Flight below minimum safe altitude
What this relates to:
Minimum altitudes are calculated in relation to the highest terrain or obstacle within a specified area, allowing a buffer for error. Flight below minimum safe altitude occurs when an instrument-flying aircraft infringes this buffer. This includes:
- Infringement of the buffer during departure, en route or approach phases in Switzerland or in foreign airspaces under the responsibility of Skyguide
- Flown approach path for Swiss aerodromes too low (below glide path)
- This safety performance indicator includes both flights below minimum safe altitude (MSA) and flights below minimum vectoring altitude (MVA). The generic term ‘minimum safe altitude’ is used for simplicity.
Note: The rate is calculated in relation to IFR movements as this type of occurrence only concerns IFR flights.
Example:
Due to strong wind shears, an approaching aircraft descends faster than expected and finds itself for a short while below the standard glide slope. Alerts are triggered both on the air traffic control side and in the cockpit, and the pilot corrects the approach path.
Remarks on the 2024 figures:
As in past years, more detailed monitoring of the type of procedures and/or clearance deviations has been developed this year with the aim of showing more precisely where the main problems are. Due to some inconsistencies with past data, trends and comparisons with the past should be considered with caution.
There were 67 flights below minimum safe altitude reported in 2024, which shows a significant increase in comparison with the previous year (29). As mentioned above, however, this trend requires future confirmation. Most of the occurrences took place in the vicinity of the two largest aerodromes, which handle most of the IFR traffic in Switzerland. There were 31 incidents reported in the vicinity of Zurich Airport and 19 in the vicinity of Geneva Airport.
One approach below glide path on 20 April in Zurich is being investigated by the Swiss Transportation Safety Investigation Board (STSB).
The main contributing factor is aircraft deviating from their Standard Instrument Departure (SID) route, followed by level or route deviations during approach. Six occurred when the aircraft was on or intercepting the instrument landing system (ILS), which is usually noticed very quickly by pilots or by ATC.
