RNAV Solutions Blog

On 30 June, 2009 at around 1:50 a.m. local time, Yemenia Flight 626 entered the Indian Ocean – crashing off the north coast of Grande Comore, Comoros – just a few miles from the airport. All on board lost their lives, with the exception of a single, 12-year-old girl who was found clinging to a piece of debris, among bodies and wreckage.

The Airbus A310 aircraft was attempting a challenging visual approach procedure, at Moroni – an airport with high terrain variation (from Sea Level to 7,700ft) with Mount Karthala immediately next to the airport. The airport is known among pilots in the Middle East and Africa as being challenging – especially if prevailing winds at the airport are such that the only runway with a published instrument approach – Runway 02 with an ILS – cannot be used.

Sadly, at the time of the accident – and even now – there is no instrument approach to Runway 20 at Moroni. This is due to the proximity of Mount Karthala to the runway and the local topography. If such an instrument approach is limited only to straight-line segments it’s very difficult to produce a usable design aircraft can safely fly.

But what if an instrument approach track could be curved, to avoid high terrain and place the aircraft on final approach in a safe position to make a landing? This opens a new range of possibilities – and makes a new level of safety in air transport possible. Not only at places like Moroni, but at airports around the world.

The Radius-to-Fix (RF) Leg Gives New Capabilities

Developed and tested by The Mitre Corporation (a non-profit technology corporation set-up by the US-government) and a number of avionics manufacturers, the curved, RF (Radius-to-Fix) leg capability was created. This allows arrivals or departures to be developed, allowing for special terrain, traffic or ATC considerations.

During development, studies showed remarkable conformance to the lateral and vertical paths tested during approach. Tracking does not have to be perfect, because of additional “buffer” airspace taken into account, either side of the track. In strong winds, tracking can deviate slightly and still remain well within the constraints.

RF Legs in Real-Life Approaches

Let’s look at a recent application of RF legs at a challenging island airport with high terrain, similar in nature to Moroni. And see how a similar process might be used to create a new instrument approach at Moroni, taking advantage of today’s navigation technology.

Seychelles is such a challenging island destination with high terrain close to the runway.

Prior to Rwy 13 at Seychelles having an RNP-AR (Required Navigation Performance – Authorization Required) approach available, aircraft were often forced to perform a visual procedure to runway 13. While there was a straight-in RNAV approach to Runway 13, the approach minima were quite high – and took the aircraft over high terrain – which could trigger GPWS warnings under certain conditions and speeds.

What might be required to resolve such challenging approach issues? Answer: an approach with a gentle turn, avoiding high terrain nearby (a 3,000 ft mountain is located next to the airport) and placing the aircraft on final approach – nicely lined up with the runway. Lower minima than any existing Seychelles approach would be nice, too.

Given the topography, the only option was to use an RF-leg to follow the Seychelles shoreline, shown below;

Even though this approach ‘points’ the aircraft at a 3,000ft mountain as it starts the approach turn, the RF-leg and special RNP-AR procedures guarantee the aircraft safely navigates around the mountain, lined-up with Runway 13 in plenty of time for the pilot to take over visually and make a stable final approach and landing.

The Rwy 13 RNP approach minimum for aircraft operating to RNP 0.11 (Boeing 777 for example) is 410 feet. This is actually lower than the ILS approach for Rwy 31, at the opposite-end.

Following a period of testing, in day-visual conditions, Emirates received regulatory approval to conduct the approach in full IMC (Instrument Metrological Conditions) at night.

The RNP-AR approach at Seychelles is now part of Emirates daily operations. And has significantly increased safety, at what was a challenging destination for the airline and its pilots.

The development of RF legs on final approaches made new RNP approaches like these possible. Airlines, business jet operators and governments are looking increasingly to approaches with RF-legs to solve some of the more challenging approaches scenarios, faced at airports like Moroni.

Had such an approach been in place at Moroni – with a qualified operator, crew and aircraft, it’s very likely the crew and passengers, who lost their lives when the A310 aircraft hit the water at Moroni, would still be alive today.

Further proof that today’s sophisticated RNAV approaches enhance operational capability, reduce costs and save lives.

Seychelles RNP 13 Approach (shown below), developed by Emirates, increases safety and is used by operators almost every day. A 3,000ft mountain lies very close to the runway

Moroni (shown below) has no instrument approach to Rwy 20. Visual tracks are shown. Note how the topography of the shoreline is similar to Seychelles – and would allow an instrument approach with an RF leg

Safety events during an approach often involve a common missing factor; one that’s getting easier to remedy with today’s on-board navigation. The missing factor is often, approach path predictability.

In any human activity, knowing what to expect allows us to properly prepare. Increased predictability during arrival results in a safer approach to the runway. Because it allows the crew to properly; prepare the Flight Management System, brief for the approach, achieve the correct speed at the right time and configure the aircraft.

Increased approach path predictability also reduces pilot and ATC stress at what is often a busy time. This allows all concerned to focus on the bigger safety picture – due to a more accurate view of what’s likely to occur in the near future around the airport.

Special RNAV approaches, containing curved sections (RF-legs) are helping Emirates increase safety at challenging airports. This new Seychelles approach to Rwy 13 (shown above) uses new RNAV capabilities. Approaches like this are now an everyday occurrence at Emirates.

In years past, aircraft had to rely on ground-based navigation aids. This limited our ability to deliver positive guidance all the way to the runway threshold. But today’s Airbus and other aircraft ‘carry the navigation with them.’ Having ADIRS and GNSS mean we no longer need any ground-based equipment to achieve positive guidance to almost any runway.

Increasing Safety and Efficiency

Making positive lateral and vertical guidance available to the crew, for all runway-ends, is a goal at Emirates. Because it enhances safety, reduces crew task workload and increases efficiency – by reducing the number of missed approaches and often saving track miles.

Toward our goal of increasing approach predictability, Emirates has developed, tested and implemented over ten all-new RNAV approaches at destinations including; Seychelles, Sana’a (Yemen) Karachi (Pakistan) and Khartoum (Sudan).

These destinations were targeted for RNAV approach development because the existing methods of achieving an arrival on the runways used, were somewhat lacking. For example, a particular runway might have only had a visual circling procedure – from a VOR approach. Or, in some cases no published approach at all. This might force the crew to perform a visual procedure, at night, which statistically is more prone to error than an instrument approach.

Examples

These new RNAV approaches have resulted in a significant drop in approach Air Safety Reports (ASR), and a measurable increase in efficiency. For example;

RNAV Safety: a 75+% drop in approach-related Air Safety Reports at Sana’a

Previously at Sana’a, Emirates pilots filed at least 4 ASRs a month, associated with visual-circling procedures (which were the only option at Sana’a before RNAV arrived). Working together with Yemeni authorities, Emirates designed and implemented RNAV approaches to each runway end. Today, only one ASR a month is typically received – often unrelated to instrument approach guidance.

RNAV Efficiency: over 20 track miles saved for each Seychelles approach – with lower approach minima than ILS

At Seychelles, together with local authorities, Emirates developed a RNP (Required Navigation Performance) ‘Special Authorization’ approach, where before there was no instrument approach at all. With a new, curved sector taking the aircraft safely around a mountain to the threshold of Runway 13, this is the first approach of its kind in the Mid-East/Africa region.

What’s more, the new RNAV approach has lower minima than the existing ILS for the opposite-end runway.

New RNAV (RNP) approach at Seychelles for Rwy 13, developed by Emirates. This new RNP approach has lower minima than the existing ILS at the opposite-end.

Efficiency has also increased at Sana’a. The average track mile reduction, for each approach there is approximately 30 miles. Over a year, these daily savings contribute to significant cost reduction.

RNAV Cost Savings: Reduced Training and Crew Costs

While some cost is involved with additional crew training in the simulator and development of special procedures, RNAV benefits in terms of safety, efficiency and lower approach minima far outweigh initial costs.

Once positive lateral and vertical guidance is brought to all runways at an airport, significant cost savings may be possible by placing the airport in a lower risk category. This can often save money by eliminating dedicated simulator sessions, ground training or carrying additional crew members – while maintaining an overall higher level of safety.

Required Navigation Performance – Authorization Required (RNP-AR)

Certain approaches Emirates has developed have required special regulatory approval. These special approaches are a small minority, and are deemed ‘Special’ by an authority, due to one or more of the following;

•    RNP value of 0.3 or less
•    a curved path to the runway – radius-to-a-fix (RF) legs
•    missed approach requiring less than RNP 1.0
•    secondary obstacle area not used (increases operational effectiveness)

This Special class of RNAV approaches can sometimes come with additional training requirements. However, once initial training is complete the qualification is maintained as part of regular recurrent training (similar to a pilot’s Category II/III landing qualification).

Emirates developed special FCOM and QRH guidance for this type of approach, as there are additional items for pilots to consider before and during an approach of this type.

With permission of local authorities in USA and Australia, more RNAV ‘Specials’ will become available to Emirates. Each Special RNAV approach offers the opportunity for significant safety and efficiency benefits, when compared with previous instrument or visual procedures.

RNAV Visual Approach

This is a hybrid type of RNAV approach. Where RNAV guidance is used for the initial portion to position the aircraft on final approach – at which point the pilots carry on with visual reference to the runway. We find this is often a good way to introduce RNAV operations to an airport that currently has none.

Because the introduction of such a visual approach is typically a lot faster than a full RNAV approach. After this stage is complete, we develop and introduce an full RNAV instrument approach, building on the success of the RNAV visual procedure.