Existing CAT I Ground Based Augmentation Systems (GBAS) could be suitable for CAT II approaches


Preliminary flight tests performed in an Airbus A330 simulator indicate that the existing GBAS CAT I system could be suitable for CAT II operations, provided that anomalous ionospheric errors occurring during solar storms can be mitigated (either operationally or technically). This could improve access to GBAS runways in low visibility for airlines that have invested in GBAS CAT I systems.

GBAS is a new precision approach landing system using GNSS navigation with differential correction provided by a ground station. It has been certified on certain GBAS equipped aircraft for CAT I operation. To assess its suitability for CAT II operations, tests in an Airbus A330 Level D full motion flight crew training simulator were performed by EUROCONTROL in cooperation with the Technical University of Berlin. The purpose of the tests was to investigate whether non- detected worst case errors within the certification boundary of a GBAS CAT I system, would be acceptable in a CAT II or OTS CAT II operation. The errors applied in the simulation under CAT II conditions were the same kind of errors as the ones used for CAT I aircraft airworthiness certification programs.

Additionally, similar assessment was performed for the new approach classification, termed “Other Than Standard” (OTS) CAT II, which allows operation of CAT II approved aircraft down to a decision height (DH) of 100ft on runways not meeting full Cat II requirements. OTS CAT II relaxes the approach lighting requirements which are compensated by a higher required Runway Visual Range (RVR) and higher guidance performance requirements (i.e. autoland). GBAS CAT I suitability to support OTS CAT II and CAT II operation would provide a new potential benefit for airlines that have invested in a GBAS CAT I system in the form of increased access to runways served by GBAS in low visibility conditions.

The tests have shown that the existing GBAS CAT I standardised system would be suitable for CAT II and OTS CAT II operations. The tests indicated that vertical errors of up to +/-10 metres (i.e. the worst case GBAS CAT I vertical error considered for aircraft certification) are operationally acceptable for CAT II operations. The landing point was always well within the landing box criteria used for the certification of autoland systems. However, even though the lateral deviations were easily detected, pilots expressed more difficultly in detecting the vertical errors. Not all pilots detected errors exceeding 10m.

To confirm the ability of the pilots to detect exceptional errors outside the certification boundaries, a few additional tests in CAT I conditions were performed with vertical biases up to 35m. These errors could be caused by anomalous ionospheric events. Preliminary results show that pilots performed a go around at a very late stage. For CAT II operations with a 100ft decision height, these kind of errors have to be mitigated by other means.

The visual cues were sufficient at the decision height (DH) to conduct the OTS CAT II operation. The perception was that due to the increased RVR required for OTS CAT II, the visual cues at DH were even better in OTS CAT II conditions than in standard CAT II conditions.
Figure 1: Airbus A330 Level D training and research flight simulator
The next steps are to investigate means for detecting and mitigating errors caused by ionospheric anomalies, and to include GBAS into CAT II and OTS CAT II EASA regulation.


  HTML Full article Includes interesting videos of simulated low visibility approaches.


David De Smedt
Navigation Expert, pilot A320
EUROCONTROL Navigation Unit
Sylvie Grand-Perret
Focal Point Precision Landing and Take Off (LATO)
EUROCONTROL Navigation Unit
  Last validation: 17/06/2010