Over the weekend I flew an IFR training flight from Oxford to Southampton. This involved conducting a NDB approach to runway 20 at Southampton and then returning to Oxford for an asymmetric ILS approach for Runway 19. These approaches are used when a visual approach is not possible.
After conducting these approaches, I thought it would be nice to write a short piece explaining them in more detail. Although, I will caveat this, by saying that I do not profess to be an expert, merely a curious professional.
The NDB (Non-Directional Beacon)
I will begin by explaining what an NDB is and how an approach is conducted using this facility. A Non-Directional Beacon (NDB) is a low- to medium-frequency radio transmitter that radiates signals in all directions.
Aircraft are equipped with an Automatic Direction Finder (ADF), which indicates the direction of the NDB relative to the aircraft by means of a needle on a cockpit display. Pilots can track either towards the beacon (following the head of the needle) or away from it (following the tail), depending on the requirements of the procedure.
Although this may sound straightforward, in practice it can be challenging. Pilots must manage radio communications, configure the aircraft for the approach, complete checklists, brief passengers, and maintain situational awareness. To help manage workload, pilots often rely on mnemonics. A commonly used phrase is “push the head, pull the tail.”
In simple terms, if the needle is displaced to one side of the desired track, the pilot applies a correction by turning in the appropriate direction to regain the required bearing to or from the station. However, in practice, pilots track a desired bearing (QDM/QDR) rather than simply “chasing the needle,” often applying corrections for wind drift.
How does the pilot receive distance information?
ADF equipment alone does not provide distance information. Therefore, an NDB approach is often supplemented by Distance Measuring Equipment (DME).
DME works by transmitting a signal from the aircraft to a ground-based transponder in the UHF frequency range. The system measures the time taken for the signal to travel to the station and back, providing the pilot with a slant range distance to the DME station. This allows the pilot to identify specific points along the approach.
Conducting the NDB approach
To conduct a procedural approach, pilots use an approach chart (plate), which contains all the necessary information, including initial, intermediate, and final approach fixes, as well as distances, altitudes/heights, and the vertical profile.
In a typical NDB approach, the pilot tracks towards the beacon. Upon reaching the beacon (station passage), and assuming an acceptable intercept angle, the pilot will then follow the published outbound track and descend to a specified platform altitude.
At a defined DME distance, the pilot will turn onto the final approach track, and at another specified distance, commence descent. The descent is usually anticipated slightly before the published point to ensure a smooth and stabilised profile.
I have copied the approach plate for the NDB(L)/DME Runway 20 approach into Southampton below. In this case, upon reaching the beacon, a Category A aircraft will track outbound 038 degrees whilst descending to a platform altitude of 1800 ft. On reaching 7.2 nautical miles from the DME, a turn is then made to intercept the final approach track of 205 degrees. The descent into the runway then begins, assuming the final approach track has been established, at 5.7 nautical miles from the DME.
Limitations of the NDB approach
Although NDB approaches are still in use, they have several disadvantages. They are less accurate than modern systems and are susceptible to various sources of interference, such as thunderstorms, coastal refraction, and terrain effects.
As a result, more precise systems such as the Instrument Landing System (ILS) have been developed, providing both lateral and vertical guidance for a safer and more reliable approach.
The ILS “Instrument Landing System”
The Instrument Landing System (ILS) is a more precise and reliable approach than an NDB. Unlike an NDB/DME-based approach, an ILS approach provides both lateral and vertical guidance to the pilot.
The pilot intercepts the localiser, which provides lateral (side-to-side) guidance, and the glide slope, which provides vertical guidance along the descent path. These indications are typically displayed on a flight instrument, showing a vertical needle (for lateral guidance) and a horizontal needle (for vertical guidance).
When the aircraft is correctly established on the ILS, both needles will be centred. If either needle is displaced, the pilot must make corrections to re-centre it by flying towards the needle to regain the correct path.
The objective is to maintain the indications within half-scale deflection for both lateral and vertical guidance. If this limit is exceeded, the approach is considered unstable and a missed approach should be executed.
As with an NDB approach, the relevant ILS procedure is published on an approach chart (plate), which provides all the necessary information for conducting the approach safely.
Are these the only types of Approaches?
These are just two types of approach that a pilot can adopt out of many. For instance, a pilot can use GPS to utilise an RNP approach or can adopt a localiser only approach utilising the ILS localiser without glideslope guidance. I will explain these in a later approach. I should add that I am still learning and do not consider myself to be an aviation expert, but hopefully these provide a short introduction into some of the techniques that pilots use to land safely in lower visibility or cloudy conditions.
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