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5.4.2 Procedures: General.
5.4.2.1 Prior to beginning the minimum control speed tests, the applicant should verify which engine’s failure will result in the largest asymmetric yawing moment (i.e., the “critical” engine). This is typically done by setting one outboard engine to maximum power or thrust, setting the corresponding opposite engine at idle, and decelerating with wings level until full rudder is required. By alternating power or thrust on/power or thrust off from left to right, the critical engine can be defined as the idle engine that requires the highest minimum speed to maintain a constant heading with full rudder deflection.
5.4.2.2 For propeller-driven airplanes, VMCA, VMCG, and VMCL (and VMCL-2, as applicable) should be determined by rendering the critical engine(s) inoperative and allowing the propeller to attain the position it automatically assumes. However, for some engine/propeller installations, a more critical drag condition could be produced as the result of a failure mode that results in a partial power condition that does not activate the automatic propeller drag reduction system (e.g., autofeather system). One example is a turbopropeller installation that can have a fuel control failure, which causes the engine to go to flight idle, resulting in a higher asymmetric yawing moment than would result from an inoperative engine.
In such cases, in accordance with § 25.149(a), the minimum control speed tests must be conducted using the most critical failure mode. For propeller-driven airplanes where VMCA is based on operation of a propeller drag reduction system, VMCA should also be defined with the critical engine at idle to address the training situation where engine failure is simulated by retarding the critical engine to idle. If VMCA at idle is more than one knot greater than for the engine failure with an operating drag reduction system, the idle engine VMCA should be included in the normal procedures section of the AFM as advisory information to maintain the level of safety in the aforementioned training situation.
5.4.2.3 AFM values of VMCA, VMCG, and VMCL (and VMCL-2, as applicable) should be based on the maximum net power or thrust reasonably expected for a production engine. These speeds should not be based on specification power or thrust, since this value represents the minimum power or thrust guaranteed by the engine manufacturer, and the resulting minimum control speeds will not be representative of what could be achieved in operation.
The maximum power or thrust used for scheduled AFM minimum control speeds should represent the high side of the tolerance band, but may be determined by analysis instead of tests.
5.4.2.4 When determining VMCA, VMCL and VMCL-2, consideration should be given to the adverse effect of maximum approved lateral fuel imbalance on lateral control availability. This is especially of concern if tests or analysis
5-26 show that the lateral control available is the determining factor of a particular VMC.
5.4.2.5 For changes to approved designs, the effect of any aerodynamic or propulsive changes on compliance with 25.149 must be assessed per § 21.20. For example, for design changes involving an increase in engine thrust, the effect of the higher thrust on minimum control speeds must be specifically evaluated, and, if found to be not negligible, must be accounted for.
5.4.3 Procedures: Minimum Control Speeds—Air (VMCA).
5.4.3.1 In showing compliance with the VMCA requirements, the following two conditions should be satisfied:
5.4.3.1.1 The stabilized (static) condition where constant heading is maintained without exceeding a 5° bank angle, and
5.4.3.1.2 The dynamic condition in which control is maintained without exceeding a heading change of 20°.
Note: Separate tests are usually conducted to show compliance with these two conditions.
5.4.3.2 Static Test Procedure and Required Data.
5.4.3.2.1 To determine VMCA, use the configuration specified in § 25.149, except that VMCA is normally determined at minimum weight in order to minimize the stall speed and because static VMCA decreases with increased weight if a 5° bank angle is used. The requirement of § 25.149(c) that VMCA not exceed 1.13 VSR is based on VSR at maximum sea level takeoff weight. With the critical engine inoperative, the corresponding opposite engine should be adjusted to maximum takeoff power/thrust, and the airspeed decreased until heading can just be maintained with full rudder and no more than a 5° bank into the operating engine. For airplanes with more than two engines, the inboard engine(s) may be set to any power or thrust necessary to assist in developing the desired level of asymmetric power or thrust, or to achieve the desired flight path angle (normally level flight).
5.4.3.2.2 If the maximum asymmetric power or thrust that is permitted by the AFM operating limitations was maintained at the test day VMCA, and the rudder pedal force did not exceed the limit specified in § 25.149(d), the resulting speed may be used as the single value of VMCA for the airplane. If, at the option of the applicant, the AFM value of VMCA is to vary with pressure altitude and temperature, the test day minimum control speed and the corresponding power or thrust should be used to calculate an equivalent yawing moment coefficient (CN). This CN value may then be used to
5-27 calculate VMCA as a function of takeoff power or thrust, thus permitting VMCA to be scheduled as a function of pressure altitude and temperature for takeoff data expansion and presentation in the AFM. (See appendix F of this AC for further discussion of VMCA correction.)
5.4.3.2.3 If maximum allowable takeoff power or thrust could not be developed at the flight test conditions, but maximum rudder deflection was achieved, then the VMCA value corresponding to sea level standard day maximum asymmetric power or thrust may be calculated from the CN attained at the test value of VMCA. Extrapolation using this constant CN method should be limited to 5 percent of the test day asymmetric power or thrust, and should only be permitted if the rudder pedal force at the test day VMCA was not more than 95 percent of the limit value specified in § 25.149(d). For extrapolation beyond 5 percent power or thrust, a more rigorous analysis, using all the applicable stability and control terms, should be made. (See appendix F of this AC for further discussion of VMCA correction.)
5.4.3.2.4 If VMCA could not be achieved due to stall buffet, or excessive rudder pedal force, a parametric investigation should be undertaken to determine whether VMCA is limited by stall speed, maximum rudder deflection, or maximum allowable rudder pedal force. (See appendix G of this AC.)
5.4.3.3 Dynamic Test Procedures and Required Data.
5.4.3.3.1 After the static VMCA tests have been completed, dynamic engine cuts should be evaluated at a series of decreasing airspeeds to show that sudden engine failure at any speed down to the static VMCA value meets the requirements of § 25.149. The dynamic VMCA test is conducted by applying the maximum approved power/thrust to all outboard engines, stabilizing at the test airspeed, and then cutting fuel to the critical engine.
The pilot must be able to recover to a straight flight condition (constant heading) with an angle of bank of not more than 5°.
Without deviating more than 20 degrees from the original heading, While maintaining the test airspeed, without reducing power/thrust on the operating engine(s), and Without exceeding the rudder pedal force limit of § 25.149(d).
5.4.3.3.2 In accordance with § 25.149(d), the airplane may not assume any dangerous attitude, nor require exceptional piloting skill, alertness, or strength. The maximum bank angle achieved during the tests may exceed 5° provided the airplane characteristics comply with this qualitative requirement. If the dynamic tests result in a VMCA greater than the static value, the increment between the static and dynamic VMCA at the same altitude should be added to the sea level extrapolated value. If the dynamic value is less than the static value, the static VMCA should be used for the AFM data expansion.
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5.4.3.3.3 If static VMCA is near stall speed at the minimum practicable test weight, or if the thrust-to-weight ratio (T/W) results in a trimmed pitch attitude of more than 20°, it is not feasible to attempt to accurately define a quantitative value of VMCA using a sudden engine cut because of the dynamics of the rapid pitch down maneuver required, and the hazard associated with a potential spin entry. Additionally, an extreme nose up attitude followed by an engine cut is not representative of an operational takeoff engine failure. Since § 25.107(e)(1)(ii) requires VR to be not less than 1.05 VMCA, and there is some additional speed increase prior to lift off, a transport airplane is typically never airborne below approximately
1.08 VMCA. Therefore, instead of using the dynamic method to define VMCA for these aircraft with high T/W or stall speed coincident with VMCA, it is more appropriate for a dynamic engine cut to be evaluated only for acceptable controllability, and at a more representative speed. For these airplanes, a dynamic engine cut should be evaluated at an airspeed of either 1.08 VSR or 1.1 VMCA (static), whichever is greater. During the entry to, and recovery from this maneuver, all the requirements of § 25.149(d) must be met.
5.4.3.3.4 For airplanes with rudder travel-limited VMCA that have increased power or thrust engines installed, with no changes to the airframe’s geometric layout or dimensions, it may not be necessary to conduct dynamic VMCA flight testing if the power or thrust has not increased more than 10 percent above the level at which dynamic VMCA had previously been demonstrated. (See appendix F of this AC.)
5.4.4 Procedures: Minimum Control Speed-Ground (VMCG)—§ 25.149(e).
5.4.4.1 It must be demonstrated that, when the critical engine is suddenly made inoperative at VMCG during the takeoff ground roll, the airplane is safely controllable if the takeoff is continued. During the demonstration, the airplane must not deviate more than 30 feet (25 feet prior to amendment 25-42) from the pre-engine-cut projected ground track. The critical engine) for ground minimum control speed testing should be determined during the takeoff ground run using techniques similar to these described in paragraph 5.4.2. If there is a significant difference in left and right rudder deflection, the loss of asymmetric propeller disc loading, due to near zero angle-of-attack during the takeoff roll, could result in the critical engine being on the opposite side of the airplane relative to the airborne minimum control speed tests.
5.4.4.2 Work up tests may be conducted by abruptly retarding the critical engine to idle to determine the airplane asymmetric control characteristics and provide data from which an estimate of VMCG can be made. Due to the engine spindown characteristics with the critical engine retarded to idle, the speed will not, in general, be representative of the VMCG speed that would be obtained with a fuel cut. Therefore, the certification tests for
5-29 VMCG should be conducted using fuel cuts. Starting from a speed comfortably above the estimated VMCG and with the maximum takeoff power or thrust level to be certified, several fuel cuts should be made at decreasing calibrated airspeeds to establish the minimum airspeed at which the lateral deviation is less than or equal to 30 feet. VMCG is determined for zero crosswind conditions. However, in light crosswind test conditions the VMCG value determined should be that which is appropriate to the adverse crosswind or, at the applicant’s option, may be corrected to a zero crosswind value using runs made on reciprocal headings.
5.4.4.3 During determination of VMCG, engine failure recognition should be provided by the pilot:
Feeling a distinct change in the directional tracking characteristics of the airplane, or Seeing a directional divergence of the airplane with respect to the view outside the airplane.
5.4.4.4 Directional control of the airplane should be accomplished by use of the rudder only. All other controls, such as ailerons and spoilers, should only be used to correct any alterations in the airplane attitude and to maintain a wings level condition. Pilot input to controls to supplement the rudder effectiveness should not be used. Care should also be taken not to inadvertently apply brake pressure during large rudder deflections, as this will invalidate the test data.
5.4.4.5 VMCG testing should be conducted at the most critical weight in the range where VMCG may impact AFM V1 speeds.
5.4.4.6 VMCG testing should be conducted at aft CG and with the nose wheel free to caster, to minimize the stabilizing effect of the nose gear. If the nose wheel does not caster freely, the test may be conducted with enough nose up elevator applied to lift the nose wheel off the runway.
5.4.4.7 VMCG testing should not be conducted on runways with excessive crowning (i.e., cross-runway slope) unless the effects of such crowning are determined to be conservative.
5.4.4.8 For airplanes with certification bases prior to amendment 25-42, VMCG values may be demonstrated with nose wheel rudder pedal steering operative for dispatch on wet runways. The test should be conducted on an actual wet, smooth (i.e., not grooved or PFC) runway. The test(s) should include engine failure at or near a minimum VEF associated with minimum VR to demonstrate adequate controllability during rotation, liftoff, and the initial climbout. The VMCG values obtained by this method are applicable for wet or dry runways only, not for icy runways.
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5.4.5 Procedures: Minimum Control Speed during Approach and Landing (VMCL)— § 25.149(f).
5.4.5.1 This section is intended to ensure that the airplane is safely controllable following an engine failure during an all-engines-operating approach and landing. From a controllability standpoint, the most critical case usually consists of an engine failing after the power or thrust has been increased to perform a go-around from an all-engines-operating approach.
Section 25.149(f) requires the minimum control speed to be determined that allows a pilot of average skill and strength to retain control of the airplane after the critical engine becomes inoperative and to maintain straight flight with less than 5° of bank angle. Section 25.149(h) requires that sufficient lateral control be available at VMCL to roll the airplane through an angle of 20°, in the direction necessary to initiate a turn away from the inoperative engine, in not more than five seconds when starting from a steady straight flight condition.
5.4.5.2 Conduct this test using the most critical of the all-engines-operating approach and landing configurations or, at the option of the applicant, each of the all-engines-operating approach and landing configurations.
The procedures given in paragraphs 5.4.3.2 and 5.4.3.2.3 for VMCA may be used to determine VMCL, except that flap and trim settings should be appropriate to the approach and landing configurations, the power or thrust on the operating engine(s) should be set to the go-around power or thrust setting, and compliance with all VMCL requirements of § 25.149(f) and (h) must be demonstrated.
5.4.5.3 In accordance with § 25.149(f)(5) for propeller driven airplanes, the propeller must be in the position it achieves without pilot action following engine failure, assuming the engine fails while at the power or thrust necessary to maintain a 3° approach path angle.
5.4.5.4 At the option of the applicant, a one-engine-inoperative landing minimum control speed, VMCL(1 out), may be determined in the conditions appropriate to an approach and landing with one engine having failed before the start of the approach. In this case, only those configurations recommended for use during an approach and landing with one engine inoperative need be considered. The propeller of the inoperative engine, if applicable, may be feathered throughout. The resulting value of VMCL(1 out) may be used in determining the recommended procedures and speeds for a one-engine-inoperative approach and landing.
5.4.6 Procedures: Minimum Control Speed with Two Inoperative Engines during Approach and Landing (VMCL-2)—§ 25.149(g).
5.4.6.1 For airplanes with three or more engines, VMCL-2 is the minimum speed for maintaining safe control during the power or thrust changes that are likely
5-31 to be made following the failure of a second critical engine during an approach initiated with one engine inoperative.
5.4.6.2 In accordance with § 25.149(g)(5) for propeller driven airplanes, the propeller of the engine that is inoperative at the beginning of the approach may be in the feathered position. The propeller of the more critical engine must be in the position it automatically assumes following engine failure.
5.4.6.3 Conduct this test using the most critical approved one-engine-inoperative approach or landing configuration (usually the minimum flap deflection), or at the option of the applicant, each of the approved one-engine-inoperative approach and landing configurations. The following demonstrations should be conducted to determine VMCL-2:
5.4.6.3.1 With the power or thrust on the operating engines set to maintain a -3° glideslope with one critical engine inoperative, the second critical engine is made inoperative and the remaining operating engine(s) are advanced to the go-around power or thrust setting. The VMCL-2 speed is established by the procedures presented in paragraphs 5.4.3.2 and 5.4.3.3 below for VMCA, except that flap and trim settings should be appropriate to the approach and landing configurations, the power or thrust on the operating engine(s) should be set to the go-around power or thrust setting, and compliance with all VMCL-2 requirements of § 25.149(g) and (h) must be demonstrated.
5.4.6.3.2 With power or thrust on the operating engines set to maintain a -3° glideslope, with one critical engine inoperative:
1. Set the airspeed at the value determined in paragraph 5.4.6.3.1 above and, with zero bank angle, maintain a constant heading using trim to reduce the control force to zero. If full trim is insufficient to reduce the control force to zero, full trim should be used plus control deflection as required; and
2. Make the second critical engine inoperative and retard the remaining operating engine(s) to minimum available power or thrust without changing the directional trim. The VMCL-2 determined in paragraph 5.4.6.3.1 is acceptable if constant heading can be maintained without exceeding a 5° bank angle and the limiting conditions of § 25.149(h).
3. Starting from a steady straight flight condition, demonstrate that sufficient lateral control is available at VMCL-2 to roll the airplane through an angle of 20° in the direction necessary to initiate a turn away from the inoperative engines in not more than five seconds. This maneuver may be flown in a bank-to-bank roll through a wings level attitude.
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5.4.6.4 At the option of the applicant, a two-engines-inoperative landing minimum control speed, VMCL-2(2 out), may be determined in the conditions appropriate to an approach and landing with two engines having failed before the start of the approach. In this case, only those configurations recommended for use during an approach and landing with two engines inoperative need be considered. The propellers of the inoperative engines, if applicable, may be feathered throughout. The values of VMCL-2 or VMCL-2(2 out) should be used as guidance in determining the recommended procedures and speeds for a two-engines-inoperative approach and landing.
5.4.6.5 Autofeather Effects.
Where an autofeather or other drag limiting system is installed, and will be operative at approach power settings, its operation may be assumed in determining the propeller position achieved when the engine fails. Where automatic feathering is not available, the effects of subsequent movements of the engine and propeller controls should be considered, including fully closing the power lever of the failed engine in conjunction with maintaining the go-around power setting on the operating engine(s).
6-1 CHAPTER 6. FLIGHT: TRIM
6.1 Trim—§ 25.161.
6.1.1 Explanation.
Adequate trim capability should be provided for any flight condition that it is reasonable to assume will be maintained steadily for any appreciable time.
6.1.2 Procedures.
6.1.2.1 The trim requirements specify the ranges of speed and the airplane configurations at which the airplane must be able to maintain trim.
6.1.2.2 All weights, from the minimum in-flight weight to the maximum takeoff weight, should be considered. For airplanes with unpowered controls, the lowest weight may be more critical since this results in the lowest airspeed.
6.2 [Reserved.]
7-1 CHAPTER 7. FLIGHT: STABILITY
7.1 General—§ 25.171. [Reserved]
7.2 Static Longitudinal Stability and Demonstration of Static Longitudinal Stability— §§ 25.173 and 25.175.
7.2.1 Explanation.
7.2.1.1 Static Longitudinal Stability—§ 25.173.
7.2.1.1.1 Compliance with the general requirements of § 25.173 is determined from a demonstration of static longitudinal stability under the conditions specified in § 25.175.
7.2.1.1.2 The requirement is to have a pull force to obtain and maintain speeds lower than trim speed, and a push force to obtain and maintain speeds higher than trim speed. There may be no force reversal at any speed that can be obtained, except lower than the minimum for steady, unstalled flight or, higher than the landing gear or wing flap operating limit speed or VFC/MFC, whichever is appropriate for the test configuration. The required trim speeds are specified in § 25.175.
7.2.1.1.3 When the control force is slowly released from any speed within the required test speed range, the airspeed must return to within 10 percent of the original trim speed in the climb, approach, and landing conditions, and return to within 7.5 percent of the trim speed in the cruising condition specified in § 25.175 (free return).
7.2.1.1.4 The average gradient of the stick force versus speed curves for each test configuration may not be less than 1 lb for each 6 knots for the appropriate speed ranges specified in § 25.175. Therefore, after each curve is drawn, draw a straight line from the intersection of the curve and the required maximum speed to the trim point. Then draw a straight line from the intersection of the curve and the required minimum speed to the trim point. The slope of these lines must be at least 1 lb for each 6 knots. The local slope of the curve must remain stable for this range.
7.2.1.2 Demonstration of Static Longitudinal Stability—§ 25.175.
This section specifically defines the flight conditions, airplane configurations, trim speed, test speed ranges, and power or thrust settings to be used in demonstrating compliance with the longitudinal stability requirements.
7-2
7.2.2 Procedures.
7.2.2.1 Stabilized Method.
7.2.2.1.1 For the demonstration of static longitudinal stability, the airplane should be trimmed in smooth air at the conditions required by the regulation. Aft CG loadings are generally most critical. After stabilizing at the trim speed, apply a light pull force and stabilize at a slower speed. Continue this process in increments, the size of the speed increment being dependent on the speed spread being investigated, until reaching the minimum speed for steady, unstalled flight or the minimum speed appropriate for the configuration. A continuous pull force should be used from the trim speed on each series of test points to eliminate hysteresis effects. At the end of the required speed range, the force should be gradually relaxed to allow the airplane to return slowly toward the trim speed and zero stick force.
Depending on the amount of friction in the control system, the eventual speed at which the airplane stabilizes will normally be less than the original trim speed. The new speed, called the free return speed, must meet the requirements of § 25.173.
7.2.2.1.2 Starting again at the trim speed, and with the airplane in trim, push forces should be gradually applied and gradually relaxed in the same manner as described in paragraph 7.2.2.1.1 above.
7.2.2.1.3 The above techniques result in several problems in practice. One effect of changing airspeed is a change of altitude, with a corresponding change in Mach number and power or thrust output. Consequently, a reasonably small altitude band, limited to ±3,000 feet, should be used for the complete maneuver. If this altitude band is exceeded, regain the original trim altitude by changing the power or thrust setting and flap and gear position as necessary, but without changing the trim setting. Then continue the push or pull maneuver in the original configuration. Testing somewhat beyond the required speed limits in each direction assures that the resulting data covers at least the required speed ranges. It will also be noted in testing that while holding force constant at each data point, the airspeed and instantaneous vertical speed vary in a cyclic manner. This is due to the long period (phugoid) oscillation. Care should be exercised in defining and evaluating the data point, since it may be biased by this phugoid oscillation. Averaging these oscillating speeds at each data point is an acceptable method of eliminating this effect. Extremely smooth air improves the quality of the test data. In-bay and cross-bay wing fuel shift is another issue experienced in some airplanes. In-bay fuel shift occurs rapidly with pitch angle; therefore, consideration should be given to testing with fuel loadings that provide the maximum shift since it is generally destabilizing. Slower, cross-bay fuel shift, or burn from an aft tank, can influence the measured stability but usually only because of the time required to obtain the data points. This testing induced instability
7-3 should be removed from the data before evaluating the slope of the stick force versus speed.
7.2.2.2 Acceleration-Deceleration Method.
7.2.2.2.1 Trim at the desired airspeed and note the power or thrust setting. Without changing pitch trim, increase power or thrust to accelerate the airplane to the extreme speed of the desired data band. Using elevator control as needed, maintain approximately a constant altitude. Then, without changing pitch trim, quickly reset the power or thrust to the original power setting and allow the airplane to decelerate at a constant altitude back to the original trim speed. Obtain longitudinal static stability data during the deceleration to trim speed with the power and the pitch trim position the same as the original trim data point.
7.2.2.2.2 Obtain data below the trim speed in a similar manner, by reducing power or thrust to decelerate the airplane to the lowest speed in the data band.
Using elevator control as needed without changing pitch trim, maintain approximately a constant altitude. Then, without changing pitch trim, quickly reset the power to the original power setting, and record the data during the level flight acceleration back to trim speed. If, because of thrust/drag relationships, the airplane has difficulty returning to the trim conditions, small altitude changes within ±2,000 feet can also be used to coax an airplane back to trim speed. Level flight is preferred, if possible.
Obtain speed and elevator stick force data approximately every 10 knots of speed change.
7.2.2.2.3 The resulting pilot longitudinal force test points should be plotted versus airspeed to show the positive stable gradient of static longitudinal stability and that there are no “local” reversals in the stick force versus airspeed relationship over the range of airspeeds tested. This plot should also show the initial trim point and the two return-to-trim points to evaluate the return-to-trim characteristics. (See figure 7-1 below.)