ILS-RNP hybrid approach offers ‘best of both worlds’: Air Berlin

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Air Berlin’s Boeing 737 fleet chief and pilot Michail Tounas says a hybrid approach the German carrier is using at Innsbruck airport “combines the best of both worlds” between traditional radio-based ILS and the new RNP procedure.

The airline started using a required navigation performance-based precision approach for the Alpine gateway’s runway 26 in 2012, lowering the decision height from 1,500ft for the standard instrument landing system to around 700ft under the satellite navigation RNP procedure, and cutting visibility from 3,700m to 2,400m. But since December 2013, the airline has been employing a hybrid approach which has further reduced the decision height, to 360ft, thus enabling the carrier to land in weather conditions characterised by lower cloud ceilings.

Innsbruck is located in a valley between mountain ranges peaking at 10,500ft, while the airport has a field elevation of 1,900ft.

A main improvement of the RNP operations was a simplified missed approach procedure for runway 26. This allowed pilots to climb to a safe altitude while continuing to fly through the valley – roughly along the Inn river – before turning around and following the same flightpath in the opposite direction to a holding area overhead the Rattenberg non-directional beacon.

If pilots cannot see the runway at decision height during a standard ILS approach, they have to make a 200° climbing left turn for a return to Rattenberg NDB. The radius during that turn – over rising terrain – must not exceed 0.9nm, which equates to a 25° bank angle at an airspeed of 155kt.

A missed approach under RNP operations is hence much less demanding than the ILS procedure. The RNP employment also reduced the glidepath angle from nearly 3.8° to 3.5°. This allows pilots to extend wing flaps and landing gear at a later point during the approach and thus cut noise and fuel consumption.

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Short final approach to Innsbruck’s runway 26

But greater lateral precision of the ILS signal over the RNP flightpath – especially in the final approach segment – led air navigation service provider Austro Control to revert to the former technology for the development of a hybrid procedure. While the ILS localiser and glidepath signals create a funnel-like approach zone with increasing precision as the aircraft comes closer to the airport, the RNP procedure – which is based on GPS positioning and an onboard navigation databank – calculates a box-shaped approach corridor, says Tounas.

The width of that corridor is determined by minor, accepted tolerances in the GPS data. But it means that from the decision height onward, the pilots have to visually continue the approach.

A number of obstacles are located in the vicinity of Innsbruck airport. As the RNP procedure does not provide sufficient separation to these structures, pilots must be able see them, which in turn determines the decision height. But by moving the localiser transmitter – its approach path is now located slightly further south than before – Austro Control was able to provide sufficient separation to the obstacles through the ILS.

The transmitter adjustment also reduced the offset angle between the localiser signal and runway. During the original ILS procedure, aircraft were tracking 254° toward the runway – which has a 259° heading – with pilots having to make a 5° right turn around 1.5nm from the threshold to line up with the centreline. Today, the localiser creates a 255° approach track.

The simplified RNP missed approach procedure has meanwhile been kept for the hybrid navigation mode dubbed “LOC R 26”, with the letter R standing for RNP.

It is Europe’s first certified hybrid landing approach procedure, says Air Berlin. The carrier is employing it along with a number of other operators.

Tounas says that “a large part” of Air Berlin’s 737 fleet has been equipped with the FMS software module and navigational database for RNP operations.

The airline’s narrowbody fleet comprises 10 737-700s and 39 737-800s as well as 11 Airbus A319s, 32 A320s and 11 A321s, Flightglobal’s Ascend Online database shows.

No decision has been made on whether the CFM International CFM56-powered A320-familly aircraft will be equipped for RNP operations, says Tounas.