Basic Instrument Flying
Attitude Instruments Flying
A. INTRODUCTION
The purpose of this section is to provide a brief review of the techniques of attitude instrument flight, including stall and stall recovery maneuvers.
Attitude instrument flying is an extension of the concept of attitude flying. The establishment of a specific pitch and bank attitude, accompanied by a designated power setting, will cause predictable aircraft performance. Therefore, if pitch, bank and power are determined through reference to the flight instruments and the desired performance is confirmed by these instruments, the definition and technique of attitude instrument flight is clearly evident.
There are three basic ingredients to attitude instrument flying:
- scan;
- interpretation; and
- aircraft control.
The human body is subject to sensations which are unreliable when interpreting the aircraft's actual attitude; therefore, the pilot must learn to disregard these sensations and control the aircraft through proper scan and interpretation of the flight instruments.
Proper scan is vital to the instrument pilot. Of course, instrument flying requires that certain instruments be used more often during particular maneuvers. This is called selective radial scan. During a constant airspeed climb, for instance, the altimeter is less important than the airspeed indicator. Under instrument meteorological conditions, the pilot uses the attitude indicator to determine the aircraft's pitch and re-establish an attitude that will correct the airspeed to the desired value.
The attitude indicator replaces the normal outside visual references; therefore, it is the principal attitude control instrument for the radial scan. When scanning, the pilot should regard the attitude indicator as the hub of a wagon wheel", and the other instruments as spokes. (see Attitude Indicator is Centre of Scan figure, on the right).
The second important ingredient in instrument flying is proper instrument interpretation. The attitude indicator provides an artificial horizon to replace the natural one; hence, proper interpretation is extremely important.
The last ingredient, aircraft control, results from scan and interpretation. It is simply a matter of applying the proper control pressures to attain the desired aircraft performance. These pressures are the same as in visual flight except that smaller and smoother control inputs are required.
B. INSTRUMENT FLYING CONCEPT
The concept of control and performance attitude instrument flying can be applied. to any aspect of instrument flight. Under this concept, instruments are divided into three broad categories: control, performance and navigation.
1. CONTROL INSTRUMENTS: Control instruments indicate attitude of the aircraft and power (thrust/drag) being supplied to the aircraft. These instruments are calibrated to permit adjustments in definite amounts. They include the attitude indicator and engine control instruments (tachometer, manifold pressure, RPM, EPR).
2. PERFORMANCE INSTRUMENTS: Performance instruments indicate the actual performance of the aircraft, which can be determined from
the airspeed/mach, turn-and-bank, vertical speed indicators, altimeters, heading indicator, turn co-ordinator, magnetic compass.
3. NAVIGATION INSTRUMENTS: Navigation instruments indicate the position of the aircraft in relation to a particular navigational aid that has been selected. These can include NDB, VOR, ILS, INS, GPS, Loran-C, and OMEGA.
C. ATTITUDE AND POWER CONTROL
Proper control of aircraft attitude is the result of knowing when and how much to change attitude, and then smoothly changing it by a definite amount. Aircraft attitude control is accomplished by proper use of the attitude indicator. The attitude indicator provides an immediate, direct and corresponding indication of any change in aircraft pitch and/or bank attitude.
Pitch changes are accomplished by changing the pitch attitude of the reference line by set amounts in relation to the horizon bar. These changes are made in bat widths or degrees, depending upon the type of attitude indicator. On most attitude indicators a bat width represents approximately 2º of pitch change.
Bank changes are accomplished. by changing the bank attitude or bank pointers by set amounts in relation to the bank scale. Normally, the bank scale is graduated by 0º, 10º 20º, 30º, 60º and 90º, and this scale may be located at the top or bottom of the attitude indicator. Generally, an angle of bank that approximates the degrees to be turned is recommended; however, it should not exceed 30º in instrument flight. The TAS and the desired rate of turn are factors to be considered.
Proper power control results from the ability to smoothly establish or maintain desired airspeeds in co-ordination with attitude changes. Power changes are accomplished by throttle adjustment and with reference to the power indicators. Little attention is required to ensure that the power indication remains constant once it is established, because these indications are not affected by such factors as turbulence, improper trim or inadvertent control pressures.
D. TRIM TECHNIQUE
The aircraft has been correctly trimmed when it maintains a desired attitude with all control pressures neutralized. It is much easier to hold a given attitude constant by relieving all control pressures. In addition, more attention can then be devoted to the performance and navigation instruments and other cockpit duties.
First, apply control pressure to establish a desired attitude and then adjust the trim so that the aircraft will maintain that attitude when the flight controls are neutralized. Trim the aircraft for co-ordinated flight by centering the ball of the turn-and-slip indicator. This is done by using rudder trim in the direction the ball is displaced from center.
Changes in attitude, power or configuration may require a trim adjustment. Independent use of trim to establish a change in aircraft attitude invariably leads to erratic aircraft control and is not recommended. Smooth and precise attitude changes are best attained by a combination of control pressures and trim.
E. SCAN TECHNIQUE
Scanning, or cross checking as it is sometimes known, is the continuous and logical observation of flight instruments. A methodical and meaningful instrument scan is necessary to make appropriate changes in aircraft attitude and performance.
The control and performance concept of attitude instrument flying requires that the pilot establish an aircraft attitude and power setting on the control instruments which should result in the desired aircraft performance. The pilot must be able to recognize the requirements for a change in attitude or power or both. By cross checking the instruments properly (scan), the pilot can determine the magnitude and direction of adjustment required to achieve the desired performance.
Scan can be reduced to the proper division of attention and interpretation of the flight instruments. Attention must be efficiently divided between the control and performance instruments and in a sequence that will ensure comprehensive coverage of the flight instruments. The pilot must quickly interpret what he or she sees when looking at the instruments and must become familiar with the factors to be considered in dividing his or her attention properly.
A factor influencing scan technique is the characteristic manner in which instruments respond to changes of attitude and power. The control instruments provide direct and immediate indications of attitude and power changes. Changes in the indications on the performance instruments will lag slightly behind changes of attitude or power. This lag is due to inertia of the aircraft and the operating principles and mechanisms of the performance instruments.
To develop the technique of always referring to the correct instrument at the appropriate time, you must continually ask yourself these questions:
- What information do I need?
- Which instruments give me the needed information?
- Is the information reliable?
As mentioned earlier, the attitude indicator is the only instrument that the pilot should observe for any appreciable length of time. It is also the instrument that the pilot should observe the greatest number of times. An example of a scan demonstrates this; the pilot glances from the attitude indicator, then a glance at the airspeed indicator, back to the attitude indicator, and so forth (wagon wheel technique or radial scan). Of course different phases of flight will require slightly different scan techniques. This is called selective radial scan since the pilot will use particular instruments to carry out a particular task.
A correct or incorrect scan can be recognized by analyzing certain symptoms of aircraft control. Symptoms of insufficient reference to the control instruments are readily recognizable. The pilot should have some definite attitude and power indications in mind that should be maintained. If the performance instruments fluctuate erratically through the desired indications, then the pilot is probably not referring sufficiently to the control instruments. This lack of precise aircraft control is called chasing the indications.
Too much attention to the control instruments can be recognized by the following symptoms - if the pilot has a smooth, positive and continuous control over the indications of the control instruments but large deviations are observed to occur slowly on the performance instruments, a closer scan of the performance instruments is required.
The indications on some instruments are not as eye-catching as those on other instruments. For example, a 4º heading change is not as obvious as a 300 to 400-feet per-minute change on the vertical-speed indicator. Through deliberate effort and proper habit, the pilot must ensure that all the instruments are included in the scan. If this is accomplished, deviations on the performance instruments should be observed in their early stages.
A correct scan results in the continuous interpretation of the flight instruments, which enables the pilot to maintain proper aircraft control at all times. Remember, rapidly looking from one instrument to another without interpretation is of no value. Instrument systems and the location of the flight instruments vary. Pilot ability also varies. Therefore, each pilot should develop their own rate and technique of checking the instruments which will ensure a continuous and correct interpretation of the flight instruments.
F. ADJUSTING ATTITUDE AND POWER
The control and performance concept of attitude instrument flying requires the adjustment of aircraft attitude and power to achieve the desired performance in relation to the capabilities of your aircraft. A change of aircraft attitude and/or power is required when any indication other than that desired is observed on the performance instruments. However, it is equally important for the pilot to know what to change and how much of a pitch, bank or power change is required.
The phrase "Attitude plus power equals performance" summarizes the philosophy behind instrument flying. In other words, an aircraft's performance is the product of attitude and power. Performance is expressed in terms of airspeed, altitude, rate of climb or descent, or other criteria. If either attitude or power is changed, a change in performance will result.
The pilot knows what to change by understanding which control instrument to adjust to achieve the desired indications on the performance instruments. Bank attitude control is used to maintain a heading or a desired angle of bank during turns. Power control, in conjunction with a slight attitude change, may be used for maintaining or changing the airspeed while at a constant altitude. Power may also be used to establish a rate of climb or descent at a given airspeed or trim setting.
How much to adjust the attitude or power or both is, initially, an estimate based on familiarity with the aircraft and the amount the pilot desires to change the indications on the performance instruments. After making a change of attitude or power, the pilot should observe the performance instruments to see if the desired change has occurred. If it has not, further adjustment is required.
To sum up, instrument flight is a continuous process of:
- establishing an attitude and power setting on the control instruments;
- trimming;
- scanning, and
- adjusting.
These procedural steps can be applied to any instrument maneuver and should result in precise attitude instrument flying.
