The Human Factor: The Two Challenge Rule
Setting Stable Approach Criteria
This tool eliminates any necessity for the PNF to make a judgment call about the PF’s state or ability to fly the airplane. Only a slight modification is necessary to apply the Two Challenge Rule to an unstable approach in either single- or dual-piloted airplanes. First, define the flight parameter deviation callout criteria. The values would vary depending on the type of aircraft and operating environment, but typical criteria might be:
Airspeed — Exceeding approach velocity minus 5 knots or plus 10 knots
Vertical Speed — Greater than 1,000 feet per minute
Bank Angle — Greater than 10 degrees
Pitch Attitude — Exceeding 5 degrees nose up or down
Glideslope Deviation — Greater than one dot
Localizer Deviation — Greater than one half-dot
Configuration — Not in the specified configuration for an approach, usually gear down and approach flaps
Power — Exceeding normal approach power settings plus or minus 20 percent
Next, specify the stable approach height (SAH) below which the airplane must be within these parameters. Until recently, most airlines and corporate jet operators have used 1,000 feet above threshold elevation for instrument conditions and 500 feet when visual. However, based on the preliminary findings of this study, some operators are now specifying a stable approach is required below 1,000 feet whether on instruments or visual. It is important to set deviation criteria and stable approach heights that are realistic for your type of aircraft and flight operation. While it is obviously dangerous to specify a very low altitude as the SAH or to allow too great a deviation before requiring a callout, it is also counterproductive to set too high an altitude or excessively restrictive criteria because pilots will not follow unrealistic SOPs. In a flight operation with multiple pilots, it would be good to get input from the pilots as to what they think should be used. This will result in a stable approach SOP that is realistic for that flight operation, and pilots who have been asked for their input are more likely to adhere to the resulting SOP, even if it is not exactly what they recommended.
Using the Two Challenge Rule
Even with a comprehensive stable approach SOP in place, pilots will still be tempted by the sirens’ song that they can get back within the specified parameters in just a few more seconds. The SOP should specify that any time the PF or PNF notices a parameter outside stable approach criteria while lower than the stable approach height, he will state the deviation. The PF must respond “Correcting,” and there should be immediate evidence that the PF is doing so. If the deviation continues or gets worse, the PF or PNF would make a second callout. If the PF is not able to return immediately to stable approach criteria after the second callout, the PF or PNF should state, “Go around!” A single pilot should still make the callouts out loud, although if passengers are present, the callout can be made mentally.
My favorite example of the effectiveness of the Two Challenge Rule came from an F-14 Tomcat radar intercept officer (RIO — the guy in back, “Goose” for all you Top Gun fans). He learned the Two Challenge Rule at a Navy Aircrew Coordination Training (ACT) class I led. Several years later, I ran into him at Naval Air Station Oceana, and he told me if he had not learned the Two Challenge Rule, he would not be alive. He had been on a training mission south of Key West, Florida, with a new F-14 pilot at the controls when the pilot lost control of the airplane while doing simulated combat maneuvers. The RIO asked the pilot twice if he had control of the airplane, and when he did not get a positive response, he pulled the ejection handle. Four seconds after their parachutes opened, they hit the water. He said if he had not learned the Two Challenge Rule during ACT training, he would have asked the pilot if he had control one more time.
If the Two Challenge Rule can save two lives in a simulated combat situation, it can surely help professional and general aviation pilots take positive action to abandon an unstable approach early in the approach when a go-around is relatively simple to accomplish. Beyond that, having and using a stable approach SOP will lead each pilot to fly more accurate approaches by forcing us to recognize and deal with deviations while they are still small. This in turn will lead to better, safer landings. So, ultimately, a stable approach SOP combined with the Two Challenge Rule will result in less need to ever do a go-around. What’s not to like about that?