U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Transition to Unfamiliar Aircraft Date: 6/29/15 Initiated by: AFS-800 AC No: 90-109A Change:
1. PURPOSE. This advisory circular (AC) is intended to help plan the transition to any unfamiliar fixed-wing airplanes, including type-certificated (TC) and/or experimental airplanes.
It provides information and guidance to owners and pilots of experimental, simple, complex, high-performance, and/or unfamiliar airplanes. It also provides information to flight instructors who teach in these airplanes. This information and guidance contains recommendations for training experience for pilots of experimental airplanes in a variety of groupings based on performance and handling characteristics. This AC does not address the testing of newly built experimental airplanes. The current edition of AC 90-89, Amateur-Built Aircraft and Ultralight Flight Testing Handbook, provides information on such testing. However, if a pilot is planning to participate in a flight test program in an unfamiliar and/or experimental airplane, this AC should be used to develop the skills and knowledge necessary to safely accomplish the test program utilizing the guidance found in AC 90-89.
2. CANCELLATION. AC 90-109, Airmen Transition to Experimental or Unfamiliar Airplanes, dated March 30, 2011, is canceled.
3. BACKGROUND.
a. Transitioning between Aircraft. The Federal Aviation Administration (FAA) has recognized a need to devote resources to preventing accidents occurring because of inadequate training when transitioning between aircraft types. Specifically, accidents resulting from loss of aircraft control or situational awareness frequently result from pilot unpreparedness for challenges presented by the aircraft. Pilots transitioning to unfamiliar aircraft require specific training in the new aircraft’s systems and operating characteristics to include normal, abnormal, and emergency procedures.
b. Experimental Airplanes. The experimental airplane community is an important part of the civil aviation industry in the United States; some of aviation’s greatest technological achievements were developed by amateur airplane builders. The amateur builder community is foundational to General Aviation (GA) in the United States. Historically, experimental airplane flight operations represent a small percentage of flight hours, but a significant percentage of GA accidents.
c. High-Performance and Complex Airplanes. Many contemporary and legacy GA aircraft have features such as retractable landing gear, turbocharging, pressurization, sophisticated avionics, autopilots, and turbine powerplants, all of which add operational
Par 3 Page 2 complexity. Furthermore, the National Airspace System (NAS) itself has become complex, requiring pilots to be proficient in operating high-performance aircraft in a complex air traffic environment.
d. Examples and Data. The predominant factor in fatal airplane accidents is pilot performance, particularly when transitioning to an unfamiliar airplane. In order for the recreational, educational, and experimental benefits of airplanes to flourish, both the FAA and the GA industry agree on the need for improvements in safety. Through collaboration between the FAA, GA, and amateur-built community, the recommendations developed in this AC mitigate some of the risks found in transitioning to unfamiliar airplanes. The recommendations are also applicable to pilots transitioning into an unfamiliar TC’d aircraft.
4. DEFINITIONS. The following are terms defined for use in this AC.
a. Hazard. A condition or factor that has the potential to cause harm or damage.
b. Risk. The likelihood and severity of the harm or damage caused by a hazard.
c. Risk Mitigation. Controls implemented to reduce the likelihood and/or severity of the harm or damage caused by a hazard.
5. DISCUSSION.
a. Flight Standardization Board (FSB). The development of this AC used an existing model of risk management (RM) in airplane operations. In large and turbojet-powered airplanes and certain aircraft in different aircraft categories, the FAA establishes an FSB; the FSB evaluates airplanes as they are completing their certification process to determine the specific requirements for training, checking, and operation in revenue service. The operations component of the FSB studies the airplane’s systems, performance, limitations, and procedures and flies the airplane to determine what requirements need to be established. The maintenance component of the FSB reviews the systems, structures, and powerplants to determine specific maintenance procedures, training, tools, and processes required to maintain the airplane in revenue service.
The final report of the FSB establishes the baseline training and procedural requirements for an operator to fly and maintain the airplane.
b. Tabletop Group. The development of this AC used the same general considerations of an FSB, but in a “tabletop” fashion. The FAA assembled experts from within the FAA and industry to develop these recommendations to pilots of experimental and/or unfamiliar airplanes.
Participants included personnel from the FAA’s Flight Standards Service (AFS), Aircraft Certification Service (AIR), and the Office of Accident Investigation and Prevention (AVP) organizations and representatives from the Experimental Aircraft Association (EAA), Aircraft Owners and Pilots Association (AOPA), and the National Association of Flight Instructors (NAFI). These participants brought substantial experience in airplane operations and maintenance. Using the tabletop methodology, the group established “families,” or categories of airplanes with similar handling, performance, configuration, or complexity, and identified the knowledge and skill required to safely fly an airplane of that category.
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c. Reviewing the Risks. Since pilots may come from a variety of backgrounds, the tabletop group determined that the best course of action would be to first identify the hazards presented by each category of airplane. Once the hazards were identified, the risks could be assessed. With the hazards identified and the risks assessed, we can establish mitigations to reduce the risk of harm from those hazards. Prior to flying an unfamiliar airplane, all pilots should review the hazards and risks outlined in this AC, and complete the training recommended before operating the airplane. Accident data has shown that there is as much risk in “moving down” in performance as “moving up.” For example, consider a pilot who has substantial experience in high-performance corporate, airline, or military airplanes. The knowledge and skills used to safely fly at high speeds, high altitudes, and over long flights will, by themselves, not prepare the pilot for the challenges of a low-inertia, high-drag airplane. Likewise, a pilot with considerable experience limited to light, low-powered airplanes will need specific training to successfully transition to high-performance airplanes.
d. Additional Assistance. Kit vendors, aircraft owners, and type clubs may offer additional assistance related to the training and operation of the airplane. See the EAA and AOPA Web sites:
• EAA, The Spirit of Aviation, http://www.eaa.org.
• AOPA Online, http://www.aopa.org.
• FAA Safety Team, http://faasafety.gov.
e. Flight Advisor Program. Another source of education and guidance is the EAA Flight Advisor Program. Under this program, the EAA Flight Advisor helps prepare the pilot for flight in a newly built or restored aircraft, or to transition to an unfamiliar aircraft. Additional information is on the EAA Web site referenced above.
NOTE: All pilots should consider the first flight in any particular experimental airplane a test flight, and should refer to AC 90-89, as well as this AC.
f. Airplane Flight Training. The choice of airplane and instructor used for this flight training is very important. To accomplish the best training, use the specific airplane that you plan to routinely operate, with a well-qualified instructor who, preferably, has recent experience in the specific make and model. The second best choice would be in the same make and model as the one the pilot is planning to fly. For pilots intending to operate an experimental aircraft, a third choice is to fly an airplane with similar characteristics, which may include a TC’d airplane (see Appendix 2, Categorizing Designs or Models). Due to construction or rigging differences, note that non-TC’d airplanes of the same model may have different handling and performance characteristics. Figure 1, Recommend Airplane Transition Training Approach, is a decision chart that summarizes the recommended approach.
Par 5 Page 4 FIGURE 1. RECOMMENDED AIRPLANE TRANSITION TRAINING APPROACH
(1) Current accident analysis indicates that subsequent owners and/or pilots of experimental and/or unfamiliar airplanes have a higher accident rate during initial flight time than did the original owner/pilot. Therefore, the recommendation is that the subsequent owners/pilots of experimental airplanes receive airplane-specific training before operating the airplane.
(2) Analysis indicates that pilots conducting operations in challenging conditions without significant experience in make/model of aircraft are at a greater risk for fatal accidents. Several type clubs, such as Lancair Owners and Builders Organization (LOBO) and Cirrus Owners and Pilots Association (COPA), recommend that pilots accumulate significant day visual flight rules (VFR) flight time prior to conducting more challenging operations such as a night or instrument flight rules (IFR) flight.
(3) The new buyer of an experimental and/or unfamiliar airplane may not fully understand the challenges of transitioning to a new airplane, particularly one which has characteristics outside his or her previous aviation experience. The FAA recommends that owners engaged in selling experimental airplanes provide a copy of this AC to prospective buyers, or refer the buyer to the FAA Web site to download a copy. In addition, an owner considering allowing another pilot to fly his or her experimental and/or unfamiliar airplane should consult this AC, along with the prospective pilot, to prepare the new pilot to safely operate the airplane. The EAA provides a checklist for the prospective buyer of an experimental aircraft (refer to the EAA Web site).
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g. How To Use This AC. Pilots transitioning to unfamiliar airplanes need to develop a training strategy for mitigating the risks of operation of the new airplane. Some of these risks are inherent to any airplane that is unfamiliar to a pilot, while others are specific to the handling characteristics, performance, configurations, systems, operation, and maintenance considerations of the new airplane.
(1) To develop the training strategy appropriate to the airplane the pilot is transitioning to, the pilot should first review paragraphs 1 through 7 of this AC. These paragraphs establish the basic training considerations for transitioning to any unfamiliar experimental or TC’d aircraft.
Pilots should then assess the characteristics of their particular airplane in light of the families of characteristics provided in Appendix 1. For convenience, Appendix 2, Categorizing Designs or Models, provides a Reverse Lookup Table for many common experimental amateur-built airplanes. Each family of characteristics has guidance provided in Appendices 3 through 9.
NOTE: An airplane may fit in more than one family. For example, a high-inertia, low-drag airplane might also employ nontraditional configuration or controls. In this case, the training strategy should reflect the considerations of both families.
(2) With the aid of a certificated flight instructor (CFI), preferably with recent experience in the new make and model, the pilot should develop the training strategy as a function of the guidance provided in the appropriate appendix or appendices. These families are somewhat wide in scope and capture aircraft that display varying levels of the specific characteristics of that family. For example, a Piper Cub exhibits low-inertia/high-drag characteristics when compared to the average GA airplane available for rental, such as a Cessna C-172 or Piper PA-28.
However, the Piper Cub does not display these characteristics as strongly as certain other aircraft within that family, such as the Kolb Firefly. The pilot will have to make an assessment of the criticality of the characteristics in developing a model-specific training strategy. Variances in construction may affect certain critical characteristics of the airplane. The assessment process is not complete until fully understanding the specific handling and performance characteristics of the aircraft in question.
(3) In addition to the general guidance contained in paragraphs 1 through 7 and the appropriate family-specific guidance contained in Appendices 3 through 9, Appendix 10, Transitional Guidance, provides guidance for transitions to higher-performance and complex airplanes.
6. CONSIDERATIONS COMMON TO ALL AIRPLANES.
a. Hazard Identification, Risk Assessment, and Risk Mitigation. All flying comes with inherent risks. It is up to all pilots to mitigate those risks. A proven approach when transitioning into an unfamiliar airplane is the RM approach used by professional test pilots. RM is the process by which:
• Hazards are identified;
• An assessment is made of the risks involved;
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• Mitigating procedures are implemented to reduce or eliminate the risks; and
• A conscious decision is made, at the appropriate level, to accept residual risks.
(1) Hazard Identification. The pilot determines the specific safety hazard, or list of hazards, for review. For example, a significant hazard is a loss of control such as stall/spin, or a loss of directional control on takeoff or landing.
(2) Risk Assessment. Each risk assessment should first analyze the two elements of risk:
the severity of the hazard and the likelihood of occurrence. Every time a person flies, there are several specific hazards experienced which have potentially fatal consequences. This risk is tolerable because the likelihood of experiencing this hazard is extremely low.
(a) Another way to look at the situation is using the term “exposure” in place of “likelihood.” A specific hazard may be fatal, but the exposure to the hazard may be small. For example, inadvertently flying from visual meteorological conditions (VMC) into instrument meteorological conditions (IMC): on a clear day, the exposure to this hazard is zero. Another example is stalling on the turn from base to final; the potential consequence of this hazard can be fatal, but exposure is only a small fraction of every flight.
(b) Always consider hazard severity and likelihood of occurrence to get a realistic understanding of risk for your flight. Try to think of all possible hazards and remember that the lack of historical data on a particular hazard does not preclude the hazard from being applicable to your airplane.
(3) Risk Mitigation. Risk mitigation is taking action(s) that result in a systemic reduction in the exposure to a risk and/or the likelihood of its occurrence. They should be actions the pilot can control. The following items are examples, but are by no means all-inclusive, of considerations in mitigating risks.
(a) Obtain specific training in your airplane or a similar airplane.
(b) Use specific safety equipment (e.g., parachute, helmet, or fire extinguishers).
(c) Evaluate the condition of the airplane, including such things as total time, cycles, inoperative components, and maintenance and inspection history.
(d) Review the FAA-issued operating limitations and the designer’s/vendor’s operational information/recommendations.
(e) Plan your flights for a conservative buildup of maneuvers. Do not take on more maneuvering or conditions, such as cross-wind takeoffs and landings, than you are ready for.
(f) Review your flight environment (temperatures, winds, visibility, etc.).
(g) Use performance predictions: what level of performance do you expect from the airplane today? Takeoff distance is just one example.
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(h) Limit the number of people who fly with you until you become more familiar with the airplane, the exception being instruction.
(i) Evaluate your own health, fitness, and fatigue level. (IMSAFE checklist, see the current edition of the Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25.
(j) Establish personal minimums for items such as duty day, weather minimums, and currency, etc., based on your personal experience level.
(4) Acceptable Risk. Even after applying risk mitigations, there will always be residual risk. The pilot will need to decide if this residual risk is acceptable depending upon the nature of the flight. If the residual risk is unacceptable, apply additional mitigations until the risk is acceptable or the pilot decides not to conduct the operation.
b. Stall Characteristics. The airplane may not provide adequate natural or artificial stall warning to the pilot. The airplane may produce unexpected, violent, or extreme attitude changes that can result in disorientation, out-of-control flight, and difficulty recovering from the stall.
Regardless of the airplane’s stall warning and stall behavior, the stall may result in significant altitude loss. While this discussion is intended for pilots of experimental aircraft, the content applies to pilots flying TC’d aircraft as well.
(1) TC’d small airplanes require adequate stall warning, and fairly benign stall and stall recovery characteristics. The warning must be obvious and readily discernable by the pilot under all conditions, and provide enough of an airspeed margin for recognition and avoidance. The allowable change in airplane attitude caused by a stall is very limited and recovery must not require unusual skill, strength, or alertness. Information about stall characteristics may be published in the Airplane Flight Manual (AFM).
(2) There are no rules for stall behavior with experimental airplanes. Some experimental airplanes can be flown in a carefree manner with the stick all the way back, while others can depart controlled flight dramatically without any perceptible warning. These behaviors might be inherent in the airplane design, due to its construction, the result of improper flight control rigging, or due to a host of other reasons. Since amateur-built airplanes are built by individuals, there can be a wide variation in the stall behavior of identical models. It is essential that every experimental airplane pilot be aware of the variety of possible stall characteristics among experimental airplanes and become familiar with the characteristics of his or her airplane.
(3) Whether the stall warning is a natural airframe buffet or a warning device triggered by an angle of attack (AOA) sensor, it needs to be several knots faster than the stall speed, but not so much faster that it becomes a nuisance or the pilot ignores it. A sufficient stall margin should exist for all flap and landing gear configurations and provide reliable warning even when maneuvering. In addition to the airspeed margin between warning and stall speeds, the warning must be obvious enough to ensure the pilot notices its occurrence. A weak horn in a noisy cockpit or gentle buffet masked by turbulence can go unnoticed.
(4) There are measures you can take to help avoid an inadvertent stall due to insufficient stall warning. Ensure the airplane’s construction meets the kit vendor’s specifications. Consult the kit vendor to determine whether your airplane’s stall warning is representative of the design.
Par 6 Page 8 Consider adding stall warning devices or stall characteristic improvements (with the manufacturer’s concurrence) such as a warning horn, AOA system, or stall strips. Ensure that existing and new systems receive proper calibration regardless of what stall warning system, if any, you install; you should receive training in stall warning recognition in your airplane from a qualified instructor. Practice stall warning recognition and stall avoidance until you are proficient.
NOTE: Any modification should be thoroughly evaluated during design and flight test.
(5) The airplane’s response to control inputs usually degrades at slower speeds, requiring larger deflections to achieve the desired response. These larger control surface deflections can actually precipitate a stall, making it incumbent on the pilot to become familiar with their airplane’s behavior in case it has this characteristic. Ideally, every airplane would stall with a clearly defined, slight pitch break without rolling off to one side, but that is not always the case with experimental airplanes. Some planes can abruptly drop a wing 45 degrees or more.
Applying an intuitive opposite aileron control input may actually aggravate this roll-off and induce a yaw in the same direction. This is not a good situation when you consider that a stalled wing and a yaw rate are the two ingredients required for a spin.
(6) It is also possible, even for a seemingly carefree-handling airplane, to achieve what some have called a deep stall, where there is not sufficient nose-down pitch authority to break the stall, possibly creating an unrecoverable situation. Some airplanes can pitch nose-up before the stall, resulting in a rapid stall entry unless the pilot counters with a conscious forward yoke/stick motion.
(7) An airplane that stalls wing-level, but results in an excessive altitude loss, can lead to trouble should an inadvertent stall occur near the ground. Airplanes with carefree handling at minimum flying speed can inspire a false confidence if the descent rate is high at low airspeed.
Some experimental airplanes have very powerful engines turning large propellers; while these planes might enjoy a performance advantage, stalling with a high power setting may introduce significant attitude, rate, and acceleration variations, even violently disorienting motions. This is a possible accident scenario during a go-around when the pilot rapidly applies power without controlling the pitch and yaw tendencies.
(8) There are steps you can take if your airplane’s stall characteristics are beyond what you consider benign:
• If experimental, ensure the airplane is built according to the kit vendor’s specifications or consult the kit vendor to determine whether your airplane’s stall behavior is representative of the design;
• Receive training in your airplane on stall avoidance and recovery from a qualified instructor, preferably with recent experience in the make and model; and
• Adhere to stall-free flying by establishing and honoring minimum airspeeds for all flight phases.
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(9) It is possible for an airplane to have normal stall warning traits and still have undesirable stall characteristics. Conversely, an airplane can be lacking in stall warning but have a non-threatening stall. Either of these situations could be acceptable or not, but the pilot needs to know whether his or her airplane behaves this way and how to best fly that plane to maximize safety. Stall recovery should be easy to perform any time during the warning phase or after the stall. The airplane should respond as expected to intuitive control inputs with no tendency for a control-induced secondary stall. Knowing whether your airplane performs this way is essential knowledge for the safe operation of your plane.
(10) While some altitude loss may occur, excessive altitude loss following a stall is obviously problematic. Ensure the airplane’s construction meets the kit vendor’s specifications.
Consult the kit vendor to determine whether your airplane’s stall characteristics and recovery performance are representative of the design. Ensure that your engine and propeller are operating properly. Practice stall entries and recoveries, after receiving instruction, at a safe altitude. Learn how to optimize your plane’s stall recovery performance without entering a secondary stall. Take note of the worst case altitude lost, and keep this figure in mind when maneuvering or flying in gusty conditions at low altitude.
(11) The following list is not all-inclusive, and it is up to every pilot to determine his or her airplane’s sensitivity to these factors through education, training, and practice under safe, controlled conditions. Some other issues that can significantly affect an airplane’s stall characteristics include:
• Weight;
• Center of gravity (CG);
• Wing contamination such as a minor “ding” or dead insects near the leading edge, or icing present on skin surfaces (particularly for laminar flow wing designs);
• Sideslip/yaw (which can raise the stall speed and result in a rolling/yawing departure);
• Rate of speed decay (deceleration); and
• Normal acceleration (G Load) increase.
c. Recommended Ground and Flight Training.
(1) Know your airplane’s systems, limits, and recommended procedures before you begin flying; paying special attention to all onboard flight instruments, avionics, and navigation equipment, whether legacy or modern. If experimental, consult the kit vendor for advice. This is especially important for technically advanced aircraft (TAA) equipped with sophisticated avionics or flight automation systems (e.g., autopilot) you might not be familiar or competent with. Some automated flight systems, like AOA indicators, can enhance the pilot’s stall margin awareness. In 2012, the General Aviation Joint Steering Committee (GAJSC) placed increased emphasis on promoting the installation of AOA indication systems on GA aircraft. Type club members and owner/builders of your airplane model can provide valuable information about stall behavior and stall margin awareness equipment. Internet forums and chat rooms may provide valuable information, but remember that participants may or may not have the technical expertise you seek, and their airplanes may exhibit different stall behavior than yours.
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(2) A thorough airplane check-out by a qualified instructor, preferably one with recent experience in your airplane make and model, is always a good idea. If you built the airplane yourself, consider obtaining this training from the kit vendor or owner’s group, preferably in your airplane. If you purchased your plane from a previous owner, learn all you can from him or her. Periodically practice stall avoidance, entry, and recovery at a safe altitude after you have received enough instruction to feel comfortable. Stall recognition and recovery should not be self-taught. Your first experience should not come from an inadvertent stall that catches you by surprise.
d. Additional References (current editions).
• AC 23-8, Flight Test Guide for Certification of Part 23 Airplanes.
• AC 61-98, Currency Requirements and Guidance for the Flight Review and Instrument Proficiency Check.
• The Advanced Pilot’s Flight Manual, Kershner, Iowa State University Press.
7. STABILITY AND CONTROLLABILITY. An airplane may exhibit positive, neutral, or negative stability characteristics. Neutral or negative stability can result in significantly increased workload for the pilot, leading to distraction from other necessary piloting tasks. Worse, it can create a pilot-induced oscillation scenario, or even result in an uncontrollable situation.
a. Definitions. The following definitions apply to this section.
(1) Controllability. The ease or difficulty of changing an airplane’s flight condition.
(2) Maneuverability. How quickly the flight condition can be changed
(3) Stability. The airplane’s tendency to remain at its current steady flight condition or to return to its steady flight condition after being disturbed.
b. Discussion.
(1) An airplane that exhibits strong static stability is reluctant to change its flight condition. Too much static stability can mean high control forces and difficulty when trying to make changes to the airplane’s altitude or flight path. Too little static stability could make an airplane feel “twitchy” or overly sensitive to control inputs or atmospheric upsets like wind gusts. This could also make the airplane difficult to fly precisely.
(2) Once the airplane is disturbed from its steady condition, dynamic stability comes into play. A positively stable airplane will return to its pre-disturbed flight condition upon removal of the disturbance. It might do this slowly or quickly, with or without oscillations. An airplane with negative dynamic stability will develop larger deviations from its original flight condition following a disturbance. Again, this can happen slowly or quickly, with or without oscillations.
Obviously, an airplane with negative dynamic stability could be difficult, if not impossible, to fly.
(3) The following are a few stability modes, along with their effect on piloting:
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(a) Negative longitudinal static stability. If the airplane deviates from its trimmed airspeed, the deviation will increase until the airplane stalls or exceeds the never-exceed speed (VNE). The pilot must continuously monitor the airspeed indicator and make corrective pitch control inputs to correct any airspeed deviation.
(b) Negative longitudinal dynamic stability (phugoid). If the airplane deviates slower (or faster) from its trimmed airspeed, it will accelerate (or decelerate) beyond its trimmed airspeed, then decelerate (or accelerate) again beyond its trimmed airspeed in ever-increasing airspeed and altitude deviations until it stalls, exceeds VNE, or impacts the ground. The pilot cannot rely on the airplane to self-correct even minor airspeed deviations caused by control input, wind gust, thermal activity, etc. This can result in fatigue, as the pilot must continuously suppress the excursions.
(c) Negative longitudinal dynamic stability (short period). Short period is a pitch oscillation caused by a change in AOA; excursions grow larger with each oscillation. Unlike the phugoid, these oscillations can occur very fast with each cycle taking only 1 or 2 seconds. Due to the high frequency of these rapid pitch direction reversals, the short period is felt as G Load excursions that can become intolerable, despite the seemingly small initial pitch attitude changes.
There are presently no known experimental airplane designs that exhibit this behavior, but should it occur, the excursions will increase with every cycle until the airplane either stalls or exceeds a structural limit. This is not a pilot-induced oscillation, as the airplane will continue its oscillation without pilot control input. Suppressing a negative short period is likely to be extremely challenging due to the requirement for perfectly timed counter pitch control inputs of the proper size.
(d) Negative spiral stability. An airplane in a bank will continue to increase its bank angle unless the pilot prevents it by applying opposite roll control. This is particularly dangerous during night and IMC flight unless the pilot closely monitors the airplane’s attitude when turning. Any roll-off due to this characteristic is usually subtle, depriving the pilot of cues such as rolling motion, changes in wind noise, and cockpit control position.
(e) Negative lateral-directional stability. A Dutch roll is a yawing, rolling oscillation caused by sideslip or relative wind coming from the left or right of the airplane’s nose.
Excursions grow larger in yaw, roll, or both with each oscillation. There are presently no known experimental amateur-built airplane designs that exhibit this behavior, but should it occur, the excursions will increase with every cycle until the airplane either departs controlled flight or exceeds a structural limit. Suppressing the Dutch roll requires well-timed, properly sized counter control inputs. The most effective control is usually the rudder; however, aileron may also be effective in some airplane designs.
(4) TC’d airplanes must exhibit positive static and dynamic stability, but experimental airplanes may or may not. Just because an experimental airplane shows positive stability traits during cruise flight does not preclude it from becoming unstable in the landing pattern or vice versa. Pilots might not notice a mildly unstable airplane, because the attentive pilot is always making small control inputs for minor deviations, often without even realizing he or she is doing so. The hazard with this kind of airplane is when the pilot is distracted and doesn’t initially notice a deviation in altitude or path, and must then make a large correction, which now distracts
Par 7 Page 12 him or her from other piloting responsibilities. Some examples of documented instabilities in experimental airplanes during some flight regimes include:
(a) Following a small airspeed deviation slower than the trimmed speed, the airplane will continue to decelerate until it stalls, unless the pilot intervenes. This is an example of negative longitudinal static stability.
(b) Following a small airspeed deviation faster than the trimmed speed, the airplane will continue to accelerate until it exceeds VNE, unless the pilot intervenes. This is an example of negative longitudinal static stability.
(c) Following a small rudder pedal displacement, the airplane will continue to yaw to a larger sideslip angle, unless the pilot intervenes with opposite pedal displacement. This is an example of negative directional stability.
(d) Following a deviation (either faster or slower) in airspeed, the airplane alternates slowing and speeding up with each oscillation larger than the previous one, until it either stalls or exceeds VNE. This is an example of negative longitudinal dynamic (phugoid) stability.
(e) After establishing the airplane in a bank angle, it continues to roll toward increasing bank, unless the pilot applies opposite aileron. This is an example of negative spiral stability.
(5) These examples pertain to several experimental airplane designs, hundreds of which have been flying for years. Although this statement may appear to minimize the concern, it is intended to illustrate that some experimental airplanes have behaviors that can be kept in check under normal circumstances, but can rapidly develop into an emergency situation unless the pilot remains absolutely vigilant at all times. A common analogy is balancing a broomstick in the palm of your hand: it is possible, but doing it successfully while copying a clearance or programming a Global Positioning System (GPS) is much more difficult, and if it tilts too far, no amount of effort or concentration can save it.
(6) The airplane’s control system plays a major role in the pilot’s impression of its stability, controllability, and maneuverability. An airplane with weak stability but high control force requirements might feel more stable because the pilot would have to apply a substantial force to the controls to cause a small control surface deflection. If that small deflection, however, resulted in an unexpectedly large airplane response, the pilot might have to spend an inordinate effort to keep the responses manageable. Likewise, an airplane with very low control forces and highly maneuverable response can easily lead to over-controlling with a series of alternating inputs in an attempt to arrest airplane excursions. Both scenarios of airplane-pilot coupling are pilot-induced oscillations and can escalate very rapidly into an out-of-control situation.
(7) The freedoms allowed in experimental airplane design and also in individual builders’ construction decisions may significantly influence the airplane’s stability and controllability. The following list provides some examples of what may affect stability and controllability; all have an effect that can range from negligible to disastrous: