Instrument Training: Instrument Interpretation

The "mystics" of instrument flight

Instrument interpretation is a subject that doesn't receive the attention it deserves. Like my four-step scan procedure ("Instrument Training," February, April, and June 1998), it is a key element for minimizing cockpit workload and maximizing pilot confidence. You learn how to analyze specific flight instrument deviations and make the appropriate attitude and/or power correction.

Here is a typical example. You've had a long day but you can't chock the wheels until you fly an instrument approach to IFR minimums and land. Tuning and identifying the navigation radios is difficult because of turbulence and noise - rain pelting the windshield and non-stop ATC communications pounding your eardrums. When instrument deviations occur, you must make quick, efficient corrections that will eliminate deviations and keep your flight path within tolerances. Do not overlook the significance of this task. Far too often, instructors teach procedures that work well in smooth air or in a simulator but fail miserably in turbulence or strong wind gradients.

Bank Considerations

Instrument interpretation doesn't apply to bank because you have only one primary bank instrument for any given maneuver, never two. This instrument is the directional gyro or horizontal situation indicator (HSI) for maintaining a heading, the turn coordinator (TC) for maintaining a specific turn rate, or the attitude indicator (AI) for a steep-bank turn.

If the TC's trend is incorrect during step two of my four-step scan procedure, or if the primary bank instrument is incorrect during step three, you have nothing to interpret. You know immediately what you must do, so you look at the AI and change your bank angle.

Pitch With One Pitch Requirement

Instrument interpretation doesn't apply when only one pitch requirement exists because you have only one primary pitch instrument, never two. This instrument is the airspeed indicator (ASI) for maintaining a specific airspeed, the altimeter for maintaining a specific altitude, or the vertical speed indicator (VSI) for maintaining a specific vertical speed.

If the VSI's trend is incorrect during step two of the scan procedure, or if the primary pitch instrument is incorrect during step three, you have nothing to interpret. You know immediately what is necessary, so you look at the AI and change your pitch attitude.

Pitch With Two Pitch Requirements

Jackpot! Here come the "mystics." When two pitch requirements exist, two pitch instruments become primary, and instrument interpretation becomes mandatory. You must think before you act. You cannot make an immediate decision as you did with the previous bank and pitch considerations. You must interpret the two, primary-pitch-instrument indications and then make an appropriate control input. Two pitch-instrument combinations occur - airspeed and altitude, and airspeed and vertical speed. In both cases, throttle (power) controls airspeed, and pitch controls altitude or vertical speed (Figure 1).

Please note that throttle status also tells you whether one or two pitch instruments are primary. When your throttle position is fixed at climb, cruise, or descent power, only one pitch instrument is primary. When your throttle position is variable - you can move it to maintain airspeed - two pitch instruments are primary.

To simplify the interpretation of two primary pitch instruments, think in terms of airspeed and flight path - the aircraft's trajectory. Your flight path is high when your altitude is too high during level flight, when vertical speed is too high during a constant-rate climb, or when vertical speed is too low during a constant-rate descent. Your flight path is too low when your altitude is too low during level flight, when vertical speed is too low during a constant-rate climb, or when vertical speed is too high during a constant-rate descent. ILS approaches require constant-rate descents.

You use one of four control corrections to correct airspeed and/or flight path errors while you are scanning two primary pitch instruments.

1. Coordinated elevator and throttle inputs to maintain an airspeed. If you increase or decrease power and coordinate your elevator input to maintain airspeed, your flight path will change.
2. Coordinated elevator and throttle inputs to maintain a flight path. If you increase or decrease power and coordinate your elevator input to maintain your flight path, your airspeed will change.
3. Elevator input only. If you increase or decrease your pitch attitude, your flight path will change as will airspeed, altitude, and vertical speed. And yes, if your pitch attitude increases or decreases excessively, or for a prolonged period, the resulting airspeed and flight path would be unacceptable, but that doesn't occur in this case. These are small corrections, momentarily applied while the airplane is flying at normal speeds.
4. Throttle input only. If you increase or decrease power, your flight path will change, as will altitude and vertical speed. Airspeed changes will be minor providing you trimmed the elevator properly before you changed your power setting.

You use one of these four corrections after you interpret the two primary pitch instruments. To make this interpretation, you must look for one of two situations - either airspeed or flight path is incorrect, or both airspeed and flight path are incorrect.

When either airspeed or flight path is incorrect, you use coordinated elevator and throttle inputs - you simultaneously change pitch attitude and power. Because these control actions are compounded (throttle or elevator input affects all pitch instruments), you should use small inputs.

If airspeed is correct but flight path is incorrect, use coordinated elevator and throttle inputs to maintain airspeed while correcting flight path. Your hands will move in opposite directions. If your right hand moves the throttle forward, your left hand must move the yoke aft, or if your right hand moves the throttle aft, your left hand must move the yoke forward.

When the flight path is correct but airspeed is incorrect, use coordinated elevator and throttle inputs to maintain flight path while correcting airspeed. This time your hands will move in the same direction. If your right hand moves the throttle forward, your left hand must move the yoke forward - once when power is increased, and again as airspeed increases. If your right hand moves the throttle aft, your left hand must move the yoke aft - once when power is reduced, and again when airspeed decreases. You should always check your elevator trim when airspeed stabilizes.

You must master these elevator-throttle coordination skills to fly instruments proficiently. Practice both tasks until you can perform them subconsciously. My four-step scan procedure assumes that you did this during initial instrument training, and that's why you don't consider the airspeed indicator until step three.

When both airspeed and flight path are incorrect, you use elevator input or throttle input for the correction, not both. If airspeed and flight path are both high, reduce power slightly. When your flight path is correct, capture altitude or vertical speed with elevator, and when airspeed decreases, capture it with power. If airspeed and flight path are both low, increase power slightly. When your flight path is correct, capture altitude or vertical speed with elevator, and when airspeed increases, capture it with power.

When airspeed is high and flight path is low, increase pitch slightly. When your flight path is correct, capture altitude or vertical speed with elevator and capture airspeed with power. If airspeed is low and flight path is high, decrease pitch slightly. When your flight path is correct, capture altitude or vertical speed with elevator and capture airspeed with power. Check your elevator trim when airspeed stabilizes.

You must also evaluate what turbulence and wind gradient changes are doing to your flight path before you make an impulsive control input. Sometimes, when errors occur these adverse factors automatically correct the situation. When this level of thinking occurs, you are in total command of the situation.

Think about and practice these interpretation procedures whenever possible. When they become automatic, your control inputs will be minimized and you'll have more time to manage your navigation, communication, and aircraft system's requirements.