recently certificated rotorcraft. The high VNE speeds of current rotorcraft result in relatively high values for power-off VNE. Speeds lower than that specified in the regulation have been found acceptable through a finding of equivalent safety if the selected power-off VE is equal to or greater than the power-off speed for best range.
In any case, the power-off VE must be a high enough speed to be practical. A demonstration is required of the deceleration from the power-on VNE or OEIVNE to the power-off NE: The transition must be made in a controlled manner with normal pilot reaction and skill.
b. Procedures. The tests to substantiate the different VE speeds are ordinarily conducted during the flight characteristics flight tests. The flight test procedures are discussed for the various limiting areas in earlier paragraphs of this document. Static stability test techniques are covered in Paragraph 86 and the vibration test techniques in Paragraph 110.
721. § 27.1509 ROTOR SPEED.
a. Explanation.
(1) General. This rule requires minimum and maximum power-off rotor speeds to be established as operating limitations. It also specifies the appropriate margins below and above these limits which must be substantiated structurally and by flight tests. In addition to addressing power-off limits, the rule requires that minimum power-on RPM be established as an operating limit, and it specifies conditions, by reference, for establishing a minimum appropriate power-on speed.
(2) Power-off Limits. The power-off or autorotational RPM limits are established by design and substantiated by structural testing. Limits are confirmed during flight testing. Critical components must be designed for RPM values at least 5 percent above and below the maximum and minimum approved RPM values respectively. This 5 percent conservative speed requirement is in addition to the other structural safety factors built into the design requirements. A transient limit lower than the minimum in-flight RPM (power-off) will be defined to cover the final phase of a total power-off landing. Maximum weight is ordinarily critical for both tests. At low RPM, high coning angles can produce high stress levels in blade bending. Large flapping angles or controllability problems may also develop. At high RPM values, centrifugal forces on the blades are at their highest and stress levels on rotating components such as blade grips may be critical. If a particular model has a very large weight spread between minimum and maximum gross weights, the applicant may elect to specify two ranges of power-off RPM dependent upon weight. This may be needed to ensure adequate power-off rotor RPM with collective full down without requiring the very low power-off rotor speeds at maximum weight, a condition which would be inappropriate for operation of the rotorcraft in service. Transient power-off RPM ranges may also be approved if needed for engine failure conditions; however, these transients must also be substantiated structurally and in flight.
956 Par 720
7/30/97 АС 27-1A
(3) Power-on Limits. The minimum power-on rotor speed must be established so that it is no less than the minimum rotor speed which has been established structurally. The minimum power-on speed also cannot be less than those values achieved during any of the critical maneuvers during flight test substantiation of the rotorcraft. A 5 percent margin between the substantiated value and the limit value is not required as in the power-off case. This rule also makes reference to § 27.33(a)(1) and (b)(1) for establishing the minimum power-on value. The reference to Paragraph (a)(1) is intended to ensure that the minimum power-on RPM value is low enough to accommodate the RPM values which will occur as a result of power changes and flight maneuvers expected in service. The reference to (b)(1) establishes the requirement that the minimum power-on RPM can not be greater than the minimum RPM used to determine the appropriate setting of the main rotor high pitch stop.
Although the maximum power-on value is not specifically referred to in this section, it must be established as a limitation per § 27.309. In addition, for compliance with the requirements of § 27.141(b) regarding smooth transition from power-on to power-off flight, the power-on maximum limit should not be greater than the power-off maximum limit and the power-on minimum limit should not be less than the power-off minimum limit.
(4) Transient Limits. Transient limits must be substantiated and approved in a similar manner. Transient limits may be outside of the steady state "red-line" limits.
b. Procedures.
(1) Tests for substantiation of stress and vibration at the 5 percent underspeed and overspeed conditions in autorotation are ordinarily conducted as a part of the flight strain survey. For purposes of finding compliance with this rule, it is suggested that as a minimum, FAA/AUTHORITY certification personnel witness applicable portions of the test program and monitor telemetry or flight recorded data, as necessary, to verify compliance with this rule. Tests at maximum weight and at a relatively light weight condition are normally sufficient. Tests must be conducted at speeds up to VNE (power-off) at 105 percent of maximum RPM and 95 percent of minimum RPM. It is also appropriate to investigate speeds to 1.1 VNE (power-off) at maximum and minimum power-off RPM values. The normal low pitch stop may need to be downrigged in order to achieve the high RPM values at high speed. This feature should be coordinated with the manufacturer prior to the flight strain survey to ensure necessary conditions are achieved. It may be difficult to obtain minimum power-off RPM prior to encountering retreating blade stall at combinations of high weight, high collective pitch, low rotor speed, and high forward speed. In this case VNE (power-off) can either be decreased in accordance with § 27.1505(c) or the low RPM range can be evaluated in a transient manner during engine failure testing at high speed. Any condition in which blade stall is suspected should, of course, be investigated with a great deal of caution and build-up testing is recommended. The transient low RPM limit for power-off landings may be Par 721 957
tested only during actual power-off landings. In that case, the 5 percent margin is not required.
(2) Testing for suitable minimum and maximum power-on RPM values may be conducted during the designated FAA/AUTHORITY flight test program. The combined engine and governor response must allow accomplishment of all appropriate flight maneuvers without exceeding minimum or maximum power-on rotor limits. As in the power-off case, appropriate transient ranges and limits may be approved when properly substantiated. Transient ranges should be evaluated using similar methods and techniques to those described above. Power-on RPM determination must include not only rotor system considerations but engine and drive system characteristics as well. It is important to remember that all power-on ranges must be eligible under the Part 33 engine approval and that the power-off range must include adequate margins from potentially hazardous drive system phenomena, such as drive shaft whirl modes.
958 Par 721
722. RESERVED.
Intentionally Left Blank Par 722 959
723. § 27.1519 (Amendment 27-21) WEIGHT AND CENTER OF GRAVITY.
a. Explanation. This rule requires that weight and center of gravity (CG) combinations which are substantiated structurally and also found satisfactory during flight tests (per §§ 27.25 and 27.27) must be established as operating limits. A related portion in § 27.1583(c) further requires that weight and CG limitations be entered in the RFM limitations section. Both maximum and minimum weight must be established as operating limitations along with the corresponding longitudinal and lateral centers of gravity for each condition. Weight and CG limits are discussed in more detail in Paragraphs 43 and 44 of this AC.
b. Procedures.
(1) The results of shifts in center of gravity with fuel burn should be evaluated.
If it is possible to take off within the approved loading envelope and subsequently burn fuel to a condition which is significantly beyond the approved weight/CG envelope, then there should be appropriate instructions in the loading and/or operating procedures of the RFM to avoid this condition.
(2) Typical loading conditions should not result in weight/CG combinations outside of approved limits. A minimum of two loadings, appropriate to the rotorcraft configuration, should be evaluated. These should include critical combinations of maximum/minimum variables for fuel, passengers, and crew. If this results in loading outside approved limits, special interior placarding or cautionary information should be provided in appropriate sections of the RFM.
724. § 27.1521 (Amendment 27-14) POWERPLANT LIMITATIONS.
a. Explanation.
(1) This rule requires that the various parameters and operating conditions listed under each type of powerplant operation be evaluated and established as operating limitations. The procedures for establishing and verifying each powerplant limitation are discussed in the powerplant section of this AC. This rule requires that powerplant limitations be established for two specific types of operation or power ratings; takeoff and continuous. Additional limitations are required to account for engine and transmission cooling and minimum required fuel grade.
(2) Paragraph (e) requires that for turboshaft engines, a limit engine torque be established in addition to the other limiting parameters listed under each type of operation in Paragraphs (b) and (c). Compliance with this paragraph requires that a torque limit be established for each approved engine rating (i.e., takeoff, continuous, etc.) even though not specifically stated in the rule.
960 Par 723
(3) For rotorcraft equipped with two or more turboshaft engines and seeking approval for one-engine-inoperative (OEI) ratings, the same parameters required for the takeoff and continuous ratings should be established as limitations for each approved OEl rating (i.e., maximum rotational speed, time, gas temperature, and torque).
Section 27.923 includes requirements for qualification of the rotor drive system for 2½-minute and 30-minute OEl powers. Section 27.1501(a) requires that any information necessary for safe operation must be established as limitations. Thus the establishment of OEI powerplant limitations is required even though not specifically addressed in § 27.1521 (through Amendment 27-14).
(4) It is important to differentiate between the rotorcraft powerplant limitations and the engine limitations as established under Part 33. For some parameters, these two limits may be identical, but frequently the engines will be capable of exceeding the maximum limitations substantiated for the combined powerplant installation. Limitations established according to this rule may not exceed the engine limitations established in accordance with Part 33 but may be less than the Part 33 limits as desired by the applicant.
b. Procedures.
(1) Determine the limiting parameters for each required power rating according to the requirements of Part 27, Subpart E, Powerplant. (See applicable paragraphs of this AC for detailed procedures.)
(2) Provide the limitations established according to this rule to the rotorcraft crew through placards in accordance with § 27.1541, instrument markings in accordance with § 27.1549 and in the Rotorcraft Flight Manual Limitations Section in accordance with § 27.1583(b). (See Paragraphs 763 and 781 of this AC.)
724A. § 27.1521 (Amendment 27-23) POWERPLANT LIMITATIONS.
a. Explanation. Amendment 27-23 added §§ 27.1521(g), (h), and (i) that establish and define the powerplant limitations associated with OEI power ratings. The new sections introduce the term "OEl" to emphasize and clarify the limitations on the use of the 2½-minute and 30-minute power ratings. Amendment 27-23 added the introductory phrase "unless otherwise authorized." In order to authorize use of these emergency ratings, additional qualification tests, or other adequate safety measures have been instituted. The sections set forth specific limitations on the use of these emergency ratings. These changes were intended to avoid misconceptions regarding the eligibility of these ratings. Section 27.1521(1) establishes and defines a new continuous OEI power rating using terminology similar to that developed for the 2½-minute and 30-minute power ratings. These new sections ensure proper recognition in the powerplant limitations listing required by § 27.1583(b).
Par 724 961
b. Procedures. All of the policy material pertaining to this section remains in effect. Additionally, the following procedures should be considered:
(1) Sections 27.1521(g) through (i) require limitations for OEl operation for multi-turbine engine powered rotorcraft. The same parameters required for the takeoff and continuous ratings should be established as limitations for each approved OEI rating (i.e., maximum rotational speed, time, gas temperature, and torque).
Section 27.923 includes requirements for qualification of the rotor drive system for 2½-minute and 30-minute, and continuous OEl powers. Section 27.1501(a) requires that information necessary for safe operation should be established as limitations.
Thus the establishment of OEI powerplant limitations is required even though not specifically addressed in § 27.1521.
(2) It is important to differentiate between the rotorcraft powerplant limitations and the engine limitations as established under Part 33. For some parameters, these two limits may be identical, but frequently, the engines will be capable of exceeding the maximum limitations substantiated for the combined rotorcraft drive system. Limitations established according to this rule may not exceed the engine limitations established in accordance with Part 33 but may be less than the Part 33 limits as desired by the applicant.
724B. § 27.1521 (Amendment 27-29) POWERPLANT LIMITATIONS.
a. Explanation. Amendment 27-29 adds §§ 27.1521(j) and (k). The new §§ 27.15210) and (k) introduce the 30-second and 2-minute OE power rating limitations, respectively. These paragraphs define the limitations on the use of the 30-second and 2-minute power ratings using terminology similar to that developed for the 2½-minute and 30-minute power ratings. Additionally, these paragraphs require the ability to detect any damage which occurs due to the use of either 30-second or 2-minute OEI limits and requires that the procedures to inspect for such damage be provided in the instructions for continued airworthiness for either the engine and/or the airframe.
b. Procedures. All of the policy material pertaining to this section remains in effect. Additionally, the following procedures should be considered:
Sections 27.1521() and (k) require limitations for 30-second/2-minute OEl operation for multi-turbine engine powered rotorcraft. The same parameters required for the takeoff and continuous ratings should be established as limitations for each approved OEI rating (i.e., maximum rotational speed, time, gas temperature, and torque). These new ratings can only be approved as a rating in conjunction with the other. That is, a rotorcraft with a 30-second OE rating must also have a 2-minute OEI rating and vice-versa. The 30-second and 2-minute OE ratings are also limited to use for continued operation of the remaining engine(s) upon failure or precautionary shutdown of an engine. Upon the use of 30-second or 2-minute OEl ratings an inspection for 962 Par 724A
damage to the airframe and/or engine may need to be conducted. The inspection should be accomplished per the procedures furnished by the airframe and engine manufacturers and any damage occurring due to the use of these new ratings should be detected using these inspection procedures. Section 27.923 includes requirements for qualification of the rotor drive system for 30-second and 2-minute OEI powers.
Section 27.1501(a) requires that information necessary for safe operation should be established as limitations. The limitation information provided in this paragraph should be provided in the flight manual. This includes a possible requirement for an inspection prior to further flight after the use of either 30-second or 2-minute OE rating.
Par 724B 963
725. RESERVED.
Intentionally Left Blank 964 Par 725
726. § 27.1523 MINIMUM FLIGHTCREW.
a. Explanation.
(1) This rule requires that the minimum crew necessary to show compliance with the requirements of Part 27 or for safe operation of the rotorcraft be established as an operating limitation.
(2) The determination of minimum crew requirements is typically based on a subjective pilot assessment of the crew requirements for safe operation of each rotorcraft design. Certain regulations, such as the requirements for instrument flight rules (IFR), have specific quantitative differences between single-pilot and two-pilot requirements. However, most often the minimum crew requirement will be based on more subjective considerations such as location of necessary controls, pilot workload to accomplish required tasks, type of operation, and overall complexity of the rotorcraft design.
(3) Minimum crew requirements for the same type design may vary with the kind of operation. Many rotorcraft have been approved for a single-pilot crew for visual flight rules (VFR) operations but require a two-pilot crew for IFR operations. Other kinds of operations that may require more than one crewmember to meet type certification requirements are night operations, operations into known icing conditions, operations in falling and blowing snow, extended overwater operations, and external load operations.
(4) It is important to distinguish between the minimum crew requirements for compliance with Part 27 type certification regulations and the minimum crew requirements of the various operating regulations (Parts 61, 91, 121, 133, 135, and
137). A rotorcraft may be type certified for a minimum crew of one and still be required to have a crew of two or more by the operating regulations for certain types of operation or by the workload associated with an operating environment. Therefore, an applicant should carefully consider the possible operational uses of any rotorcraft design and become familiar with the applicable operating regulations as well as the type certification requirements early in the design process.
(5) Although the rotorcraft configuration is typically certified with the pilot-in-command station in the right seat, the left seat may be used for the pilot-in-command if, in addition to the flight controls required to control the rotorcraft, the following are included for the pilot: throttle control including ability to shut down all engines, airspeed indication, altitude indication, rotor and engine RPM, and engine torque and exhaust gas temperature. The authority should evaluate a change to the pilot-in-command station.
(6) The applicant is encouraged to contact the responsible type certification office as early in the design phase as possible to initiate the qualitative assessment Par 726 965
process. Cockpit layout drawings, instrument panel mockups, and full-scale cockpit mockups can be used to determine if required controls are accessible and to begin the pilot workload assessment for certain operations.
b. Procedures.
(1) General.
(i) A systematic evaluation and test plan is required for any new or modified rotorcraft. The methods for showing compliance should emphasize the use of acceptable analytical, simulation, and flight test techniques. The crew complement should be studied through a logical process of estimating, measuring, and then demonstrating the workload imposed by a particular cockpit design. When the minimum crew requirements have been determined, they should be included in the limitations section of the Rotorcraft Flight Manual in accordance with § 27.1583(d).
(ii) Appropriate analysis should be conducted by the applicant early in the design process. The specific methods) of analysis should be selected on the basis of its predictive validity, sensitivity, reliability, applicability to the particular cockpit configuration, and availability of a suitable reference for comparison.
(2) Analytical Approach.
One analytical approach defines workload as a percentage of the time available to perform tasks (Time Line Analysis). This process may be applied to an appropriate set of flight segments in which operationally important time constraints can be identified. This method is useful for evaluation of cockpit changes relating to overt pilot work such as control movements and data inputs. The generally accepted practice involves careful selection of the limited set of flight scenarios and time segments that represent the range of operational requirements (including the range of normal and nonnormal procedures.) Time line analysis yields useful data when tasks must be performed within operationally significant time constraints. The adequacy of this method is very much dependent on an accurate determination of the time available.
Absolute standards are not available for interpretation of obtained time required scores, but such records can be used to identify high or simultaneous workload demands for later testing in a simulator or aircraft, and comparisons can be made with overt workload demands in proven aircraft. However, the impact of cockpit changes on planning and decisionmaking is difficult to quantify by this method.
(¡i) The most frequently used basis for deciding that a new design is acceptable is a comparison of a new design with previous designs proven in operational service. By making specific evaluations using the acceptable human factors techniques, and comparing new designs to a known baseline, it is possible to proceed with confidence that the changes incorporated in the new designs accomplish the intended result. When the new cockpit is considered, certain components may be 966 Par 726
proposed as replacements for conventional items, and some degree of rearrangement may be contemplated. New avionics systems may need to be fitted into existing panels, and newly automated systems may replace current indicators and controls. As a result of this evolutionary characteristic of the cockpit design process, there is frequently a reference cockpit design, which is usually a conventional aircraft that has been through the test of operational usage. If the new design represents an evolution, improvement attempt, or other deviation from this reference cockpit, the potential exists to make direct comparisons. Service experience should be researched to assure that any existing problems are understood and not perpetuated.
(ili) If preliminary analyses by the certification team identify potential problem areas, these areas should receive more extensive evaluation and data collection in order to verify compliance with § 27.1523. These concerns should be adequately addressed in the manufacturer's demonstration plan when submitted to the FAA/AUTHORITY.
(iv) If the new design represents a significant change in level of automation or pilot duties, analytic comparison to a reference design may have lessened value. Without a firm data base on the time required to accomplish both normally required and contingency duties, more complete and realistic simulation and flight testing will be required.
(3) Testing.
(i) In the case of the minimum crew determination, the final decision is reserved until the rotorcraft has been flown by experienced flight test pilots trained and current in the aircraft. More assurance is derived from actual flight tests than from earlier simulator tests or other synthetic or computer model procedures.
(її) The test program should address the workload functions and factors listed below. For example, an evaluation of communications workload should include the basic workload required to properly operate the aircraft in the environment for which approval is sought. The goal is to evaluate workload with the proposed crew complement during realistic operating conditions, including representative air traffic and weather.
(A) Basic workload functions. The following basic workload functions are considered:
(1) Flight path control.
(2) Collision avoidance.
(3) Navigation.
Par 726 967