How Pilots Use Air Navigation to Fly
Air navigation is accomplished by various methods. The method or system that a pilot uses for navigating through today's airspace system will depend on the type of flight that will occur (VFR or IFR), which navigation systems are installed on the aircraft, and which navigation systems are available in a certain area.
Dead Reckoning and Pilotage
At the most simple level, navigation is accomplished through ideas known as dead reckoning and pilotage. Pilotage is a term that refers to the sole use of visual ground references. The pilot identifies landmarks, such as rivers, towns, airports, and buildings and navigates among them. The trouble with pilotage is that often, references aren't easily seen and can't be easily identified in low visibility conditions or if the pilot gets off track even slightly. Therefore, the idea of dead reckoning was introduced.
Dead reckoning involves the use of visual checkpoints along with time and distance calculations. The pilot chooses checkpoints that are easily seen from the air and also identified on the map and then calculates the time it will take to fly from one point to the next based on distance, airspeed, and wind calculations. A flight computer aids pilots in computing the time and distance calculations and the pilot typically uses a flight planning log to keep track of the calculations during flight.
With aircraft equipped with radio navigation aids (NAVAIDS), pilots can navigate more accurately than with dead reckoning alone. Radio NAVAIDS come in handy in low visibility conditions and act as a suitable backup method for general aviation pilots that prefer dead reckoning. They are also more precise. Instead of flying from checkpoint to checkpoint, pilots can fly a straight line to a "fix" or an airport. Specific radio NAVAIDS are also required for IFR operations.
There are different types of radio NAVAIDS used in aviation:
- ADF/NDB: The most elementary form of radio navigation is the ADF/NDB pair. An NDB is a nondirectional radio beacon that is stationed on the ground and emits an electrical signal in all directions. If an aircraft is equipped with an automatic direction finder (ADF), it will display the aircraft's position in relation to the NDB station on the ground. The ADF instrument is basically an arrow pointer placed over a compass card-type display. The arrow always points in the direction of the NDB station, which means that if the pilot points the aircraft in the direction of the arrow in a no-wind situation, he will fly directly to the station. The ADF/NDB is an outdated NAVAID, and it's a system prone to errors. Since its range is line-of-sight, a pilot can get erroneous readings while flying in mountainous terrain or too far from the station. The system is also subject to electrical interference and can only accommodate limited aircraft at once. Many are being decommissioned as GPS becomes the primary navigation source.
- VOR: Next to GPS, the VOR system is probably the most commonly used NAVAIDS in the world. VOR, short for VHF Omnidirectional Range, is a radio-based NAVAID that operates in the very-high-frequency range. VOR stations are located on the ground and transmit two signals -- one continuous 360-degree reference signal and another sweeping directional signal.
- The aircraft instrument (OBI) interprets the phase difference between the two signals and displays the results as a radial on the OBI (omni-bearing indicator) or HSI (horizontal situation indicator), depending on which instrument the aircraft uses. In its most basic form, the OBI or HSI depicts which radial from the station the aircraft is located on and whether the aircraft is flying toward or away from the station.
- VORs are more accurate than NDBs and are less prone to errors, although the reception is still susceptible to line-of-sight only.
- DME: Distance Measuring Equipment is one of the most simple and valuable NAVAIDS to date. It's a basic method using a transponder in the aircraft to determine the time it takes for a signal to travel to and from a DME station. DME transmits on UHF frequencies and computes slant-range distance. The transponder in the aircraft displays the distance in tenths of a nautical mile.
- A single DME station can handle up to 100 aircraft at one time, and they usually co-exist with VOR ground stations.
- ILS: An instrument landing system (ILS) is an instrument approach system used to guide aircraft down to the runway from the approach phase of flight. It uses both horizontal and vertical radio signals emitted from a point along the runway. These signals intercept to give the pilot precise location information in the form of a glideslope -- a constant-angle, stabilized descent path all the way down to the approach end of the runway. ILS systems are widely in use today as one of the most accurate approach systems available.
The global positioning system has become the most valuable method of navigation in the modern aviation world. GPS has proven to be tremendously reliable and precise and is probably the most common NAVAID in use today.
The global positioning system uses 24 U.S. Department of Defense satellites to provide precise location data, such as aircraft position, track, speed, and to pilots. The GPS system uses triangulation to determine the aircraft's exact position over the earth. To be accurate, a GPS system must have the ability to gather data from at least three satellites for 2-D positioning, and 4 satellites for 3-D positioning.
GPS has become a preferred method of navigating due to the accuracy and ease of use. Though there are errors associated with GPS, they are rare. GPS systems can be used anywhere in the world, even in mountainous terrain, and they aren't prone to the errors of radio NAVAIDS, such as line-of-sight and electrical interference.
Practical Use of NAVAIDS
Pilots will fly under visual flight rules (VFR) or instrument flight rules (IFR), depending on the weather conditions. During visual meteorological conditions (VMC), a pilot might fly by using pilotage and dead reckoning alone, or he might use radio navigation or GPS navigation techniques. Basic navigation is taught in the early stages of flight training.
In instrument meteorological conditions (IMC) or while flying IFR, a pilot will need to rely on cockpit instruments, such as a VOR or GPS system. Because flying in the clouds and navigating with these instruments can be tricky, a pilot must earn an FAA Instrument Rating to fly in IMC conditions legally.
Currently, the FAA is emphasizing new training for general aviation pilots in technologically advanced aircraft (TAA). TAA are aircraft that have advanced highly technical systems on board, such as GPS. Even light sport aircraft are coming out of the factory with advanced equipment these days. It can be confusing and dangerous for a pilot to attempt to use these modern cockpit systems in-flight without additional training, and current FAA training standards haven't kept up with this issue.
The FAA's updated FITS program finally addressed the issue, although the program is still voluntary.