Accounting for winds and non-standard conditions requires testing procedures and data reduction methods that reduce the data to standard atmospheric conditions.
(ii) Reciprocating Engine-Powered Airplanes of More Than 6,000 lbs. (2730 kg) Maximum Weight and Turbine-engine Powered Airplanes. Performance tests should be conducted in the range of atmospheric conditions that will show compliance with the selected weight, altitude, and temperature limits. See paragraph 19 of this advisory material for guidance on extrapolation of takeoff data and paragraph 29 for extrapolation of landing data.
(2) Standard Atmosphere. The Standard Atmosphere is identical to the International Civil Aviation Organization (ICAO) Standard Atmosphere for altitudes below 65,000 feet.
Appendix 7, figure 1, gives properties of the Standard Atmosphere in an abbreviated format.
(3) Installed Power. The installed propulsive horsepower/thrust of the test engine(s) may be determined using the applicable method described in Appendix 1, based on the power approved during airplane certification. The methods in Appendix 1 account for installation losses and the power absorbed by accessories and services. Consideration should also be given to the accuracy of the power setting instruments/systems, and the pilot's ability to accurately set the power/thrust.
(4) Propeller Cutoff. In general, if the airplane will be approved with an allowable cutoff for the propeller, then the performance flight testing should be done using the most critical propeller diameter. In most cases, this is expected to be the minimum diameter propeller allowed.
For normal, utility, and acrobatic category airplanes only, a 2 percent margin was the allowed reduction in propeller diameter. Historically, the 2 percent margin was selected as being the maximum permissible reduction in diameter of a given propeller that will not noticeably reduce performance. Service history has shown this to be an acceptable margin without additional flight testing.
(5) Flight Procedures. The flight procedures must not be unduly sensitive to less than ideal atmospheric conditions. The atmospheric conditions "reasonably expected to be encountered in service" may be different depending on the class of aircraft, but should cover at least the maximum demonstrated crosswind component established in compliance with § 23.233(a).
(6) Flight Test Data. For calibrated engines, test day power would be the calibrated test day power. For uncalibrated engines, an acceptable method is to assume that the test day power is the upper tolerance chart brake horsepower. See Appendix 1 for further discussion. The performance data required by § 23.1587 is dependent on the horsepower assumed for the various temperature and altitude conditions. Refer to Appendix 1, which deals both with test data reduction and expansion.
(7) Humidity Correction. See Appendix 1.
b. Procedures. See Appendix 1.
c. Time Delays. The reasonable time delays, required by § 23.45e(5)(c), for different procedures are covered in respective sections, such as accelerate-stop and landing.
d. Operation on Unpaved Runways.
(1) Small airplanes operations from grass runways. For airplanes less than 6,000 lbs.
(2730 kg) maximum weight, the factors given below may be quoted in the flight manual, as an alternative to the scheduling of data derived from testing or calculation. It should be noted that these factors are intended to cover the range of airplane types in this category and are necessarily conservative. Manufacturers are, therefore, encouraged to produce and schedule their own data in accordance with (1) and (2) above to obtain optimized performance for their airplane.
Takeoff - Dry Grass: 1.2 Landing - Dry Grass: 1.2
Notes: 1. If the grass is known to be wet, the factors should be
Takeoff: 1.3 Landing: 1.6
2. The above data are for a known smooth, flat runway. If the runway is not smooth, the grass is very long or very short, higher factors may be warranted.
Very short grass = golf course length Very long grass = unmowed field or high interstate highway median grass length
(2) Airplanes with 6,000 lbs. (2730 kg) maximum weight or more. Operations on other than smooth, dry, hard runway surfaces require specific approval and the scheduling of information on the effect of those surfaces on takeoff and landing distances in the flight manual. To obtain approval for takeoff and landing operations on unpaved runway surfaces, compliance with the following must be shown:
(i) Each type of surface must be defined so that it can be recognized in operations in service. The identification must include specification of all characteristics of the surface necessary for safe operation, such as:
(A) Surface and sub-base bearing strength;
(B) Thickness, compactness, and aggregate of the surface material; and
(C) Surface condition (for example, dry or wet)
(ii) It must be determined that the airplane can be operated on each defined surface without hazard from likely impingement or engine ingestion of any foreign objects that constitute parts of the surface.
(iii) If any special procedures or techniques are found to be necessary, these must also be determined and scheduled.
(iv) The takeoff and landing performance on each defined surface must be determined in accordance with §§ 23.53 and 23.75, as modified in (3) below.
(3) Takeoff and Landing Data. Takeoff and landing data must be determined and scheduled for each type of unpaved surface for which approval is requested.
(i) The test runways on which the takeoff and landing distance measurements are conducted should be chosen to be representative of the worst characteristics (that is, high rolling friction, low braking friction) of each of the types of runway under consideration.
(ii) In establishing the operation limitations for a particular type of unpaved runway, the runway’s load bearing characteristics, rolling and braking friction, and impingement and ingestion characteristics, must be considered.
17. SECTION 23.49 STALLING SPEED.
a. Explanation.
(1) 61 Knot Stall Speed. The 61 knot stalling speed applies to the maximum takeoff weight for which the airplane is to be certificated.
(2) Background. Since many of the regulations pertain to performance, handling qualities, airspeed indicator markings, and other variables that are functions of stall speeds, it is desirable to accomplish the stall speed testing early in the program so the data are available for subsequent testing. Because of this interrelationship between the stall speeds and other critical performance parameters, it is essential that accurate measurement methods and careful piloting techniques be used. Most standard airplane pitot-static systems have not been found to be acceptable for stall speed determination. These tests require the use of properly calibrated instruments and usually require a separate test airspeed system, such as a trailing bomb, a trailing cone, or an acceptable nose or wing boom. The stall speed determinations necessary for marking the airspeed indicator are in terms of indicated airspeed (IAS) corrected for instrument error. The other stall speeds are in terms of calibrated airspeed (CAS). Thus, a production airspeed system should be available during stall speed measurements to determine stall speeds in terms of IAS.
(3) Stall Definition. Section 23.49(d) requires the VS0 and VS1 speeds to be determined using the procedures specified in § 23.201. See Part 1 and § 23.49 for definitions of VS0 and VS1. Section 23.201(b) defines when the airplane can be considered stalled for airplane certification purposes when one of three conditions occurs, whichever occurs first. The conditions are:
(i) Uncontrollable downward pitching motion;
(ii) Downward pitching motion resulting from the activation of a device (for example, stick pusher); or,
(iii) The control reaches the stop.
For those airplanes where the control reaches the stop, VS is considered to be the minimum speed obtained while the control is held against the stop. Elevator limited airplanes may or may not develop a minimum steady flight speed. See figure 17-1 for a graphic representation of stall speed time histories for various configurations. The time the control is held against 13
the stop for stall speed determination should be a minimum of 2 seconds and consistent with the time against the stop for stall characteristics testing § 23.201. Additionally, for airplanes with a stall barrier system, stick pusher operation has been considered as the stall speed. The term "uncontrollable downward pitching motion" is the point at which the pitching motion can no longer be arrested by application of nose-up elevator and is not necessarily the first indication of nose-down pitch.
(4) Reciprocating Engine Throttle Position. For reciprocating engine airplanes, the stalling speed is that obtainable with the propellers in the takeoff position and the engines idling with throttles closed. As an alternative to "throttles closed," the regulations allow the use of sufficient power to produce zero propeller thrust at a speed not more than 10 percent above the stalling speed. The regulations do not allow any alternative to the use of "propellers in the takeoff position," nor is any alternative intended except that the use of a feathered propeller in certification stalling speed tests is acceptable only when it has been determined that the resulting stalling speed is conservative (higher). If the stalling speed tests are to be conducted with the propellers delivering zero thrust, a dependable method of determining inflight thrust, such as a propeller slipstream rake, should be available in flight. The practice of establishing zero thrust r.p.m. by calculation is also acceptable. One calculation method is given in subparagraph (5) below. Analytical corrections may be acceptable if satisfactory accounting is made for the effects of propeller efficiency, slipstream, altitude, and other pertinent variables.
(5) Zero-Thrust R.P.M. Calculation.
(i) Zero-thrust r.p.m. can be calculated by using the propeller manufacturer's propeller coefficient curves. The thrust will be zero when the propeller thrust coefficient is zero for the particular propeller blade angle. Using the propeller coefficient curves, obtain or construct a chart like figure 17-2, where:
CT = thrust coefficient C P = power coefficient ß = blade angle setting J = advance ratio
TRIM SPEED
1.1 Vs
NOT ELEVATOR
LIMITED ENTRY- RATE SLOPE NOSE TRIM : DOWN SPEED : PITCH
1.1 Vs:
AIRSPEED ELEVATOR ELEVATOR
REACHES CONTROL
STOP LIMITED ENTRY:
RATE :
SLOPE TRIM SPEED
1.1 Vs
ARTIFICIAL BARRIER (PUSHER SYSTEM) ENTRY- RATE SLOPE PUSHER FIRED
TIME - SECONDS FIGURE 17-1 STALL SPEED
(ii) The propeller blade is usually against the low pitch stop position, in the speed range of interest. Knowing the blade angle setting, the advance ratio J can be determined to give zero-thrust for the particular propeller under consideration. Knowing the value of J for zero-thrust, the propeller r.p.m. for various velocities can be calculated as follows:
Propeller r.p.m. = 101.27 V JD
Where: V = airplane true airspeed in knots J = advance ratio D = propeller diameter in feet
(c) The calculated velocities and propeller r.p.m. for zero-thrust can be plotted as shown in figure 17-3.
(6) Turbopropeller Thrust. For turbopropeller airplanes, § 23.49(e)(2) requires the propulsive thrust not be greater than zero during stall speed determination or, as an alternative to zero thrust, if idle thrust has no appreciable effect on stall speed, stall speed can be determined with the engines idling. If the airplane has a flight idle position, this would be the appropriate throttle position. Flight test experience has shown that some turbopropellerpowered airplanes may demonstrate a relatively high positive propeller thrust at the stall speed with the engines at flight idle. This thrust condition may yield an unconservative (lower) stall speed; therefore, just as for piston-powered airplanes, some dependable method to determine zero thrust should be available for comparison of zero thrust stall speed and flight idle stall speed or for determination of zero thrust stall speed. Residual jet thrust should be considered. Comparisons of zero thrust stall speed and flight idle stall speed should be investigated at high and low altitudes. Use of feathered propellers is acceptable if the feathered stall speeds are found to be conservative (higher).
(7) Fixed Shaft Turboprops. Experience on some fixed-shaft turboprop installations indicates that stall speeds can be evaluated at mid-altitudes and appear to be totally conservative. However, if stalls are conducted at altitudes of 5,000 feet or below, the stall speed can increase dramatically. This occurs because the propeller drag characteristics are a function of true airspeed, and as true airspeed decreases, the drag goes up substantially and the flow behind the propeller on wing-mounted engines causes premature inboard wing airflow separation.
In addition, if the horizontal tail and the elevator are exposed to the same flow, the elevator power is decreased and tends to compound the problem. It is recommended that stall speeds be re-evaluated at low altitudes on all fixed shaft turboprops to assure that the stall speeds have not increased.
J - ADVANCE RATIO
CP - POWER COEFFICIENT CT =.10 CT=.08 CT=.06 CT=.04 CT=.02 CT=0 B=10 O B=15 O B=2 0O B=2 5O B=30 O
FIGURE 17-2 PROPELLER COEFFICIENTS
V-TAS IN KNOTS P R O P E L L E R R P M
FIGURE 17-3 - ZERO THRUST
b. Procedures.
(1) Instrumentation.
(i) Test Systems. As previously mentioned, the production airspeed system is normally not sufficiently predictable or repeatable at high angles of attack to accurately measure the performance stall speeds of an airplane. However, a production airspeed system should be installed during stall speed tests to define the airspeed indicator markings required by § 23.1545.
The performance stall speed test system utilized in a type certification program should be calibrated to a minimum speed at least as low as the predicted minimum stall speed anticipated on the test airplane. Test systems that have been utilized to accurately define the performance stall speeds include, but are not limited to the following:
(A) Boom Systems. Swivel-head, boom-mounted, pitot-static systems with sufficient free-swivel angle to cover the stall angle-of-attack range of the airplane have been found to be acceptable. Some angle-of-attack compensated fixed pitot heads have also been found to be acceptable over a wind tunnel defined angle-of-attack range. In all wing-mounted boom systems, the boom mounted static source should be at least one chord length ahead of the wing leading edge. On nose-boom mounted systems, it has been generally accepted that the static source should be at least one and one-half fuselage diameters ahead of the nose. All boom systems should be installed in a manner which assures that the boom and boom pitot-static head are structurally sound (both static and dynamic) within the proposed operating range.
(B) Pitot-Static Bombs. Pitot-static bombs that are stable through the stall maneuvers have been found to provide acceptable data.
(C) Trailing Cones. A trailing cone static source dynamically balanced with a swivel head pitot source, or dynamically balanced with a fixed pitot source of proven accuracy in the stall angle-of-attack range has been acceptable. The stability of the cone should be verified during stall tests and throughout its intended operating range.
The length of the cone may need to be adjusted on individual airplane installations to assure cone stability.
(ii) Lag Equalization. All of the systems described in paragraph (i) could involve the use of long lengths of pressure tubing, and the associated pressure lags then occur whenever either speed or altitude, or both, are changed. Probably the most important consideration in these installations (on most small general aviation airplanes) is that the test pitot-static systems should be dynamically balanced. This is easily accomplished experimentally by putting both the total head and static orifices in a common chamber and varying the pressure in the chamber at a rate corresponding to a 2,000 to 3,000 feet-per-minute rate of descent. Various volumes are inserted in the total head line until the airspeed indicator has no tendency to move in either direction from zero during the simulated rate of descent. This method results in approximately the same volume in both systems, and, for the same size tubing, the Reynolds Number of the flow through both lines will be the same. A dynamically balanced airspeed system has equal lag in both the total and static sides. Use of a balanced system simplifies the interpretation of recorded stall time histories.
(iii) Lag Correction. When a balanced test airspeed system is used, it is often unnecessary to determine the actual amount of lag present. When such a determination is necessary, a method of accounting for lag errors is described in NASA Reference Publication 1046, "Measurement of Aircraft Speed and Altitude," by W. Gracey, May 1980.
(2) Test.
(i) Stall Speed. The actual test should be commenced with the airplane in the configuration desired and trimmed at approximately 1.5 VS1 or the minimum speed trim, whichever is greater. The airplane should be slowed to about 10 knots above the stall, at which time the speed should be reduced at a rate of one knot per second or less, until the stall occurs or the control reaches the stop. Where exact determination of stalling speed is required, entry rate should be varied to bracket one knot per second, and data should be recorded to allow the preparation of time histories similar to those shown in figure 17-1.
The indicated airspeed at the stall should be noted, using the production airspeed system.
Both the indicated airspeeds and the calibrated stall speeds may then be plotted versus entry rate to determine the one knot per second values.
(ii) Bomb. When using a bomb, caution should be used in recovering from the stall so that the bomb is not whipped off the end of the hose.
(iii) Weight and C.G. The stalling speed should be determined at all weight and c.g.
positions defining the corners of the loading envelope to determine the critical condition.
Data should be recorded so that the weight and c.g. at the time of the test can be accurately determined. This can often be done by recording the time of takeoff, time of test, time of landing, and total fuel used during the flight.
(iv) Power and Configuration. The stall should be repeated enough times for each configuration to ensure a consistent speed. If a correction is to be made for zero thrust, then the stall speed and power at several power settings may be recorded for later extrapolation to zero thrust.
(v) Control Stops. The elevator up stop should be set to the minimum allowable deflection. Flap travels should be set to minimum allowable settings.
(3) Data Reduction. The correction involves:
(i) Correction for airspeed error. IAS to CAS (correct for instrument as well as position error) when CAS is required.
(ii) Correction for weight. Multiply the test calibrated stall speed times the square root of the standard weight divided by the test weight.
VS = VST W W S T
Where: VS = Stall speed (CAS) VST = Test stall speed (CAS) 19
WS = Standard weight (lbs.)
WT = Test weight (lbs.)
(CAUTION — Do not use for minimum steady flight speed)
(iii) The correction for weight shown above applies only where the c.g. is not also changing with weight. Where c.g. is changing with weight, such as between forward regardless and forward gross, stall speed should account for this. A straight line variation between the measured stall speeds for the two weight and c.g. conditions has been found to be an acceptable method.
18. SECTION 23.51 TAKEOFF SPEEDS.
a. Explanation. The primary objective of this section is to determine the normal takeoff speeds for non-weight, altitude, and temperature limited airplanes. For WAT limited airplanes, the objective is to determine the takeoff speed schedules for all takeoff configurations at weight, altitude, and temperature conditions within the operational limits selected by the applicant.
b. For normal, utility, and acrobatic category airplanes, the rotation speed, (VR) in terms of inground effect calibrated airspeed, must be selected by the applicant. VR is constrained by § 23.51(a), as follows:
(1) For multiengine landplanes, VR must not be less than the greater of 1.05 VMC or 1.10 VS1;
(2) For single-engine landplanes, VR must not be less than VS1; and
(3) For seaplanes and amphibians taking off from water, VR may be any speed that is shown to be safe under all reasonably expected conditions, including turbulence and complete failure of the critical engine.
c. For normal, utility, and acrobatic category airplanes, the speed at 50 feet.
(1) Multiengine 50-foot Speed. For multiengine airplanes, § 23.51(b)(1) requires the speed at the 50-foot point to be the higher of the following:
(i) A speed that is shown to be safe for continued flight (or land back, if applicable) under all reasonably expected conditions, including turbulence and complete engine failure; or
(ii) 1.1 VMC; or
(iii) 1.2 VS1.
(2) Single Engine 50-foot Speed. For single-engine airplanes, § 23.51(b)(2) requires the speed at the 50-foot point to be the higher of the following:
(i) A speed that is shown to be safe under all reasonably expected conditions, including turbulence and complete engine failure; or
(ii) 1.2 VS1.
(3) Takeoff Speed Investigations - General. Investigation of the acceptability of the takeoff speed and of the associated takeoff procedure should include a demonstration that controllability and maneuverability in the takeoff configuration are adequate to safely proceed with the takeoff in turbulent crosswind conditions and maximum approved lateral imbalance.
(4) Single-engine Airplane Takeoff Speeds. The takeoff speed investigation should include a demonstration that controllability and maneuverability following engine failure at any time between lift-off and the 50-foot point are adequate for safe landing.
(5) Multiengine Airplane Takeoff Speeds. For multiengine airplanes, the investigation should include a demonstration that the controllability and maneuverability following critical engine failure at any time between lift-off and the 50-foot point are adequate for either safe landing or for safe continuation of the takeoff. There will be some combinations of weight, altitude, and temperature where positive climb at the 50-foot height with one engine inoperative is not possible. Because of this, a satisfactory re-land maneuver should be demonstrated. Rotation speed should be scheduled so that the speed at 50 feet is in accordance with § 23.51(b)(1).
(6) Multiple Takeoff Weights. For those multiengine airplanes for which takeoff distance data are to be approved for a range of weights, and for which the takeoff distance is based upon takeoff speeds that decrease as the weight decreases, the investigations of paragraph (3) of this section also should include consideration of the minimum control speed, VMC. The
1.2 VS design limit imposed on VMC by § 23.149 is intended to provide a controllability margin below the takeoff speed that is sufficient for adequate control of the airplane in the event of engine failure during takeoff. Hence, to maintain the intended level of safety for the lower takeoff speeds associated with the lighter takeoff weights, investigation of the acceptability of such speeds for compliance with § 23.51(b)(1) should include demonstration of acceptable characteristics following engine failure at any time between liftoff and the 50-foot point during takeoff in accordance with the established takeoff procedures.
(7) Complete Engine Failure. The term "complete engine failure" has been consistently interpreted to require that, for multiengine airplanes that meet the powerplant isolation requirements of § 23.903(c) in the takeoff configuration, only one engine needs be made inoperative in the specified investigations.
d. Commuter Category Airplanes.
(1) Takeoff Speeds. The following speed definitions are given in terms of calibrated airspeed. The AFM presentations are required, by § 23.1581(d), in indicated airspeed (IAS).
(i) Section 23.51(c)(1) - Engine Failure Speed (VEF). The engine failure speed VEF is defined as the calibrated airspeed at which the critical engine is assumed to fail and must be selected by the applicant. VEF cannot be less than 1.05 VMC, as determined in § 23.149. Ground controllability should also be determined to be adequate at VEF to ensure meeting the requirements of § 23.51(c)(1), that is, speed adequate to safely continue the takeoff. During the demonstration, the airplane's ground run should not deviate more than 30 feet from the pre-engine-cut projected ground track. VMCg determined under 14 CFR Part 25, § 25.149(d) is acceptable in lieu of 1.05 VMC. At the applicant's option, in crosswind conditions, the runs may be made on reciprocal headings or an analytical correction may be applied to determine the zero crosswind deviation. If 21
nose wheel steering is an integral part of the rudder system and is required to be operative, then nose wheel steering may be active. Otherwise, control of the airplane should be accomplished by use of the rudder only. If the applicant elects to use VMCg, then the nosewheel steering must be disconnected as required in § 25.149(d). 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. Use of those controls to supplement the rudder effectiveness should be avoided.
(ii) Section 23.51(c)(l) - Takeoff Decision Speed (V1). The takeoff decision speed V1 may not be less than VEF plus the speed gained with the critical engine inoperative during the time interval between VEF and the instant at which the pilot recognizes the engine failure. This is indicated by pilot application of the first decelerating device, such as brakes, throttles, spoilers, and so forth, during accelerate-stop tests. The applicant may choose the sequence of events. V1 should include any airspeed system errors determined during accelerate-takeoff ground runs. Refer to the requirements of § 23.1323(c).
(iii) Section 23.51(c)(2)- Rotation Speed (VR).
(A) The rotation speed, VR in terms of in-ground effect calibrated airspeed, must be selected by the applicant. VR is constrained by § 23.51(c)(2) as follows:
(1) V1; or
(2) 1.05 VMC determined under § 23.149(b); or
(3) 1.10 VS1; or
(4) The speed that allows attaining the initial climbout speed, V2, before reaching a height of 35 feet above the takeoff surface in accordance with § 23.57(c)(2).
(B) Early rotation, one-engine inoperative abuse test.
(1) In showing compliance with § 23.51(c)(5), some guidance relative to the airspeed attained at a height of 35 feet during the associated flight test is necessary. As this requirement dealing with a rotation speed abuse test only specifies an early rotation (VR-5 knots), it is assumed that pilot technique is to remain the same as normally used for an engine-out condition. With these considerations in mind, it is apparent that the airspeed achieved at a height of 35 feet can be somewhat below the normal scheduled V2 speed. However, the amount of permissible V2 speed reduction should be limited to a reasonable amount, as described in paragraphs (2) and (3) below.
(2) In conducting the flight tests required by § 23.51(c)(5), the test pilot should use a normal/natural rotation technique as associated with the use of scheduled takeoff speeds for the airplane being tested. Intentional tail or tail skid contact is not considered acceptable. Further, the airspeed attained at a height of 35 feet during this test is required to be not less than the scheduled V2 value minus 5 knots. These speed limits should not be considered or utilized as target V2 test 22