The techniques used for navigation in the air will depend on whether the aircraft is flying under the visual flight rules or the instrument flight rules. In the latter case, the pilot will navigate exclusively using his 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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2 In Flight 3 Navigation Aids |
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 he is not permitted to use, 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 with 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 his 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 the E-6B flight computer - a type of slide rule - to calculate initial headings.
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 that he needs to use, the next step is to calculate the flight times for each leg. This is necessary to perform accurate dead reckoning.
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.
The final stage is to note over which areas the route will take him, and to make a note of all of the things he needs to do - which ATC units to contact, the appropriate frequencies, and so on. Another important thing is to note which pressire setting regions will be entered, so that the pilot can ask for the QNH (air pressure) of the areas he will be flying through. 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. The more work a pilot can do on the ground prior to departure, the less trouble he will have in the air.
Once in flight, the pilot must take pains to stick to his 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 what he sees on the ground with his map, to ensure that the track is being followed. If the wind is different from that expected, the pilot must adjust his 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 keep him on track.
If, for example, at the halfway stage of a leg the pilot determines that he is two degrees off course, he knows that by adjusting his heading by twice this angle, four degrees, he will arrive back on track exactly 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 established for his 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 E-6B in flight is usually imptactical, 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°, which is adequately accurate. However the pilot must be extra vigilant when flying diversions that he knows where he is.
Some diversions can be temporary - for example to skirt around a local storm cloud. In such cases, the pilot can fly away from his 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.
The good pilot should use all the means at his disposal to help navigate. Many GA aircraft are fitted with a variety of radio navigation aids, such as ADF and VOR.
ADF, Automatic Direction Finding, 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 his 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 (VHF Omni-range) 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, many VOR stations also transmit ATIS reports 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 him establish his position, or can actually tell the pilot his position, depending on the level of ATC service the pilot is receiving.
The use of GPS navigation in aircraft is becoming increasingly common, but currently no aviation authority has approved GPS for use as a primary navigation instrument. Until this happens, pilots are meant to use GPS only as a means of confirming their position obtained by conventional means.
See also: Instrument Landing SystemFlight Planning
In Flight
Navigation Aids