In years gone by, Controlled Flight into Terrain (CFIT) has been a high-risk category which was largely mitigated with EGPWS and other ground collision alert systems, but have we reduced our risks beyond the technical solutions? During the pandemic it was highlighted from multiple aviation bodies that the industry must be guarded against an increase in human errors, and to be aware of issues emerging that were once considered latent and reduced to acceptable levels. So, what threats have emerged post pandemic which could increase the exposure to CFIT risk?
The SIRM reviewed a couple of case studies which related to CFIT and incorrect barometric pressure settings during the landing phase of flight and highlighted the importance of basic good practice and reinforced the danger of assumptions.
#SIRM 30 Case Study – Close Call!
An aircraft positioned for a required navigation performance (RNP) approach was provided by Air Traffic Control (ATC) with a QNH of 1011hPa (hectopascal pressure unit), despite the ATIS broadcasting 1001hPa the crew elected to set 1011hPa and commenced descent towards the airfield and planned approach path.
At approximately 1.5miles from touchdown ATC received a minimum safe altitude warning (MSAW) and moments later alerted the crew that they had passed through the decision altitude. ATC asked the flight crew to confirm that they were visual with the runway, at which point the pilot flying executed a go-around. 6ft was indicated on the radio altimeter. The aircraft was repositioned for a second approach, which was made using the same incorrect pressure setting, however on this occasion and with an increase in airfield lighting, the crew were able to gain visual references and correct their approach trajectory utilizing the Precision Approach Path Indicators (PAPI).
Lessons
- The relative proximity to the intended flight path may prevent systems such as EGPWS from providing terrain collision alerts.
- Variations in expected, anticipated, and received data should be challenged (ATIS vs what was passed as a pressure).
- Radar Altimeter (RA) may not function as anticipated, particularly if there is low terrain prior to the runway surface (call outs could be at very low altitudes).
- Technical solutions reliant on the barometric reading will not provide warnings if the pressure is incorrectly set (e.g., flight management system (FMS) will assume that the aircraft is on the intended path).
- Aerodromes with RNP approaches should revise policies for using airfield lighting, especially during periods of intermittent or variable weather. Globally we are seeing safety threats emerging from outdated EGPWS databases, increases in GNSS outages and increase in human error.
- Following a go-around from an event where height discrepancy could have been a factor, a check of pressure (hPa) should be considered as SOP (even more so if there were significant differences between the ATIS, the flight crew expectation based on the pre-flight weather report, and the pressure which was subsequently passed by ATC)
Safety Enhancements
- Airbus is working on introducing an Altimeter Setting Monitor (AltSM) component, which will compare the Aircraft Captain’s altimeter output to a GPS derived altitude.
- Communication between ATC and Flight Crew needs to be improved and maintained, in what is likely to be the most challenging phase of flight, read backs are critically important and should not be marginalized. Verification of pressure from more than one source should be normalized practice with any variances in data challenged and questioned.
- Risk analysis should be conducted by airlines to identify where the most likely source of error or incorrect altimeter setting is likely to occur within the sphere of the operation.
- See the full Airbus report
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