| In this climbing right turn (above), the "wedges" (center screen) have moved apart, and the "snakes" are out of their holes. The airspeed snake burrows downward (left side), while the altitude snake jumps up (on the right side), and the heading snake shows a right turn on the HSI. |
| In this steep right turn, the "heading snake" (magenta arrow at top) shows where the airplane will be headed in six seconds--080 degrees. |
I call flight directors wedges, because a wedge tapers to a thin edge in order to align or split something. My use of the word is a stretch of the imagination, but flight director symbols are tapered and they will align an airplane with a course or glidepath and split that reference down the middle.
The upper wedge, shaped like a wide inverted-V, is the director. It appears when the flight director is turned on. The lower wedge, which looks like the sides of a wide triangle, represents the airplane. It performs just like the airplane symbol that you see on any attitude indicator, and it is always in view regardless of the flight director's status: on or off. With the flight director on, you maneuver the lower wedge so that it stays in tight formation with the upper wedge.
The flight director's upper wedge works in parallel with the autopilot. If you use only the autopilot and ask it to perform a function such as turning to a new heading or intercepting a course, the airplane will do it. If you use only the flight director for
the same task, and follow the guidance given, the airplane will do it. If you use both the flight director and the autopilot, the airplane will do it, and the flight director will mirror those actions.
Here's the problem: The brightly colored wedges on the attitude indicator will capture your attention. You will fixate on them. That's a mistake. The flight director is a supporting reference, not a primary reference. You must learn to look beyond the wedges and continually monitor the airplane's attitude. That's not easy; it takes practice.
Here's an example: I was getting an instrument checkride at night in a de Havilland Caribou when I flew for the Army. All went well, so the check pilot decided he would have some fun. On my last approach, he pulled out his Zippo cigarette lighter, lit it, reached over, and held it in front of my attitude indicator. That was a major distraction. I had to look past the flame and concentrate on the attitude indicator. That's what you must do when using a flight director.
The first step for instrument flying--and flying in general--is attitude and power. That requires the attitude indicator, not the wedges. To blindly follow the flight director or the autopilot is dangerous, because they can act erroneously. Many pilots have inadvertently flown through a course because the flight director or autopilot did not make the intercept. Caution! If that occurs, turn that stuff off and hand-fly the airplane. You don't have time to assess the problem.
Many glass-cockpit primary flight displays (PFDs) have trend vectors for airspeed, altitude, heading, and turn rate. These colored vectors indicate what that particular value will be in six seconds. I call them snakes because they hide in holes on the PFD and jump out when airspeed, altitude, or heading is changing. They are a critical part of your instrument scan, but like the flight director, they are--with one exception--supporting references, not primary references.
My four-step instrument scan procedure (see "Scanning the Glass," January 2008 AOPA Flight Training) is a perfect way to illustrate the snakes' importance. Step one of the scan is attitude, power, and trim. Concentrate on the attitude indicator and establish the proper attitude and power setting. Rough trim the elevator if necessary.
Step two requires you to scan for trend of motion (qualitative information). You're not looking for numbers; you're asking yourself if the airplane is doing what you want it to do: climbing, descending, turning, or flying straight and level.
In a conventional cockpit you use the turn coordinator and the vertical speed indicator for that purpose. In a glass cockpit you use the snakes. If you do not see the heading indicator's snake, you're not turning. Otherwise, it tells you what your heading will be in six seconds.
If you do not see the altimeter's snake, you are flying level. Other-wise, it tells you what your altitude will be in six seconds. If the trend of motion is not correct, return to step one, adjust attitude and power, then return to step two and reevaluate trend of motion.
Step three requires you to scan the numbers (quantitative information). Are you complying with your personal desires, charted requirements, or air traffic control instructions? If constant airspeed, altitude, and heading are required, you must read the respective values and all three snakes should be back in their holes.
The exception I mentioned is the heading indicator's snake. If a constant-rate turn is required, you refer to two reference marks located on either side of the heading indicator's lubber line above the compass rose. If the head of the snake is on the first mark, you're in a half-standard-rate turn; if its head is on the second mark, you're in a standard-rate turn. That snake, when used to determine turn rate, is the primary bank reference.
I do not use the term primary instruments, because it is grossly misleading. There are always three primary references for any instrument maneuver: one for pitch, one for bank, and one for power. They give you quantitative information as discussed above. However, there is not and never has been anything primary about them. They are tertiary, number three on the scanning priority list. Students see the term primary instrument and assume those instruments are the first priority. Do that and instrument flying will be very frustrating. Contrary to popular opinion, it's quite simple to determine the primary reference for any maneuver. Just ask yourself what numbers must be maintained.
Step four is used when the airplane is in stabilized flight, which is 90 percent of the time during a normal instrument flight. Now monitor all the PFD indications and trim the airplane for hands-off flight. You repeat the four-step scan procedure only when you must redirect the airplane, meaning that you change attitude or power.
Transitioning to the glass cockpit from a conventional cockpit can be frustrating because you must stop eye movement to read numbers, interpret colors, and monitor the wedges and snakes. Give it a chance. You'll soon find that it's easy to do, and the advantages will amaze you.
However, to attain the proper confidence level, make certain that you understand the airplane's electrical system and the redundancy that is provided for powering the PFD and multifunction display (MFD). Flight instructors, when you simulate failures of the PFD, MFD, or related components, never use a circuit breaker. In a demonstration of Murphy's Law, they will wear out and fail when you need them the most. Always follow the manufacturer's instructions for simulated failures.
Ralph Butcher, a retired United Airlines captain, is the chief flight instructor at a California flight school. He has been flying since 1959 and has 25,000 hours in fixed- and rotary-wing aircraft. Visit his Web site.
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Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
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