The Inverted-V gives you nine pieces of information. Three come from the AI - pitch attitude, bank attitude, and bank angle. The VSI gives you two - trend and rate of vertical movement. The remaining four come from the TC - trend and rate of turn, rate of roll (an indirect indication), and rudder coordination. (If your airplane has a turn and bank instead of a turn coordinator, you don't get the indirect rate of roll information.)
I call the TC and VSI "fine tuners" because TC movement becomes apparent before DG (directional gyro) or HSI (horizontal situation indicator) movement; VSI movement becomes apparent before altimeter movement; and both instruments - the TC and VSI - indicate trend of movement immediately. The actual turn rate is delayed slightly, and the actual vertical speed is delayed up to six seconds, depending on the amount of attitude change.
You don't use specific numbers in Step Two, only trend of movement, and you should understand the importance of differentiating between qualitative and quantitative information. Qualitative implies trend of movement without numerical reference. Examples are climbing slightly, descending rapidly, turning left slightly, or not turning. Quantitative implies a specific number, an actual measurement like the exact altitude, airspeed, heading, vertical speed, or turn rate. Specific numbers, such as 5,000 feet, 135 degrees, or 500 fpm down, are used in Steps Three and Four of the scan procedure only. Serious instrument flying errors occur if you make "the numbers" your only scanning priority.
The Music and Circular Scan Techniques
Scanning the instruments involves eye movement and mental interpretation. To develop proper eye movement, I teach two techniques called the music scan and the circular scan. These terms refer to eye movement only.
The Music Scan commands your eyes to follow linear paths that connect the AI to individual instruments (Figure 2). Start at the AI, move to a specific instrument, and return to the AI. Repeat that sequence using another instrument. You use the music scan whenever you redirect the aircraft - change attitude or power.
The Music Scan reminds you to think of a musical beat - and one and two, and one and two?. To establish the proper timing, refer to Figure 3 and repeat this musical beat sequence out loud using the slow side of your normal speech rate. When you say "and," look at the AI; when you say "one," look at the TC; and when you say "two," look at the VSI.
The Circular Scan (Figure 4) commands your eyes to follow a continuous path that forms an oval as it proceeds from one flight instrument to the next. This scan is less fatiguing than the music scan. You use it when you are not redirecting the aircraft - when attitude and power are constant, which occurs 98 percent of the time during a normal instrument flight.
Evaluating Trend of Movement
Step Two of the scan procedure is a two-step procedure. You evaluate the aircraft's trend of movement, and you validate the AI.
While music scanning the Inverted-V, ask yourself if the fine-tuning instruments indicate the desired trend of movement, considering the attitude and power setting that you've selected. If not, return to the AI and make the necessary corrections to attitude and/or power. If trend of movement is correct, validate the AI.
Performing a level turn is a good example. You look at the AI and establish the bank angle that should yield a standard rate turn. Next, you music scan the Inverted-V. The TC should indicate a standard rate turn, and the VSI should not indicate a climb or descent. If the VSI indicates a slight descent, you must return to the AI, increase pitch attitude slightly, and again music scan the Inverted-V for trend of movement.
Validating the Attitude Indicator
While you music scan the Inverted-V for trend of movement, you must validate AI operation. It must be operating correctly. Only then can you follow Step One's requirement to watch the AI while you redirect the aircraft. The AI is the only cockpit instrument that you can "stare" at - if you validate its operation. Yes, you do "stare" at the AI, and you'll do this more often than you might think. Examples are Step One of the scan procedure; when you make quick, repetitive maneuvers; when you encounter severe turbulence; and when you try to overcome a bad case of vertigo.
To validate the AI, you use Triangles of Knowledge (Figure 5). These triangles are based on comparison of independent information sources, and they're your basic insurance policy for instrument flying. They make you think in terms of independent aircraft systems. Remember, the AI is a pitch and bank instrument usually powered by the vacuum system, the TC is a bank instrument usually powered by the electrical system, the VSI is a pitch instrument powered by the static air system, and the magnetic compass is a bank instrument powered by the earth's magnetic field.
The Triangle of Knowledge rule states that if one of three independent sources of similar information differs from the other two, that source is erroneous. The Bank Triangle evaluation begins with the AI and the TC, bank information powered by the vacuum system and the electrical system, respectively. If the instruments disagree, you resolve it by looking at your third independent bank instrument - the magnetic compass. If you're not holding a steady course, you're turning, which means you're in a bank.
If the AI indicates a left bank, the TC indicates a right turn, and the magnetic compass indicates a right turn (the numbers are getting bigger), the AI or the vacuum system has failed because the TC and compass agree. But, if the magnetic compass indicates a left turn (the numbers are getting smaller), the TC or the electrical system has failed.
You cannot use the DG or HSI in the Bank Triangle because it, too, is powered by the vacuum system, which may be inoperative. You should check the airplane's suction gauge, and if the vacuum system is working, you can use the DG or HSI instead of the magnetic compass.
The Pitch Triangle evaluation begins with the AI and the VSI, pitch information powered by the vacuum and static-air system respectively. If the instruments disagree, you resolve the problem by looking at your third independent pitch instrument. You don't have one unless the aircraft has a second altimeter with its own static system, which might be a smart investment for aircraft owners who fly a lot of IFR. Typically, your only solution is to activate the aircraft's alternate static source, if one exists, and then use the altimeter to solve the disagreement.
The Pitch Triangle is not as critical as the Bank Triangle. If you release elevator pressure, the aircraft will attempt to regain the angle-of-attack (airspeed) that existed when you last trimmed the elevator. Even though you must ignore the physical sensations of flight when flying instruments, airflow noise, engine noise, and control pressures will signal gross airspeed variations.
The bank situation is critical because releasing aileron pressure will not counteract spiral instability. A graveyard spiral - a steep spiral with continually increasing bank angle, airspeed, and G-load - can develop quickly.
In a steep spiral you cannot raise the aircraft's nose with elevator input until you've reduced the bank angle considerably. Unusual attitude recovery training teaches you the proper recovery technique.
Caution! If you see an instrument disagreement, don't attempt to resolve the conflict by moving the flight controls. Freeze the controls and apply the Triangles of Knowledge rule while the attitude deviations are minor. In other words - stop and think. There is absolutely no reason for a pilot to lose control of an aircraft just because an instrument or system fails.
In June's "Instrument Training" I'll discuss Steps Three and Four of my four-step scan procedure. Until then, practice flying straight-and-level using the Inverted-V. No, you don't need the altimeter and DG or HSI to do this. If the TC's miniature airplane and the VSI's needle are both horizontal, you are doing an absolutely perfect job of flying straight-and-level. Knowing your actual altitude and heading has nothing to do with this important but often difficult maneuver.
Related Articles
