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the ski itself is satisfactory. This is true also in the case of a single set of skis where no type certificate is involved.

(1) Approval of the ski model. The strength of all skis must be substantiated before they may be used on certificated aircraft, whether or not the designer or manufacturer desires to obtain a type certificate for the skis.

(2) Approval of an airplane equipped with approved skis. Certain airplane models are already approved with certain specific approved skis installed. The owner of a certificated airplane of some such model wishing to install skis, need only install skis of the model with which airplanes of his model are approved and his airplane will be approved with the skis installed, upon the satisfactory completion of an inspection of the installation by a FAA representative. Should changes in the landing gear be necessary to accommodate the skis, the owner, of course, must make the changes in accordance with the change data approved by the Administrator. If the airplane is of a model which has not been approved with the installation of skis of the particular approved model it is desired to install, the procedure hereinafter outlined should be followed:

(i) Technical data showing any changes in the landing gear should be submitted to the Administrator for approval. This is not often necessary, as skis are usually designed to attach to the axles in place of the wheels.

(ii) Upon approval of the change data, if any, the installation must pass a satisfactory inspection by a representative of the Administrator.

(iii) During this inspection, the representative will obtain the weight of the ski installation and the weight of the wheel installation which has been replaced.

(iv) Upon completion of a satisfactory inspection, the representative will witness takeoffs and landings, and other demonstrations if deemed necessary, of the airplane equipped with skis. The characteristics of the airplane equipped with skis must be acceptable to the Administrator's representative.

(v) If the airplane inspected and tested is a standard airplane of a certain model and the skis installed are approved under a type certificate and manufactured under a production certificate or if the skis are manufactured under an

approved type certificate, all airplanes of this model will be considered eligible for approval when equipped with skis of the model installed on the airplane inspected. The aircraft specification will identify the approval accordingly.

(vi) If the skis installed are not approved under an approved type certificate or were not manufactured under a production certificate, each airplane so equipped must undergo the tests above in order to be eligible for approval. The notes on the pertinent aircraft specification will list this distinction.

(c) Increase in gross weight. An increase in gross weight will naturally require that the structure be able to withstand greater loads in flying and landing. There is usually involved, therefore, a study of the original design data and the preparation of a partial strength analysis, preferably by the manufacturer of the airplane. Likewise, the flying characteristics will be affected, so flight tests are usually required. Increases in weight are often accompanied by changes in weight distribution, the effects of which are covered in § 18.1-2 (c) Increases in gross weight for specific industrial purposes, such as crop dusting, may be permitted, provided that the requirements of Part 8 (restricted category) of this subchapter (i. e. the Civil Air Regulations) have been complied with.

(d) Change in weight distribution. (1) Any change in the location of items having considerable weight or the addition of new items (equipment, etc.) may have serious effects on the flight behavior of an airplane. When any changes in weight distribution are made, it is the repair agency's responsibility to determine by computation or reweighing, whether or not the approved limits, which appear on the airplane specification in the case of later models, will be exceeded. If they are, approval cannot be granted unless the structure is substantiated for the new limits and it is shown that the new limits are in compliance with all flight requirements as proved by means of a flight test.

(2) It is of the greatest importance to realize that flight characteristics not only become worse gradually with rearward displacement of the center of gravity, but that a condition sometimes exists or will finally be reached where a small change in weight will have very large effects. For this reason, care

should be taken not to install items to the rear of the rearward c. g. limit for which the aircraft was originally approved. Removal of items forward of the forward c. g. limit for which the aircraft was originally approved will have a similar effect. Increasing the weight forward of the wing will tend to disturb the balancing in flight and might make the landing conditions dangerous. Naturally, the effects of weight changes will be greater near the nose and the tail. Reasonably small changes within the portion covered by the wing are not usually serious from a balance or stability standpoint. For details regarding weight and balance procedure. see § 18.30-16.

(e) Installation of new items. In addition to the effects on weight and weight distribution discussed in this section there is a danger that a piece of new equipment, if improperly installed will cause local loads which might seriously damage the airplane structure. It should be remembered that in flight maneuvers and in landing, it is possible to develop inertia forces such that an item will impose a load of several times its own weight on the supporting structure. For instance, a 35-pound storage battery supported by a fuselage cross tube will have the effect of weight of an applied load of over 100 pounds on the cross tube during a hard landing. If the cross tube was not originally designed for this load, it will probably fail or bend. The greatest danger arises when such a partial failure occurs in landing, without being noticed, as the structure might then fail completely during some subsequent flight.

(f) Alterations or modifications of control surfaces and systems. (1) Any change in the size of control surfaces affects the loading conditions for the airplane structure and therefore requires additional strength analyses, static tests, or both. Flight tests are usually required also. One should be particularly warned against making minor changes on control surfaces, since the original design often just meets certain requirements for flutter prevention. No balancing weights should be removed or added without consulting the manufacturer and finally obtaining an Aviation Safety agent's approval. The importance of retaining the proper balance and rigidity of airplane control surfaces during repair and maintenance work

cannot be underestimated. In order to preclude the occurrence of flutter in a control surface, a degree of static and/or dynamic balance is established for each model of aircraft. Failure to check and retain the original or maximum allowable degree of balance may allow serious hazards to safe flight to exist in an airplane.

(2) The development of new materials and construction techniques has made possible lighter control surfaces, for a given area, on many postwar airplanes than were used on some older airplanes. The effect of using these lighter surfaces, insofar as flutter is concerned, is to make the surface more sensitive to weight changes. Repair can cause a greater change in weight distribution, or center of gravity, than would be the case with the heavier surfaces. Since control surfaces are, in some models, only balanced to the degree necessary to obtain flutterfree operation up to the maximum speed for which the airplane is designed in the original, undamaged condition, repairs to these lighter surfaces should be carefully considered from the standpoint of how they affect the balance of the surface. This is true whether or not it has a balance weight. Therefore, any structural repairs to control surfaces should be performed with due consideration of the effect of such rework on the balance of the surface: the repair work should be done in a manner so that the original or maximum allowable static unbalance is not exceeded; or counter-balance forward of the hinge line should be added, when possible, to retain the original or maximum allowable static unbalance. When the static unbalance cannot be brought within the limits and in the manner prescribed by the manufacturer, the control surface must be discarded.

(3) Special attention is called to the indiscriminate application of extra coats of dope or paint to a control surface. In some instances, the application of extra coats of dope or paint has resulted in unsatisfactory control surface balance conditions. Proper maintenance of control surface balance conditions may require removal of dope or paint down to the base coat prior to application of new finish coatings.

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(4) In line with proper maintenance for the prevention of flutter, the "water- | tightness" of the original control surface should be retained. Instances of flutter have occurred due to the high unbalance

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of a control surface arising from trapped water or ice within the surface. Therefore, either the original "water-tightness" or water drainage provisions should be checked and retained during maintenance work.

(5) In addition to improperly balanced surfaces, improperly maintained trim tab systems can contribute greatly to the possibility of flutter occurrence. Trim tabs, in most cases, are not balanced and flutter is prevented by providing a rigid trim tab mechanism. In order to keep the weight of the main surface aft of the hinge line as light as possible, trim tabs and their actuating * mechanisms are built as lightly as possible. This fact together with the vibratory nature of airflow at the tab can create a mechanism which is highly susceptible to wear and deformation. In service use, therefore, the tab rigidity may be decreased to an unsafe value.

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(6) In maintaining trim tabs and their actuating mechanism, only a minor amount of wear can be tolerated, in comparison to the rest of the airplane. The maximum free-play that is acceptable is 22% of the chord, measured at the trailing edge. For example, a tab which has a chord of four inches at one point along its span, would have a permissible free-play of 4" x 0.025 or 0.100" (total motion up and down) measured at that location on the trailing edge. Any slack in excess of this amount should be eliminated by overhauling the system.

(g) Fairing and cowling modifications. (1) Although changes in fairing can usually be made without impairing the airworthiness of the aircraft, it has been found that certain airplanes are very sensitive to slight changes in body lines, windshield designs, and filleting. Obviously, any change in engine cowling affects the cooling and thereby introduces possibilities of malfunctioning or failure of the engine.

(2) Low wing airplanes are usually supplied with wing-fuselage fillets which prevent tail buffeting. Any alteration of such fillets may be dangerous. The manufacturer should be consulted regarding such changes.

(h) Appliance installations. The aircraft and appliance manufacturer's instructions for installation of equipment should be closely followed. In particular, the recoil from flares should be proUvided for and, in the case of position lights, where certain angular limits are

required, the light manufacturer's mounting instructions should be followed.

(i) Batteries. Batteries should be installed in accordance with the instructions contained in §§ 3.682 through 3.684, 4a.571, and 4b.621 of this chapter. It should be noted that dry batteries are not considered satisfactory for the operation of position lights.

(j) Powerplant changes. (1) The powerplant installation on a certificated aircraft has been thoroughly checked in accordance with rules based on a study of accident and service records covering several years of operation of all types of aircraft. These records show that many accidents are due to improper installation in small details concerning the fuel or oil system, cowling, manifolding, and items of a like nature. It is, therefore, unwise to make any changes without careful consideration.

(2) When changes appear to be necessary in the powerplant installation, the manufacturer of the aircraft should be consulted. If the manufacturer has no data concerning such a change, an Aviation Safety agent should be consulted to determine if the airworthiness of the airplane will be adversely affected by making the change.

(k) Propeller changes. With reference to propeller changes, the pertinent aircraft specification states the type of propeller which is approved. A large diameter propeller should not be used without investigation, because it may result in unsatisfactory ground clearance and resultant damage to the tips. Furthermore, a change to a propeller of appreciably different diameter, either larger or smaller, or different pitch, might in some cases result in unsafe performance.

§ 18.30-18 Periodic and 100-hour inspections (FAA rules which apply to § 18.30 (c)).

(a) General. The inspecting agency shall employ an inspection form as a checklist while performing a periodic or 100-hour inspection. Such form may be developed by the mechanic, repair station, or the manufacturer similar to the example given in appendix D,' or provide the scope and detail of the items of inspection set forth in subdivisions (i) through (xi) of subparagraph (1) of this paragraph.

"Appendix D not filed with the Office of the Federal Register.

(1) Inspection procedures. Prior to inspection, all necessary inspection plates, access doors, fairing and cowling shall be opened or removed and the aircraft and engine thoroughly cleaned to properly reveal the actual condition of the parts being inspected. Airworthiness of the aircraft shall be determined by thoroughly inspecting the pertinent items in subdivisions (i) through (xi) of this subparagraph in accordance with instructions contained in the aforementioned subdivisions, manufacturers' inspection procedures, supplemental service information, and standard inspection practices. The aircraft shall conform with FAA aircraft specifications, airworthiness directives, or other approved data before being checked as airworthy.

(i) Fuselage and hull group. The fuselage and/or hull shall be carefully inspected for general condition; fabric or skin for deterioration, distortion, other evidence of failure, and security of attachment of fittings. The various systems and components installed in this group shall be checked to assure that they are properly installed with no apparent defects and are operating satisfactorily. When applicable, the same general inspection procedures will apply to lighter-than-air craft and a determination made of the condition of the envelope, gas bags, ballast tanks, etc. Rotary-wing-type aircraft or other craft utilizing rotor drive shafts or other similar systems shall have the shafts inspected in accordance with the manufacturer's maintenance manual.

(ii) Cabin and cockpit group. The cabin and cockpit shall be checked for cleanliness and/or loose equipment which might foul the controls; seats and safety belts for condition and apparent defects; windows and windshields for deterioration or breakage; instruments for condition, mounting, marking, and, where practicable, proper operation; flight and engine controls for proper installation and operation; batteries for installation and proper charge; the various systems for installation, general condition, apparent and obvious defects and security of attachment. The above inspection procedure will also apply to the control car of lighter-than-air craft.

(iii) Engine and nacelle group. All necessary engine cowling shall be removed and a visual inspection shall be made of the entire engine section for evidence of excessive oil, fuel, or hy

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draulic leaks. Any and all leaks shall be traced to their origin so that they may be corrected. All studs and nuts shall be checked for tightness or obvious defects. The general internal condition of the engine shall be determined by checking cylinder compression, condition of screens and sump drain plugs for foreign material and metal particles. Cylinders with weak compression shall be removed and the internal conditione and clearances checked for tolerances. The engine mount shall be inspected for cracks, tightness of mounting, and security of engine attachment to mount.ac The flexible vibration dampeners shall be examined to insure they are in good condition. The engine controls shall be all examined for defects, proper travel, and e safetying; lines for leaks; and hoses and clamps for condition and tightness. Exhaust stacks shall be checked for cracks or other defects and satisfactory attachments. Accessories shall be examined for apparent defects and security of mounting. The various systems shall be inspected for proper installation, general condition, defects, and attachment. Cowling shall be inspected for cracks or other defects. On rotary-wing-type aircraft the main rotor transmission gear box shall be inspected for obvious defects as outlined in the manufacturer's maintenance manual.

(iv) Landing gear group. The landing gear shall be examined for general condition and security of attachment of all units. Shock-absorbing devices shall be in good condition and oleo fluid level shall be proper height. All linkage, trusses, and members shall be inspected for evidence of undue or excessive wear, fatigue, distortion, and security of attachment. The retracting and locking mechanisms, when installed, shall be examined for satisfactory operation. Hydraulic lines shall be checked for leakage and electrical systems for chafing and proper operation of switches. The wheels shall be removed and examined for cracks or other defects, condition of bearings, tires for wear or cuts, and brakes for proper adjustment. floats or skis are installed, they shall be inspected for security of attachment, general condition, and any obvious or apparent defects.

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(v) Wing and center section group. The airworthiness of the wing and center section group shall be determined by thoroughly inspecting the complete

assemblies for general condition, fabric or skin for deterioration, distortion, other evidence of failure, and for security of attachment. This inspection shall include the various systems installed which make up a complete wing assembly. Rotary-wing-type aircraft shall be inspected in accordance with the manufacturer's maintenance manual.

(vi) Empennage group. The complete empennage assemply shall be inspected for general condition; fabric or skin for deterioration, distortion, other evidences of failure, and for security of attachment. Components and systems which make up the complete assembly shall receive the same attention and it shall be determined that they are installed properly and operating satisfactorily. Lighter-than-air craft shall be inspected in the same manner. Helicopters shall have the tail rotors inspected in accordance with the manufacturer's maintenance manual.

(vii) Propeller group. All parts of the propeller shall be carefully examined for cracks, nicks, bends, or oil leakage, if hydraulically controlled. All bolts shall be properly torqued and safetied. The propeller anti-icing devices shall be checked for proper operation or obvious defects. The control mechanism shall operate satisfactorily, be securely mounted, and the controls shall operate through full range of travel.

(viii) Radio group. Radio and electronic equipment shall be inspected for installation and security of mounting. Wiring and conduits shall be checked for proper routing and security of mounting to prevent short-circuiting and to insure that there are no other obvious defects. Bonding and shielding will be determined to be properly installed and in good condition. All antennas shall be inspected for condition and security and, if installed, trailing antenna mechanism shall be inspected for proper operation.

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(static and idle r. p. m.), magneto drop, fuel and oil pressure, cylinder and oil temperatures in accordance with the manufacturer's recommendations.

(xi) Aircraft maintenance record entries-(a) Periodic or 100-hour inspections. Where the aircraft is found to be in an airworthy condition after a periodic or 100-hour inspection, the mechanic, repair station, or manufacturer releasing the aircraft to service shall enter the following notation, inserting the type of inspection (i. e., 100-hour or periodic) in the aircraft maintenance records over the mechanic's signature and certificate number, repair station's or manufacturer's name, signature of authorized personnel, certificate number, and include time in service, and date of inspection:

I certify that this aircraft has been inspected in accordance with a ---

(Insert type) inspection and was determined to be in airworthy condition.

(b) Unairworthy aircraft. Where the aircraft is found unairworthy because of needed maintenance or repairs or is found not to meet the requirements of the applicable specifications, or airworthiness directives, or other approved data, such unairworthy items may be corrected by the inspecting mechanic, repair station, or manufacturer. In the event required maintenance is to be performed by a person other than the one who conducted the periodic inspection, the person making the periodic inspection shall provide the aircraft owner or lessee with a signed and dated copy of a list of the discrepancies and make the following statement in the aircraft maintenance records over the mechanic's signature and certificate number, the repair station's or manufacturer's name, signature of authorized personnel, certificate number, and include aircraft time in service and date of inspection:

I certify that this aircraft has been inspected in accordance with a periodic inspection and a list of the discrepancies and unairworthy items dated (date) has been provided for the aircraft owner or lessee. The aircraft owner or his agent shall obtain a ferry flight authorization, in accordance with Part 1 of this subchapter, prior to ferrying an aircraft for the purpose of obtaining required maintenance or correcting discrepancies.

[21 F. R. 3181, May 15, 1956]

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