increasing availability of real-time satellite based position fixes will eventually relegate dead reckoning to a backup role used primarily during outages of high quality satellite signals. See also Dead reckoning.

Air navigation begins with a flight plan. For a flight in visual meteorological conditions (VMC), a simple plan may start with drawing a course line on a specially designed aeronautical chart between the point of departure and the destination. From the chart and the course line, the pilot, using a protractor, determines the true course (in degrees clockwise from true north), magnetic variation, and distance to be flown, usually in nautical miles. Following a preflight weather briefing, the pilot has the necessary facts to prepare a simple flight plan: wind direction and velocity, which were included in the briefing; true course, which was determined from the course line on the chart; and true airspeed, which is calculated by applying the corrections for temperature and altitude to the indicated airspeed. With the wind, true course, and true airspeed, the pilot can construct a wind triangle to determine the effect of the wind on speed and heading. With the planned groundspeed and the distance to be flown in miles, the pilot can compute the flight time and the fuel required. Extra fuel is added to assure an adequate reserve upon landing.

The process assumes greater complexity for a higher-performance aircraft, for a longer flight, or for a flight in adverse weather. The flight plan of a commercial jet is commonly prepared on a large specially programmed computer.

Air navigation is three-dimensional, and selection of a flight altitude is an important part of the planning process. Light, piston-engine aircraft are usually operated at altitudes of 10,000 ft (3000 m) or less, while jet aircraft commonly operate at much higher altitudes, where they may take advantage of the jet's increased fuel economy. During flight planning for large turboprop and jet aircraft, the time en route and the fuel used at various altitudes or over different routes are often compared to select the optimum altitude and route.

Aircraft altitude may be obtained from a barometric altimeter, a radio altimeter, or the Global Positioning System (GPS). A barometric altimeter translates the linear pressure changes experienced during climbs or descents into an indication of aircraft altitude. A radio altimeter (also known as a radar altimeter) shows actual altitude above the terrain immediately below the aircraft. The Global Positioning System is designed to provide accurate altitude information, unaffected by the errors found in barometric altimetry.

All navigation requires special maps or charts such as topographic maps intended for pilotage or radio navigation charts intended to be used with ground radio aids. Aeronautical maps such as sectional charts show terrain contours and the elevation of significant terrain and other obstacles, such as mountain peaks, buildings, and television towers.

The operational phase of air navigation commences with the preflight cockpit check of clocks, radios, compasses, and other flight and navigation equipment.

After the flight is on course toward the first fix, the pilot's primary navigational task is to assure that the aircraft stays on course. If navigation is by pilotage, the pilot frequently checks the aircraft's position by reference to topographic features on the aeronautical chart being used, and the aircraft's heading is corrected as necessary to follow the course. Using dead reckoning, the pilot estimates the time that the aircraft is expected to be over the next fix, based on the speed thus far made good and the speed anticipated over the next segment.

The most common form of en route navigation over land masses in the western world uses a network of very high-frequency (VHF) omni-directional radio range (VOR) stations. VOR transmitters are spaced at strategic intervals, averaging about 100 mi (160 km) between stations within the United States, but sometimes much farther apart. Designated routes between VOR stations are referred to as airways.

VOR transmitters emit a directional signal which can be received and interpreted by suitably equipped aircraft. The pilot can then determine the direction from the ground station to the aircraft, measured in degrees magnetic and referred to as a radial. Distance from the station is displayed when a distance-measuring equipment (DME) transmitter is collocated with the VOR. An airway is defined by a series of VOR stations along an airway route. Modern automatic pilots can follow the selected VOR radial automatically, leaving the human pilot to monitor the flight and make necessary adjustments as succeeding stations are selected along the route. See also Autopilot; Distance-measuring equipment; VOR (VHF omni-directional range).

A high-altitude jet starts descending miles from the landing airport. Un-pressurized aircraft carrying passengers fly much lower, but their descent must be more gradual to minimize passenger discomfort in adjusting to the change in atmospheric pressure. Regardless of the aircraft type, the pilot estimates as accurately as possible the time and point to start descending. Every instrument approach for landing has its own chart, containing such things as the radio frequency and identification of the aids to be used, altitudes to be observed at various points during the approach, heading of the final approach course, minimum weather conditions required for the approach, missed approach instructions, and airport altitude.

The preferred aid for instrument landing use is ILS. It consists of two separate radio beams: one called the localizer is aligned very precisely with the runway centerline, and the other called the glide slope is projected upward at a slight angle from the landing touchdown point on the runway. By following the two beams very closely on the cockpit instruments, the pilot can line up with the runway and descend safely for landing, regardless of weather.

The navigation systems discussed thus far—VOR for en route use and ILS for terminal use—are sometimes augmented or replaced by other navigational equipment and systems. These systems are particularly useful for long-distance flights or when more conventional navigational aids are minimal or nonexistent. The table lists the principal frequency bands that are used by the electronic systems discussed, and indicates which systems are internationally standardized.

*Internationally standardized systems.

Sponsored Links

Top

air navigation, science and technology of determining the position of an aircraft with respect to the surface of the earth and accurately maintaining a desired course (see navigation).

Visual and Instrument Flight

The simplest and least sophisticated way to keep track of position, course, and speed is to use pilotage, a method in which landmarks are noted and compared with an aeronautical chart. Whether these landmarks are observed visually or on radar, this technique of air navigation is usually called flying under visual flight regulations (VFR). These establish the minimum weather conditions under which pilotage is permissible.

Pilotage is not satisfactory for long trips, especially over water or terrain lacking distinctive features. In these cases, or when weather conditions do not permit navigation by visual reference, planes must fly according to instrument flight regulations (IFR), which require that the aircraft be equipped with the necessary position-finding instruments and that the pilot be trained in operating those instruments. Also required under IFR is the filing of a flight plan with air traffic control authorities at the departure point. The aircraft is then cleared for a given course and a given altitude. Air traffic controllers monitor the craft until it reaches its destination.

Aircraft Instruments

Light aircraft, flown by pilotage, typically have a simple set of navigational instruments, including an airspeed indicator (see pitot static system), an aneroid altimeter, and a magnetic compass. For supersonic and hypersonic aircraft the airspeed indicator is altered to show the airspeed as a Mach number, which is the ratio of the speed of an aircraft to the speed of sound. Advanced aircraft also use electronic systems to give the pilot highly accurate positional information for use during landing. The Instrument Landing System enables an airplane to navigate through clouds or darkness to an airport's runway; the Microwave Landing System, installed in U.S. airports beginning in 1988, is capable of landing the plane automatically, although the pilot always has the option of overriding manually.

Other navigational aids include the radio altimeter, a radar device that indicates the distance of the plane from the ground; the ground-speed indicator, which operates by measuring the Doppler shift in a radio wave reflected from the ground; and, in commercial airliners, the flight management computer, which can display altitude, speed, course, wind conditions, and route information, as well as monitor the airplane's progress through the airway. Other similar systems use inertial devices such as free-swinging pendulums and gyroscopes as references in determining position. These automated and semi automated procedures free the pilot from many of the activities previously necessary for navigation and thus allow the pilot to concentrate on actually flying the aircraft. Another device which is useful in this way is the automatic pilot, which interprets data on direction, speed, attitude, and altitude to maintain an aircraft in straight, level flight on a given course at a given speed.

Airways and Radio Ranges

Basic to air traffic control are special air routes called airways. Airways are defined on charts and are provided with radio ranges, devices that allow the pilot whose craft has a suitable receiver to determine the plane's bearing and distance from a fixed location. The most common beacon is a very high frequency omni directional radio beacon, which emits a signal that varies according to the direction in which it is transmitted. Using a special receiver, an air navigator can obtain an accurate bearing on the transmitter and, using distance-measuring equipment (DME), distance from it as well.

The system of radio ranges around the United States is often called the VORTAC system. For long distances other electronic navigation systems have been developed: Omega, accurate to about two miles (3 km); Loran-C, accurate to within .25 mi (.4 km) but available only in the United States; and the Global Positioning System (GPS), a network of 24 satellites that is accurate to within a few yards and is making radio ranging obsolete.

The principles of air navigation are the same for all aircraft, big or small. Air navigation involves successfully piloting an aircraft from place to place without getting lost, breaking the laws applying to aircraft, or endangering the safety of those on board or on the ground.

Air navigation differs from the navigation of surface craft in several ways: Aircraft travel at relatively high speeds, leaving less time to calculate their position en route. Aircraft normally cannot stop in mid-air to ascertain their position at leisure. Aircraft are safety-limited by the amount of fuel they can carry; a surface vehicle can usually get lost, run out of fuel, then simply await rescue. There is no in-flight rescue for most aircraft. And collisions with obstructions are usually fatal. Therefore, constant awareness of position is critical for aircraft pilots.

The techniques used for navigation in the air will depend on whether the aircraft is flying under the visual flight rules (VFR) or the instrument flight rules (IFR). In the latter case, the pilot will navigate exclusively using instruments and radio navigation aids such as beacons, or as directed under radar control by air traffic control. In the VFR case, a pilot will largely navigate using dead reckoning combined with visual observations (known as pilotage), with reference to appropriate maps. This may be supplemented using radio navigation aids.

The first step in navigation is deciding where one wishes to go. A private pilot planning a flight under VFR will usually use an aeronautical chart of the area which is published specifically for the use of pilots. This map will depict controlled airspace, radio navigation aids and airfields prominently, as well as hazards to flying such as mountains, tall radio masts, etc. It also includes sufficient ground detail - towns, roads, wooded areas - to aid visual navigation. In the UK, the CAA publishes a series of maps covering the whole of the UK at various scales, updated annually. The information is also updated in the notices to airmen, or NOTAMs.

The pilot will choose a route, taking care to avoid controlled airspace that is not permitted for the flight, restricted areas, danger areas and so on. The chosen route is plotted on the map, and the lines drawn are called the track. The aim of all subsequent navigation is to follow the chosen track as accurately as possible. Occasionally, the pilot may elect on one leg to follow a clearly visible feature on the ground such as a railway track, river, highway, or coast.

Adjustment of an aircraft's heading to compensate for wind flow perpendicular to the ground track

When an aircraft is in flight, it is moving relative to the body of air through which it is flying; therefore maintaining an accurate ground track is not as easy as it might appear, unless there is no wind at all — a very rare occurrence. The pilot must adjust heading to compensate for the wind, in order to follow the ground track. Initially the pilot will calculate headings to fly for each leg of the trip prior to departure, using the forecast wind directions and speeds supplied by the meteorological authorities for the purpose. These figures are generally accurate and updated several times per day, but the unpredictable nature of the weather means that the pilot must be prepared to make further adjustments in flight. A general aviation (GA) pilot will often make use of either the E6B flight computer - a type of slide rule - or a purpose-designed electronic navigational computer to calculate initial headings.

The primary instrument of navigation is the magnetic compass. The needle or card aligns itself to magnetic north, which does not coincide with true north, so the pilot must also allow for this, called the magnetic variation (or declination). The variation that applies locally is also shown on the flight map. Once the pilot has calculated the actual headings required, the next step is to calculate the flight times for each leg. This is necessary to perform accurate dead reckoning. The pilot also needs to take into account the slower initial airspeed during climb to calculate the time to top of climb. It is also helpful to calculate the top of descent, or the point at which the pilot would plan to commence the descent for landing.

The flight time will depend on both the desired cruising speed of the aircraft, and the wind - a tailwind will shorten flight times, a headwind will increase them. The E6B has scales to help pilots compute these easily.

The point of no return, sometimes referred to as the PNR, is the point on a flight at which a plane has just enough fuel, plus any mandatory reserve, to return to the airfield from which it departed. Beyond this point that option is closed, and the plane must proceed to some other destination. Alternatively, with respect to a large region without airfields, e.g. an ocean, it can mean the point before which it is closer to turn around and after which it is closer to continue. Similarly, the Equal time point, referred to as the ETP, is the point in the flight where it would take the same time to continue flying strait, or track back to the departure aerodrome. the ETP is not dependant on fuel, but wind, giving a change in ground speed out from, and back to the departure aerodrome. In Nil wind conditions, the ETP is located halfway between the two aerodromes, but in reality it is shifted depending on the wind speed and direction.

The aircraft that is flying across the Ocean for example, would be required to calculate ETPs for one engine inoperative, depressurization, and a normal ETP; all of which could actually be different points along the route. For example, in one engine inoperative and depressurization situations the aircraft would be forced to lower operational altitudes, which would affect its fuel consumption, cruise speed and ground speed. Each situation therefore would have a different ETP.

Commercial aircraft are not allowed to operate along a route that is out of range of a suitable place to land if an emergency such as an engine failure occurs. The ETP calculations serve as a planning strategy, so flight crews always have an 'out' in an emergency event, allowing a safe diversion to their chosen alternate.

The final stage is to note which areas the route will pass through or over, and to make a note of all of the things to be done - which ATC units to contact, the appropriate frequencies, visual reporting points, and so on. It is also important to note which pressure setting regions will be entered, so that the pilot can ask for the QNH (air pressure) of those regions. Finally, the pilot should have in mind some alternative plans in case the route cannot be flown for some reason - unexpected weather conditions being the most common. At times the pilot may be required to file a flight plan for an alternate destination and to carry adequate fuel for this. The more work a pilot can do on the ground prior to departure, the easier it will be in the air.

In many respects this is similar to VFR flight planning except that the task is generally made simpler by the use of special charts that show IFR routes from beacon to beacon with the lowest safe altitude (LSALT), bearings (in both directions) and distance marked for each route. IFR pilots may fly on other routes but they then have to do all of these calculations themselves with the LSALT calculation being the most difficult. The pilot then needs to look at the weather and minimum specifications for landing at the destination airport and the alternate requirements. The pilot must also comply with all the rules including their legal ability to use a particular instrument approach depending on how recently they last performed one.

In recent years, strict beacon-to-beacon flight paths have started to be replaced by routes derived through Performance Based Navigation (PBN) techniques. When operators are developing flight plans for their aircraft, the PBN approach encourages them to assess the overall accuracy, integrity, availability, continuity and functionality of the aggregate navigation aids present within the applicable airspace. Once these determinations have been made, the operator develops a route that is the most time and fuel efficient while respecting all applicable safety concerns — thereby maximizing both the aircraft's and the airspace's overall performance capabilities.

Under the PBN approach, technologies are able to evolve over time (ground beacons become satellites become...) without requiring the underlying aircraft operation to be recalculated. As well, navigation specifications used to assess the sensors and equipment that are available in an airspace can be cataloged and shared to inform equipment upgrade decisions and the ongoing harmonization of the world's various air navigation systems.

Once in flight, the pilot must take pains to stick to plan, otherwise getting lost is all too easy. This is especially true if flying in the dark or over featureless terrain. This means that the pilot must stick to the calculated headings, heights and speeds as accurately as possible, unless flying under visual flight rules. The visual pilot must regularly compare the ground with the map, (pilotage) to ensure that the track is being followed although adjustments are generally calculated and planned. Usually, the pilot will fly for some time as planned to a point where features on the ground are easily recognized. If the wind is different from that expected, the pilot must adjust heading accordingly, but this is not done by guesswork, but by mental calculation - often using the 1 in 60 rule. For example a two degree error at the halfway stage can be corrected by adjusting heading by four degrees the other way to arrive in position at the end of the leg. This is also a point to reassess the estimated time for the leg. A good pilot will become adept at applying a variety of techniques to stay on track.

While the compass is the primary instrument used to determine one's heading, pilots will usually refer instead to the direction indicator (DI), a gyroscopically driven device which is much more stable than a compass. The compass reading will be used to correct for any drift (precession) of the DI periodically. The compass itself will only show a steady reading when the aircraft has been in straight and level flight long enough to allow it to settle.

Should the pilot be unable to complete a leg - for example bad weather arises, or the visibility falls below the minima permitted by the pilot's license, the pilot must divert to another route. Since this is an unplanned leg, the pilot must be able to mentally calculate suitable headings to give the desired new track. Using the E6B in flight is usually impractical, so mental techniques to give rough and ready results are used. The wind is usually allowed for by assuming that sine A = A, for angles less than 60° (when expressed in terms of a fraction of 60° - e.g. 30° is 1/2 of 60°, and sine 30° = 0.5), which is adequately accurate. A method for computing this mentally is the clock code. However the pilot must be extra vigilant when flying diversions to maintain awareness of position.

Some diversions can be temporary - for example to skirt around a local storm cloud. In such cases, the pilot can turn 60 degrees away his desired heading for a given period of time. Once clear of the storm, he can then turn back in the opposite direction 120 degrees, and fly this heading for the same length of time. This is a 'wind-star' maneuver and, with no winds aloft, will place him back on his original track with his trip time increased by the length of one diversion leg.

 Radio navigation

Good pilots use all means available to help navigate. Many GA aircraft are fitted with a variety of navigation aids, such as Automatic direction finder (ADF), inertial navigation, compasses, radar navigation, VHF omni directional range (VOR) and GNSS.

ADF uses non-directional beacons (NDBs) on the ground to drive a display which shows the direction of the beacon from the aircraft. The pilot may use this bearing to draw a line on the map to show the bearing from the beacon. By using a second beacon, two lines may be drawn to locate the aircraft at the intersection of the lines. This is called a cross-cut. Alternatively, if the track takes the flight directly overhead a beacon, the pilot can use the ADF instrument to maintain heading relative to the beacon, though "following the needle" is bad practice, especially in the presence of a strong cross wind - the pilot's actual track will spiral in towards the beacon, not what was intended. NDBs also can give erroneous readings because they use very long wavelengths, which are easily bent and reflected by ground features and the atmosphere. NDBs continue to be used as a common form of navigation in some countries with relatively few navigational aids.

VOR is a more sophisticated system, and is still the primary air navigation system established for aircraft flying under IFR in those countries with many navigational aids. In this system, a beacon emits a specially modulated signal which consists of two sine waves which are out of phase. The phase difference corresponds to the actual bearing relative to true north that the receiver is from the station. The upshot is that the receiver can determine with certainty the exact bearing from the station. Again, a cross-cut is used to pinpoint the location. Many VOR stations also have additional equipment called DME (distance measuring equipment) which will allow a suitable receiver to determine the exact distance from the station. Together with the bearing, this allows an exact position to be determined from a single beacon alone. For convenience, some VOR stations also transmit local weather information which the pilot can listen in to, perhaps generated by an Automated Surface Observing System.

Prior to the advent of GNSS, Celestial Navigation was also used by trained navigators on military bombers and transport aircraft in the event of all electronic navigational aids being turned off in time of war. Originally navigators used an astrodome and regular sextant but the more streamlined periscopic sextant was used from the 1940s to the 1990s. From the 1970s airliners used inertial navigation systems, especially on inter-continental routes, until the shooting down of Korean Air Lines Flight 007 in 1983 prompted the US government to make GPS available for civilian use.

Finally, an aircraft may be supervised from the ground using surveillance information from e.g. radar or multi-lateration. ATC can then feed back information to the pilot to help establish position, or can actually tell the pilot the position of the aircraft, depending on the level of ATC service the pilot is receiving.

The use of GNSS in aircraft is becoming increasingly common. GNSS provides very precise aircraft position, altitude, heading and ground speed information. GNSS makes navigation precision once reserved to large RNAV-equipped aircraft available to the GA pilot. Recently, more and more airports include GNSS instrument approaches. GNSS approaches consist of either overlays to existing non-precision approaches or stand-alone GNSS non-precision approaches.

• ADS-B
• Aeronautical chart
• Aircraft
• Air safety
• RAIM
• Great-circle distance
• ETOPS/LROPS
• Flight instruments
• Flight management system
• Flight planning
• Instrument Landing System
• Air traffic obstacle
• NavTool Software
• Radio navigation
• SIGI
• Spherical trigonometry
• Transatlantic flight
• Wind triangle