Appendix 4 A4-3
6. The performance requirements contained herein include not only the performance requirements necessary for the certification of an airplane, but also the complementary performance operating limitations as applicable under Parts 40, 41, and 42 of the Civil Air Regulations. In promulgating this new performance code, the Board intends that the resulting level of safety will be generally similar to the level of safety established by the performance code as expressed by the provisions now contained in Parts 4b and 40 (or 41 or 42 as appropriate) for reciprocating engine airplanes. To attain this, many of the performance provisions have been modified for better applicability to turbine-powered airplanes, some in the direction of liberalization, others in the direction of improvement in the required performance.
7. A significant change being made is the introduction of full temperature accountability in all stages of performance, except the landing distances required. The introduction of full temperature accountability will insure that the airplane’s performance is satisfactory irrespective of the existing atmospheric temperature. The performance requirements heretofore applicable did not give sufficient assurance in this respect.
8. The reason for omitting the direct application of temperature accountability in the requirement for landing distances is that this stage of performance always has been treated in a highly empirical fashion whereby temperature effects are taken into account indirectly together with the effects of other operational factors. Long range studies on rationalization of airplane performance so far have not yielded a satisfactory solution to the landing stage of performance.
The Board hopes, however, that continued studies will result in a solution of this problem in the near future.
9. The introduction of full temperature accountability has necessitated a complete reevaluation of the minimum climb requirements. Since the prescribed climb must now be met at all temperatures rather than to be associated with standard temperature, the specific values of climb have been altered. In each instance, the change has been in the downward direction because, although the previous values were related to standard temperature, a satisfactory resultant climb performance was attained at temperatures substantially above standard. While values of minimum climb performance specified in the new code will tend to increase the maximum certificated weights of the airplane for the lower range of temperatures, they will limit these weights for the upper range of temperatures, giving adequate assurance of satisfactory climb performance at all temperatures.
10. In considering the various stages of flight where minimum values of climb have been heretofore established, the Board finds that in two of the stages (all-engines-operating en route and one-engine-inoperative en route) the establishment of minimum values of climb is unnecessary because, in the case of the all-engines-operating stage, it has been found not to be critical and the case of the one-engine-inoperative stage is now more effectively covered by the en route performance operating limitations.
11. Considering that the minimum climbs being prescribed affect mainly the maximum certificated weights of the airplane but not the maximum operating weights, the Board, in adopting the new performance code, places considerable emphasis on the ability of the airplane to clear obstacles on takeoff and during flight. To this end, criteria for the takeoff path, the en
Appendix 4 A4-4 route flight paths, and the transition from takeoff to the en route stage of flight have been prescribed to reflect realistic operating procedures. Temperature is fully accounted for in establishing all flight paths and an expanding clearance between the takeoff path and the terrain or obstacles is required until the en route stage of flight is reached.
12. In order to insure that the objectives of the prescribed performance are in fact realized in actual operations, the manufacturer is required to establish procedures to be followed in the operation of the airplane in the various conditions specified in the regulation. These procedures, each designed for a specific airplane, will permit the operator to utilize the full performance capabilities of the airplane more readily than if the regulations prescribed all-inclusive procedures. The use of these procedures in determining compliance with the requirements governing takeoff, en route, and landing stages, will also add considerable flexibility to the regulation.
13. The new performance requirements established more clearly than heretofore which of the performance limitations are conditions on the airworthiness certificate of the airplane. In addition to the maximum certificated takeoff and landing weights, there are included limitations on the takeoff distances and on the use of the airplane within the ranges of operational variables, such as altitude, temperature, and wind. Since these limitations are in the airworthiness certificate, they are applicable to all type operations conducted with the airplane.
14. The new performance code contains values for minimum climb expressed as gradients of climb, in percent, rather than as rates of climb, in feet per minute, as has been the case heretofore. The Board believes that the gradient of climb is more direct in expressing the performance margins of the airplane. Use of the gradient eliminates the influence of the stalling speed on the required climb. Heretofore, higher rates of climb were required for airplanes with higher stalling speeds. The only differentiation in the new code with respect to the required climb is between two and four-engine airplanes. This type of differentiation is of long standing in the regulations, being applicable to the one-engine-inoperative stage of flight. It is now being expanded to the takeoff and approach stages.
15. The new performance requirements contained herein are based on the best information presently available to the Board. It is realized, however, that due to the present limited operating experience with turbine-powered transport airplanes, improvement in the requirements can be expected as a result of the direct application of the code to specific designs of new airplanes.
There are certain areas in the new requirements where additional refinement of details might be advisable. This is so particularly in the case of the requirements pertaining to the landing stage of flight. It is anticipated that, after further study of the regulation and especially after its application in the design, certification, and operation of forthcoming turbine-powered airplanes, the desirability of changes may become more apparent. It is the intent of the Board to consider without delay such changes as might be found necessary. Only after the provisions of this Special Civil Air Regulation are reasonably verified by practical application will the Board consider incorporating them on a more permanent basis into Parts 4b, 40, 41, and 42 of the Civil Air Regulations.
Appendix 4 A4-5
16. This Special Civil Air Regulation is not intended to compromise the authority of the Administrator under section 4b.10 to impose such special conditions as he finds necessary in any particular case to avoid unsafe design features and otherwise to insure equivalent safety.
17. Interested persons have been afforded an opportunity to participate in the making of this regulation (21 F.R. 6091), and due consideration has been given to all relevant matter presented.
18. In consideration of the foregoing, the Civil Aeronautics Board hereby makes and promulgates the following Special Civil Air Regulation, effective August 27, 1957.
a. Contrary provisions of the Civil Air Regul ations notwithstanding, all turbine-powered transport category airplanes for which a type certificate is issued after the effective date of this Special Civil Air Regulation shall comply with the following:
(1) The provisions of part 4b of the Civil Air Regulations, effective on the date of application for type certificate; and such of the provisions of all subsequent amendments to part 4b, in effect prior to the effective date of this special regulation, as the Administrator finds necessary to insure that the level of safety of turbine-powered airplanes is equivalent to that generally intended by part 4b.
In lieu of sections 4b.110 through 4b.125, and 4b.743 of part 4b of the Civil Air Regulations, the following shall be applicable:
Performance
4T.110 General.
(a) The performance of the airplane shall be determined and scheduled in accordance with, and shall meet the minima prescribed by, the provisions of sections 4T.110 through 4T.123. The performance limitations, information, and other data shall be given in accordance with section 4T.743.
(b) Unless otherwise specifically prescrib ed, the performance shall correspond with ambient atmospheric conditions and still air. Humidity shall be accounted for as specified in paragraph (c) of this section.
(c) The performance as affected by engine power and/or thrust shall be based on a relative humidity of 80 percent at and below standard temperatures and on 34 percent at and above standard temperatures plus 50°F. Between these two temperatures the relative humidity shall vary linearly.
(d) The performance shall correspond with the propulsive thrust available under the particular ambient atmospheric conditions, the particular flight conditions, and the relative humidity specified in paragraph (c) of this section. The available propulsive thrust shall correspond with engine power and/or thrust not exceeding the approved power and/or thrust less the installational losses and less the power and/or equivalent thrust absorbed by the accessories
Appendix 4 A4-6 and services appropriate to the particular ambient atmospheric conditions and the particular flight condition.
4T.111 Airplane configuration, speed, power, and/or thrust; general.
(a) The airplane configuration (setting of wing and cowl flaps, air brakes, landing gear, propeller, etc.), denoted respectively as the takeoff, en route, approach, and landing configurations, shall be selected by the applicant except as otherwise prescribed.
(b) It shall be acceptable to make the airplane configurations variable with weight, altitude, and temperature, to an extent found by the Administrator to be compatible with operating procedures required in accordance with paragraph (c) of this section.
(c) In determining the accelerate-stop distances, takeoff flight paths, takeoff distances, and landing distances, changes in the airplane’s configuration and speed, and in the power and/or thrust shall be in accordance with procedures established by the applicant for the operation of the airplane in service, except as otherwise prescribed. The procedures shall comply with the provisions of subparagraphs (1) through (3) of this paragraph.
(1) The Administrator shall find that the procedures can be consistently executed in service by crews of average skill.
(2) The procedures shall not involve methods or the use of devices which have not been proven to be safe and reliable.
(3) Allowance shall be made for such time delays in the execution of the procedures as may be reasonably expected to occur during service.
4T.112 Stalling speeds.
The minimum steady flight speed at which the airplane is controllable, in
(a) The speed V S0 shall denote the calibrated stalling speed, in knots, with:
(1) Zero thrust at the stalling speed, or engines idling and throttles closed if it is shown that the resultant thrust has no appreciable effect on the stalling speed;
(2) If applicable, propeller pitch controls in the position necessary for compliance with subparagraph (1) of this paragraph;
(3) The airplane in the landing configuration;
(4) The center of gravity in the most unfavorable position within the allowable landing range;
Appendix 4 A4-7
(5) The weight of the airplane equal to the weight in connection with which V S is being used to determine compliance with a particular requirement.
(b) The speed V S1 shall denote the calibrated stalling speed, or the minimum steady flight speed at which the airplane is controllable, in knots, with:
(1) Zero thrust at the stalling speed, or engines idling and throttles closed if it is shown that the resultant thrust has no appreciable effect on the stalling speed;
(2) If applicable, propeller pitch controls in the position necessary for compliance with subparagraph (1) of this paragraph; the airplane in all other respects (flaps, landing gear, etc.) in the particular configuration corresponding with that in connection with which VS1 is being used;
(3) The weight of the airplane equal to the weight in connection with which V S1 is being used to determine compliance with a particular requirement.
(c) The stall speeds defined in this section shall be the minimum speeds obtained in flight tests conducted in accordance with the procedure of subparagraphs (1) and (2) of this paragraph.
(1) With the airplane trimmed for straight flight at a speed of 1.4 V S and from a speed sufficiently above the stalling speed to insure steady conditions, the elevator control shall be applied at a rate such that the airplane speed reduction does not exceed one knot per second.
(2) During the test prescribed in subparagraph (1) of this paragraph, the flight characteristics provisions of section 4b.160 of part 4b of the Civil Air Regulations shall be complied with.
4T.113 Takeoff; general
(a) The takeoff data in sections 4T.114 through 4T.117 shall be determined under the conditions of subparagraphs (1) and (2) of this paragraph.
(1) At all weights, altitudes, and ambient temperatures within the operational limits established by the applicant for the airplane.
(2) In the configuration for takeoff (see sec. 4T.111).
(b) Takeoff data shall be based on a smooth, dry, hard-surfaced runway, and shall be determined in such a manner that reproduction of the performance does not require exceptional skill or alertness on the part of the pilot. In the case of seaplanes or float planes, the takeoff surface shall be smooth water, while for skiplanes it shall be smooth dry snow. In addition, the takeoff data shall be corrected in accordance with subparagraphs (1) and (2) of this paragraph for wind and for runway gradients within the operational limits established by the applicant for the airplane.
Appendix 4 A4-8
(1) Not more than 50 percent of nominal wind components along the takeoff path opposite to the direction of takeoff, and not less than 150 percent of nominal wind components along the takeoff path in the direction of takeoff.
(2) Effective runway gradients.
4T.114 Takeoff speeds.
(a) The critical-engine-failure speed V 1, in terms of calibrated air speed, shall be selected by the applicant, but shall not be less than the minimum speed at which controllability by primary aerodynamic controls alone is demonstrated during the takeoff run to be adequate to permit proceeding safely with the takeoff using average piloting skill, when the critical engine is suddenly made inoperative.
(b) The minimum takeoff safety speed V 2, in terms of calibrated air speed, shall be selected by the applicant so as to permit the gradient of climb required in section 4T.120 (a) and (b), but it shall not be less than:
(1) 1.2 V S1 for two-engine propeller-driven airplanes and for airplanes without propellers which have no provisions for obtaining a significant reduction in the one-engineinoperative power-on stalling speed.
(2) 1.15 V S1 for propeller-driven airplanes having more than two engines and for airplanes without propellers which have provisions for obtaining a significant reduction in the one-engine-inoperative power-on stalling speed;
(3) 1.10 times the minimum control speed V MC , established in accordance with section 4b.133 of part 4b of the Civil Air Regulations.
(c) If engine failure is assumed to occur at or after the attainment of V 2 , the demonstration in which the takeoff run is continued to include the takeoff climb, as provided in paragraph (a) of this section, shall not be required.
4T.115 Accelerate-stop distance.
(a) The accelerate-stop distance shall be the sum of the following:
(1) The distance required to accelerate the airplane from a standing start to the speed V1;
(2) Assuming the critical engine to fail at the speed V 1, the distance required to bring the airplane to a full stop from the point corresponding with the speed V1.
(b) In addition to, or in lieu of, wheel brakes, the use of other braking means shall be acceptable in determining the accelerate-stop distance, provided that such braking means shall have been proven to be safe and reliable, that the manner of their employment is such that
Appendix 4 A4-9 consistent results can be expected in service, and that exceptional skill is not required to control the airplane.
(c) The landing gear shall remain extended throughout the accelerate-stop distance.
4T.116 Takeoff path. The takeoff path shall be considered to extend from the standing start to a point in the takeoff where a height of 1,000 feet above the takeoff surface is reached or to a point in the takeoff where the transition from the takeoff to the en route configuration is completed and a speed is reached at which compliance with section 4T.120(c) is shown, whichever point is at a higher altitude. The conditions of paragraphs (a) through (i) of this section shall apply in determining the takeoff path.
(a) The takeoff path shall be based upon procedures prescribed in accordance with section 4T.111(c).
(b) The airplane shall be accelerated on or near the ground to the speed V 2 during which time the critical engine shall be made inoperative at speed V1 and shall remain inoperative during the remainder of the takeoff.
(c) Landing gear retraction shall not be initiated prior to reaching the speed V 2
(d) The slope of the airborne portion of the takeoff path shall be positive at all points.
(e) After the V 2 speed is reached, the speed throughout the takeoff path shall not be less than V2 and shall be constant from the point where the landing gear is completely retracted until a height of 400 feet above the takeoff surface is reached.
(f) Except for gear retraction and propeller feathering, the airplane configuration shall not be changed before reaching a height of 400 feet above the takeoff surface.
(g) At all points along the takeoff path starting at the point where the airplane first reaches a height of 400 feet above the takeoff surface, the available gradient of climb shall not be less than
1.4 percent for two-engine airplanes and 1.8 percent for four-engine airplanes.
(h) The takeoff path shall be determined either by a continuous demonstration takeoff, or alternatively, by synthesizing from segments the complete takeoff path.
(i) If the takeoff path is determined by the segmental method, the provisions of subparagraphs (1) through (4) of this paragraph shall be specifically applicable.
(1) The segments of a segmental takeoff path shall be clearly defined and shall be related to the distinct changes in the configuration of the airplane, in power and/or thrust, and in speed.
Appendix 4 A4-10
(2) The weight of the airplane, the configuration, and the power and/or thrust shall be constant throughout each segment and shall correspond with the most critical condition prevailing in the particular segment.
(3) The segmental flight path shall be based on the airplane’s performance without ground effect.
(4) Segmental takeoff path data shall be checked by continuous demonstrated takeoffs to insure that the segmental path is conservative relative to the continuous path.
4T.117 Takeoff distance. The takeoff distance shall be the horizontal distance along the takeoff path from the start of the takeoff to the point where the airplane attains a height of 35 feet above the takeoff surface as determined in accordance with 4T.116.
4T.118 Climb; general. Compliance shall be shown with the climb requirements of sections 4T.119 and 4T.120 at all weights, altitudes, and ambient temperatures, within the operational limits established by the applicant for the airplane. The airplane’s center of gravity shall be in the most unfavorable position corresponding with the applicable configuration.
4T.119 All-engine-operating landing climb. In the landing configuration, the steady gradient of climb shall not be less than 4.0 per cent, with:
(a) All engines operating at the available takeoff power and/or thrust;
(b) A climb speed not in excess of 1.4 V S0.
4T.120 One-engine-inoperative climb.
(a) Takeoff; landing gear extended. In the takeoff configuration at the point of the flight path where the airplane’s speed first reaches V2, in accordance with section 4T.116 but without ground effect, the steady gradient of climb shall be positive with:
(1) The critical engine inoperative, the remaining engine(s) operating at the available takeoff power and/or thrust existing in accordance with section 4T.116 at the time the airplane’s landing gear is fully retracted;
(2) The weight equal to the airplane’s weight existing in accordance with section 4T.116 at the time retraction of the airplane’s landing gear is initiated;
(3) The speed equal to the speed V 2.
(b) Takeoff; landing gear retracted. In the takeoff configuration at the point of the flight path where the airplane’s landing gear is fully retracted, in accordance with section 4T.116 but without ground effect, the steady gradient of climb shall not be less than 2.5 percent for two-engine airplanes and not less than 3.0 percent for four-engine airplanes, with:
Appendix 4 A4-11
(1) The critical engine inoperative, the remaining engine(s) operating at the takeoff power and/or thrust available at a height of 400 feet above the takeoff surface and existing in accordance with section 4T.116;
(2) The weight equal to the airplane’s weight existing in accordance with section 4T.116 at the time the airplane’s landing gear is fully retracted;
(3) The speed equal to the speed V 2.
(c) Final takeoff. In the en route configuration, the steady gradient of climb shall not be less than 1.4 percent for two-engine air-planes and not less than 1.8 percent for four-engine airplanes, at the end of the takeoff path as determined by section 4T.116, with:
(1) The critical engine inoperative, the remaining engine(s) operating at the available maximum continuous power and/or thrust;
(2) The weight equal to the airplane’s weight existing in accordance with section 4T.116 at the time retraction of the airplane’s flaps is initiated;
(3) The speed equal to not less than 1.25 V S1.
(d) Approach. In the approach configuration such that VS1 does not exceed 1.10 VS0, the steady gradient of climb shall not be less than 2.2 percent for two-engine airplanes and not less than 2.8 percent for four-engine airplanes, with:
(1) The critical engine inoperative, the remaining engine(s) operating at the available takeoff power and/or thrust;
(2) The weight equal to the maximum landing weight;
(3) A climb speed in excess of 1.5 V S1.
4T.121 En route flight paths. With the airplane in the en route configuration, the flight paths prescribed in paragraphs (a) and (b) of this section shall be determined at all weights, altitudes, and ambient temperatures within the limits established by the applicant for the airplane.
(a) One engine inoperative. The one-engine-inoperative net flight path data shall be determined in such a manner that they represent the airplane’s actual climb performance diminished by a gradient of climb equal to 1.4 percent for two-engine airplanes and 1.8 percent for four-engine airplanes. It shall be acceptable to include in these data the variation of the airplane’s weight along the flight path to take into account the progressive consumption of fuel and oil by the operating engine(s).
(b) Two engines inoperative. For airplanes with four engines, the two-engine-inoperative net flight path data shall be determined in such a manner that they represent the airplane’s actual climb performance diminished by a gradient of climb equal to 0.6 percent. It shall be acceptable
Appendix 4 A4-12 to include in these data the variation of the airplane’s weight along the flight path to take into account the progressive consumption of fuel and oil by the operating engines.
(c) Conditions. In determining the flight paths prescribed in paragraphs (a) and (b) of this section, the conditions of subparagraphs (1) through (4) of this paragraph shall apply.
(1) The airplane’s center of gravity shall be in the most unfavorable position.
(2) The critical engine(s) shall be inoperative, the remaining engine(s) operating at the available maximum continuous power and/or thrust.
(3) Means for controlling the engine cooling air supply shall be in the position which provides adequate cooling in the hot-day condition.
(4) The speed shall be selected by the applicant.
4T.122 Landing distance. The landing distance shall be the horizontal distance required to land and to come to a complete stop (to a speed of approximately 3 knots in the case of seaplanes or float planes) from a point at a height of 50 feet above the landing surface. Landing distances shall be determined for standard temperatures at all weights, altitudes, and winds within the operational limits established by the applicant for the airplane. The conditions of paragraphs (a) through (f) of this section shall apply.
(a) The airplane shall be in the landing configuration. During the landing, changes in the airplane’s configuration, in power and/or thrust, and in speed shall be in accordance with procedures established by the applicant for the operation of the airplane in service. The procedures shall comply with the provisions of section 4T.111(c).
(b) The landing shall be preceded by a steady gliding approach down to the 50-foot height with a calibrated air speed of not less than 1.3 VS0.
(c) The landing distance shall be based on a smooth, dry, hard-surfaced runway, and shall be determined in such a manner that reproduction does not require exceptional skill or alertness on the part of the pilot. In the case of seaplanes or float planes, the landing surface shall be smooth water, while for skiplanes it shall be smooth dry snow. During landing, the airplane shall not exhibit excessive vertical acceleration, a tendency to bounce, nose over, ground loop, porpoise, or water loop.
(d) The landing distance shall be corrected for not more than 50 percent of nominal wind components along the landing path opposite to the direction of landing and not less than 150 percent of nominal wind components along the landing path in the direction of landing.
(e) During landing, the operating pressures on the wheel braking system shall not be in excess of those approved by the manufacturer of the brakes, and the wheel brakes shall not be used in such a manner as to produce excessive wear of brakes and tires.
Appendix 4 A4-13
(f) If the Administrator finds that a device on the airplane other than wheel brakes has a noticeable effect on the landing distance and if the device depends upon the operation of the engine and the effect of such a device is not compensated for by other devices in the event of engine failure, the landing distance shall be determined by assuming the critical engine to be inoperative.
4T.123 Limitations and information.
(a) Limitations. The performance limitations on the operation of the airplane shall be established in accordance with subparagraphs (1) through (4) of this paragraph. (See also sec.
4T.743.)
(1) Takeoff weights. The maximum takeoff weights shall be established at which compliance is shown with the generally applicable provisions of this regulation and with section 4T.120 (a), (b), and (c) for altitudes and ambient temperatures within the operational limits of the airplane (see subparagraph (4) of this paragraph).
(2) Landing weights. The maximum landing weights shall be established at which compliance is shown with the generally applicable provisions of this regulation and with sections 4T.119 and 4T.120(d) for altitudes and ambient temperatures within the operational limits of the airplane (see subparagraph (4) of this paragraph).
(3) Takeoff and accelerate-stop distances. The minimum distances required for takeoff shall be established at which compliance is shown with the generally applicable provisions of this regulation and with sections 4T.115 and 4T.117 for weights, altitudes, temperatures, wind components, and runway gradients, within the operational limits of the airplane (see subparagraph (4) of this paragraph).
(4) Operational limits. The operational limits of the airplane shall be established by the applicant for all variable factors required in showing compliance with this regulation (weight, altitude, temperature, etc.). (See secs. 4T.113(a) (1) and (b), 4T.118, 4T.121, and 4T.122.)
(b) Information. The performance information on the operation of the airplane shall be scheduled in compliance with the generally applicable provisions of this regulation and with sections 4T.116, 4T.121, and 4T.122 for weights, altitudes, temperatures, wind components, and runway gradients, as these may be applicable, within the operational limits of the airplane (see subparagraph (a)(4) of this section). In addition, the performance information specified in subparagraphs (1) through (3) of this paragraph shall be determined by extrapolation and scheduled for the ranges of weights between the maximum landing and maximum takeoff weights established in accordance with subparagraphs (a) (1) and (a) (2) of this section. (See also sec. 4T.743.)
(1) Climb in the landing configuration (see sec. 4T.119);
(2) Climb in the approach configuration (see sec. 4T.120(d));
Appendix 4 A4-14
(3) Landing distance (see sec. 4T.122).
Airplane Flight Manual
4T.743 Performance limitations, information, and other data.
(a) Limitations. The airplanes’ performance limitations shall be given in accordance with section 4T.123(a).
(b) Information. The performance information prescribed in section 4T.123(b) for the application of the operating rules of this regulation shall be given together with descriptions of the conditions, air speeds, etc. under which the data were determined.
(c) Procedures. For all stages of flight, procedures shall be given with respect to airplane configurations, power and/or thrust settings, and indicated air speeds, to the extent such procedures are related to the limitations and information set forth in accordance with paragraphs
(a) and (b) of this section.
(d) Miscellaneous. An explanation shall be given of significant or unusual flight or ground handling characteristics of the airplane.
In lieu of sections 40.70 through 40.78, 41-27 through 41.36(d), and 42.70 through
42.83, of Parts 40, 41, and 42 of the Civil Air Regulations, respectively, the following shall be applicable:
Operating Rules
40T.80 Transport category airplane operating limitations.
(a) In operating any passenger-carrying transport category airplane certificated in accordance with the performance requirements of this regulation, the provisions of sections 40T.80 through 40T.894 shall be complied with, unless deviations therefrom are specifically authorized by the Administrator on the ground that the special circumstances of a particular case make a literal observance of the requirements unnecessary for safety.
(b) The performance data in the AFM shall be applied in determining compliance with the provisions of sections 40T.81 through 40T.84. Where conditions differ from those for which specific tests were made compliance shall be determined by approved interpolation or computation of the effects of changes in the specific variables if such interpolations or computations give results substantially equaling in accuracy the results of a direct test.
40T.81 Airplane’s certificate limitations.