Airplane Navigation System

[Antonio Brittenham]

Thebasic principles of air navigation are identical to general navigation, which includes the process of planning, recording, and controlling the movement of a craft from one place to another.[1]

Successful air navigation involves piloting an aircraft from place to place without getting lost, not 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. Additionally, 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 visual flight rules (VFR) or 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 former 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 or satellite based positioning systems.

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.

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 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 (also critical point), is the point in the flight where it would take the same time to continue flying straight, or track back to the departure aerodrome. The ETP is not dependent 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 windspeed 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.

Instrument flight rules (IFR) navigation is similar to visual flight rules (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 must perform all such calculations themselves; the LSALT calculation is 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. Pilots 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.

Under the PBN approach, technologies evolve over time (e.g., ground beacons become satellite beacons) without requiring the underlying aircraft operation to be recalculated. Also, 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.

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.

Another reason for not relying on the magnetic compass during flight, apart from calibrating the Heading indicator from time to time, is because magnetic compasses are subject to errors caused by flight conditions and other internal and external interferences on the magnet system.[2]

Many GA aircraft are fitted with a variety of navigation aids, such as Automatic direction finder (ADF), inertial navigation, compasses, radar navigation, VHF omnidirectional range (VOR) and Global navigation satellite system (GNSS).

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 magnetic north (in some cases 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. A VOR which is co-located with a DME is usually a component of a TACAN.

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 multilateration. 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.

Civilian flight navigators (a mostly redundant aircrew position, also called 'air navigator' or 'flight navigator'), were employed on older aircraft, typically between the late-1910s and the 1970s. The crew member, occasionally two navigation crew members for some flights, was responsible for the trip navigation, including its dead reckoning and celestial navigation. This was especially essential when trips were flown over oceans or other large bodies of water, where radio navigation aids were not originally available. (satellite coverage is now provided worldwide). As sophisticated electronic and GNSS systems came online, the navigator's position was discontinued and its function was assumed by dual-licensed pilot-navigators, and still later by the flight's primary pilots (Captain and First Officer), resulting in a downsizing in the number of aircrew positions for commercial flights. As the installation of electronic navigation systems into the Captain's and FO's instrument panels was relatively straight forward, the navigator's position in commercial aviation (but not necessarily military aviation) became redundant. (Some countries task their air forces to fly without navigation aids during wartime, thus still requiring a navigator's position). Most civilian air navigators were retired or made redundant by the early 1980s.[3]

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