Safely Flying Non-Precision Instrument Approaches

Development of the earliest radio navigation systems started in the 1920s and 1930s. Initially, only the lateral course was supported by a radio navigation aid through systems such as Localiser (LOC), Non-Directional Beacons (NDB), and VHF Omni-Range (VOR). These systems provided, and continue to provide, guidance data for non-precision approaches.

With the growth of the air-transport system in the 1970s, it became necessary to reduce the number of accidents occurring due to lack of vertical guidance in approach, as well as to enable more consistent operations in poor weather.

Instrument-based Landing Systems (ILS) satisfy the requirement to provide both lateral and vertical (glide-slope) guidance, and therefore quickly became standard equipment at airports during the early 1970s. The inclusion of glide-slope guidance created what has become known as ‘precision approaches’. Later in that decade, the Microwave Landing System (MLS) was developed to reduce ILS -beam distortion and multi-path errors; but although it is in operation today, MLS has never gained a significant commercial aviation foothold and is only in limited service.

Historically, with the ground-based technologies described above providing the guidance, it was easy to differentiate between precision approaches and non-precision approaches simply on the basis of whether glide-slope guidance information was provided or not.

With the increase in Flight Management System (FMS) capability through the 80’s and 90’s, and especially with the introduction of Global Positioning System (GPS) equipment into civil aviation, the simple distinction between precision and non-precision approaches used earlier is no longer possible. These on-board technologies have rapidly become very sophisticated and are progressively enabling vertical and lateral approach guidance at a similar level to that of an ILS precision approach.

The first enhancement of these non-ILS/MLS instrument approaches came in the 1980s, with the replacement of the step-down technique (“dive & drive”) by Continuous Descent Final Approach (CDFA).

Today, the majority of non-ILS/MLS approaches are flown using a barometric vertical guidance, for which QNH setting and temperature are key factors and this must be taken into consideration by the crew. The most sophisticated instrument approaches use geometric vertical guidance based on an augmented GPS signal to create ‘ILS-like’ approaches.In addition, various new GPS based techniques offer sufficient accuracy, even to the point of taking the industry beyond the traditional ‘straight-line’ approaches and enabled curved approaches. As a result of all this development, some airports may have several approach charts available for a given runway as shown in (fig.1).

In addition, each chart can present several minima. Therefore, pilots must be familiar with the charting from their provider in order to ensure correct understanding of approach charts.

Whatever the type of technology, we can state that with the introduction of the CDFA technique, all approaches now share two common characteristics:

• Descent profiles of instrument approaches have become similar: vertical guidance is provided and there is no level-off required at minima
• If the required visual references are not acquired by the applicable minima, or indeed lost after, a missed approach must be initiated.

(fig.1) VOR and RNAV (GNSS) approach charts for LFPG RWY 08L.

ICAO Controlled Flight Into Terrain (CFIT) studies have shown that once some form of vertical guidance is added to approaches, the safety margin is increased by a factor of 8. As a consequence, a focus was placed at Airbus in recent years to offer some guidance on the vertical path for all instrument approaches.

If we now discount ILS and MLS approaches, there are different guidance modes available on Airbus aircraft to fly all types of instrument approaches, from TRK/FPA to managed modes offering guidance on both the lateral and vertical trajectory.

Depending on the approach type, the crew has to select the appropriate one (fig.2). Managed modes are recommended, but selected mode might be useful in case of system or equipment failures.

It is worth recalling that in selected mode, the Flight Path Angle (FPA) easily permits to follow the published descent gradient, but the pilot must still ensure that the vertical trajectory relative to the touchdown point is precisely followed.

The creation of new approach modes that have lateral and vertical profiles independent of navaids followed the introduction of the Flight Management System (FMS) in the 1980s and of the GPS in the 1990s. The objective was to standardize the way of flying all approaches down to the published approach minima, whatever the airport, and whatever the equipment on the ground. The FLS (FMS Landing System) is part of that concept and today, it is an Airbus option offering a solution to fly 99% of approaches that are not ILS/MLS, with a barometric vertical profile.

It offers lateral and vertical guidance for a straight-in instrument approach, referenced from the aircraft position, along a trajectory retrieved from the FMS navigation database.

(fig.2) Guidance modes available to fly non-ILS/MLS approaches not based on augmented GPS signal

Ultimately, what is needed to safely fly an approach is a clear picture of what it represents in terms of:

• The aircraft capabilities and crew qualifications (e.g. RNP-AR)
• The approach type
• The approach lateral axis, including potential offset with the runway axis (or FLS anchor point position)
• Vertical profile (barometric and temperature considerations)
• Applicable minima
• Aircraft guidance mode
• The recovery scenario in case of system failures or deviations exceedance.

For a flight crew, after possibly long hours of flight or a busy day’s flying schedule, the objective is to perform the most appropriate approach available at the airport according to the weather, aircraft capability, crew knowledge and training.

To fly a non-ILS/MLS approach using managed guidance requires a valid FMS data base. If not, then selected guidance must be used.

The FMS data base is considered validated if the provider is compliant with Regulatory requirements and/or validated by the Operator (depending on FMS standards and approach types).

In addition, because instrument approaches that are not ILS/MLS may not be flown on a daily basis they require good preparation both on ground and in flight.

Before the flight commences, GPS coverage (Receiver Autonomous Integrity Monitoring (RAIM) / Autonomous Integrity Monitored Extrapolation (AIME)) at destination must be checked if approach requiring GPS only is expected.

When in flight, the crew should ensure that the status of the aircraft is compliant with the technical requirement to fly the approach. In accordance with SOP, the FMS waypoints have to be checked versus the applicable chart to ensure that the correct approach has been selected and that the aircraft will fly the charted trajectory. During the descent preparation, the crew must define and agree on the aircraft guidance mode depending on the approach type and applicable minima. For this purpose, the cross-reference table published in FCOM is helpful (fig.5).

The action plan to fly the approach must also consider threats and errors management, e.g. vertical profile, visual segment after minima and offset.

During the descent, the flight crew should check that the navigation accuracy is compliant with the approach type and use the guidance mode that was intended to be flown, as per SOP.

(fig.5) Example of a cross-reference table, as available in A320 FCOM PRO-NOR-SOP-APPROACH-APPROACH GENERAL.

With the increasing precision of the navigation means used to fly any approach (e.g. GPS positioning) and the improved reliability of aircraft on-board systems, there is an observed tendency of crews to delay the go-around decision perhaps because of increased confidence in the aircraft automation to guide them below the published minima. This tendency translates into a significant reduction of the safety margins, especially with respect to flying without visual references below the minima.

Data has shown that if visual conditions were not achieved at the minima but were still expected, some crews waited a little bit longer, hoping for visibility to improve before they made the decision to go-around. This means that they were now flying unsafely below minima with no visual references. Likewise, if visibility is good at minima but it then reduces, some crews may decide to continue the approach, hoping for an improvement in the visibility. This tendency could also be reinforced if pilots are not go-around minded.

In reality, any “negotiation” with the visibility requirement from the minima and below for any approach is a drift into danger.