Incorrect Altimeter Setting

This summer, Pilot Standards published revisions to the SOP that dictate an altimeter crosscheck at transition altitude. These changes are intended to counteract the incorrect altimeter settings upon landing that have been detected 13 times between June and November of this year by the Flight Data Analysis (FDA) software. Despite the small number, these events are nonetheless alarming to see due to the hazardous potential of an incorrect altimeter setting.

One example of the dangerous nature of an incorrect altimeter setting is highlighted in the article AINsight: Six Feet from Disaster by Stuart “Kipp” Lau, which discusses a near-miss incident with an A320 due to an incorrect altimeter setting on approach. The pilots were flying the Area Navigation (RNAV) GPS approach and after being cleared with an altimeter setting of 1001, the pilots repeated back 1011. ATC did not catch the incorrect read-back twice as the pilots continued their descent. As a result, the aircraft was more than 200 feet below the published approach. Despite the incorrect altimeter setting, the pilots received no alerts in the cockpit to warn them of the imminent danger. ATC finally received a minimum safety altitude warning (MSAW) and informed the pilots who then initiated a go-around. The lowest altitude registered was six feet above terrain at 0.8 nautical miles (NM) from the runway threshold. This situation demonstrates the dangers of a missed altimeter setting where pilots can be completely unaware of their surroundings.

When looking at aviation as an industry, many accidents or incidents are caused by a string of small events that bypass defense or control measures, which left unchanged result in a larger accident.

This idea was documented by James T. Reason with the Swiss Cheese Model which demonstrates the importance of multi-layer safety procedures that prevent a chance occurrence from dictating whether an accident will occur. In the Swiss Cheese Model, holes in the cheese represent a weakness in safety procedures where an accident becomes more likely to occur. In the A320 situation, there were many control layers such as ATC read-back, pilot procedures, and aircraft equipment that did not prevent the near-miss from occurring. However, the ATC equipment which registered an MSAW and the pilot’s training to quickly initiate a go-around acted as the last barriers which prevented disaster.

One of the most important goals of the FDA program is to observe the flight data in detail to close the holes in the Swiss Cheese by putting control measures in place before an accident occurs. While these missed altimeter settings are exceedingly rare in Omni’s operations, they still do occur. The addition of the altimeter crosscheck at transition altitude will decrease the probability of these events and ensure Omni continues to fly safer operations far into the future.