The Human Factor: The Two Challenge Rule

As I was talking with Dr. Martin Smith about the research he and his associates at Presage Group were doing on unstable approaches, he commented that visual approaches "were a little more seductive than instrument approaches in terms of continuing with an approach that is unstable." Dr. Smith said even though all of the participants in the study were professional pilots and most were flying scheduled short-haul passenger service under instrument flight rules, 80 percent of the unstable approaches reported in the study occurred during visual ­meteorological conditions.

The Presage data shows pilots flying an approach in VMC are more likely to be flying the airplane manually, seem less aware of operational and environmental threats, are less likely to view those threats as unmanageable, are less likely to rely on expert knowledge of instruments and procedures and have lower confidence in their companies’ go-around policies. As Dr. Smith said: “That’s some seduction! It’s almost like they’re hand-flying a Cherokee 140 on a CAVU summer day with not a worry in the world.” Dr. Smith worried that due to the normalization of deviance mentioned in my previous article, these ­nonprescribed, deviant behaviors may have become the accepted way to conduct VMC approaches within the industry.

This made perfect sense to me and was illustrated by my own experience that I related last month about an ILS approach in a Learjet to Houston’s William P. ­Hobby Airport at 4 a.m. After considering Dr. Smith’s comments, I realized my approach was no more stable after we broke out of the clouds than it was a few seconds before when we were still in the clouds. The only difference was that I was now flying an unstable approach visually rather than on the instruments. Like the pilots in the study, once I was in VMC, I had more confidence in my ability to compensate for the unstable approach and disregarded standard operating procedures that required a go-around in that situation.

The Sirens’ Song

My experience illustrates the powerful “sirens’ song” that tempts us into continuing an approach that is unstable, especially when operating in VMC. The sirens were depicted as enchanting, partly human creatures in Greek mythology that enticed sailors to crash their ships with their alluring music. Their song has been described as lulling the listener into a fatal lethargy with an appeal that is hard to resist but if heeded will lead to a bad outcome. It is amazing how well this description fits what happens to a pilot on an unstable approach. The sirens’ song tells us if we just continue the approach a little bit farther, we will be able to get things back under control. This leads to a fatal lethargy in which the pilot does not take decisive action to respond to the obvious danger but continues the unstable approach, hoping everything will fall into place. The bad outcome can be anything from a hard landing to a fatal crash.

Odysseus instructed his sailors to put beeswax in their ears and then tie him firmly to the mast so he could safely listen to the sirens’ song without being able to jump overboard to try to swim to the shore. While rope or beeswax might not be of much help to a pilot, something called the Two Challenge Rule could be very useful in overcoming the temptation to continue an unstable approach, hoping that everything will turn out well in the end.

The Two Challenge Rule was developed after research and actual accidents showed approximately 25 percent of copilots would allow a captain to continue even a radically unstable approach due to the incapacitation of the captain as long as that incapacitation was not obvious. It states that the copilot or pilot not flying (PNF) would assume control of the airplane if the captain or pilot flying (PF) did not respond appropriately after two callouts. For example, if the airplane begins to descend below the glideslope, the PNF would state, “Half-dot low and increasing.” The desired response is “Correcting,” followed by the appropriate action to return to the desired flight path or configuration. If there is no response from the PF and/or the airplane continues to sink below the glideslope, the PNF would state, “One dot low and increasing.” If there was still no response or effective correction from the PF, the PNF would state, “I have the controls” and initiate a missed approach.

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:

AirspeedExceeding approach velocity minus 5 knots or plus 10 knots

Vertical SpeedGreater than 1,000 feet per minute

Bank AngleGreater than 10 degrees

Pitch AttitudeExceeding 5 degrees nose up or down

Glideslope DeviationGreater than one dot

Localizer DeviationGreater than one half-dot

ConfigurationNot in the specified configuration for an approach, usually gear down and approach flaps

PowerExceeding 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?


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