SECTION 40. OPERATING LIMITATIONS
718. § 27.1501 (Amendment 27-14) GENERAL.
Explanation. This section simply requires specified operating limitations in addition to any other information necessary for the safe operation of the rotorcraft to be determined. Secondly, it requires that this pertinent information be made readily available to the crew members as required in the various sections of this subpart.
719. § 27.1503 AIRSPEED LIMITATIONS: GENERAL.
a. Explanation. This section requires that a safe operating speed range be established for all rotorcraft. If the safe operating speed range varies with operating conditions (rotor speed, power, etc.), ambient conditions (altitude and/or temperature), rotorcraft configuration (gross weight, center of gravity, and/or external equipment), or type of operation (in ground effect (IGE), instrument flight rules (IFR), etc.), airspeed limitations that correspond with the most critical combinations of these factors must be established.
b. Procedures.
(1) Airspeed Limitations. The airspeed limitations for each critical combination of factors are established by tests or analyses and verified by flight test. The following are airspeed limitations that are typically required depending on the particular rotorcraft design:
(i) VNE (Power-On). See Paragraph 720.
(ii) VNE (One-Engine-Inoperative (OEI)). See Paragraph 720.
(iii) VNE (Power-Off). See Paragraph 720.
(iv) VLo (Maximum Airspeed for Landing Gear Operation). Compliance with structural, handling qualities, and controllability requirements should be demonstrated at the airspeed limit.
(V) VLE (Maximum Airspeed Landing Gear Extended). If this airspeed limit differs from the maximum gear operation speed, compliance with the applicable structural, handling qualities, and controllability requirements should be demonstrated.
(vi) Low Speed Flight Limitation. It is permissible for the applicant to establish minimum airspeed operating limitations as a function of weight, altitude, and temperature as long as there is still a practical flight envelope.
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(vii) MINI (Minimum FR Speed). The minimum speed for which compliance with the IF handling qualities requirements has been demonstrated should be established as a limit for IFR operations.
(viii) Maximum Sideward and Rearward Flight Speed. The maximum demonstrated sideward flight or crosswind hover and rearward flight or tailwind hover airspeeds should be provided in the RFM. If these maximum speeds resulted from a control margin limitation, they should be included in the airspeed limitations section of the RFM. If adequate control margin remained for the critical combination of rotorcraft configuration and ambient conditions, the maximum demonstrated sideward or rearward flight airspeeds should be included in either the performance section or the limitations section of the RFM as the applicant desires.
(ix) Maximum Airspeeds for Special Configurations or Special Equipment.
Standard configuration airspeed limits frequently have to be reduced for specific changes or external modifications. The following are examples of special equipment or configurations that have required additional airspeed limitations:
(A) Doors open or doors off.
(B) External hoist/cargo hook (stowed).
(C) Fixed or emergency flotation gear.
(D) External avionics equipment (large antennas, wires, etc.)
(E) External fuel tanks.
Skid pad or ski equipment modifications to standard skid type landing gear.
(x) Maximum Airspeeds after Failure of Required Equipment. Rotorcraft that require auxiliary equipment such as stability augmentation systems to comply with FAR requirements throughout the approved operating envelope frequently require airspeed limitations following failure of part or all of this system in order to comply after the failure. The following are examples of auxiliary equipment that have required maximum airspeed limitations after failure of all or part of the system.
(A) Stability Augmentation Systems (SAS).
(B) Automatic Flight Control Systems (AFCS).
(C) Fly-by-Wire Elevator Systems (FBW).
(D) Air Data Computer Systems (ADC).
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(2) Groundspeed Limitations. Although not specifically required by this "airspeed limitations" regulation, it may be necessary to establish "groundspeed" limitations for wheel-gear-equipped rotorcraft and maximum landing touchdown groundspeeds for utility type, float-gear-equipped rotorcraft. These wheel gear limitations are required to show compliance with the ground-handling characteristic requirements, structural strength requirements, or the ground-loads requirements.
However because of the operational similarity of groundspeed limits to airspeed limits, it is a common practice to include groundspeed limitations under the airspeed limitations heading in the flight manual. For this reason, groundspeed limitations are included in this paragraph of the AC. Groundspeed limitations should be established with adequate safety margins to account for the possible inaccuracies associated with the necessity for the pilot to estimate groundspeed from indicated airspeed and available wind speed and direction information during actual operations. The following are examples of groundspeed limitations that have been required during past type certification programs:
Maximum Groundspeed for Takeoff or Landing. The maximum acceptable groundspeed that can sately be used for wheel gear equipped rotorcraft takeoff and landing maneuvers should be determined based on landing gear limitations or ground controllability limitations. This speed should be fast enough to account for landing touchdown speeds at the maximum approved density altitude for normal takeoff and landing.
(ii) Maximum Groundspeed for Brake Application. The maximum speed at which the wheel brakes may be applied without exceeding maximum brake energy capabilities should be determined for wheel equipped rotorcraft. This speed should be verified by test throughout the approved takeoff and landing envelope of the rotorcraft.
The critical combination of gross weight and density altitude for brake energy considerations may be determined by analysis to minimize the required amount of testing. The maximum brake application groundspeed should be high enough to encompass brake application during landing at the maximum approved density altitude.
Other Groundspeed Limitations. For some rotorcraft designs with skid type landing gear, it may be necessary to establish a maximum landing touchdown speed for normal operations to comply with structural requirements. Optional equipment configurations such as float equipment, skis, etc., which are attached to conventional landing gear skids may require maximum landing groundspeed limits that are less than the limit for the basic rotorcraft. Rather than limitations, operational information may be sufficient for skid type landing gear and for utility or float landing gear.
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720. § 27.1505 (Amendment 27-21) NEVER-EXCEED SPEED.
a. Explanation.
(1) General. This rule requires the never-exceed speed (VNE) for both power-on and power-off flight to be established as operating limitations. The rule specifies how to establish and substantiate these limits.
(2) Power-on Limit.
The all-engines-operating VNE is established by design and substantiated by flight tests. The VNE limits are the most conservative value that demonstrates compliance with the structural requirements (§ 27.309), the maneuverability and controllability requirements (§ 27.143), the stability requirements (§$ 27.173 and 27.175), or the vibration requirements (§ 27.251). The power-on VNE will normally decrease as density altitude or weight increases. A variation in rotor speed may also require a variation in the NE. The regulation restricts the number of variables that are used to determine the VNE at any given time so that a single pilot can readily ascertain the correct VNE for his flight condition with a minimum of mental effort.
Rotorcraft that are equipped with airdata computers or other similar equipment are allowed to vary as many parameters as desired if the final result is no more than two parameters that define the VNE displayed to the pilot in an unambiguous manner.
These rotorcraft must also have a method for determining VNE that complies with the regulation in the event the airdata computer system fails. This method is usually more conservative than the automatic system because of the limitation in the number of parameters that can be varied.
(u) To ensure compliance with the structural requirements (§ 27.309), vibration requirements (§ 27.251), and flutter requirements (§ 27.629), the all-engines-operating VNE should be restricted so that the maximum demonstrated main rotor tip Mach number will not be exceeded at 1.11 VNE for any approved combination of altitude and ambient temperature. Previous rotorcraft cold weather tests have shown that the rotor system may exhibit several undesirable and possibly hazardous characteristics due to compressibility effects at high advancing blade tip Mach numbers.
As the center of pressure of the advancing rotor blade moves aft near the blade tip due to the formation of localized upper surface shock waves, rotor system loads may increase, the rotor system may exhibit an aerodynamic instability such as rotor weave, rotorcraft vibration may increase substantially, and rotorcraft static or dynamic stability may be adversely affected. Which, if any, of these adverse characteristics are exhibited at high rotor tip Mach numbers is dependent on the design of each particular rotor system. FAA/AUTHORITY experience with high advancing blade tip Mach number has shown that different types of rotor systems (articulated, semi-rigid, rigid, etc.) have various adverse characteristics. Therefore, it has been FAA/AUTHORITY policy to establish VNE so that it is not more than 0.9 times the maximum speed substantiated for advancing blade tip Mach number effects for the critical combination of altitude, 954 Par 720
approved power-on rotor speed, and ambient temperature conditions. This policy was incorporated as a specific regulatory requirement with Amendment 27-21 to § 27.1505.
High main rotor tip Mach numbers obtained power off at higher than normal main rotor rotational speeds should not be used to establish the maximum power-on tip Mach number VNE limit. In addition, since the onset of adverse conditions associated with high tip Mach numbers can occur with little or no warning and amplify very rapidly, no extrapolation of the maximum demonstrated main rotor tip Mach number VNE limitation should be allowed.
A maximum speed for use of power in excess of maximum continuous power (MCP) should be established unless structural requirements have been substantiated for the use of takeoff power (TOP) at the maximum approved NE airspeed. TOP is intended for use during takeoff and climb for not more than 5 minutes at relatively low airspeeds. However, FAA/AUTHORITY experience has shown that pilots will not hesitate to use TOP at much higher than best-rate-of-climb airspeeds unless a specific limitation against TOP use above a specified airspeed is included in the RFM. Structural and fatigue substantiations have not normally included loads associated with the use of TOP at VNE; thus, a TOP airspeed limitation should be established from the structural substantiation data to preclude the accumulation of damaging rotor system and control mechanism loads through intentional use of the TOP rating at high airspeeds.
(iv) A one-engine-inoperative (OEI) VNE is generally established through flight test and is usually near the Vu or VNE of the rotorcraft. It is the highest speed at which the failure of the remaining engine must be demonstrated. For rotorcraft with more than two engines, the appropriate designation would be "one-engine-operating" VE and would be that speed at which the last remaining engine could be failed with satisfactory handling qualities. It is possible, although believed improbable, that a rotorcraft with more than two engines could have different VNES depending upon the number of engines still operating. It is recommended that the OEVNE not be significantly lower than the OEl best range airspeed. A multiengine rotorcraft may require an OEIVNE if the handling qualities following the last remaining engine failure are not satisfactory or if the rotor speed decays below the power-off transient limits at the all-engine-operating VNE-
(3) Power-off Limits. A power-off VNE may be established either by design or flight test and should be substantiated by flight tests. A power-off VNE is generally required if the handling qualities or stability characteristics at high speed in autorotation are not acceptable. A limitation of the power-off VNE may also be used if the rotorcraft has undesirable or objectionable flying qualities, such as large lateral-directional oscillations, at high autorotational airspeeds. The power-off VNE must meet the same criteria for control margins as the power-on VNE. The regulation requires that the power-off NE be no less than the speed midway between the power-on VNE and the speed used to comply with the rate of climb requirements for the rotorcraft. When the regulation was written, rotorcraft VNE speeds were significantly lower than those of Par 720 955
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.
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(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.
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722. RESERVED.
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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.
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(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).
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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