Frequently Asked Questions (FAQ) on Safety nets

Are there any ‘common mode’ failures between ground-based and airborne safety nets (and/or normal operations)?

Pressure altitude information from aircraft transponders is crucial for the operation of ground-based and airborne safety nets, as well as automated support systems during normal operations.

Detection of partial or complete transponder failure is therefore crucial. While this is widely regarded as being the sole responsibility of ATC, considering the hazardous consequences of transponder failure for normal operations as well as for the operation of ground-based and airborne safety nets, its detection should be a shared responsibility between pilots and controllers, possibly supported by appropriate alerts in the cockpit and at the CWP.

In order to mitigate the possible hazards resulting from the absence of transponder emissions by a flight, or their reception by the ground systems, the automatic generation of flight plan tracks should be considered either when a track already exists and disappears or when a track is not created at a position for which an estimate exists.

Can nuisance alerts generated from aircraft whose RVSM status is “unknown” be suppressed?

STCA should use the RVSM (Reduced Vertical Separation Minima) approval or non-approval status of aircraft and apply the appropriate vertical separation minima of 1,000 or 2,000 feet. The RVSM status of an aircraft is “unknown” when the aircraft track is not yet, or no longer, correlated with its flight plan. In this case STCA should assume RVSM approval to avoid alerts that would be operationally inappropriate.

How are terrain and obstacles defined in MSAW?

When MSAW was first implemented the only option was to model the terrain and obstacles with a limited number of polygons. More recently digital terrain elevation data has become readily available and MSAW benefits from using such data: the closer the definitions follow the terrain profile, the better the alerting performance will be. However, care should be taken not to lose MSAW protection in valleys where surveillance coverage is reduced.

Although the majority of MSAW systems do not allow its direct importation, the benefits of digital terrain elevation data can be partly achieved in conventional MSAW implementations as well. EUROCONTROL has developed a tool (PolyGen) that converts digital terrain elevation data into a set of polygons

How are the operational requirements for ground-based safety nets defined?

Operational requirements are usually established by a multi-disciplinary team at the beginning of an implementation or replacement project. In order to ensure that the requirements are completely defined, the chapters 3 of the EUROCONTROL Guidance Material for STCA, MSAW, APM and APW contain checklists that can be used during the requirements capture phase of a project. This is an essential prerequisite for optimising safety nets to operate in the local operational environment.

How can flight data improve safety nets' performance?

The sections 3.3.3 in the EUROCONTROL Specifications for STCA, MSAW, APM and APW contain general information about the use of flight data. Further details are provided in the supporting EUROCONTROL Guidance Material.

How can the number of nuisance alerts around airports be reduced?

Common practice is to define inhibition areas around airports to prevent nuisance alerts for each aircraft that lands or takes off.

How do you ensure the effectiveness of ground-based safety nets after implementation?

Firstly, the performance of safety nets needs to be continuously monitored. This can be achieved by regularly inspecting safety nets log files. However, as this can be very time consuming, it is more likely to require the use of automated monitoring tools.

Secondly, any detected anomalies or degradation in safety nets performance needs to be addressed. This is likely to require retuning of the safety net and may also trigger, for example, changes in airspace design in order to remove any detected hot spots.

Thirdly, and most importantly, it is essential to obtain feedback from active controllers on safety net performance.

EUROCONTROL can provide assistance in establishing appropriate processes for ensuring the effectiveness of safety nets in a specific operational environment.

How do you know if ground-based safety nets are sufficiently optimised?

It is vital to have the correct balance between warning times and nuisance alerts. However, as this balance is dependent on local circumstances, there is no generally agreed performance indicator that can be measured to provide a definitive answer to this question. Sharing of experiences is an effective way of benchmarking safety nets performance and obtaining ideas or help to enhance their performance. EUROCONTROL’s SPIN (Safety Nets Performance Improvement Network) Sub-Group is the forum where this is possible.

How does APW differ from RAI, DAIW and CAIT?

Area Proximity Warning (APW), Airspace Penetration Warning (APW), Restricted Area Intrusion (RAI), Danger Area Infringement Warning (DAIW) and Controlled Airspace Infringement Tool (CAIT) are all different names for system elements that have the same purpose: to warn the controller about an unauthorised penetration of an airspace volume.

How is APM configured?

A typical APM system has an alerting threshold defined by a funnel shape; aircraft below (and sometimes above) the approach funnel produce an alert. The funnel does not extend to touchdown as during the very final stages of approach aircraft are too close to the ground for APM to transmit a timely alert. APM has parameters which can be defined to both set the funnel shape and to minimise the number of nuisance alerts.

How should APM operate alongside MSAW?

APM and MSAW are different but complementary functionalities (and therefore often implemented in one system element, usually called MSAW). In the final approach phase MSAW will generate nuisance alerts. The APM functionality prevents this. APM then continues to provide protection until close to the runway.

Is APW set up differently to protect restricted or controlled airspace?

Guidance for setting up APW to protect restricted airspace is described in the EUROCONTROL Guidance Material for APW.

APW used to protect controlled airspace tend to be harder to tune as General Aviation (GA) traffic often operates close by. This can lead to an intolerable level of nuisance alerts (for example GA aircraft do not generally maintain strict flight levels, resulting in an erratic vertical track, and nuisance alerts particularly for traffic below the controlled areas). To keep nuisance alerts to a minimum it may therefore be necessary to both modify the APW volumes to differ from the shape of the published controlled airspace and set the look ahead times to only alert after an unauthorised aircraft penetrates controlled airspace (i.e. set the look ahead times to zero).

Should STCA process tracks without altitude information?

Firstly, aircraft not equipped with a pressure altitude reporting transponder are not normally allowed to fly in controlled airspace and therefore should not usually be eligible for the generation of STCA alerts.

Secondly, aircraft not equipped with a pressure altitude reporting transponder but allowed to fly in controlled airspace (e.g. military aircraft) could be assigned a block flight level and this could be taken into account for generation of STCA alerts. The justification for including such functionality depends on the operational environment.

Thirdly, aircraft flying in controlled airspace and suffering a transponder failure should receive special attention. Some systems allow the actual flight level to be manually entered and this could be taken into account for generation of STCA alerts. However, as transponder failure has become a rare event, the justification for including such functionality is decreasing.

Should STCA use the Cleared Flight Level input by the controller?

Using the CFL, STCA can predict conflicts along the most likely vertical profile of the aircraft. Doing so has the following advantages: it provides earlier warning of conflicts at the cleared flight level, and it reduces nuisance alerts. However, the downside is that conflicts are alerted very late in case of level busts.

Another approach to achieve similar benefits and reduced drawbacks is to use multiple predictions (linear plus level off at next flight level). Furthermore, the use of aircraft parameters such as Selected Altitude down linked via Mode S radars could also be used to a similar effect.

Should VFR or military flights be eligible for MSAW?

As a general rule, MSAW alerts for flights that are not in radio communication with ATC are operationally irrelevant. The SSR (mode 3/A) codes normally assigned to such flights can be used to automatically inhibit alerts. In case of IFR flights cleared for a visual approach, common practice is for the controller to manually inhibit alerts for these flights.

What is the fundamental difference between ATC (decision support) tools and ground-based safety nets?

ATC tools are designed and used to increase the overall performance of ATC (often by providing a combination of capacity, efficiency and safety benefits). Safety nets on the other hand are exclusively used to increase safety (by adding system safety defences: a single failure should never cause an accident).

ATC tools are designed and proven to be dependable (as they are intended or accepted to alter the controller’s way of working). Safety nets on the other hand are designed for maximum effectiveness during hazardous situations and proven to have no impact on normal operations (as they are not intended or accepted to alter the controller’s way of working).

Both ATC tools and safety nets are integral parts of the ATM system and therefore subject to ESARR4 or equivalent requirements. However, the fundamental different nature of safety nets led the EUROCONTROL Safety Regulation Commission (SRC) to publish Explanatory Material on Ground Based Safety Nets. This document aims to assist EUROCONTROL Member States, National Supervisory Authorities (NSAs) and Air Navigation Services Providers (ANSPs) in adequately addressing the relevant requirements during the development, implementation and operational use of ground based safety nets.

Further information:

What should controllers do in case of a ground-based safety nets alert?

In general terms, following alerts controllers should without delay take action to ensure that the increased risk to flight safety is properly addressed. The ICAO Procedures for Air Navigation-Air Traffic Management (PANS-ATM, ICAO Doc 4444) contains specific provisions for STCA and MSAW. These provisions are transposed into requirements on procedures in the EUROCONTROL Specifications for STCA, MSAW, APM and APW.

Why are both ground-based and airborne safety nets needed?

The purposes of ground-based and airborne safety nets are identical: to provide timely warning of increased risk to flight safety. Having complementary and independent safety nets that address the same risk not only adds to the robustness of the overall system, it also provides mitigation for the following;

  • airborne safety nets are not mandated for all aircraft, can fail during flight and aircraft are allowed to continue flying several days with certain failures;
  • ground-based safety nets are not mandated or voluntarily implemented universally and aircraft can fly in airspace in which no Air Traffic Services (ATS) are provided;
  • airborne safety nets and ground-based safety nets are subject to different technical limitations (access to ‘own aircraft’ data and access to contextual information).

Safety nets serve as another pair of eyes.

Why are ground-based safety nets needed?

Despite the high levels of safety in ATM systems, sometimes even the safest systems fail. Safety nets are the last line of defence that provide an additional system safety margin over and above the inherently safe provision of Air Traffic Services (ATS). Ground-based safety nets are needed to provide controllers with timely warnings of an increased risk to flight safety:

  • potential or actual infringement of separation minima
  • proximity to terrain or obstacles
  • potential or actual infringement of the required spacing to unauthorised airspace volumes.
Why does STCA not always trigger before TCAS?

In many conflict geometries STCA will trigger well before TCAS. However, in the case of sudden unexpected manoeuvres, TCAS may trigger before STCA.

Also, when approaching the cleared flight level with a high vertical rate, TCAS may trigger before STCA or STCA may even be designed to not trigger at all.

Why is APM needed for ILS-equipped runways?

Deviations from the ILS glide slope are rare but can occur because of equipment failure or pilot error (see examples accessed via the links below). Additionally non precision approaches may be required due to, for example, maintenance or unavailability of equipment. Controlled Flight Into Terrain (CFIT) accidents occurring in the approach and landing phase of flight are mostly associated with non-precision approaches.

More information on CFIT can be accessed here:

Why is MSAW needed in areas with flat terrain and without obstacles?

The majority of Controlled Flight Into Terrain (CFIT) accidents occur in hilly or mountainous terrain so the use of MSAW is predominantly associated with these areas. However, about one in three CFIT accidents occur on flat terrain making the use of MSAW applicable to these areas as well.

More information on CFIT can be accessed here:

Why is STCA needed in the TMA if all SIDs and STARs are strategically de-conflicted?

Although SID (Standard Instrument Departure) routes and STAR (Standard Instrument Arrival) routes are designed to avoid conflicts, potential losses of separation are still possible in the TMA, for example, due to a failure to follow ATC instructions, unauthorised entry by an uncontrolled aircraft in controlled airspace, or equipment malfunction.

In an area of high density traffic operations such as a TMA, the margins for error are further reduced and it is important for the controller to be warned quickly of an actual or potential loss of separation.

On the other hand, to be effective in the TMA, STCA must be tuned to account for the lower separation standards and the significant frequency of turns, climbs and descends that aircraft undertake in this airspace. Tuning STCA correctly for a busy TMA is a challenging but not impossible task.