Rule of 10’s

Rule of 10’s

Try this technique to maintain precise control when hand-flying an approach.

Flying an accurate instrument approach is an exercise in precision. Whether flying an ILS or a nonprecision approach, it takes tight control of aircraft pitch and heading to arrive at the Missed Approach Point (MAP) in a position to land if the runway is visible. Yet many times IFR training launches into approach navigation without first teaching or reviewing the vital skill of attitude instrument flight. Consequently, many pilots “chase the needles” on navigation displays instead of precision attitude flying. The result is often an airplane off heading and/or altitude at the MAP, and a tendency to wander off the safe approach path when transfixed solely on needles instead of basic aircraft control.

Even some highly experienced IFR pilots become “needle-chasers” in the final stages of an instrument approach — especially if they have become dependent on autopilots or flight directors and are flying a “raw data” approach. As I tell my instrument students, however, flying an approach is done by flying proper attitudes, headings, speeds and rates of descent. The navigation instruments are a backup; they just tell us where to aim the airplane using the primary flight instruments.

For the novice IFR student; the newly minted instrument pilot; the experienced IFR pilot wanting to fly tighter, more accurate approaches, I present the “Rule of Tens.”

What is it?

The Rule of Tens is a technique instrument pilots may use to maintain precise pitch, heading, airspeed and vertical speed control when flying an IFR approach. It is especially helpful in low IFR and in windy or turbulent conditions near the ground. Specifically, the Rule of Tens defines a range, or bracket, for each control and performance variable. Staying within the brackets leads to a more accurate, safer approach, and eliminates wild excursions close to minimums that often result from hand-flown needle-chasing.

The Rule of 10s first applies to the attitude indicator. Once established on the approach, pitch attitude should never vary outside a 10-degree range. In most piston twin and light turbine airplanes the pitch attitude necessary to maintain an ILS glideslope or GPS glidepath is slightly below the horizon, from 3 to 5 degrees nose down. This results in a 500- to 600-foot rate of descent at typical piston-twin approach speeds. If you find yourself low on the glideslope, you may elect to pitch the nose slightly upward to regain guidance (with or without a power change as needed). In no case, however, should you pitch the nose higher than the horizon, because without a significant increase in power this will likely result in a dangerously low airspeed and an out-of-trim condition that will destabilize your approach.

Conversely, if you are high on glidepath, do not lower the nose below 10 degrees below the horizon. This is actually a quite steep descent, and will result in 1,000 foot-per-minute descent or more.

Consequently, you have a maximum 10-degree range of allowable pitch attitudes for stabilized precision during an approach. Any greater excursions result in unsafe airspeeds and the likelihood of “blowing through” the glideslope, to be out of trim and either dangerously low or unacceptable high before reaching the missed approach point.

The higher descent rate required for a traditional nonprecision approach requires a slightly lower pitch attitude, about 5 degrees below the horizon. This results in an 800-1,000 foot-per-minute rate of descent. Use the corrections range: level on the horizon at a maximum “up” limit, 10 degrees below as a maximum “down.” The Rule of Tens still applies.

Heading

Similarly, precise heading control is vital to maintaining course guidance, especially when following a localizer as it gets increasingly sensitive closer to the airport. The Rule of Tens for heading means maintaining heading within 5 degrees either side of your inbound course. This provides a 10-degree wide range of headings to avoid large intercept angles that will likely cause you to “fly through the needle” to the other side.

Most heading bugs define this 10-degree spread, 5 degrees either side of the selected heading. Stay within the limits of the heading bug to precisely follow the heading Rule of Tens inbound on the approach. As you determine a wind correction angle, set your heading bug to the necessary, revised heading. From there, use the limits of the heading bug as your maximum heading changes to reacquire or maintain course guidance. If you need to correct left, align the white “lubber line” with the left edge of the heading bug. If you need to correct to the right, align the lubber with the right edge of the bug.

Speed and Vertical Speed

The Rule of Tens also gives us ranges of safe airspeed and vertical speed to monitor the results of our attitude-flying inputs.

If you hold a constant pitch attitude, airspeed will remain constant, assuming you don’t change power or the position of landing gear or flaps. This is your target airspeed for the approach. If you need to correct UP by briefly raising the pitch to the horizon, you’ll get a roughly 5-knot reduction in airspeed in most cases. Correct DOWN by lowering the nose to 10 degrees down for a moment, and the speed will increase about 5 knots from target. Hence there is a 10-knot range of airspeeds to use as a backup to pitch control.

Now, let’s look at vertical speed. Normally it will indicate a 500 to 600 feet-per-minute descent on a precision approach, maybe as much as 800 to 1,000 feet per minute on a nonprecision approach. If you’re descending more rapidly than desired, for instance, you’re starting to go below the glideslope, raise the pitch to the horizon and confirm that your vertical speed indicator shows reduced to zero rate of descent. You don’t go up, you just don’t go down until you’ve reacquired glideslope. When the glideslope needle centers, re-establish your descent with pitch and cross-check the result with vertical speed and airspeed.

If you’re high on glidepath, or flying a nonprecision profile, the steepest descent you should allow is 10 degrees nose low, which will result in about 1,000 feet per minute down in light twin airplanes in the approach configuration. Keep it even shallower in the last 500 feet. If you’re high go down, but don’t go down too rapidly, until the glideslope centers and you return to the normal pitch attitude, with predictable airspeed and vertical speed results.

Therefore, there’s a Rule of 10s for the vertical speed as well: from “zero Tens” (level flight) to “10 Tens” (1,000 feet per minute descent) as a maximum range for flying an approach.

Using the Rule

OK, let’s have an example. I’m flying a Beech Baron 58 inbound on an ILS approach. My pitch is at 3 degrees nose down, my heading dead on, gear is down and flaps are at approach in configuration at my target airspeed (120 knots) and 600 feet-per-minute descent. My glideslope and localizer needles are centered.

Now the wind changes, and I’m blown left of course. Seeing the needle begin to move to the right, I adjust heading to 5 degrees right of my selected heading, as indicated by the right edge of the heading bug. This provides a slight intercept angle that I’ll hold until the needle again centers. Meanwhile I keep my attitude scan going to hold my precise pitch, and the glideslope remains centered. Airspeed and vertical speed remain constant.

The localizer returns to center but a slight increase in headwind causes me to drift below glideslope. I raise the nose to the horizon line and notice my vertical speed go to zero, with a slight reduction in airspeed. If the airspeed falls more than 5 knots below my target, I’ll add power while holding the pitch correction. When the glideslope centers again, I’ll set power back where it should be and lower the nose to 3 degrees down. Fly that attitude and crosscheck heading, airspeed and vertical speed.

Want some reinforcement? Fly a coupled ILS or WAAS glide path approach. You’ll see your autopilot does drift with wind changes, and for corrections uses the Rule of 10s. It’s probably closer to the Rule of Fives or even Threes, but you get the point. Clearly, successful approaches to minimums result in maintaining small tolerances (five units to each side of target) in all axes of movement.

Rules Violation

If your deviation from course guidance or glidepath is so great that you can’t re-center the needles without violating the Rule of Tens, this is grounds for missing the approach. Low altitude and converging with the earth is no place to be making wild corrections or massive airspeed changes. Don’t delay your decision. Miss the approach, set up and try it again.

This method both supports and is dependent on flying a stabilized approach. A stabilized approach means establishing and trimming for the target airspeed, and setting the aircraft configuration before or as you cross the final approach course inbound. Maintaining that speed (within a small range during corrections) and configuration until the missed approach point or when you can transition to a visual landing. A few hundred feet above decision height isn’t the time to be changing flap positions, power settings or trimmed airspeeds, any one of which is destabilizing to an approach.

“Chasing the needles” is a common, and dangerous, way to fly an instrument approach. The Rule of Tens is a technique you can use to command extremely precise approaches while avoiding the hazards of overcontrolling and becoming destabilized close to the ground. 

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