AINsight: Pilot’s Guide To Avoiding Unstable Approaches

 - October 23, 2020, 12:38 PM

Unstable approaches are a popular topic with aviation safety professionals. For nearly 20 years, the industry has largely tried to either explain the issue away, academically, or manage it through policies and procedures. Rarely do we talk about flying the aircraft.

Academic discussions focus on the frailties and limitations of the human mind that may contribute to an unstable approach. Included are complex psychological aspects such as continuation bias, cognitive lockup, task switching, and the framing effect. Another human factor issue contributing to the problem is SOP non-compliance.

Other discussions focus on managing the issue by establishing policies and procedures to promote the stabilized approach concept and support the go-around decision once an approach becomes destabilized.

Missing from these conversations are meaningful discussions about the common pitfalls of an unstable approach such as environmental factors and approach design elements. Likewise, the ability to recognize and recover from a high-energy state before initiating the approach and understanding the appropriate use of automation will help contribute to a stable approach.

Flying a stable approach begins with the approach briefing, often hundreds of miles from the destination, preferably before top of descent. This is where the crew—both the pilot flying and pilot monitoring—can identify some of the common traps (threats and hazards).

If any of these “gotchas” are identified, brief it, have a plan to mitigate that threat, and recap it. This threat-forward approach helps pilots build a shared mental model.

Environmental Factors

The first set of threats is environmental.

High-elevation airports tend to have higher unstable approach rates. A higher elevation translates into higher true airspeeds, increased rates of descents, and increased turning radius/ground track. In addition to the approach, there are other issues with operating at a high-elevation airport to include the landing flare, final power reduction to idle, and brake temperatures after landing.  

Another environmental factor is strong and/or gusty winds. When it “blows hard” the crew must determine the appropriate approach speed additives and discuss momentary deviations in airspeed (what is acceptable?). Unrelated, this would be a good time to brief windshear warning indications and recovery procedures.

Nonstandard Instrument Approaches

The next set of threats relate to nonstandard instrument approach procedures. These procedures include a runway without an instrument approach, any approach where there is a short distance from the final approach fix (FAF) to the runway threshold, approaches with a steep vertical path or glideslope, and runways with a steep PAPI/VASI or no PAPI/VASI. Each of these is subtle and if not properly briefed can lead to an unstable approach.

Before getting into the gotchas with nonstandard instrument approaches, let us discuss the hazards of flying into a runway that has no instrument approach. Often at smaller airports, there may be only one instrument approach procedure available. Keep in mind, the prevalence of unstable approaches is higher in VMC versus IMC.

In general—if winds are not a factor—it is safer to fly an instrument approach since it will provide terrain clearance and both lateral and vertical guidance to the runway. Flying direct to a runway, based on convenience, without an approach is a set up for disaster. (Read about the crash of Dale Earnhardt Jr.’s Cessna Citation Latitude.)

Approaches designed with a short distance from the FAF to the RW are challenging and can contribute to an unstable approach. An example is KABQ ILS RW 08, where the distance between the FAF and runway threshold is only 2.9 nm. The glideslope passes through that fix at only 945 feet above field elevation (AFE). To be stable at 1,000 feet AFE, the aircraft must be fully configured and “on speed” before crossing the FAF.

Another gotcha is an approach with a steep vertical path or glideslope. Any path greater than 3.0 degrees requires a higher rate of descent to maintain the proper path. Again, the aircraft must be fully configured and on speed to meet stabilized approach criteria. Examples of approaches with steep vertical paths include KSAN RW27 (3.5 degrees), KELP RW04 (3.42 degrees), and EGLC (5.5 degrees).

Similarly, a runway that has a PAPI/VASI path that is steeper than 3.0 degrees can be a challenge. On occasion, these runways have a standard ILS glideslope and transitions to the steeper VASI/PAPI path. High rates of descent on short final are common on these approaches.

Making Heads and Tails of an Unstable Approach

Operators employing a flight data monitoring program are beginning to gain a better understanding of unstable approaches. Flight data analysis can not only detect the unstable approach but also provide some insight into the common traits or precursors of unstable approaches.

One useful technique is to break the unstable approach events into either indicative or consequential events. An indicative event would involve a high-energy state, either excessive speed or rate of descent. Consequential events are associated with late configuration (gear and flap) changes during the approach – often there is a correlation between indicative and consequential events due to excessive speed.

Another trait of an unstable approach occurs well before the traditional stabilized approach gates. Again, these events relate to a high-energy state and the pilot(s) attempting to “get down and slow down.” These events can occur 3,000 to 5,000 feet AFE and involve an open or level change descent mode, idle thrust, and speed brakes extended.

These events are useful during a debrief to demonstrate how the approach was not only unstable, but there was little chance to ever become stable during the progression of the approach. Often it is easier to break off the approach early rather than attempt to salvage an approach that begins in a high-energy state.

Another flight data discovery is to look at mode control panel (MCP) settings and thrust settings during or after the highly unstable approach. A common trait is that the MCP altitude and heading are often not set correctly. Often, the MCP altitude will not be set to the missed approach altitude and the MCP heading will not be set correctly.

In extreme cases, the thrust and throttles remain at idle throughout the entire approach. These findings validate similar studies where the pilots have “tunnel vision” during an unstable approach, focusing primarily on “fixing” an unstable approach rather than being procedurally compliant.

Awareness of environmental factors, nonstandard approach elements, and energy management should help pilots plan for, brief, and fly a stable approach. Understanding these pitfalls can help pilots fly safer approaches and reduce the risk of approach and/or landing accidents.

Pilot, safety expert, consultant, and aviation journalist Stuart “Kipp” Lau writes about flight safety and airmanship for AIN. He can be reached at stuart.lau3@gmail.com.