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4 Step Instrument Scan

Remember to validate your trap door

A good instrument scan procedure means that the pilot is always looking at meaningful information. Telling a student to "keep your eyes moving and scan all the instruments" does not fulfill this objective. That action will cause you to look at nonessential information, which wastes time, causes poor performance, and decimates self-confidence.

I never observed instrument-pilot eye movement until I started teaching in flight simulators. That was an education. Some students' eyeballs moved like lemons and oranges spinning inside a slot machine; some fixated on the wrong instruments; and some moved too slowly. I could never make these observations in an airplane because the student wore a view-limiting device, and I was watching for traffic when flying in visual conditions and watching the instruments when in instrument conditions.

Students with slow eye movement performed best. They usually knew when and where to look for the task at hand, and when eye movement stopped, they were always looking at the attitude indicator. These pilots were relaxed and they had more time for proper aircraft management.

I subsequently researched all aspects of instrument flight training. Information was abundant, but conflicts existed. I wanted a method that would be easy for pilots to remember and use, as well as one that was compatible with my maximum confidence/minimum workload training philosophy.

In the August issue of AOPA Flight Training, I described an instrument scan procedure for student pilots (see "Elementary Instrument Flying: Emergency procedures for student pilots"). That procedure was designed for emergency use by private pilots who usually receive the minimum amount of instrument instruction as required by the federal aviation regulations for pilot certification. If additional training time is allotted, a proper instrument scan procedure can be mastered.

I learned to fly instruments in 1959 using the needle, ball, and airspeed method. In 1965 the U.S. Army taught me the U.S. Air Force method of scanning primary and supporting instruments. This method, supported by the FAA, is currently taught in most general aviation flight training programs. When the Air Force and the U.S. Navy transitioned to jets, however, the control and performance method was developed.

All three scanning methods have been taught to thousands of excellent instrument pilots, but you must consider one factor. Military pilots are highly motivated individuals who train on a full-time basis. Each pilot masters what is taught in his or her own way. After graduation, they continue to fly full time, and their instrument flying skills become deeply ingrained.

General aviation doesn't work that way. Intense, full-time training may be OK for a homogeneous group of individuals such as those in college or university flight training programs, but not for a heterogeneous group of persons with varied ages, backgrounds, learning abilities, and non-aviation career commitments. These people cannot train or fly full time, and to many of them, instrument training and flying can mean total frustration.

My success with GA instrument students is based on innovation and common sense. It starts with my four-step scan procedure that uses new terms to help students identify and remember specific objectives.

Step One complies with the basic axiom of flight: Attitude plus power equals performance. You set a reasonable attitude and power setting for the desired maneuver. To do this, you concentrate on the attitude indicator, set engine power by sound, and then glance at the engine's tachometer or manifold pressure gauge to fine-tune your power setting. Adjust the elevator trim coarsely if pressures become excessive.

Do me a favor. Never use the term attitude indicator when discussing instrument flying. Call it the "trap door," because if you don't give it the attention that it requires, you'll think that the bottom fell out of your airplane.

Step Two requires you to scan the trap door and the fine-tuners-the turn coordinator (TC) and the vertical speed indicator (VSI). I call these instruments the fine-tuners because of their sensitivity. The TC will show a turn before the heading indicator reacts; the VSI will show a climb or descent before the altimeter reacts.

Use a linear scan pattern to scan these three instruments. Your eyes move from the trap door to the TC, back to the trap door, down to the VSI, and back to the trap door. I call this scan pattern the inverted-V; I call the required eye movement the music scan. Using a slow speech rate, say the following musical beat: "and one and two, and one and two, and..."-that's how fast your eyes should move. When you say "and," look at the trap door; when you say "one," look at the TC; when you say "two," look at the VSI.

Your first priority while music scanning the inverted-V is trend of motion-qualitative information. In Step One, you selected a specific attitude and power setting. Now, ask yourself if you are happy with the fine-tuners' indications. You are not concerned with specific numbers. You are only evaluating whether or not the airplane is doing what you want it to do-turning or not turning, flying level, climbing, or descending.

Your second priority is to validate the trap door. Many instructors will tell you to keep your eyes moving at all times so as not to fixate on a single instrument. This is not correct. You must concentrate on the trap door when doing repetitive maneuvering, flying in severe turbulence, and trying to recover from spatial disorientation.

To validate the trap door, you must think in terms of aircraft systems. When scanning the inverted-V you are comparing electrical and vacuum bank information (TC and trap door), and you are comparing static-air and vacuum pitch information (VSI and trap door). If a bank or pitch disagreement exists, you must refer to an independent bank or pitch system in order to resolve the conflict.

To resolve bank conflicts, use the magnetic compass. It will agree with either the trap door or the TC. The instrument that disagrees is the problem. (You cannot use the heading indicator unless you check the suction gauge and confirm proper operation of the vacuum system.)

To resolve pitch conflicts, use the alternate static air system and the altimeter. It will agree with either the trap door or the VSI. Again, the instrument that disagrees is not reliable. (You cannot use the altimeter or the airspeed indicator unless you replace the normal static-air system with the alternate static-air system if you have one.)

Validating the trap door is excellent instrument-flying insurance. When a pitch or bank conflict develops, freeze, count to 10, and use system analysis to resolve the problem before you make a false assumption and lose control of the airplane. If you determine that an instrument is erroneous, cover its face so that you aren't distracted by the incorrect information it's displaying.

If an instructor now tells you not to stare at the trap door, you can say, "You're wrong. I know how to validate it, and if it's working properly, I can stare at it all day long."

Step Three requires you to music scan the primary instruments. Folks, this is the most misleading term in aviation. The primary instruments are the tertiary instruments-the third level of importance. Let's ignore the confusion that results from discussing the ramifications of the tertiary instruments. Just scan the instruments that have the numbers that you are trying to maintain-quantitative information. Are you complying with your personal desires, what your navigation charts require, or what ATC requires? If you music scan those instruments and the trap door you will automatically scan the tertiary instruments. If during Step Two or Step Three you do not like what you see, return to Step One and change attitude and/or power. Return to Step Two for trend of motion and for trap-door validation. Return to Step Three for the numbers.

In Step Four, you transition to a circular scan and fine-trim the elevator trim (and rudder, if rudder trim is available). Imagine an oval pattern that sits over all six flight instruments. This is the circular scan pattern. The linear music scan is very fatiguing, but it is only used when you are redirecting the airplane-changing attitude and/or power. The circular scan is more relaxing, and it allows you to make small corrections as you evaluate all six flight instruments. You continue the circular scan pattern as long as the airplane is in stabilized, straight-and-level flight.

Now go to work and learn how easy it is to scan the instruments, validate the trap door, and maintain aircraft control. Caution! Never practice my four-step scan procedure when you are flying solo. Always have a safety pilot with you who can look out for traffic.

Ralph Butcher, a retired United Airlines captain, is the chief flight instructor at a California flight school. He has been flying for 43 years and has 25,000 hours in fixed- and rotary-wing aircraft. Visit his Web site.

Want to know more?

The following resources additional information on topics discussed in this article.

  • Review AOPA Pilot's 1998 "Instrument Insights" series on the basics of instrument flight.
  • Read Ralph Butcher's "Elementary Instrument Flying: Emergency procedures for student pilots" from the August 2003 AOPA Flight Training.
  • Read other AOPA Flight Training articles about flying-and learning to fly-under instrument flight rules.
  • Read the AOPA Air Safety Foundation's Single-Pilot IFR Safety Advisor publication, as well as the foundation's Weather Strategies and Weather Tactics publications.

Links to these resources are available on AOPA Online.

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