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Air navigation

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.

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. This may be supplemented using radio navigation aids.

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Contents

Flight Planning

The first step in navigation is deciding where one wishes to go. A private pilot will usually plan a flight using a map 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.

When an aircraft is in flight, it is moving relative to the body of air it is flying in, 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. Therefore 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 reported 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 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.

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The primary instrument of navigation is the magnetic compass. This aligns itself to magnetic north, which does not coincide with true north, so the pilot must also allow for this, called the magnetic variation. The variation that applies locally is also shown on the flight map. Once the pilot has calculated the actual headings, 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 E-6B has scales to help pilots compute these easily.

Additional calculations depending on the aircraft and the terrain may include the point of no return for an overwater flight (the point before which it is closer to turn around and after which it is closer to continue), and single enging flight characteristics in the event of a loss of one of a twin's engines in flight.

The final stage is to note over which areas the route will go, and to make a note of all of the things to be done - which ATC units to contact, the appropriate frequencies, and so on. Another important thing is 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 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.

IFR planning

In many respects this is similar to VFR 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 Flight

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 over featureless terrain. This means that the pilot must stick to the calculated headings, heights and speeds as accurately as possible. The visual pilot must constantly compare the ground with the map, to ensure that the track is being followed. 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. A good pilot will become adept at applying a variety of techniques to stay on track.

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.

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. 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 fly away from track at an angle of 60 for a noted period, fly another 60 once clear of the problem for the same period, and be back on track at the end. This works regardless of wind direction, because the effect of the wind will cancel out on the two flown legs, but will affect the time estimates for the overall leg by adding a period equal to half of the total flight time for the additional two legs.

Navigation Aids

Good pilots use all means available to help navigate. Many GA aircraft are fitted with a variety of radio navigation aids, such as Automatic Direction Finding (ADF), VHF Omni-range (VOR) and Global Positioning System (GPS).

ADF uses non-directional beacons (NDB) 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.

VOR is a more sophisticated system which is still the primary air navigation system established for aircraft flying under IFR. 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.

Finally, an aircraft may be supervised from the ground using radar. ATC can then feedback 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 GPS navigation in aircraft is becoming increasingly common. GPS provides very precise aircraft position, altitude, heading and ground speed information. GPS makes navigation precision once reserved to RNAV equipped large aircraft to the GA pilot. Recently, more and more airports include GPS instrument approaches. GPS approaches consist of either overlays to existing non-precision approaches or stand alone GPS non-precision approaches.

See also: Instrument Landing System, Air safety

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Last updated: 06-01-2005 23:03:28
Last updated: 08-17-2005 19:09:45