What is VOR/DME vs RNAV vs ILS approaches

What is VOR/DME vs RNAV vs ILS Approach (Description)

To Aviate an Aircraft to the final approach for landing, pilots must know their position (with their distance in some cases) related to a fixed point in the ground, this point may be a radio station emitting a special radio signal from its antenna and located in the ground or a navigational system guiding pilots through their approach for landing in a specified runway using one of the Types of Precession approach or Non-Precession Approach methods.

You can also, reading this article IFR vs VFR Flight Rules, What is the difference? for more information on flights using these approaches.

In this article, you will get to know the main technical requirements for VOR, RNAV, and ILS approaches, how they operate, and in which conditions they are used.

What is the VOR/DME Approach

VOR stands for Very High-Frequency Omni-directional range, in aviation, this short-range radio navigation system allows aircraft with the required receiving unit to determine its position or to fly along a chosen course related to a fixed VOR ground station on the ground.

VOR Operation

VOR operates by a ground station transmitting a Very High frequency (VHF) radio signal band from 108.00 MHz to 117.95 MHz using its VOR VHF antenna.

This radio signal emitted by the VOR ground navigation antenna consists of two modulated signals:

  1. Amplitude modulated VHF signal
  2. Frequency modulated VHF signal

One signal of these is a signal with a constant phase emitted around all directions functions as a reference signal, and the other signal is a varying phase signal, dependent on the direction of the transmission. Sometimes, this signal carries additional signals like, a signal transmitting a station identifier in Morse code, which a pilot can get from the ATC during his approach or on the time of planning and submitting his flight plan to get a clearance, this identifier is transmitted using aviation alphabet phonetics along with other information in publications like Aviation NOTAMs.

When the Aircraft receives this signal through its VHF antenna, the onboard VOR navigation system demodulate it for two base signals, using the differential phase between the constant phase signal known as the reference signal and the varying phase signal which is dependent on the direction of the transmission to find the bearing from the VOR station to the receiver aircraft related to the magnetic north.

The encoded information is then transferred through one of the common four indicators used for VOR navigation:

  1. OBI, known as Omni-bearing indicator, consist of OBS (Omni Bearing Selector), CDI (Course Deviation Indicator), and a TO-FROM indicator
  2. Radio Magnetic Indicator (RMI)
  3. Horizontal Situation Indicator (HSI)
  4. On-board area navigation (RNAV) system display

How to fly a VOR approach

A VOR approach is considered to be a Non-precision Instrument flight rules (IFR) approach. To fly this approach, a pilot needs an aircraft equipped with a VOR navigation system and flying in VOR-enabled airdrome airspace.

When a pilot gets near the airport, he sets the Airport’s VOR frequency in his instrument and starts to receive its signal.

If the station is transmitting an identification signal, the pilot will then listen to the Morse code or the voice signal of the station’s identifier to ensure he is tuned to the right frequency.

If a pilot wants to approach the VOR station of an airdrome from due east then the aircraft will have to fly due west to reach the station. The pilot will use the OBS to rotate the compass dial until the number 27 (270°) aligns with the pointer (called the primary index) at the top of the dial. When the aircraft intercepts the 90° radial (due east of the VOR station) the needle will be centered and the To/From indicator will show “To”.

The pilot will continue to fly in that direction until he passes over the VOR station, after passing the station the To/From indicator will be changed to show “From”. The pilot will then start counting the time from the passing point to determine the distance to start the procedure turn to align the aircraft to the runway to the final approach fixed point, the pilot will take a 180° turn and use the OBS to rotate the compass dial until the number 09 (90°) aligns with the pointer the needle will be centered and the To/From indicator will show “To”.

The pilot will then continue his descent until he levels out on the MDA (Minimum Decision Altitude), and continue to fly until he encounters the runway visual reference to start the final descent on the final approach fixed point using a visual landing.

If the pilot couldn’t encounter the runway’s visual references until he reached the missed approach point (MAP), the pilot must fly a Go-around and fly the VOR approach again.

VOR/DME approach

Some VOR stations are augmented with Distance Measuring Equipment (DME), hence called VOR/DME NAV aid facility.

If the aircraft is equipped with DME equipment, the pilot will be able to read his distance from the VOR/DME station that he selected its frequency. This will enable the pilot to fly a VOR/DME approach using the distance to determine, the distance to start the procedure turn, the final approach fixed point, and the missed approach point (MAP) without the need of counting the time after passing the VOR station.

If available, DME equipment could be used by the pilot to enhance his visual awareness even if the approach isn’t required it.

What is the RNAV Approach

RNAV is considered to be a Non-precession approach using IFR, an RNAV navigation system is equipped to the aircraft to enable it to use Area navigation to aviate.

Using onboard systems like Inertial Reference System (IRS), or external positioning data like the Global Positioning System (GPS), the aircraft will use this data from these resources along with at least one VOR/DME, or VOR/VOR link to calculate a direct route between Waypoints (Virtual VOR), unlike The VOR navigation method which requires a direct route from a VOR station to another.

The RNAV method explained above enabled the aircraft to fly Lateral navigation (LNAV), and some of these systems enhanced with a GPS data link enabling the aircraft to fly LNAV/VNAV navigation, VNAV stand for Vertical navigation.

How to Fly RNAV approach

The pilot will fly using a calculated flight route until he reached the Decision Altitude (DA) on which, he will continue to final in case he encounters visual references of the runway.

What is ILS Approach

In aviation, the instrument landing system (ILS) is a radio navigation system that is considered as Precession approach Instrument flight rules, provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather.

In its original form, it allows an aircraft to approach until it is 200 feet (61 m) over the ground, within a 1⁄2 mile (800 m) of the runway. At that point the runway should be visible to the pilot; if it is not, they perform a missed approach. Bringing the aircraft this close to the runway dramatically improves the weather conditions in which a safe landing can be made. Later versions of the system, or “categories”, have further reduced the minimum altitudes.

ILS uses two directional radio signals, the localizer (108 to 112 MHz frequency), which provides horizontal guidance, and the glideslope (329.15 to 335 MHz frequency) for vertical. The relationship between the aircraft’s position and these signals is displayed on an aircraft instrument, often additional pointers in the attitude indicator. The pilot attempts to maneuver the aircraft to keep these indicators centered while they approach the runway to the decision height. Optional markers provide distance information as the approach proceeds, including the middle marker placed close to the position of the decision height. ILS may also include high-intensity lighting at the end of the runways.

How to Fly ILS Approach

Pilots will select the ILS navigation system on the runway and will follow the Localizer and glideslope for ideal decent when the pilot reaches the Deaccession Altitude that is dependent on the ILS category installed on the runway.

The pilot may disconnect the autopilot after reaching minimums or continue an automatic ILS landing if the installed system on the aircraft allows it.

Conclusion VOR/DME vs RNAV vs ILS

VOR/DME vs RNAV, both of these approaches are categorized as Non-Precession Approaches, for VOR navigation the Aircraft fly a direct path from VOR to VOR. And RNAV makes the aircrafts capable to fly directly from one waypoint to another.

ILS is categorized as a Non-Precession Approach which makes the pilot to decent to a low DA because of its precise positioning and alignment to the landing runway.

ILS VOR is a combination of both systems to add a DME to a running ILS system for a more precise approach.

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