AC 27-1B, Chg 4 9/17/2009 AC 27.49. § 27.49 (Former § 27.73) (Amendment 27-44) Performance at Minimum Operating Speed.
(For § 27.73 prior to Amendment 27-44, see AC 27.73)
a. Explanation. Amendment 27-44 adds a requirement to determine out-of-groundeffect (OGE) hover performance. Once reserved for special missions, OGE operations are now a common practice.
(1) The word "hover" applies to a helicopter that is airborne at a given altitude over a fixed geographical point regardless of wind. Pure hover is accomplished only in still air. For the purpose of this manual, the word "hover" will mean pure hover.
(2) The regulatory requirement for hover performance, § 27.49, refers to hover in-ground-effect (IGE) and OGE. Hover OGE is the absence of measurable ground effect. Hover OGE is established when the power required to hover is the same at different heights above the ground.
(3) The objective of hover performance tests is to determine the power required to hover at different gross weights, ambient temperatures, and pressure altitudes.
Using non-dimensional power coefficients (Cp) and thrust coefficients (Ct) for normalizing and presenting test results minimizes the amount of data required to cover the helicopter's operating envelope.
(4) Hover performance tests must be conducted over a sufficient range of pressure altitudes and weights to cover the approved ranges of those variables for takeoff and landing. Additional data should be acquired during cold ambient temperatures, especially at high altitudes, to account for possible Mach effects.
(5) The IGE hover ceiling for which data should be obtained and subsequently presented in the flight manual should be the same height consistent with the minimum hover height demonstrated during the takeoff tests. Refer to AC 27.51 for the procedure to determine this hover height.
b. Procedures.
(1) Two methods of acquiring hover performance data are the tethered and the free flight techniques. The tethered technique is accomplished by tethering the rotorcraft to the ground using a cable and load cell. The load cell and cable are attached to the ground tie-down and to the rotorcraft cargo hook. The load cell is used to measure the rotorcraft's pull on the cable. Hover heights are based on skid or wheel height above the ground. During tethered hover tests, the rotorcraft should be at light gross weight. The rotorcraft will be stabilized at a fixed power setting and rotor speed at the appropriate skid or wheel height. Once the required data are obtained, power should be varied from the minimum to the maximum allowed at various rotor RPM. This technique will produce a large COp spread. The load cell reading is recorded for each Page B - 24 2
stabilized point. The total thrust the rotor produces is equal to the rotorcraft's gross weight plus the weight of the cables and load cell plus cable tension. Care must be taken that the cable tension does not exceed the cargo hook limit or load capacity of the tie-down. For some rotorcraft, it may be necessary to ballast the rotorcraft to a heavy weight in order to record high power hover data. IGE hover performance may be affected by the composition and slope of the surface beneath the rotorcraft. Therefore, the tests should be conducted over a smooth, level, hard surface.
(2) The pilot maintains the rotorcraft in position so that the cables and load cell are perpendicular to the ground. To ensure the cable is vertical, two outside observers, one forward of the rotorcraft and one to one side, can be used. Either hand signals or radio can be used to direct the pilot. The observers should be provided with protective equipment. Positioning can also be accomplished by attaching two accelerometers to the load cell, which sense angle or movement along the longitudinal and lateral axes.
Any displacement of the load cell will be reflected on instrumentation in the cockpit, and by reference to this instrumentation, the rotorcraft can be maintained in the correct position. Increased caution should be utilized as tethered hover heights are decreased because the rotorcraft may become more difficult to control precisely. The tethered hover technique is especially useful for OGE hover performance data because the rotorcraft's internal weight is low and the cable and load cell can be jettisoned in the event of an engine failure or other emergency.
(3) To obtain consistent data, the wind velocity should be less than 3 knots as there are no accurate methods of correcting hover data for wind effects. To minimize inaccuracies due to hysteresis, collective movement should be made in only one direction. Rotorcraft with high downwash velocities may tolerate higher wind velocities.
The parameters usually recorded at each stabilized condition are:
(i) Engine and transmission torque.
(il) Rotor speed.
(ili) Ambient and engine temperatures, such as Measured Gas Temperature (MGT).
(iv) Pressure altitude.
(v) Fuel used (or remaining).
(vi) Load cell reading.
(vii) Generator(s) load.
(vill) Wind speed and direction.
(ix) Hover height.
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AC 27-1B, Chg 4 9/17/2009 As a technique, it is recommended the rotorcraft be loaded to a center of gravity (CG) near the hook to minimize fuselage angle changes with varying powers. All tethered hover data should be verified by a limited spot-check using the free flight technique.
The free flight technique in AC 27.49.b. (4) will determine if any problems, such as load cell malfunctions, have occurred. The free flight hover data must fall within the allowable scatter of the tethered data.
(4) If there are no provisions or equipment to conduct tethered hover tests, the free flight technique is also a valid method. The disadvantage of this technique, as the primary source of data acquisition, is that it is very time consuming. In addition, a certain element of safety is lost OGE in the event of an emergency. The rotorcraft must be re-ballasted to different weights to allow the maximum C/Cp spread. When using the free flight technique, either as a primary data source or to substantiate the tethered chried che sue, appy dra fight over tests shed parameters ed a use in the to verify any CG effects. Applicants must account for any rotorcraft stability augmentation system that may influence hover performance.
(5) Comprehensive hover performance tests are typically conducted at low, intermediate (~7000 feet Hd), and high altitude test sites, with prepared landing surfaces, in conjunction with takeoff, landing, controllability, and maneuverability testing. Alternatively, a predicted hover performance model developed for high altitude may be used if verified by limited flight-testing. The extrapolation guidelines in AC
27.45.b. (2) are still applicable. These higher altitude hover tests could typically be conducted in conjunction with the limited controllability tests. If the applicant is able to demonstrate to the FAA/AUTHORITY a method to provide a reliable hover reference, it is acceptable to conduct OGE tests without ground reference. Hover performance can usually be extrapolated up to a maximum of 4,000 feet above the highest test site altitude.
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AC 27.51. § 27.51 TAKEOFF.
a. Explanation. Section 27.51 details the conditions under which takeoff data must be obtained. The flight manual must contain the technique(s) to be used to obtain the published flight manual takeoff procedures. Technique should not be confused with exceptional pilot skill and/or alertness as mentioned in § 27.51. Because rotorcraft differ, different pilot techniques are sometimes required to achieve the safest and most optimum takeoff performance. The recommended technique that is published in the flight manual must be determined to be one that the operational pilot can duplicate using the minimum amount of type design cockpit instrumentation and the minimum crew. Only rotorcraft takeoff techniques will be covered in this section.
b. Background.
(1) Certain special takeoff techniques are necessary when a rotorcraft is unable to take off vertically because of altitude, weight, power effects, or operational limitations.
The recommended technique used to take off under such conditions is to accelerate the rotorcraft in-ground-effect (IGE) to a predetermined airspeed prior to climbout. Takeoff tests are performed to determine the best repeatable technique(s) for a particular rotorcraft over the range of weight and altitude for which certification is requested.
(2) Utilizing the total power available to execute a takeoff may not be operationally feasible due to such items as HV or aircraft attitude constraints. In such situations, hover power required plus some power increment may be the maximum recommended for use.
(3) Wheel or skid height should be not less than that demonstrated satisfactorily for the high speed, low altitude portion of the HV curve, or that height below which ground contact may occur when accomplishing takeoff procedures.
(4) For rotorcraft fitted with wheels, a running takeoff procedure may be accepted.
c. Procedures.
(1) There are different takeoff profiles which may be used to complete a maximum performance takeoff in a rotorcraft. The manufacturer will normally determine which method is best for a particular rotorcraft. The most commonly accepted method is the hover and level acceleration technique. In this technique, the rotorcraft is stabilized in a hover at the reference height. From the stabilized hover, the rotorcraft is accelerated to the climbout airspeed using the predetermined takeoff power. When the desired climbout airspeed is achieved, the rotorcraft is rotated and the climbout is accomplished at the scheduled airspeed(s) and constant rotor RPM. Power adjustments may be accomplished to maintain the targeted power except where procedure requires high workload outside the cockpit (i.e., that portion of takeoff where Page B -25
AC 27-1B, Chg 4 9/17/2009 horizontal acceleration close to the ground has pilot scan outside the cockpit and adjustment of engine torque or temperature would require an undue increase in workload). The recommended takeoff procedure must be demonstrated to remain clear of the HV "avoid" areas without requiring exceptional piloting skill or exceptionally favorable conditions.
(2) The hover reference height is established as the minimum skid or wheel height above the takeoff surface from which a takeoff can consistently be accomplished in zero wind without contacting the runway surface. The takeoff must be accomplished with power fixed at the power required to hover at the hover reference height and must not require exceptional piloting skill to avoid runway surface contact.
AC 27.51A. § 27.51 (Amendment 27-44) Takeoff.
a. Explanation. Amendment 27-44 revised the requirement to perform the test at the most critical center-of-gravity (CG) location as opposed to the most forward CG. While it is not always the case, the forward CG is particularly critical for most rotorcraft.
Additionally, the change clarifies that the test must be performed at the maximum weight requested for takeoff for altitudes above sea level. The previous requirement stated that the test be performed at a weight selected by the applicant for altitudes above sea level. That weight was traditionally interpreted to mean the maximum requested takeoff weight for that altitude and it is now stated that way in the rule.
b. Procedures. The policy material pertaining to the procedures outlined in this section remain in effect.
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AC 27-1B, Chg 4 3/31/2014 AC 27.65. § 27.65 (Amendment 27-14) CLIMB: ALL ENGINES OPERATING.
a. Explanation.
(1) Rotorcraft other than helicopters.
(i) Section 27.65 requires that the steady rate of climb be determined for each rotorcraft other than helicopter with maximum continuous power on each engine for the range of weights, altitudes, and temperatures for which certification is requested.
Equivalent levels of safety have been found wherein the applicant was allowed to select a climb airspeed that was not the actual Vy. The selected airspeed must be consistent with the speed used to show compliance with such items as cooling, stability, etc. The rate of climb resulting from the selected climb airspeed versus that from the actual Vy shall not differ to an extent that a pilot will be encouraged, by appreciable increases in climb performance, to fly a climb airspeed different from that published in the flight
(ii) For rotorcraft other than helicopters, the climb performance data obtained above must be used to show that a minimum climb gradient can be achieved for each weight, altitude, and temperature within the range for which certification is required.
This gradient must be at least 1:10 if testing is done to determine the required takeoff distance over a 50-foot obstacle. If this option is selected, an explanation of the takeoff distance determination requirements and procedures may be found in paragraph AC 29.63 of AC 29-2C.
(III) If takeoff distance is not determined, the minimum climb gradient must be 1:6 for standard sea level conditions.
(2) For helicopters, Vy must be determined for standard sea level conditions at maximum weight using maximum continuous power on each engine. Although not required, the steady rate of climb may be determined using the procedure in paragraph AC 27.65c of this section (Procedure to Determine All-Engine-Operating Climb Performance).
[Section AC 27.65 continued on next page.] Page B - 26.2
9/30/99/ АС 27-1B
(3) For helicopters, if VNE at any altitude is less than the maximum gross weight sea level standard day condition Vy, the steady rate of climb must be determined at the climb speed(s) selected by the applicant not to exceed VNE. The climb performance must be determined from 2,000 feet below the altitude from where VNE intersects Vy up to the maximum altitude for which certification is requested. This should be done utilizing maximum continuous power on each engine with the landing gear retracted.
b. Procedure to Determine Vy.
(1) Sawtooth climbs may be used to determine Vy. If such a technique is used, climbs should be flown in pairs on opposite headings 90° to the wind at the test altitude.
This procedure will minimize any windshear effects. All testing must be done in smooth air. Windshear is usually an indication of unstable air or a temperature inversion and must also be avoided. The climbs are flown on reciprocal headings for approximately 5 minutes or through an altitude band using maximum continuous power at a constant airspeed. Periodic power adjustments may be necessary. Additional reciprocal heading climbs must also be conducted at different airspeeds above and below the airspeed at the lowest point of the power required versus airspeed curve. This technique can be repeated at different altitudes to obtain Vy throughout the altitude range.
(2) Level flight performance (speed power) may also be used to determine Vy.
The testing should be done in smooth air. The advantage of this method is that less time is required, and the accuracy is equivalent to the sawtooth climb method. The test can be repeated at various altitudes to determine the Vy throughout the altitude range desired for the rotorcraft. The test at each altitude should be conducted at a constant weight over sigma (W/o). The test is normally started at the desired W/o with maximum continuous power, or at VNE, in level flight. A series of points should be taken, reducing airspeed 10 to 15 knots between points, with the lowest speed point around 20 to 30 knots. Weight should be computed for each point and the test altitude adjusted to maintain a constant W/o. After the data are reduced to standard day conditions, the minimum power required airspeed will be the Vy speed.
(3) Prior to the flight test, the rotorcraft should be ballasted to the desired gross weight and the critical center of gravity. The airspeed should be stabilized prior to data acquisition. Data to be recorded includes time, altitude, airspeed, ambient temperature, engine parameters, torque(s), rotor RPM, fuel reading, aircraft heading, external configuration, etc. Power setting, weight, and climb airspeed should be planned prior to flight. For some turboshaft engines, temperature and/or engine speed limits may be reached prior to a limiting torque. The test team should verify that the resulting power utilized in these tests closely approximates the power producing capabilities of a minimum installed specification engine.
c. Procedure to Determine All-Engine-Operating Climb Performance.
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(1) Background. Continuous climbs are conducted at the appropriate climb airspeeds as outlined above in order to validate the rotorcraft's climb performance.
By-products are a qualitative evaluation of the rotorcraft handling characteristics in a climb and engine data to assist in the determination of installed power available.
(2) Techniques. The climbs are conducted on reciprocal headings at the established airspeed(s) through the target altitude range. The same parameters are recorded as during sawtooth climbs. The rotorcraft will usually climb very rapidly during the first few thousand feet; therefore, the data acquisition method must be timely if accurate results are expected. This procedure is usually repeated at weight extremes.
The resulting data must then be corrected for power and weight. Power and weight corrections are satisfactory, provided the test powers and weights closely approximate the target values to make the weight and power corrections small. Once this data is finalized and corrected for all the flight test variables, interpolation for intermediate weights can be made with a high degree of reliability. If the rotorcraft has any stability augmentation system, vent systems, etc., which may influence the climb performance, then it must be accounted for. Caution should be taken that anti-ice, air-conditioning, etc., are not on unless the performance is being established specifically for those conditions.
AC 27.65A. § 27.65 (Amendment 27-33) CLIMB: ALL ENGINES OPERATING.
a. Explanation. Amendment 27-33 added the requirement to determine the steady rate of climb, for helicopters, from sea level up to the maximum altitude for which certification is requested. Although not specifically stated in the rule, the rate of climb should be determined at Vy or, if VNE at any altitude is less than the maximum gross weight sea level standard day condition Vy, the steady rate of climb at these altitudes must be determined at a climb speed(s) selected by the applicant not to exceed VNE-
b. Procedures. The policy material pertaining to the procedures outlined in this section remain in effect.
AC 27.67. § 27.67 (Amendment 27-23) CLIMB: ONE ENGINE INOPERATIVE.
a. Explanation.
(1) Section 27.67 requires that for multiengine normal category rotorcraft, the steady rate of climb or descent with one engine inoperative must be determined at Vy (or at the speed for minimum rate of descent) for maximum gross weight.
(2) The rate of climb (or descent) will be determined with the critical engine inoperative and the remaining engine(s) at maximum continuous or 30-minute minimum specification installed power available values. The landing gear should be retracted if it is retractable.
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b. Procedures.
(1) The procedure discussed in paragraph AC 27.65 for all-engines-operating climb performance is also applicable to the OEI condition. For twin-engine rotorcraft that are shown not to have a "critical engine" with respect to performance characteristics, both engines may be used to simulate the appropriate single-engine power available during these tests.
(2) Adequate testing must be accomplished to determine the rotorcraft's OEI climb performance at maximum gross weight for all variations in altitude and temperature for inclusion in the Rotorcraft Flight Manual.
AC 27.71. § 27.71 (Amendment 27-44) AUTOROTATION PERFORMANCE.
All of the policy material pertaining to this section remains in effect.
a. Explanation.
(1) Performance capabilities during stabilized autorotative descent are useful tools to assist the pilot when all engines fail. This information is also useful in determining the suitability of available landing areas along a given route segment.
(2) Two speeds are of particular importance, the speed for minimum rate of descent and the speed for best angle of glide. These speeds along with glide distance information are required as flight manual entries per § 27.1587. The speed for minimum rate of descent is useful for engine failure conditions at higher altitudes and the pilot is required to perform some time-related task, engine restart, float inflation, radio calls, etc.
The speed for best angle of glide is a somewhat higher speed that is of particular use when it is necessary to reach a distant landing area. These speeds, when utilized in conjunction with appropriate rotor RPM and glide angle (or rate of descent) can be used to calculate the maximum horizontal distance available from a particular altitude assuming zero wind conditions.
(3) A third speed, recommended autorotation speed, may be provided in addition to minimum rate of descent speed and maximum glide angle speed. The recommended speed for autorotation is usually optimized to assure an effective flare capability and yet be slow enough to allow a controlled, relatively slow touchdown condition. Recommended autorotation speed is ordinarily between the minimum rate of descent and maximum glide angle speeds. The recommended autorotation speed may be provided in the Rotorcraft Flight Manual. The relationship between minimum rate of descent, best glide angle, and recommended autorotation speed is shown in figure AC 27.71-1.
(4) Forward center of gravity is usually critical; however, center of gravity effects should be spot-checked to confirm this for a given design.
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b. Procedures.
(1) Tests are conducted at speeds which bracket the anticipated speeds for minimum rate of descent and best glide angle. On a power required plot, the speed for minimum power required approximates the speed for minimum rate of descent. The speed for maximum range glide may be estimated by drawing a tangent from the origin to the power required curve.
(2) Autorotative performance tests may be conducted in conjunction with the climb performance tests. The required data are similar for both tests and it is sometimes convenient and efficient to run alternating climbs and descents through a desired altitude band. Descents should be conducted on reciprocal headings and results averaged in the same manner as climb performance tests.
(3) A reduction in rotor RPM from the normal power-on value may enhance autorotative performance. If the applicant wishes to develop autorotative performance at RPM values significantly below the governing or power-on range, the practicality of reducing and controlling RPM at the lower value and of then increasing RPM as a landing is approached, must be considered. At low weights and low density altitudes, full down collective may automatically produce lower RPM values and this condition is, of course, acceptable provided the approved power-off RPM range is not exceeded.
(4) During autorotation tests, care must be taken to make certain that no engine power is delivered to the rotor drive system since a very small amount of power can have a large effect on descent performance.
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RATE OF
DESCENT RECOMMENDED
AUTOROTATIONAL SPEED
MINIMUM BESTIGLIDE ANGLE
IRATE OF DESCENT
AIRSPEED FIGURE AC 27 71-1 AUTOROTATIONAL CHARACTERISTIOS - TYPICAL Page B - 31