Description

An Airbus A320-200 being operated by British Airways on a scheduled passenger flight from London Heathrow to Glasgow was being radar vectored in day Instrument Meteorological Conditions (IMC) towards an ILS approach to runway 23 at destination when an Terrain Avoidance and Warning System (TAWS) Mode 2 Hard Warning was received and the prescribed response promptly initiated by the flight crew with a climb to Minimum Sector Altitude.

Investigation

A Field Investigation was carried out by the UK AAIB. It found that the radar controller had wrongly believed that they had given a new heading as closing heading to the ILS LLZ when in fact this had not occurred. Focus on other traffic meant that the controller did not notice that MSA was being breached because of the unintended track.

The prompt response of the crew to the Terrain Proximity Warning resulted in the closest terrain being at 959 ft below the aircraft although it was noted that the presence of a ridge of high ground nearby could have easily reduced this clearance considerably and that the rate of terrain closure at the time the warning was generated was 6000ft per minute.

The Investigation also found that as the aircraft was being vectored for a second - and ultimately uneventful - capture of the ILS LLZ, there had been a further breach of MSA attributable to ATC Instructions in a position where the aircraft was beyond the approach plate being used by the flight crew for reference but that, in this second case, the aircraft had remained ‘terrain safe’

It was noted that Glasgow was Minimum Safe Altitude Warning (MSAW)-equipped but the system had not been deployed because of ongoing concerns in respect of operational practicality. The availability of terminal area MSA information to both flight crews and controllers was also reviewed.

The Investigation concluded that:

“The terrain at Glasgow causes difficulties in descent and approach planning for both ATC and pilots. The ATC procedures in place will keep aircraft ‘terrain safe’ if followed accurately but records show that at least three arrivals in nine years have breached the Campsie Line below MSA. In each case the actual ATC clearance, of 2,000 ft amsl, remained above the highest terrain, though safety margins were eroded. Thus, had the GPWS not operated in G-EUUR, or the crew not reacted to the warning, then this aircraft’s cleared flight path would not have resulted in a ground collision. There would, however, have been less margin for any other error, such as a mis-set QNH or a ‘level bust’ in descent. In all the reported cases, and for the vast majority of public transport aircraft, GPWS/TAWS provides a high level of protection. MSAW offers additional protection from human error and extends this protection to any transponding aircraft, though its technical complexity and high numbers of inappropriate warnings mean that it is not available for immediate deployment at Glasgow. The air traffic service provider continues to work on a technical solution for MSAW and no Safety Recommendation is made.”

The full UK AAIB Report of the Investigation was published on 4 March 2010 and may be seen at SKYbrary bookshelf: AAIB Bulletin: 3/2010 ref. EW/C2998/11/07

Further Reading

• Approach and Landing Risks
• Minimum Safe Altitude Warning (MSAW)
• Terrain Awareness
• Response to a "PULL UP" Warning
• Perception in ATC