&phis;=power-spectral density (ft./sec.)
σ=root-mean-square gust velocity,
Ω=reduced frequency, radians per foot.
(3) The limit loads must be obtained by
multiplying the A
values determined by the dynamic analysis by the following
values of the gust velocity Uσ:
(i) At speed Vc: Uσ=85
fps true gust velocity in the interval 0 to 30,000 ft. altitude and is
linearly decreased to 30 fps true gust velocity at 80,000 ft. altitude.
Where the Administrator finds that a design is comparable to a similar
design with extensive satisfactory service experience, it will be
acceptable to select Uσ at Vc less than 85 fps,
but not less than 75 fps, with linear decrease from that value at 20,000
feet to 30 fps at 80,000 feet. The following factors will be taken into
account when assessing comparability to a similar design:
(1) The transfer function of the new
design should exhibit no unusual characteristics as compared to the
similar design which will significantly affect response to turbulence;
e.g., coalescence of modal response in the frequency regime which can
result in a significant increase of loads.
(2) The typical mission of the new
airplane is substantially equivalent to that of the similar design.
(3) The similar design should
demonstrate the adequacy of the Uσ selected.
(ii) At speed VB: Uσ
is equal to 1.32 times the values obtained under paragraph (b)(3)(i) of
(iii) At speed VD: Uσ
is equal to
1/2 the values obtained under paragraph (b)(3)(i) of this appendix.
(iv) At speeds between VB
and Vc and between Vc and VD: Uσ
is equal to a value obtained by linear interpolation.
(4) When a stability augmentation system
is included in the analysis, the effect of system nonlinearities on loads
at the limit load level must be realistically or conservatively accounted
(c) Mission analysis. Limit loads
must be determined in accordance with the following:
(1) The expected utilization of the
airplane must be represented by one or more flight profiles in which the
load distribution and the variation with time of speed, altitude, gross
weight, and center of gravity position are defined. These profiles must be
divided into mission segments or blocks, for analysis, and average or
effective values of the pertinent parameters defined for each segment.
(2) For each of the mission segments
defined under paragraph (c)(1) of this appendix, values of A
and No must be determined by analysis. A
is defined as the ratio of root-mean-square incremental load
to root-mean-square gust velocity and No is the radius of
gyration of the load power spectral density function about zero frequency.
The power spectral density of the atmospheric turbulence must be given by
the equation set forth in paragraph (b)(2) of this appendix.
(3) For each of the load and stress
quantities selected, the frequency of exceedance must be determined as a
function of load level by means of the equation --
t=selected time interval.
y=net value of the load or stress.
Yone=g=value of the load or
stress in one-g level flight.
N(y)=average number of exceedances of
the indicated value of the load or stress in unit time.
Σ=symbol denoting summation over all mission segments.
=parameters determined by dynamic analysis as defined in
paragraph (c)(2) of this appendix.
P1, P2, b1,
b2=parameters defining the probability distributions of
root-mean-square gust velocity, to be read from Figures 1 and 2 of this
The limit gust loads must be read from
the frequency of exceedance curves at a frequency of exceedance of 2×10−5
exceedances per hour. Both positive and negative load directions must be
considered in determining the limit loads.
(4) If a stability augmentation system
is utilized to reduce the gust loads, consideration must be given to the
fraction of flight time that the system may be inoperative. The flight
profiles of paragraph (c)(1) of this appendix must include flight with the
system inoperative for this fraction of the flight time. When a stability
augmentation system is included in the analysis, the effect of system
nonlinearities on loads at the limit load level must be conservatively
(d) Supplementary design envelope
analysis. In addition to the limit loads defined by paragraph (c) of
this appendix, limit loads must also be determined in accordance with
paragraph (b) of this appendix, except that --
(1) In paragraph (b)(3)(i) of this
appendix, the value of Uσ=85 fps true gust velocity is
replaced by Uσ=60 fps true gust velocity on the interval 0 to
30,000 ft. altitude, and is linearly decreased to 25 fps true gust
velocity at 80,000 ft. altitude; and
(2) In paragraph (b) of this appendix,
the reference to paragraphs (b)(3)(i) through (b)(3)(iii) of this appendix
is to be understood as referring to the paragraph as modified by paragraph
[Amdt. 25-54, 45 FR 60173, Sept. 11, 1980]