Airbus estimates that approximately 4% of approaches on any given day do not meet the stable-approach criteria, and data from the International Civil Aviation Organization's line-oriented safety audits indicate that 3.4% of approaches are unstable. For the 10 million flights to, from and within the U.S. in a typical year, that amounts to 1,000 unstable approaches per day on average. Perhaps more alarming to the industry is that pilots on 97% of the questionable approaches continue on to landing.
The numbers are similar within Europe and elsewhere. Ewout Hiltermann, safety manager for KLM Cityhopper, says the airline began analyzing flight data on unstable approaches more than three years ago, finding that high approach speed, late extension of landing flaps and late arming of the lift dumper, or landing spoilers, were the top causes. As part of a weekly review of data, Cityhopper now brings in crews, union representatives, the chief pilot and the chief flight instructor for a “flight replay” of approaches validated as unstable where the crew did not perform a go-around. As part of its “just culture” approach, errors and mistakes do not result in punitive actions to the crew, except for cases of gross negligence, willful breaking of rules or illegal actions. Hiltermann says the results have been positive—pilots have no fear of discussing unstable approaches, lessons learned are immediately built into training, and the number of unstable approaches has decreased, but not to zero.
Air Canada performed a similar study using flight-data monitoring and pilot input that resulted in changes to procedures and cockpit communication, says Curtis. He notes that unstable approaches on the aircraft type for which the changes were made dropped by 50% as measured by flight data. “Surprisingly, we only saw a small increase in go-arounds,” he adds.
Martin Smith, a human factors expert who heads up the Presage Group, explains that pilots often continue an approach because they feel their airline's unstable approach policy is unrealistic, and there is often “no disincentive” to continue. Once committed to an approach, he says pilots have reduced situational awareness and talk less with one another, leading to less risk analysis of the possible outcomes. “One of the strategies for improvement is to redefine the stable-approach criteria,” he says.
Another potential solution is to take the subjectivity out of the process by installing a monitoring system that will provide alerts that trigger the crew to automatically perform a go-around, similar to wind-shear alerting systems. Two relatively new avionics products available as options—Honeywell's Stabilized Approach Monitor and Airbus's Runway Overrun Prevention System (ROPS)—are said to be helping, though neither company has released quantitative data.
The Stable Approach Monitor checks aircraft configuration, altitude and speed against pre-defined stable-approach criteria, alerting pilots both visually and aurally if landing flaps are not deployed by 950 ft. above ground level (“Flaps!”), or if the approach angle or speed is too steep or too fast, respectively. Below 450 ft., the system issues an “Unstable, Unstable” warning based on landing gear and flap positions, rate of descent and reference to a geometrical glideslope.
ROPS, available for the Airbus A380 and A320, provides visual alerts below 500 ft. and audio alerts below 200 ft. The goal is use the aircraft's total energy state to advise pilots when the landing runway will be too short, based on aircraft speed, wind, runway conditions and other factors. Ideally, an alert will trigger a go-around, but if not, the system will advise pilots to apply maximum braking and/or maximum reverse thrust. Airbus says ROPS has already prevented three potential runway excursions, but it is not providing more details.