Asiana incident at HKG after AI-314 raises concern about ILS signal protection

Hong Kong incidents

Incident 01 Apr 2020

An Asiana Airlines Boeing 747-400 freighter, registration HL7420 performing flight OZ-952 from Hanoi (Vietnam) to Hong Kong (China), performed an ILS approach to Hong Kong’s runway 07R at about 06:25L (22:25Z Mar 31st), approach advised the crew to be aware of possible glide slope fluctuations due to an aircraft in the sensitive area. However, the aircraft descended to about 800 feet AGL about 4.6nm before the runway threshold.(Aviation Herald)

Similar incidents

On 20th Oct 2018, Air India flight 314 descended to 200′ at 2.6 nm from threshold of runway 07R.

These are not isolated incidents but few amongst 6 such incidents in 18 month. Other incidents were of ILS Localiser deviation on runway 25R, the aircraft was led towards the terrain.

  • 18 Oct 2019 Virgin Atlantic B-787 LOC deviation ILS 25R
  • 29 Sep 2019 Virgin Atlantic B-787 LOC deviation ILS 25R
  • 07 Sep 2019 Etihad Airways B-787 LOC deviation ILS 25R
  • 18 Jul 2019 Ethiopian Airlines B-787 LOC deviation ILS 25R

Due to geographical constraints the airport taxiway used to reach runway 07R takes the aircrafts very close to the glideslope antenna. This could interfere with the signals thereby causing fluctuations.

Location of Glide Slope antenna
Asiana Q-952

ILS monitoring

The ILS glideslope ground transmitting equipment is monitored at two points. Automatic monitoring (field monitoring) at the glideslope transmitter and antennae checks both output power and accuracy of the ILS glideslope to ensure that the characteristics of the installation stay within the permitted tolerances. Any fault detected by the field monitoring
system should result in the defective transmitter being shut off and then the standby transmitter being activated.

If the standby transmitter is also faulty then it will also be shut down. The total period of radiation outside the specified tolerances shall be as short as practicable, consistent with the need for avoiding interruptions (such as those caused by aircraft flying over the antenna). A Category I ILS glide path facility out of tolerance transmission shall not exceed 6 seconds under any circumstances.
An additional ‘monitoring’ facility is installed in the tower and consists of an indicator panel and associated audio alarm that indicates the operating condition of the equipment. This ‘remote control and indicator’ provides the Air Traffic Controller with an indication of the operating status of the equipment within the ILS installation. It will provide a visual and audible alert if the installation ceases transmission.
Annex 10 also requires the monitor and alarm circuits to be designed to be fail-safe to ensure the navigation guidance and identification is removed and a warning provided at the designated remote control points in the event of a failure of the monitor system itself.

ILS deviations

VHHH AIC 08/12 3 February 2012

mindFly analysis

There is no doubt that the pilot is the person who is responsible for the safety of the flight. Failures are bound to happen in systems and people. The crew has to maintain situational awareness at all stages of the flight, detect and manage deviations.

The ILS system is monitored and any signal deviation or interruption is detected. The standby system is activated automatically and the information is transmitted to the air traffic control centre. Therefore, the only possibility of a signal deviation without triggering a change over is an interference of the signal sensitive zone.

Interestingly, all the incidents have another common factor, all Boeing aircrafts. Except for the latest incident which was a B-747, the five others have been B-787.

Surprisingly, the Hong Kong Aircraft Accident Investigation Authority has not published the final report for any of the incidents. In fact ever since Feb 2017, the last 21 incident final reports haven’t been published.

Accident reports and bulletins


Transponder Landing System (TLS) is a computer-generated localizer and glide slope signal. It uses standard localizer and glide slope frequencies. The ground equipment consists of a transponder interrogator, ILS frequency transmitters, and sensor arrays for detecting lateral and vertical positions. It also uses dual computers that constantly check each other and will shut the system down if differences in signal accuracy are detected.

GPS precision approaches may be imminent, but for some airports, the TLS is a readily available alternative. A TLS ground system costs about the same as an ILS (approximately $1.5 million). Its maintenance costs are comparable, too. TLS is useful in airports where there is high terrain in the vicinity and reflection of signals from large surfaces.

At the end, it is of course the pilot who has to cater for the shortcomings of any system, himself too.

About Capt. Amit Singh

I think therefore I am Airlines Operations and Safety balance expert. A former head of operations/training and safety of successful LCC's in India. An experienced member of the startup teams of these airlines has hands-on experience in establishing airlines systems and processes.

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