(ii) Caution or advisory if the autofeather is armed then is subsequently disarmed because of a system malfunction.
All annunciations should be evaluated to verify that they can be easily and quickly recognized. For critical systems, the AFM limitations should require both a satisfactory preflight check and an armed autofeather for takeoff and landing.
c. Procedures.
(1) Configuration. Prior to conducting VMC tests, rudder and aileron control travels should be set to the minimum allowable production travels. Rudder and aileron control cable tensions should be adjusted to the minimum value for use in service. The critical loading for VMC testing is generally minimum weight and maximum aft c.g.; however, each airplane design should be evaluated independently to be assured that tests are conducted under the critical loading conditions. Variable aft c.g. limits as a function of weight, tip tanks, and so forth, can cause the critical loading condition to vary from one airplane to another.
(2) Power. An airplane with a sea-level engine will normally not be able to produce rated takeoff power at the higher test altitudes. Under these circumstances, VMC should be determined at several power settings and a plot of VMC versus power will allow extrapolation to determine VMC at maximum takeoff power. See subparagraph c(6) for a further explanation of extrapolation methods. If tests are conducted at less than approximately 3,000 feet density altitude, no corrections to VMC are normally necessary. If tests are conducted above 3,000 feet density altitude, then additional tests should be conducted to allow extrapolation to sea level thrust. Because propeller thrust decreases with increasing true airspeed, VMC will increase with decreasing altitude and temperature, even at constant power.
The results of testing are used to predict the VMC for a maximum takeoff power condition at sea level unless, because of turbocharging or other reasons, some higher altitude prevails as the overall highest VMC value.
(3) Propeller Controls. All propeller controls have to stay in the recommended takeoff or approach position, as appropriate, throughout the whole procedure.
(4) Flap Settings. An applicant may want to specify more than one takeoff or landing flap setting, as appropriate, that would require VMC investigation at each flap setting.
(5) Stalls. Extreme caution should be exercised during VMC determination due to the necessity of operating with asymmetric power, full rudder and aileron at speeds near the aerodynamic stall. In the event of inadvertent entry into a stall, the pilot should immediately reduce the pitch attitude, reduce power on the operating engine(s), and return rudder and aileron controls to neutral to preclude possible entry into a spin.
(6) Static Minimum Control Speed. The test pilot should select test altitude based on the capability to develop takeoff power and consistent with safe practices. It will be necessary to determine which engine is critical to the VMC maneuver by conducting static tests with first one then the other engine inoperative to discover which one produces the higher VMC. Power should be set to the maximum available for the ambient condition. If possible, test weights should be light enough to identify the limits of directional control without stalling or being in prestall buffet.
For each test altitude condition, the following should be accomplished:
(i) Flaps and Gear. For the takeoff conditions, the gear should be retracted and the flaps in the takeoff position(s). For the landing conditions, the gear should be extended and the flaps in the landing position(s).
(ii) Trim. The airplane should be trimmed to the settings associated with normal symmetrical power for takeoff or approach (as appropriate) with all engines operating, as indicated.
(iii) Power. Set takeoff power on one engine and render the other engine inoperative.
The propeller on the inoperative engine should be windmilling, or in the condition resulting from the availability of automatic feathering or other devices.
(iv) Controls. Gradually reduce airspeed until it is no longer possible to prevent heading changes with maximum use of the directional and near maximum use of the lateral controls, or the limit control forces have been reached. No changes in lateral or directional trim should be accomplished during the speed reduction. Usually the fivedegree bank option will be used (see paragraph 48.b.(3)) to maintain straight flight. A yaw string may be used to assist the test pilot in attaining zero sideslip (or minimum sideslip).
(v) Critical Engine. Repeat steps (a) through (d) to identify which inoperative engine results in the highest minimum control speed. [If an autofeather system is installed and static VMC was determined with the propeller feathered, repeat steps (a) through (d) with the critical engine inoperative and with the propeller windmilling.]
(7) Extrapolation to Sea Level. The only VMC test data that can be extrapolated reliably are static VMC data, where most of the variables can be carefully controlled to a constant value.
Because VMC data are typically collected in ambient conditions less critical than sea level standard day, extrapolation is nearly always necessary. Therefore, the usual way to establish an AFM VMC is to extrapolate static VMC data. When VMC is determined for an airplane with an automatically feathered propeller, special techniques are required. Appendix 3 shows one method for extrapolating static VMC from test conditions to sea level standard day.
(8) Dynamic Minimum Control Speed. After determining the critical engine static VMC, and at some speed above static VMC, make a series of engine cuts (using the mixture control or idle cutoff control) dynamically while gradually working speed back toward the static speed.
While maintaining this speed after a dynamic engine cut, the pilot should be able to control the airplane and maintain straight flight without reducing power on the operating engine.
During recovery, the airplane should not assume any dangerous attitude nor should the heading change more than 20 degrees when a pilot responds to the critical engine failure with normal skill, strength, and alertness. The climb angle with all engines operating is high, and continued control following an engine failure involves the ability to lower the nose quickly and sufficiently to regain the initial stabilized speed. The dynamic VMC demonstration will normally serve as verification that the numbers obtained statically are valid. If, in fact, the dynamic case is more critical, then the extrapolated static VMC value should be increased by that increment. Frequently, the dynamic VMC demonstration will indicate a lower VMC than is obtained from static runs. This may be because the inoperative engine, during spooldown, may provide net thrust or control force peaks exceed limit values for a short period and go undetected, or, due to high yaw and pitch angles and rates, the indicated airspeed values are erroneous. Because of the multi-variable nature of the dynamic VMC demonstration, the AFM VMC value should represent the highest of the static or dynamic VMC test data, corrected to critical conditions. Specifically in test conditions with a high thrust/weight ratio, a modified procedure may be applied to avoid extreme pitch attitudes. In this case, decelerate to below VMC, all engines, accelerate with 2 x maximum takeoff power (MTOP) to a 66
representative climb pitch attitude, cut the critical engine at static VMC (verify in advance that VMC is acceptably above actual stall speed).
(9) Repeatability. Once determined, and if the dynamic VMC seems to be the critical one, the dynamic VMC should be verified by running a series of tests to determine that the speed is repeatable.
(10) AFM Minimum Control Speed Value. VMC is usually observed at either several different power settings or altitudes, or both. Sufficient test data should be obtained such that the VMC for the highest power and sea level density conditions may be determined. The VMC resulting from this extrapolation to sea level is the one entered into the AFM and marked on the airspeed indicator. If this VMC is determined with an autofeather system, the AFM required equipment list, as well as the Kind of Equipment List (KOEL), should list autofeather as a required item and the AFM would normally state the VMC with the autofeather system inoperative (propeller windmilling) in the procedures section. The procedures section should also require the autofeather to be armed (if applicable) during takeoff and landing.
(12) Safe, Intentional, One-Engine Inoperative Speed, VSSE. RESERVED
49. SECTION 23.151 ACROBATIC MANEUVERS.
a. Explanation. This regulation requires each maneuver to be evaluated and safe entry speeds established. Section 23.1567(c), which is associated with this requirement, imposes a requirement for a placard that gives entry airspeeds and approved maneuvers. If inverted flight is prohibited, the placard should so state.
b. Procedures. The applicant should fly each maneuver for which approval is sought. The FAA test pilot should then evaluate those maneuvers considered most critical.
c. Data Acquisition. A recently calibrated airspeed system, airspeed indicator, accelerometer, and tachometer should be provided by the applicant for the test airplane. The following should be recorded:
(1) Load factor;
(2) Entry airspeeds;
(3) Maximum airspeeds; and
(4) Maximum r.p.m.
50. SECTION 23.153 CONTROL DURING LANDINGS.
a. Explanation.
(1) Purpose. The purpose of this requirement is to ensure that airplanes do not encounter excessive control forces when approaching at a speed of 5 knots or 10%, whichever is lower, lower than normal landing approach speed. Also, a safe landing is required. Safe is considered to include having sufficient flare capability to overcome any excessive sink rate that may develop.
(2) Landing Requirements. Section 23.75 is a companion requirement and normally tests to determine compliance would be accomplished at the same time.
b. Procedures. The procedures applicable to § 23.75 would apply for § 23.153 except that, for turbopropeller airplanes, the flight-idle fuel flow should be adjusted to provide minimum thrust.
51. SECTION 23.155 ELEVATOR CONTROL FORCE IN MANEUVERS.
a. Explanation.
(1) Stick Force Per G. The purpose of this requirement is to ensure that the positive stick force per g levels in a cruise configuration are of sufficient magnitude to prevent the pilot from inadvertently over-stressing the airplane during maneuvering flight. The minimum maneuvering stability levels are generally found at aft c.g. loadings. Both aft heavy and aft light loadings should be considered. During initial inflight investigations, caution should be exercised in the event that pitch-up tendencies or decreasing stick force per g conditions occur.
(2) Buffet Boundaries. Low speed buffet onset may occur during high altitude investigations. A qualitative evaluation should be conducted beyond the boundary of buffet onset to ensure a capability to maneuver out of the buffet regime.
b. Procedures. Compliance with the requirements of § 23.155 may be demonstrated by measuring the normal acceleration and associated elevator stick force in a turn while maintaining the initial level flight trim speed. A descent may be required in the turn to maintain the level flight trim speed. As a minimum, the following conditions should be investigated in the cruise configuration, that is, flaps up and gear up (if retractable):
Condition POWER Wings Level Trim Speed Altitude 1 See Note Trimmed (but not to exceed VNE or
VMO/MMO) Low 2 See Note Trimmed Altitude for highest dynamic pressure (q) 3 See Note VA Low 4 See Note VA Highest attainable approved altitude
Note: 75 percent maximum continuous power or maximum power (reciprocating engine) or maximum cruise power (turbine).
TABLE 51-1 - Procedures
Compliance may be demonstrated by measuring the normal acceleration achieved with the limiting stick force (50 lbs. for wheel controls, 35 lbs. for stick controls) or by establishing the stick force per g gradient and extrapolating to the appropriate limit. Linear stick force gradients may be extrapolated up to 0.5g maximum. Non-linear stick force gradients that indicate a possible gradient lightening at higher g levels should not be extrapolated more than 0.2g.
c. Data Acquisition and Reduction. The following should be recorded for each test condition:
(1) Wt./c.g.;
(2) Pressure altitude;
(3) Outside air temperature;
(4) Engine power parameters;
(5) Trim setting;
(6) Elevator force;
(7) Normal acceleration at c.g.; and
(8) Gear/flap position.
The test data should be presented in stick force versus g plots. Figure 51-1 shows a sample plot.
Test results should be compared to the requirements of § 23.155(a).
0.51 .0 1.52 .0 2.53 .0 3.5 10 20 30 40 50 0 NORMAL ACCELERATION - G's LBS PULL
4.0 CAS= ________
c.g.= ________
FIGURE 51-1 - STICK FORCE PER G
d. Stick Force Per G, § 23.155(c). An increase in pull force should be required to produce an increase in normal acceleration throughout the range of required load factor and speed. Any reduction in control force gradient with change in load factor should not be so large or abrupt as to significantly impair the ability of the pilot to maintain control of normal acceleration and pitch rate. The local value of control force gradient should not be less than 3 lbs/g for stick-controlled 69
airplanes or 4 lbs/g for wheel-controlled airplanes. The elevator control force should increase progressively with increasing load factor.
Flight tests to satisfy the above must be performed at sufficient points to establish compliance with § 23.155(c) throughout the normal flight envelope. During these tests, the load factor should be increased until either:
(1) The intensity of buffet provides a strong and effective deterrent to further increase of load factor; or
(2) Further increase of load factor requires an elevator control force in excess of 150 lbs. for a wheel control or 125 lbs. for a stick control or is impossible because of the limitations of the control system; or
(3) The positive limit maneuvering load factor is achieved.
52. SECTION 23.157 RATE OF ROLL.
a. Explanation. The purpose of this requirement is to ensure an adequately responsive airplane in the takeoff and approach configuration.
b. Procedures.
(1) Bank Angle. The airplane should be placed in a 30-degree bank and rolled through an angle of 60 degrees. For example, with the airplane in a steady 30-degree left bank, roll through a 30-degree right bank and measure the time. Sections 23.157(b) and (d) should be accomplished by rolling the airplane in both directions.
(2) Controls. Sections 23.157(a) and (c) permit using a favorable combination of controls.
The rudder may be used as necessary to achieve a coordinated maneuver.
(3) Weight. The "W" in the formulas is the maximum takeoff weight.
53.-62. RESERVED.
Section 5. TRIM
63. SECTION 23.161 TRIM.
a. Explanation. The trim requirements ensure that the airplane will not require exceptional skill, strength, or alertness on the pilot's part to maintain a steady flight condition. The tests require the airplane to be trimmed for hands-off flight for the conditions specified. It should be noted that for single-engine airplanes, lateral-directional trim is required at only one speed, thus ground adjustable tabs are acceptable. For lateral-directional testing, the tabs may be adjusted for the test trim airspeed and readjusted for subsequent tests. For multiengine airplanes, directional trim is required for a range of speeds. If appropriate, lateral baggage loading and fuel asymmetry should be considered in this evaluation.
b. Procedures.
(1) Actuator Settings. Trim actuator travel limits should be set to the minimum allowable.
(2) Altitude and Power. Tests for trim should be conducted in smooth air. Those tests requiring use of maximum continuous power should be conducted at as low an altitude as practical to ensure attaining the required power.
(3) Weight and C.G. Longitudinal trim tests should be conducted at the most critical combinations of weight and c.g. Forward c.g. is usually critical at slow speeds and aft c.g.
critical at high speeds.
64.-69. RESERVED.
Section 6. STABILITY
70. SECTION 23.171 GENERAL.
a. Explanation.
(1) Required Stability. The stability portion of Part 23 is primarily concerned with static stability. No quantitative values are specified for the degree of stability required. This allows simple test methods or qualitative determinations unless marginal conditions are found to exist. The regulations merely require that the airplane be stable and that it have sufficient change in control force, as it is displaced from the trimmed condition, to produce suitable control feel for safe operation.
(2) Forces. The magnitude of the measured forces should increase with departure from the trim speed up to the speed limits specified in § 23.175 or up to the 40 lb. force limit specified in § 23.173. The stick force variation with speed changes should be stable. In other words, a pull force required to fly slower than trim and a push force required to fly faster than trim and the gradient should be clearly perceptible to the pilot at any speed between 1.3 VS1 and VNE or VFC/MFC. Figure 70-1 below shows an example of cruise configuration:
Vi STICK FORCE PUSH (-) PULL (+) FRSR FRSR +> 40 KNOTS OR 15%VTRIM FRSR +> 40 KNOTS OR 15%VTRIM
FIGURE 70-1 - Static Longitudinal Stability Data
Speed Range = Greater of + 40 knots (kts) or 15% VTRIM + free return speed range (FRSR)
At speeds below 1.3 VS1 for normal, utility, and acrobatic airplanes, and at speeds below
1.4 VS1 for commuter airplanes, the slope need not be stable, see Figures 70-2 and 70-3.
The pull forces can decrease in magnitude with speed decrease down to, but not including, the stall speed VS1; however, the pull force shall in no case fall below zero before the stall is reached.
Instrumented force measurements are required if there is any uncertainty in the qualitative assessment of the force gradients.
Vi STICK FORCE PUSH (-) PULL (+) FRSR +> 40 KNOTS OR 15% VTRIM VS1 1.3 VS1 VTRIM
FIGURE 70-2 - Low Speed Instabilities; Normal, Utility, and Acrobatic Airplanes
Vi STICK FORCE PUSH (-) PULL (+) FRSR +50 KNOTS VS1 1.4 VS1 VTRIM
FIGURE 70-3 - Low Speed Instabilities; Commuter Airplanes
b. Procedures. None required for this section
71. SECTION 23.173 STATIC LONGITUDINAL STABILITY.
a. Explanation.
(1) Demonstration Conditions. The general requirements of § 23.173 are determined from a demonstration of static stability under the conditions specified in § 23.175.
(2) Control Frictions. Section 23.173(b) effectively limits the amount of control friction that will be acceptable since excessive friction would have a masking effect on stability. If autopilot or stability augmentation systems are of such a design that they tend to increase the friction level of the longitudinal control system, critical static longitudinal stability tests should be conducted with the system installed. Control cable tensions should be set to the maximum.
Vi STICK FORCE PUSH (-) PULL (+) X X VS1
1.3 VS1 XX
FRSR VTRIM VNE X= FRSR +(>40 KNOTS OR 15% VTRIM)
VFC/MFC
FIGURE 71-1 – Static Longitudinal Stability Data over Complete Speed Range
A procedure that could be used is as follows:
(3) Stable Slope. Section 23.173(c) is extremely general. It requires the test pilot's best judgment as to whether the stable slope of the stick force curve versus speed is sufficiently steep so that perceptibility is satisfactory for the safe operation of the airplane.
(4) Maximum Allowable Speed. Should be taken to mean VFE, VLE, VNE, and VFC/MFC, as appropriate.
b. Procedures. Refer to paragraph 72.
72. SECTION 23.175 DEMONSTRATION OF STATIC LONGITUDINAL STABILITY.
a. Explanation. Section 23.175 requires that, for cruise configuration, static longitudinal stability tests be conducted at representative cruising speeds at high and low altitude up to VNE or VFC/MFC, as appropriate, except that the trim speed need not exceed VH. Section 23.173(a) states that static longitudinal stability must be shown at any speed that can be obtained; therefore, the 74
longitudinal stability demonstration must cover the entire range from VS1 to VNE or VFC/MFC.
Figure 72-1 shows typical coverage of the speed range in cruise with overlapping data. Midrange trim points should include speed for best endurance, range, and high-speed cruise.
(1) Trim at VS1 + (>40 kts or 15%) + an estimate of the free return speed range (FRSR), perform static longitudinal stability tests from the trim speed within the speed range ensuring that the aircraft does not stall.
(2) Determine VH at lowest altitude at maximum continuous power (MCP), perform longitudinal static stability tests within the prescribed speed range but do not exceed VNE.
(3) Select additional trim points, for example, speed for best range and endurance, and so forth, until the speed range covered by data, see figure 72-1.
(4) Go to highest operating altitude, depending on pressurization, oxygen requirements, and so forth, trim at VH, and repeat the test to a maximum speed of VFC/MFC or VNE, whichever comes first. Note that a stable slope above VNE or VFC/MFC is not required.
b. Procedures.
(1) Section 23.l75(a) Climb .
(a) Stabilized Method. The airplane should be trimmed in smooth air for the conditions required by the regulation. Tests should be conducted at the critical combinations of weight and c.g. Normally, light weight and aft c.g. are critical.
After observing trim speed, apply a pull force and stabilize at a slower speed. Continue this process in increments of 10 to 20 knots, depending on the speed spread being investigated, until reaching minimum speed for steady unstalled flight. At some stabilized point, the pull force should be gradually relaxed to allow the airplane to slowly return toward 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 be somewhat less than the original trim speed. As required by § 23.173, the new speed, called free-return speed, must be within 10 percent (7.5 percent for commuter category airplanes in cruise) of the trim speed.
Starting again at the trim speed, push forces should be applied and gradually relaxed in the same manner as previously described at speeds up to 115 percent of the trim speed, and the same determination should be made.
The flight test data band should be ± 2,000 feet from the trim altitude to minimize changes in power/thrust with altitude at a fixed throttle setting that could affect static longitudinal stability. High performance airplanes in the climb configuration sometimes require a number of iterations to stay within the data band. Or if the airplane has a very high thrust to weight ratio, it may need 3,000 feet as used for Part 25 airplanes (refer to AC 25-7).
(b) Acceleration-Deceleration Method. The stabilized flight test technique described in Section (a) above is suitable for low performance airplanes or airplane configurations with low climb performance. The acceleration-deceleration method is particularly suitable for airplanes with high cruise speed. The airplane is trimmed at the desired airspeed and the power/thrust setting noted. Power/thrust is then increased to accelerate the airplane to the extreme speed of the desired data band. The power/thrust is then reset to the original trim power setting and the airplane allowed to decelerate at a constant 75
altitude back to the original trim speed. Longitudinal static stability data is obtained during the deceleration to trim speed with the power and the elevator trim position the same as the original trim data point. The data below trim speed is obtained in a similar manner by reducing power to decelerate the airplane to the lowest speed in the data band, reset the power to trim conditions, and record the data during the level acceleration back to trim speed. If, because of thrust/drag relationships, the airplane has difficulty returning to the trim data point, small altitude changes within ± 2,000 feet can also be used to coax an airplane acceleration-deceleration back to trim speed, but level flight is preferred if possible. The data to be measured approximately every 10 knots would be speed and elevator stick force.
(2) Other Stability Test Procedures. The balance of the static longitudinal stability requirements are flown using either the stabilized method or the acceleration-deceleration method, but using the configurations, trim points, and speed ranges prescribed in § 23.175.
c. Data Acquisition and Reduction. Force readings can be made with a hand-held force gauge, fish scale, or by electronic means, and plotted against calibrated airspeed to determine compliance with the regulation. See figure 72-1 for an example of the data plot. Collect test data within a reasonable altitude band of the trim point altitude, such as ±2,000 feet. Stick force measurements must be made unless--
(1) Changes in speed are clearly reflected by changes in stick forces; and
(2) The maximum forces obtained under §§ 23.173 and 23.175 are not excessive.
70 90 110 130 150 170 190 10 5 0 5 10 15 20