In IFR conditions, navigation is performed via a network of navigational aids (NAVAIDs) which can provide vertical and lateral guidance to an aircraft. In Instrument Meteorological Conditions (IMC), there is a high degree of dependency placed on these NAVAIDs in terms of the safe navigation of the aircraft.
NAVAIDs such as VORs are comprised of ground-based stations which require regular inspection and maintenance by the relevant authority. This maintenance includes ensuring the equipment is calibrated to a high degree of accuracy.
In controlled airspace, NAVAIDs are monitored regularly by the pertinent Air Navigation Service Provider (ANSP) of that area. This monitoring is generally by default. Monitoring a NAVAID will typically consist of a communication system in place in which personnel can regularly check for failure or malfunctions of NAVAIDs. This system also allows for notifications to be made to personnel regarding an occurrence of failure of a NAVAID.
However, there are instances in which the ANSP does not have the monitoring capability for certain NAVAIDs. An unmonitored NAVAID is when there is an absence of personnel monitoring the operational status and correct functionality of a NAVAID. In this circumstance, a Notice to Airmen (NOTAM) will be issued in relation to the unmonitored status of the NAVAID. The declaration of unmonitored status will be stated as either “UNMON” or “NOT MNT” on the NOTAM.
An unmonitored NAVAID means that in the event of a system failure, there are no notifications or monitoring reports issued to the ANSP in the area. It indicates that there is no indication available as to the correct functionality of the NAVAID. This means that the flight crew has no personnel to report NAVAID failures to. With no personnel present to rectify malfunctions, the NAVAID becomes Unserviceable (U/S).
During IFR flight planning, the monitored/unmonitored status of the NAVAIDs enroute should be taken into consideration. This is particularly essential for NAVAIDs utilized during critical phases of flight (takeoff, approach, and landing).
The key factor for careful planning around monitored and unmonitored NAVAIDs is IMC conditions. In IMC conditions, there is a higher degree of reliance on NAVAID and instrument approach systems. As a result of this, the alternate arrival airport must have monitored NAVAIDs.
Unmonitored NAVAIDs can be commonly found at uncontrolled airports or airports with limited ATC service available. Uncontrolled airports or limited ATC operations are generally found at smaller, regional airports where there is no sense from either an economical or operational aspect for the airport to be controlled around the clock.
NAVAID Unserviceable (U/S)
A NAVAID that is U/S or inoperative will be declared by the accountable personnel who monitor the NAVAID. With an unmonitored NAVAID, there can be a lag in the correct operational status of the NAVAID due to the lack of personnel monitoring and verifying the functionality of the NAVAID.
As a result, there is no verification available to the flight crew as to the status of the NAVAID in relation to system failures. This can lead to confusion as to diagnosing system or component failures – flight crew may not be aware as to if on-board equipment is malfunctioning or if it’s the NAVAID. In certain flight environments where there is a high workload placed on flight crew due to weather conditions and other factors, this is particularly strenuous to identify.
The challenges faced with unmonitored NAVAIDs were highlighted in a 2000 incident involving a Boeing 767 on approach to Faleolo Airport, Samoa. The aircraft was conducting an ILS approach utilizing unmonitored glideslope equipment. The glideslope produced erroneous readings to the flight crew on the flight deck.
The aircraft descended below the correct glide path profile as a result, almost leading to a collision with terrain. Due to prompt response by the flight crew, the erroneous glideslope readings were identified and a go-around was executed.