The Control-Performance Technique for Instrument Flying
You've upgraded your airplane from the fixed-gear IFR trainer in which you earned your instrument ticket. Isn't it time you upgraded your instrument flying skills? Instead of using techniques designed for the newly-minted instrument pilot, why not transition to those developed for more experienced pilots flying faster, more capable aircraft? Contributor R. Scott Puddy discusses the benefits and how to make the transition work for you and your airplane.
Having earned your instrument rating several years ago, you have acquired a fair amount of instrument experience and a corresponding level of comfort in IMC. You also purchased an assortment of "dot com" stocks 18 months ago and cashed out before the Federal Reserve raised interest rates for the sixth time in 12 months. That venerable C-172 treated you well over the years, but you are flying more long cross-country flights these days. You have the cash, so you recently upgraded to Airplane 2.0.
Your new plane has an IO-520 up front (or one on each wing). It is fast but slippery, a nasty trait that is most apparent when you are attempting straight-and-level in IMC. Controllers used to be much more polite when you were flying your Skyhawk. You were considering requesting block altitudes for all IMC flights when you discovered that you could keep the beast more or less under control if you selected 45% power for cruise. For flights faster than that, you select "Altitude Hold" on your approach-coupled, three-axis auto-pilot.
The problem is neither you nor your airplane. When you step up to high-performance airplanes, you need to upgrade to a high-performance instrument scan. Here's how.
The FAA Way
The Primary/Supporting Scan
If your instrument instructor adhered to FAA guidance, you initially trained under the FAA's primary/supporting instrument scan regimen. Under this technique, the FAA proclaims that all six of the basic flight control instruments are created equal. Depending on the phase of flight, certain of those instruments are designated as the "primary" instruments and are to receive closer scrutiny than the other, supporting instruments. The FAA acknowledges that the attitude indicator is the only instrument that gives a direct indication of the airplane's attitude. However, the attitude indicator is never designated as a primary instrument for any single phase of flight.
Just in case you have not recently reviewed the FAA Instrument Flying Handbook (AC 61-27C), the FAA designates primary and supporting instruments as follows:
|Constant Airspeed Climb/Descent||ASI||AI/VSI||DG||AI/TC|
|Constant Rate Climb/Descent||VSI||AI||DG||AI/TC|
|Standard Rate Turn||ALT||AI/VSI||TC||AI|
AI = Attitude Indicator
DG = Directional Gyro
ALT = Altimeter
VSI = Vertical Speed Indicator
ASI = Airspeed Indicator
TC = Turn Coordinator
Would The FAA Lead You Astray?
Although this article recommends that experienced instrument pilots use an alternative scanning technique in high-performance aircraft, the primary/secondary scanning technique is appropriate for use by instrument students and inexperienced instrument pilots and is the method to use when the attitude indicator is inoperable. The FAA counsels all beginning instrument students (and the instructors who teach them) to de-emphasize use of the attitude indicator in order to develop the student's instrument scan and for reasons of safety (in case the pilot may be so unlucky as to experience a vacuum failure in IMC early in his or her instrument-flying career).
During your primary flight training, you were required to receive merely three hours of instrument training. This included exposure to straight and level flight, constant airspeed climbs and descents, turns to a heading and recovery from unusual flight attitudes solely by reference to the airplane's instruments. Also included were radio communications, the use of navigation systems and facilities and receiving radar services appropriate to instrument flight. Those subjects necessarily received limited treatment and the FAA appropriately refers to this initial instrument work as "emergency flight by reference to instruments." If you were like most students, you learned to perform the required maneuvers by fixating on the attitude indicator as though it were the only instrument on the panel.
Fixating on any one instrument is antithetical to instrument flying, which requires the development of three fundamental skills: instrument cross-check, instrument interpretation, and aircraft control. Flight instruments and the systems that support them fail from time to time. You must cross-check the instruments against one another in order to detect such a failure and to avoid unintended and undesirable aerobatic flight in IMC. In addition to calling a controller's unwanted attention to yourself, these are the kind of maneuvers from which accident reports are made. Your first task as an instrument student, therefore, was probably to unlearn the habits developed during your initial "emergency instrument training."
It is much more difficult to unlearn and relearn than it is to start from scratch. In flight-instructor jargon, the problem is called "negative transfer" or "interference." The practical implication is that scanning the flight instruments other than the attitude indicator must be given disproportionate emphasis during the initial phases of instrument training in order to overcome the student's established habit of fixating on the attitude indicator. That is one reason that we use the primary/supporting instrument scan, which relegates the attitude indicator to a supporting-actor role.
The second reason for the FAA's primary/supporting instrument scan relates to the instrument student's post-certification life expectancy. Vacuum pumps fail about every 1,000 hours or so. Unfortunately, the low-time instrument pilot does not know whether the next hour in IMC will be the hour.
If 1,000 newly minted instrument pilots were to launch for an hour's flight in the clouds, the odds are that one of them would probably end up shooting a partial-panel approach. Provided that all those pilots were trained in accordance with the FAA's Instrument Flying Handbook, the pilot who was singled out by fatigued carbon vanes should do just fine because the failed attitude indicator was merely a supporting (and not a primary) instrument. Using the FAA's primary/supporting scan allows the inexperienced or occasional instrument pilot to use a single scanning technique for both full panel and partial-panel situations.
Moving Up; Moving On
If you are moving up, then it is time to move on. Once you have gotten your wings wet in IMC, there is no reason to prepare for a once-in-a-thousand-hour emergency by acting as though the emergency condition constantly exists. The attitude indicator sits front-and-center in the standard instrument layout for a reason. Commentary from countless aviation writers to the effect that any failure of the attitude indicator should be treated as an actual emergency exists for another good reason. Commercial airliners have at least three attitude indicators installed for the same reason. The reason is this: The attitude indicator is the most important instrument on the panel. Ignoring the attitude indicator because it might someday fail is not quite as bad as setting your plane on fire to retain currency in forced landings, but ... well, you get the idea.
Using the primary/supporting scan needlessly forces you to fly your plane differently in IMC than in VMC. There you go, motoring along on an instrument flight plan in VMC. You are a well-trained pilot, so you control the airplane primarily by reference to the visual horizon. Your attention is outside the plane at least 80 percent of the time and you only occasionally glance at the directional gyro and the altimeter to confirm that you are holding the appropriate heading and altitude.
Suddenly, you encounter ... a CLOUD. According to the primary/supporting method of scanning, you should immediately attempt to control altitude by focusing primarily on the altimeter and heading by focusing primarily on the directional gyro, cross-checking the attitude indicator from time-to-time because it is a supporting instrument for both pitch and bank in straight-and-level flight. Why should you cross-check the altimeter and directional gyro only occasionally in VMC and rivet your attention on those instruments upon encountering IMC?
Attitude Instrument Flying
Yet another and more technical reason for upgrading your technique is that the primary/supporting scan contravenes the most basic and fundamental concept of instrument flying. The name of the game you are playing is "Attitude Instrument Flying." The central rule to the game is:
POWER + ATTITUDE = PERFORMANCE
That formula guarantees you that, if you select an appropriate power setting and place the airplane in a constant attitude in coordinated flight, the airplane will give predictable future performance. Your capability to predict (and hence to anticipate and correct) the airplane's future performance is the key to operating high-performance aircraft smoothly in IMC. To enforce that rule, you must be able to hold the plane in a constant attitude. To maintain a constant attitude you need to focus on the attitude indicator. The attitude indicator is the only instrument on the panel that gives instantaneous and direct information about the airplane's pitch and bank attitudes.
The need to use the attitude indicator to establish and maintain an attitude can be clarified by examining the limitations of the flight instruments. The information they provide differs greatly from one point in time to the next based on the degree to which the airplane's attitude is changing. These points in time are: (1) the past, (2) the present, and (3) the future.
Past, Present And Future...
As discussed above, the pitch control instruments in straight-and-level flight are:
Supporting: Attitude Indicator and VSI
The altimeter reacts to changes in barometric pressure and gives instantaneous information about the airplane's current altitude. The altimeter reflects the present.
The vertical speed indicator depends upon a "calibrated leak" for its indications. One result of this design is a distinct lag between a change in the airplane's attitude and related information appearing on the instrument. The VSI reflects the past.
Of the "pitch control instruments," the attitude indicator is the only one that predicts the future. It gives instantaneous and direct information about the pitch attitude of the airplane. By holding power and attitude, you can control what the resulting performance will be. Hence, if in straight-and-level flight the airplane were to pitch to a climb attitude, the attitude indicator is the only instrument on board that would allow you to correct for an altitude deviation before the airplane began a climb or a descent.
As the above discussion suggests, the limitations of the primary/supporting scan in high-performance airplanes are most evident in controlling altitude. In a Bonanza for example, if you were to focus on the altimeter as the primary means of controlling pitch you would constantly be setting off alarms at the controller's scope as you busted your assigned altitude by 200 feet or more. This is because a high-performance plane is capable of departing from its existing altitude quite rapidly. By the time you detect that an altitude deviation has occurred, the airplane can be off altitude by hundreds of feet. Moreover, deviations in altitude will distract your attention from the directional gyro and lead to deviations in heading as well.
If you use the altimeter as the primary instrument for pitch in a high-performance plane, you will constantly find yourself "behind" the plane. Although the altimeter gives information about the plane's present performance, there is a time lag associated with your need to cross-check and interpret it and the other instruments. You will constantly be reacting to what the plane has already done, or "chasing" the airplane. Your reaction, if you are like many transitioning pilots, may be to use reduced power settings in actual or simulated IMC. Unable to keep up with a high-performance plane using the FAA's primary/supporting scan, you may resort to reducing power and converting your high-performance airplane to a low-performance airplane to accommodate the limitations of your technique. That is not the answer. The answer is to change the way you fly in IMC.
Introducing The Control/Performance Scan
Although neither the FAA nor your flight instructor told you this, there is another way the control/performance scan. At first glance, the control/performance scan appears remarkably similar to the primary/supporting scan. Five of the six basic flight control instruments are treated exactly the same as before. One instrument, the attitude indicator, is singled out for special consideration. Although there are substantial similarities between the two methods, the way you will fly in IMC using the control scan will be markedly different than before.
The control/performance scan divides the panel instruments into categories that give credence to the truism that the airplane's performance is a function of power and attitude. They are:
The Control Instruments
The Performance Instruments
The Navigation Instruments
In the control/performance scan technique, the instruments that inform the pilot of the airplane's power setting (usually the manifold pressure gauge) and attitude (the attitude indicator) are designated as the "Control Instruments" and are assigned the top tier. Of course, power adjustments in cruise are relatively infrequent or certainly should be so the practical effect is that the attitude indicator rests alone atop the heap. You will make all control inputs with reference to the attitude indicator to maintain an attitude that will yield the desired indications on the "Performance Instruments."
The Performance Instruments reside in the second tier and consist of the other five familiar gauges. They are assigned "primary" or "supporting" status for each flight regime in the same manner as under the primary/supporting scan. The "primary" instruments are the ones that reflect the value the pilot is attempting to maintain. For example, in level flight at 7,500 feet, the primary pitch instrument is the altimeter, since it is the only instrument that shows 7,500 feet. In a 500-fpm constant-rate climb, the primary pitch instrument is the VSI, as it is the only instrument that shows 500 fpm. By extension, in a 90-knot constant-rate climb, the primary pitch instrument is the airspeed indicator because it is the only instrument that shows 90 knots.
...And Navigation Instruments
Within the third tier there are the "Navigation Instruments" (e.g., VOR/LOC/GS, ADF, GPS), but a discussion of this instrument group is beyond the scope of this article. Of course, if you don't know that these instruments indicate where the aircraft is and how it can get where it's going, then a quick call to your CFII to schedule some instruction is probably in order.
In sum, the control/performance concept recognizes that there is a cause-and-effect relationship between the indications maintained on the instruments in the higher tiers and the values that will result on the instruments in the lower tiers. You will use the Control Instruments to achieve the desired indications on the Performance Instruments. You will choose target indications on the Performance Instruments that will yield the desired indications on the Navigation Instruments.
That all that sounds pretty technical, so let's consider what it means in conjunction with the most usual flight regime: straight-and-level flight. Here you go again, motoring along on an instrument flight plan in VMC. You are controlling the airplane primarily by reference to the visual horizon and only occasionally glance at the panel to confirm that you are maintaining the appropriate altitude and heading.
Suddenly, you again encounter ... a CLOUD, but this time you continue to fly the airplane exactly as before. You merely substitute the visual cues of the "artificial horizon" for the visual cues of the visual horizon. You maintain a cruise power setting. You hold the airplane in a constant attitude by reference to the horizon (attitude indicator). You periodically cross-check the directional gyro and the turn coordinator on a supporting basis to confirm that you are maintaining the appropriate heading. You also cross-check the altimeter and the VSI on a supporting basis to confirm that you are holding the desired altitude.
upgrade to a more high-tech panel, you will devote even more of your attention
to the attitude indicator. For example, a flight director is a common option
in the general-aviation fleet. The altitude-hold and heading-hold features of
the flight director eliminate the need to cross-check the altimeter and
directional gyro to confirm that you are maintaining altitude and heading.
With all that information available on one instrument, the cross-check serves
simply to assure that the thing is not broken.
Later in the flight, you are still in IMC when the time comes to turn 90 degrees to the left. That will require a transition from one phase of flight (straight-and-level) to another (standard-rate level turn). The transition will take only two to three seconds. Just as your attention should be focused outside the airplane in a transition to a turn in VMC, your attention should be focused solely on the attitude indicator during the transition in IMC. This is not the time to be scanning the engine gauges. The attitude indicator is the only instrument on the panel that gives instantaneous indications of both pitch and bank. By looking at the attitude indicator while you roll into a turn, you can assure that you maintain the appropriate pitch attitude while you change the bank from 0 degrees to the 15 degrees or so required for a standard-rate turn.
It requires discipline to fixate on the attitude indicator during transitions and you may be surprised how much trouble you have in remembering to focus on a single instrument during a two-to-three-second time period. Other than lack of discipline, the problems again are "negative transfer" and "interference." Having been taught for years to scan all the instruments on the panel, you may have trouble fixating on one instrument, even if it is for only two to three seconds.
A failure to use the attitude indicator for transitions is easy enough to detect: If you depart the assigned altitude while rolling into a turn or leave an assigned heading while changing pitch, it is a sure sign that you were not looking at the attitude indicator during the transition.
The other bugaboo that frequently arises with transitions to turns is the heading bug. When assigned a new heading, some instrument pilots have a habit of adjusting the heading bug to the new heading as they roll the airplane into a bank to initiate the turn. The heading bug is attached to the directional gyro. If you are resetting the heading bug, you are looking at the directional gyro not the attitude indicator. The answer is to reset the heading bug first, and then to transition into the turn using the attitude indicator.
Once established in the turn, you once again control the airplane by holding it in a constant attitude, primarily by reference to the attitude indicator. You occasionally cross-check the altimeter and the VSI on a supporting basis to confirm that you are holding altitude, and cross-check the turn coordinator to confirm that you are turning at a standard rate. Fifteen seconds or so into the 90-degree turn, you begin to cross-check the directional gyro to avoid overshooting your new heading. About eight degrees (half the angle of bank) before reaching the new heading, you roll to straight-and-level using the attitude indicator.
...Climbs, Descents And Takeoffs
Executing climbs and descents, and transitions to and from climbs and descents using the control/performance scan, adds another requirement. In addition to using the control/performance scanning technique for instrument cross-check and instrument interpretation, you must also use the correct inputs for aircraft control. Visible moisture does not negate the fundamental principles of aerodynamics and you may have become a little lazy over the years. To fly high-performance airplanes smoothly in IMC, you need to fly correctly.
A common problem is the failure to maintain coordinated flight. Coordinated flight is essential to keeping your passengers comfortable and also to assure that the attitude you hold will yield the performance you desire. The attitude indicator reflects only pitch and bank; it does not reflect yaw. Therefore, you could maintain a wings-level (straight) attitude and nevertheless make an uncoordinated, skidding turn to the left by applying left rudder.
Excessive left rudder is the equivalent of insufficient right rudder. A high-performance single will likewise yaw to the left if you fail to input sufficient right rudder pressure when it is required due to the sometimes-ignored left-turning tendencies: 1) asymmetrical disc loading, 2) torque, and 3) prop wash. If your high-performance plane has a single IO-520 under the cowl, it has left-turning tendencies in spades in a climb. If you maintain wings-level in a climb and leave your feet on the floor, your plane will yaw dramatically to the left. In a climb, to hold a constant heading using the attitude indicator, you must center the ball with right rudder. In a descent you need left rudder, but to a lesser extent.
The left-turning tendencies are also a factor during low visibility takeoffs. On the runway, as the airplane attempts to veer into the left hedgerow, you will receive ample feedback through the right rudder pedal. The airplane will not turn left unless the nose wheel also turns left. The nose wheel is connected to the rudder pedal which tells you that the plane is attempting a left turn. You instinctively counteract with right rudder pressure to hold the airplane straight.
Upon rotation you will lose that feedback when the nose wheel breaks ground. The tendency therefore is to reduce right rudder pressure upon rotation. At the same time that the sensation of a need for right rudder pressure decreases, the actual need for right rudder pressure increases. The rotation increases the angle of attack and exacerbates the airplane's left-turning tendencies. In order to maintain coordinated flight (and a constant heading using a wings-level attitude) you need to increase right rudder input upon rotation. Otherwise, your high-performance single will turn (yaw) dramatically to the left.
You now can fly level and perform climbs and descents using the control/performance scan. But, in order to transition smoothly between those phases of flight, we need to review yet another aerodynamic principle that you learned during your primary training: static longitudinal stability.
Certification requirements compel airplane manufacturers to demonstrate that control forces will vary proportionately with changes in airspeed. As airspeed increases, you will feel the need for a proportionately greater "pitch-down" control input in order to maintain level flight. As airspeed decreases, you will feel the need for a proportionately greater "pitch-up" control input to maintain altitude. The means by which manufacturers meet the static longitudinal stability requirement is a lengthy subject that will have to wait for another article.
Meanwhile, the ramifications of immediate significance to you for flight in IMC are:
Required pitch inputs will vary proportionately with changes in airspeed; and,
Required pitch inputs will continue to change so long as airspeed is changing.
Static longitudinal stability will present a problem to you when you upgrade to high-performance planes capable of operating over a greater speed range than the instrument trainer in which you earned your rating. In an instrument trainer you might cruise climb at an airspeed of 95-100 KIAS. When you push the nose down to a level flight attitude at 8,000 feet MSL or so, indicated airspeed will increase in a short time to 105-110 KIAS, an increase of about 10 knots or about 10 percent.
In a Bonanza or other Airplane Version 2.0, you will cruise climb at around 105 KIAS and your indicated airspeed at 8,000 will be around 145-150 KIAS, an increase of 40 knots and about 40 percent. The acceleration will persist for a longer time in a high-performance airplane and there will be a corresponding increase in your workload during the transition as the required control forces constantly change.
You could partially circumvent this increased workload by selecting a lower cruise power setting. That would decrease the airspeed range (and hence the range of required pitch control inputs). It would also shorten the process of accelerating from climb speed to cruise speed (because cruise speed will be lower). Once again, there is a tendency to select lower cruise power settings in order to convert your high-performance plane to a low-performance plane so that it will fly more like the aircraft you are accustomed to piloting. Of course, reducing power for cruise is not the reason you bought Airplane 2.0: Cruising at a lower power setting could be done just as well and probably much more cheaply in Airplane 1.0.
Static longitudinal stability is also a factor during transitions from level flight to a descent. In an instrument trainer, if you push the nose forward you will experience a modest gain in airspeed and the plane will reach terminal velocity fairly quickly. If you push the nose over in a Bonanza, you will gain lots of speed over a prolonged time period. As long as airspeed is increasing, you will need to increase the "pitch-down" control input and subsequently "pitch-down" trim to counteract the airplane's static longitudinal stability. If you neglect to steadily increase the "pitch-down" control input, the Bonanza will dutifully level off just as its designers intended.
Once again, you could avoid the need for protracted changes in pitch control inputs by drastically reducing power in the descent or by lowering the gear. Most of the time, however, you would prefer to fly gradual descents at higher speeds. Bonanzas are made to go fast.
Transitions involving deceleration (such as leveling off from a descent at cruise power) present a similar problem in high-performance planes. A Bonanza is much more slippery than a C-172 and will consume more time in decelerating from descent airspeed to cruise airspeed. Throughout the transition, the required "pitch-up" control force will be increasing.
...And Putting It All Together
Each of the above situations involving protracted changes in airspeed represents a prolonged transition between phases of flight. Just as you must fixate on the attitude indicator during the two-to-three seconds that it takes to transition from straight-and-level to a standard rate turn, you must more or less fixate on the attitude indicator throughout the one to two minutes that it takes to transition from climb to cruise, from cruise to descent, or from descent to cruise.
During these transitions, you must fly by sight, not by feel. The moment you take your eyes off the attitude indicator you will literally lose sight of the small incremental changes in attitude and will instinctively, by feel, attempt to hold altitude by maintaining the same control pressures that were "correct" moments ago. As your airspeed changes, those control pressures will become incrementally incorrect and you will deviate from your desired flight path.
The fundamental concept of the control/performance scan is to focus on the attitude indicator. The requisite near fixation on the attitude indicator during prolonged transitions is much easier using the control/performance instrument scan because that is more consistent with the general manner in which you are flying the airplane.
Other than using the control/performance scan, the two skills that will help you minimize the increased workload inherent in transitions involving speed changes in high-performance planes are anticipation and trim. Each of the above scenarios is a consequence of the fundamental principles of flight. It is therefore completely predictable, for example, that required "pitch-down" forces will increase for a minute and a half or so when you level off to cruise airspeed. That should not catch you by surprise. Instead, you should plan on it.
As pitch forces increase during a prolonged transition, do not tolerate them eliminate them with trim. It requires energy to exert force. Moreover, you cannot fly smoothly using substantial control forces because the muscle groups capable of generating those forces are not the ones you use for fine motor movements. Instead, once you have eliminated substantial control pressures, you can use your fine motor skills to achieve precise attitude control.
The instrument rating, like any other FAA certificate, is a license to learn. For good reason, you were initially trained to use the FAA's primary/supporting scan. However, once you have mastered the fundamental skill of "instrument cross-check," you should consider upgrading to the control/performance scan.
The control/performance instrument-scanning technique is for accomplished instrument pilots. If you are flying or intend to fly high-performance planes in IMC, it is the technique for you because you need to be an accomplished instrument pilot to fly powerful, slippery airplanes on instruments. The technique also works well for accomplished instrument pilots flying low-performance planes.
Changing from the FAA primary/supporting scan to the control/performance scan is not learning something new, it is relearning something old. If the primary/supporting scan requires you to fly in IMC as though you were partial panel, the control/performance scan requires you to fly in IMC as though you were in VMC. The initial feeling is very reminiscent of the first few primary training flights when you learned to keep your head outside the cockpit and to control the airplane primarily by reference to the visual horizon. Once you acclimate to the change, you will fly the airplane more naturally in IMC, using the same cruise power settings you select in VMC and without having to request a block altitude.
Whether your are being propelled by an IO-520, a pair of TSIO-360s, or an O-320, if you switch to the control/performance instrument scan you will also need to preserve your primary/secondary scanning skills. You will need them to fly partial panel when not if the attitude indicator or vacuum pump fails you.