distinctive in its operation from other warning devices incorporated into the rotorcraft cockpit design.
(iii) An acceptable method of interlocking a normal landing mode and the position of the landing gear would be through the selection of some appropriate speed that is less than VLE. The syste m would be instrumented such that if the gear is not down and locked and the rotorcraft goes below the selected airspeed, the landing gear warning device would be activated.
(iv) An acceptable manual shut off capability would be one that allows disabling the warning device and yet will automatically reset itself when the landing gear is cycled or retracted, or the rotorcraft’s speed is increased above that speed selected to activate the warning device.
(v) The appropriate provisions of § 27.1309 should be used to evaluate the impact of system malfunctions.
AC 27.731. § 27.731 WHEELS.
a. Explanation. This standard requires use of approved wheels, either approved under TSO-C26 or approved under the type certificate for the aircraft. Wheels must satisfy both a design static (1g) load and design limit landing or taxiing load determined under the applicable ground load requirements. Standards for a tire installed on a wheel are contained in § 27.733.
b. Procedures.
(1) The structural design loads data shall contain both a static load and a landing and taxiing load for each wheel. These loads are determined by virtue of compliance with the standards of § 27.731(b) and (c). The ratings of the wheel shall not be exceeded. TSO -C26c contains minimum performance standards for TSO approval of aircraft wheels and wheel-brake assemblies. Ratings are assigned in accordance with this performance standard.
(2) If a wheel selected for an aircraft design has TSO-C26 approval, the wheel manufacturer will supply the rating to the aircraft manufacturer. Each wheel shall be marked as prescribed which includes a listing of the TSO number. Even though a wheel is TSO approved, the application on the aircraft (loads imposed on the wheel) requires proof that the rating is not exceeded.
(3) If a wheel selected for an aircraft design is not approved under TSO-C26, the necessary data, both detail design and assembly drawings and qualification tests and test report data, will be required to comply with the standards contained in Part 27.
Design control and inspections will be accomplished as a part of the aircraft type design. Structural substantiation and any appropriate qualification tests shall be accomplished. See §§ 27.471 through 27.497 for the ground load conditions.
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(4) The Tire and Rim Association, Inc., generally issues a yearbook listing tire and rim sizes and ratings. The dimensions and contours for aircraft wheel rims are contained in Section 9 of this yearbook.
AC 27.733. § 27.733 (Amendment 27-11) TIRES.
a. Explanation.
(1) This standard specifies both design and performance criteria for tires. The tire must fit the wheel rim. The maximum static ground reaction for the condition specified must not exceed the maximum static load rating of each tire. In addition, any tire of retractable gear systems must have adequate clearance from surrounding structure and systems as specified.
(2) Main, nose, and tail wheel tires must comply.
(3) Tire performance standards are contained in TSO-C62.
b. Procedures.
(1) The aircraft structural design loads should contain a maximum static load imposed on the tires. The load is derived for a static ground reaction assuming the design (maximum) weight and the critical center of gravity for each tire of the landing gear. The wheel loads are determined under § 27.731(b). Reduced weight but forward CG conditions may result in the highest static load on a nose wheel tire. Thus, combinations of weight and CG locations require investigation for the maximum tire load of each main, nose, and tail wheel tire.
(2) The maximum possible size of the tires considering appropriate temperatures, aging, and pressure should be obtained to check wheel well and cover clearances. Tire dimensions (for clearances) may be found in the yearbook noted in paragraph AC 27.733b(4). If the tire clearance is questionable, objects may be taped to the tire to simulate tire growth or oversize dimensions expected and the wheel retracted and rotated by hand to check for possible interferences. Minimum clearance, such as one-half inch, may be adequate as a design objective. The design drawings should be reviewed for information of correct systems installations and landing gear rigging within the wheel wells and wheel covers, if installed. If necessary to control tire sizes, specific manufacturer’s tires should be used as “required equipment” and the tire manufacturer and the part number should be specified in the design data and on the type certificate data sheet as “required equipment.”
(3) As specified in paragraph d of § 27.729 adopted by Amendment 27-21, an operation test of any retractable landing gear should be performed. During this operation test, the tire clearances described in paragraph AC 27.733b(2) should be Page D - 55
determined and recorded. Only the least or minimal clearance found, if adequate, should be recorded in the type inspection report or other appropriate type design report.
(4) The Tire and Rim Association, Inc., generally issues a yearbook listing tire and wheel rim sizes and ratings. This information is advisory as stated in the yearbook.
Section 9 concerns aircraft tires and rims. Table AP-5 in Section 9 of the yearbook concerns tires used on rotorcraft. The tire may be selected initially from the yearbook, but qualification data for the specific tires used shall be furnished with the type design data in compliance with the standards. Section 9 also contains tire size and tire growth dimensions.
(5) Aircraft Tires. Minimum performance standards for aircraft tires, excluding tail wheel tires are found in TSO-C62, Aircraft Tires. Tires meeting TSO-C62 are marked as prescribed in the standards. The load rating (reference § 27.733) is marked on the tire. TSO tires are not required but sh ould be used whenever possible. The manufacturer’s information, such as load rating, should be included in the aircraft type design structural substantiation data.
AC 27.735. § 27.735 (Amendment 27-21) BRAKES.
a. Explanation.
(1) Brakes are required for wheel landing gear aircraft. Minimum performance standards are contained in this section. During the course of the FAA/AUTHORITY flight test program and of any F&R program conducted under § 21.35, the brakes shall be used and evaluated.
(2) Design criteria are contained in this standard.
(i) The braking device must be controllable by the pilot. It is optional for the second pilot station except as may be specified under the provisions of § 27.771.
(ii) The braking device must be usable during power-off landings.
(3) Performance criteria are also contained in this standard.
(i) The brakes must be adequate to counteract any normal unbalanced torque when starting or stopping the rotor or rotors.
(ii) The brakes must be adequate to hold the rotorcraft parked on a 10° slope on dry, smooth pavement.
(4) In §§ 27.493(b)(2) and 27.497(g)(2)(ii), limiting brake torque is one ground load standard for design of the landing gear.
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(5) Although not specifically noted in a standard, the position of the brake on the wheel is important. The brake should be positioned to avoid ground contact whenever the tire is deflated.
(6) TSO-C26 contains minimum performance standards for aircraft landing wheels and wheel-brake assemblies. For rotorcraft, a wheel-brake assembly design rating is established by the manufacturer. The TSO standard for rotorcraft brakes specifies a 20° slope standard (rather than a 10° slope) for an over-pressure hydraulic brake test.
(7) The brake application device at the pilot station is subject to other structure strength standards in this Part, such as the limit pilot forces or torque specified in § 27.397.
b. Procedures.
(1) Wheel-brake assemblies approved under TSO-C26 will have various (rotorcraft) ratings as specified in the standard. One rating of TSO standard for a rotorcraft wheel-brake assembly is the kinetic energy capacity in foot-pounds at the design landing rate of absorption. The design takeoff and landing weight and rotorcraft speed in knots for brake application are a part of the equation. The brake manufacturer should furnish this rating and the two noted parameters for the selected design or designs. The ratings of selected brakes should be included in a structural design data report such as a design criteria report. The use or application of each brake design on the particular rotorcraft design should not exceed capacity of the brake or the ratings established under TSO-C26. If appropriate, the part number and manufacturer of each brake may be listed in the structural data reports as well as listed in the type design drawings.
(2) The limiting brake torque obtained from the brake manufacturer should be used in complying with § 27.493(b)(2).
(3) Compliance with the brake standards should be confirmed, demonstrated, and recorded as a part of the flight test type inspection report. This applies to TSO -C26 brakes and to brakes approved as a part of the aircraft type design.
(4) If found necessary under the provisions of § 27.771, the second pilot station should have brake control devices. The brake control devices should be listed with the other required equipment that defines the equipment necessary for a second pilot station.
(5) A brake assembly may be evaluated and approved under Part 27 as a part of the aircraft type design. TSO-approved brakes are not specifically required but are recommended. For non-TSO-approved brakes, all detail and assembly drawings, required test proposals, and test results reports may be submitted and processed as a unique part of the particular aircraft type design.
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(6) During an inspection of the landing gear, such as an engineering compliance inspection, the brake location should be checked to ensure the brake does not contact the ground when the tire is deflated. Type design drawings should control the proper location of the brake on the landing gear.
AC 27.737. § 27.737 SKIS.
a. Explanation. This standard is derived from airplane standards. Aircraft skis approved under TSO-C28 may be used on rotorcraft. TSO -C28 for aircraft skis refers to Sections 4 and 5 of National Aircraft Standards Specification 808, dated December 15, 1951, for strength and performance standards. These standards are conservative for rotorcraft ski installations.
(1) A maximum limit load rating is assigned to each ski approved under TSO-C28.
(2) This limit load rating must not be exceeded by the maximum limit ground load determined under the standards of § 27.505, Ski landing conditions.
(3) Ski mounting or installation parts used in the particular application are subject to substantiation as any landing gear member is subject to substantiation.
(4) Ski installations are also subject to flight and ground operation evaluations.
b. Procedures.
(1) The limit load rating for the ski selected shall be obtained from the ski manufacturer. This information shall be included in the design criteria and/or structural substantiation reports. The type design drawings will include the appropriate part number for the TSO-approved product and the necessary installation information.
(2) The design limit loads derived in compliance with § 27.505 shall not exceed the ski limit load rating.
(3) Skis that are not TSO approved may be approved as a part of the aircraft type design by complying with the strength and performance standards contained in TSO-C28 (NAS 808).
(4) Pads or “bear paws” installed on skid or wheel landing gear to facilitate operations in snow conditions may be approved as a part of or as an alteration to the aircraft type design. Rational design loads applicable to the particular pad design must be developed and strength substantiating data submitted proving compliance with the strength and performance standards contained in Part 27. In addition, skid landing gear may be subject to excessive vibratory loads while in flight whenever the weight and mass distribution is altered by adding “bear paws.” The effect of additional weight should be investigated. Resonant vibratory conditions should be avoided or highly damped.
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SUBPART D - DESIGN AND CONSTRUCTION FLOATS AND HULLS.
AC 27.751 § 27.751 (Amendment 27-2) MAIN FLOAT BUOYANCY.
a. Explanation.
(1) The section specifies standards for single and multiple float buoyancy in fresh water. The standard does not apply to ditching/emergency flotation devices, but to amphibian rotorcraft devices.
(2) It is a design and a performance standard. Rigid or inflatable floats may be used. Enough water tight compartments (per Amendment 27-2) rather than a specific number are required to minimize the probability of capsizing when one compartment is flooded or deflated.
b. Procedures.
(1) Excess buoyancy. A minimum of 50 or 60 percent in excess of the maximum certificated weight of the rotorcraft is required for single or multiple floats respectively. The weight of fresh water (density 62.42 pounds per cu. ft.) displaced by fully submerged float or floats (total volume at operating pressure of each float is used) should be a minimum of 50 or 60 percent greater than the maximum certificated weight of the rotorcraft.
(2) Capsizing.
(i) Each float should have enough sealed, separate and approximately equal volume compartments to minimize the probability of capsizing when the critical compartment is flooded or deflated. Five or more compartments in each float are usually necessary to meet the standard. Ten compartments per float have been employed in certain designs.
(ii) An analysis or test or combination thereof may be used, if necessary, to prove a positive margin of stability with the most “critical” compartment in one float flooded or deflated, that is ineffective.
(iii) The location of the floats, and the most critical compartment, the rotorcraft weight, mass moment of inertia, and center of gravity location are also important considerations for capsize stability.
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AC 27.753 § 27.753 MAIN FLOAT DESIGN.
a. Explanation. Loads and load distributions are specified for float design as follows:
(1) Bag floats are to be designed for:
(i) The maximum pressure differential developed at the maximum design altitude.
(ii) The vertical loads prescribed in § 27.521(a) distributed over three-fourths of the bag’s projected area.
(2) Rigid floats are to be designed for vertical, horizontal, and side loads prescribed in § 27.521 distributed along the length of the float.
b. Procedures. Structural substantiation may be accomplished by static tests or analyses using the specified loads. Substantiation should cover the float and float attachments.
AC 27.755 § 27.755 HULLS.
a. Explanation.
(1) The section requires amphibious rotorcraft with a single hull (main float design) and with auxiliary floats (outriggers) to provide a margin of positive stability great enough to minimize the probability of capsizing when any single (usually the most critical) compartment is flooded. Landing gear wheel tires may be used for stability purposes as well.
(2) Limitations for water operation are not intended by this section, but information for water operation must be included in the rotorcraft flight manual.
(3) Wave height or sea state and buoyancy relative to fresh water is not specified but is encompassed in the objective statement of § 27.751(b).
(4) Section 27.751 specifies an excess buoyancy requirement of 50 percent for single main floats (hulls) and contains a capsize/stability standard also. This section complements § 27.755 for certain hull designs.
(5) Sections 23.751, 23.755, and 23.757 concern design standards for small airplanes and may provide insight into possible rotorcraft hull designs.
b. Procedures.
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(1) The main hull must have multiple compartments. Assuming the hull has 50 percent excess buoyancy capacity, six to ten sealed compartments of approximately equal volume would allow loss of one with at least 25 percent excess capacity remaining. However, the attitude of the rotorcraft is critical with respect to capsize stability, and additional compartments may be necessary.
(2) The designer must consider separately the loss of buoyancy for each critical compartment, the aircraft center of gravity, and attitude in the water for the appropriate sea state or water height. Sea state 4, moderate, as noted in figure AC 27.801-1, is acceptable.
(3) The auxiliary floats (outrigger) must have multiple compartments. In addition, wheel tires may be used as a compartment if applicable to the design.
(4) For each critical condition under consideration, a single compartment for either the main hull or auxiliary float should be flooded or collapsed. Combined failures, one in each, are not required.
(5) Model stability (or capsize) tests are encouraged to demonstrate compliance with this section.
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SUBPART D - DESIGN AND CONSTRUCTION
PERSONNEL AND CARGO ACCOMMODATIONS AC 27.771. § 27.771 PILOT COMPARTMENT.
a. Explanation.
(1) Volumes have been written on human factors and their contribution to pilot workload and fatigue. This document cannot begin to address the myriad of considerations involved in pilot compartment design. The intent of the rule is simply to ensure that reasonable human factor engineering practices have been followed.
Equipment should be logically grouped within the pilot’s reach and view and be easy to operate. Seats should provide a reasonable level of comfort for the normal anthropometric range of pilots for a typical mission duration. Environmental considerations such as radiation from the sun through overhead windows should be addressed. Heating, cooling, and ventilation systems should be adequate for the expected range of operating conditions.
(2) Each pilot compartment and its equipment should allow the minimum flightcrew to perform their duties without unreasonable concentration or fatigue. If there is a provision/requirement for a second pilot, his station should be equipped with primary flight controls. Duplicate wheel brakes are recommended. Duplication of miscellaneous controls such as idle detent switches, RPM beep functions, nosewheel locks, and parking brakes has not been required. The need for duplicate instruments for the second pilot tends to be a function of cockpit size and panel configuration.
(3) Webster defines appurtenances as “accessory objects or apparatus.” Items such as blowers, fans, and gyros should not have noise or vibration characteristics which could contribute to pilot fatigue or distraction. Instrument panel vibration is specifically addressed in § 27.1321.
b. Procedures. Initial evaluation of the pilot compartment should be conducted on the ground. However, the cockpit assessment should be an ongoing effort throughout the flight test program. If a second pilot position is provided/required, the adequacy of controls and instruments should be evaluated under all normally expected operating conditions. If a second pilot position is not provided/required, any passenger position in the pilot compartment should be evaluated to ensure that a passenger, properly briefed by the flightcrew, can sit comfortably without inadvertent interference with normal control operations. All equipment should be operated during at least one flight of typical mission profile and duration.
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D - 63 AC 27.773. § 27.773 (Amendment 27-48) PILOT COMPARTMENT VIEW.
a. Explanation. The section outlines requirements for pilot view in fairly general terms. Requirements are purposely less stringent than for transport category rotorcraft to allow for cockpit designs ranging from fully enclosed to open to the elements. Section
27.773(b) was changed at amendment 27-48 to include the option for a ground test in lieu of a night flight test. Paragraph b.(2) below contains guidance to reflect this change.
b. Procedures.
(1) The following procedures are one acceptable means of evaluating pilot compartment field of view considering only those objects in the pilot compartment and the windshield and its support structure in nonprecipitating conditions. The applicant’s design is not required to meet these guidelines, and each design should be evaluated on its own merit. The area of visibility established in the following paragraphs will provide an acceptable level of visibility for a minimum crew of one pilot. In the event that a minimum crew of two, a pilot and copilot, is required, the second pilot should have an area of visibility equivalent to that provided for the pilot but on the opposite side. In this event, the pilot’s area of visibility to the left as shown in figure AC 27.773-1 needs only to comply to 60° left, and the copilot’s area of visibility to the right needs only to comply to 60° right.
(i) A single point established in accordance with the provisions of this paragraph constitutes the referenced eye position (i.e., a point midway between the two eyes) from which the central axis may be located. The referenced eye position is a reference datum point from which the aircrew station geometry is constructed. The referenced eye position should be located by means of ship’s coordinates that contain station reference number, water line, and butt line for both pilot and copilot, if applicable, and comply with:
(A) The pilot’s seat in a normal operating position from which all controls can be utilized to their full travel by an average subject, and which should provide for vertical adjustment of the seat of not less than 2.5 inches above and 2.5 inches below this initial vertical position.
(B) The seat back in its most upright position.
(C) The seat cushion depression being that caused by a subject weighing 170 to 200 pounds.
(D) The longitudinal axis of the rotorcraft to be that of “cruise attitude”
(0.9VH or 0.9VNE whichever is lower).
(E) The point established not beyond 1 inch to the right or left of the longitudinal centerline of the pilot’s seat.
(F) All measurements made from the single point established in accordance with this paragraph.
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(ii) A dual lens camera, as photo recorder, should be used in measuring the angles specified in the paragraphs listed below. Other methods, including the use of a goniometer, are acceptable if they produce equivalent areas to those obtained with a dual lens camera. When not using a dual lens camera, compensation should be made for one half of the distance which exists between the eyes, or 1 ¼ inches. With the referenced eye position located as indicated in paragraph AC 27.773b.(1)(i), and utilizing binocular vision and azimuthal movement of the head and eyes about a radius, the center of which is 3 and 5/16 inches behind the referenced position (this point to be known as the central axis), the pilot should have the following minimum areas of vision measured from the appropriate eye position. (See figure AC 27.773-1.)
(A) 20° forward and above the horizon between 0° and 100° left.
(B) 20° forward and below the horizon between 10° and 100° left.
(C) 20° forward and below the horizon at 10° left increasing to a point 30° forward and below the horizon at 10° right.
(D) 50° forward and below the horizon between 10° right and 135° right.
(E) 20° forward and above the horizon at 0° increasing to a point 40° above the horizon at 80° right and 100° right and then decreasing to a point 20° forward and above the horizon at 135° right.
(iii) Any vertical obstruction which falls within the minimum area of visibility outlined in paragraph AC 27.773b.(1)(ii) should be governed by the following:
(A) Between 20° right and 20° left--no vertical obstruction.
(B) Between 20° right and 135° right -- no vertical obstruction greater than 2.5 inches in width.
(C) Between 20° left and 100° left -- no vertical obstruction greater than
2.5 inches in width.
(iv) Any horizontal obstruction which falls within the minimum area of visibility outlined in paragraph AC 27.773b.(1)(ii) should be governed by the following:
(A) The area 15° forward and above the horizon between 135° right and 40° left decreasing to a point 10° above the horizon at 100° left, and 15° forward and below the horizon between 135° right and 100° left should be free from horizontal obstructions.
(B) The area above and below the horizon which is between the minimum area of vision specified in paragraphs AC 27.773b.(1)(ii) and AC
27.773b.(1)(iv)(A) is limited to one horizontal obstruction above and one below the horizon. These horizontal obstructions should not be greater than 4 inches in width. An overhead window which will provide twice as much additional visibility as that lost due to the obstruction should be located immediately above any obstruction above the horizon.
This requirement is in addition to any area of visibility specified by paragraph AC
27.773.(b)(2)(i) which may be included in the overhead window area.
(C) If the instrument panel obstructs any required area between 10° left and 10° right below 20° forward and below the horizon, a window which affords triple equivalent additional visibility should be located immediately below and between the angles of 20° left and 20° right above 65° below the horizon.
(v) For steep rejected takeoffs and steep approaches (such as to oil rigs or confined heliports), the visibility should be such that the pilot can see the touchdown pad and sufficient additional area to the side and forward to provide both an accurate approach to the touchdown point as well as a satisfactory degree of depth perception. A 5-inch head movement by the pilot forward and/or sideward of the normal position is acceptable in determining compliance.
(2) Since glare and reflection often differ with the sun’s inclination, consideration should be given to evaluating the cockpit at midday and in early morning or late afternoon. Windshields with embedded wire heating elements should be evaluated for distortion with the system both “ON” and “OFF.” Assess glare from reflected light to ensure that it does not reflect into the pilots’ eyes. Assess reflections of pilot or cockpit structures off of instrument glass that may interfere with the readability of displays or instruments. This problem is most apparent in IFR equipped aircraft (having larger instrument panels and avionic consoles) operating in VFR utility roles.
If night approval is requested, then compliance with 27.773 may be shown by either a ground or flight test. Internal lighting can be evaluated with a ground test. External lighting modifications may require a flight test. If a flight test is conducted, then the lighting system should be evaluated in likely combinations and under expected flight conditions.
(i) Internal lighting evaluations may be accomplished by a ground test with the aircraft in a dark hangar or with external light blocked to create a dark cockpit.
Locations and intensity of instrument panel and control panel light reflections should be assessed to ensure they do not interfere with the pilots’ ability to see outside the aircraft. Figure AC27.773-1 contains guidance on the areas in pilots’ field of vision where reflections should be eliminated. Although a certain amount of equipment reflection (avionics control heads, etc.) in the windshield may be unavoidable, the pilot’s normal field of view should be unobstructed. If reflections are mitigated by modifying the glare shield, then the pilots’ field of view should be revaluated. If a reflection’s effect on the pilots’ ability to see outside the aircraft is uncertain, a night flight test may be required.
(ii) External lighting evaluations should be done during night flight tests.
Evaluating external lights on the ground may result in unacceptable light entering the cockpit and causing reflections or glare due to reflection off the ground or surrounding structures. Landing and taxi lights should be exercised throughout their adjustment range (if applicable) to check for reflections, particularly in chin windows. Anticollision and strobe lights should be evaluated to ensure that frequency interaction and reflections D - 65