Step 7. If the test item operates satisfactorily, proceed to Procedure 1 (II-3). If not, resolve the problems and resteu:*t at step 1 above. Bepeat steps 1 through w until the test itein operates acceptably.

II-3 Procedure! - Glaze ice. The following test procedure provides the basis for collecting the necessary information concerning the test Item in a freezing rain environment.

Step 1. Stabilize the test item tenperature at approximately 2 ±10.

Step 2. Deliver a uniform rain spray of precooled ssater for one hour (although a tenperature of 0° С to 3°C Is ideal, a mter tenperature of 5°C (41°F) at 2.5 em/hr has proven satisfactory). Delivery can be any convenient method per 1-3.2d.

Step 3. Lower the chanber air tenperatxire to -10°C (+14°F) or as specified and maintain the rain rate until б nm of ice has accunulated on all sides and on the ipper

SuTiaoe. wind or a side spray iaay be used to aeeunulate ice on the sides.

ЮТЕ: For tests representing sea spray on marine equipment, increase the ice deposit to 37 mm, and use 75 mm in step 7.

Step 4. Adjust the chanber air tenperature to -6°C (+21°F) for two to six hours. Attempt to operate the test item and ai i subsystems at -oC.

stg-n 5: If ste 4 has resulted in failure or if the specification allows ice removal, remove the ice. If not. got to Step 7. Limit Ice removal to Integral methods plus slnple, expedient, and obvious methods. Note the effectiveness of Ice removal techniques.

Step 6. Attempt to operate the test item amd al 1 siibBysteraB at -6®C and examine for safety hazards.

Step 7. Bepeat steps 1 through 6 with a 13 nn coating of ice (75 nm for sea spray).

Step 8. Return the chaaber tenperature to anbient and restabilize the test Item

teinperattB<e. Perform a post-test operational checkout.

Step 9. Document (with photographs if necessary) the results for conparison with those obtained in II-2.2.

II-4 INFORMATION TO BE RE(X>RDED

a. Test item identification (manufaettuer, serial nuniber, etc.).

b. Previous test methods to which the test item has been subjected,

с. Results of each performance check, visual examination (and photographs, if applicable), and еогфш 180п with ths failure criteria.

(1) Pretest.

(2) During test.

(3) Post-test.

d. Length of tims required for each perfornsnce check.

e. Status of the test item for each visual examination.

f. Defects noted during visual examinations.

g. Clothing and special equipment used to set up or disaseenble the test item.

h. Test tenperatures.

i. Duration of each exposure.

j. Appropriate anthropometric measurements of personnel performing manipulation tests.

k. Tenperat\ire-time-versus data (test item and chanber).

1. Initial analysis of any failure.

I-l FUEPQSE.............. 523 Л-1

1-2 EKVIHOWMENTAL EFFECTS ................ 523.1-1

1-3 GUIDELINES FOR DETERjilNING

TEST PROCEDURES AND TEST CONDITIONS......... 523.1-3

1-4 SPECIAL CONSIDERATIONS................ 523.1-11

1-5 REFERENCES ...................... 523.1-11

SECTION II

II-l ;tf>PABATUS....................... 523,1-14

II-2 PREPARATION FOR TEST................. 523.1-16

II-3 PROCEDURES ...................... 523.1-19

11-4 INFORMATION TO BE RECORDED.............. 523.1-19

I-l.l Objective. This method seeks to reproduce the combined temperature, vibration, and other operating stresses as needed, that an externally-carried aircraft store will experience dxiring in-service flights.

EwviwuKMarrAL tofetuj-m

1-2.1 Observable effects. Possible effects of a conbination of vibration: acouatiC: and high tenperature stresses include all those effects which each of these factors can cause separately (see method 501, 514, 514). Also, the conbined environments may interact to give effects which are not predictable from the results of single-environment tests, but which do occur in actual service use.

1-2.2 Effect HJBChanigns

1-2.2.1 Relative inxx>rtance. All environmental stresses do not contribute equally to deterioration of store reliability. Analysis of service failures caused by aircraft environmental stress (reference b) has identified the four nost significant stresses causing aircraft equipment failures. These are operation, tenperature. vibration and moistise. Other environmental stresses may produce failure modes in a given type of

1-2.2.2 Tenrerature. The source of the heat that causes reliability problems in electronic conponents of aircraft stores will generally be an external surface. This heat in combination with the heat generated within the electronics causes decreased operating life or Mean-Time-to-Fallure (MT). Another stress aspect of the tenperatiSe environment Is rapid tenperatur-e charige (thermal shock). A thermal ahoek or transient registered at the outside surface of the store doss not appear as a shock to conponents somewhat thermally isolated within. Internal conponents experience thermal shock en the unit is turned on and quickly warms up to operating tenperature.

NErroD 523. 1

VETWD 523.1 VIBTO-ACOUSTIC. TEMPE31ATUBE

The tenperature of the external surface of the store tends to become that of the boundary layer air, due to high convective heat transfer at flight speeds. Bovmdary layer air tenperatvire is primarily a function of flight speed and altitude. An expression relating this tenperature to flight conditions is:

Where:

Tp = adabatic recovery tenperature (Kelvin)

Tj = ambient air tenperature (Kelvin) as a function of altitxjde R = recovery factor

к = ratio of specific heats (1.4 for air) M = Mach number

Wien the expression within the brackets, the aerodynamic heating factor, is evaluated for atmospheric air brought to rest by friction along a store with a cylindrical surface, it reduces to a function of aircraft velocity alone. The equation for a Store with a cyclindrical surface is:

Tj. = Tj (1 0. 174 M)

Higher Mach number flights tend to occur at higher, thus colder, altitudes and there is a corresponding tendency for the velocity-dependent heating effect to cancel the effect of decreaising tenperature with altitude. №еп the above expression is evaluated for normal mission profiles, flown in a standard atmosphere, 90% of the time the skin tenperature will be within the tenperature band -15°C to 35°C.

Tenperature patterns at points deep within the store will depart considerably from the corresponding skin tenperature patterns, due to thermal lag in conduction from the skin and internal heating sources such as electrical or electronic conponents. A thermal model of the store can be generated to calculate Internal tenperature patterns.

An additional thermal parameter needed to satisfy mission conditions is that of climatic departiire from the standard atmosphere, vtblch depends on the global and seasonal variations of atmospheric tenperatures (the T in the Tj fornula above) .

1-2.2.3 Vibration. Experimental evidence has shovsr. that captive flight vibratioriS are due largely to aero-acoustic loads (reference 3). This acoustic forcing function, typically consisting of broad-band random noise, is modified as it is transmitted through the store structure to the conponent. When there is sufficient

transmission of frequencies causing resonances of the unit or its connponents or strixrturai mechanisms, a vibration failure can occur. Environmental testing, using a reverberant acojstic charrfcer, tries to d-cpiioate the directional, spatial and spectal distributions of vibration expected throughout the store during captive flight.

The turbulent boundary layer is the most significant source of aero-acoustic loads because it is always present during flight and acts on the total surface frequencies of electronic conponents and structural parts. The Intensity of the t-bulent bovmdary Iser ргевёш fluctuation and resulting store vibration primarily depends on flight dynamic pressure, q. a function of flight speed and altitude:

<--2-

Wiere:

q = dynamic pressure (powids per so. ft,) к = ratio of specific heats ( 1.4 for air)

Fj, = anbient pressure as a function of altitude (pounds per sq. ft.) M = №^h nimfcer

Store vibration also includes lower frequencies (usually less than 100 Hz) mechanically transmitted from the aircraft through the stores support mechanism. Low frequency vibration is discussed in 1-3.4.2.

1-2.2.4 Operating stregg. Operating stresses are usually astimatad because the service conditions (e.g., on-time/off-time, aircraft power fluctuations) are seldom measured and recorded. This stress cannot be omitted unless the store has no operating node while carried on aircraft.

1-2.2.5 moiSt\ae. In confined environments testing, moisture often condenses on the test item during trangitior.s from low tc high tenperatures. Its presence, although uncontrolled, is tjeeful as a test condition to indicate leakage or sensitivity to moisture. Mhere humidity or corrosion problems are expected, separate tests are advised.

1-5 GuIDELIi FOR РЁГШатШ TEST РВ0СИ>(ШЕ5 AMD TEST (JMDITIONS

MOTE: The tailoring process as described in section 4 of this document should be used to determine the appropriate tests and test variables.

a. Application. This method applies to reliability-related testing of externally carried aircraft stores (table 523.1-1).

§

TABLE 52:3-1-1. TvTJical жрр11саЪ1оп8.

►-00 vo-- odo vOt4

b. Rggtrictlong. This method is intended priBsrily for the electronics and electro-mechanical assenfelies within the store.

c. Sequence. This method applies to the environmental stresses occurring in the final phases of the stores logistic cycle, and vtien used in combination with other test methods, should follow these methods.

d. Test variatior.s. Unlike the other methods in this standard, this method contains no step-by-step ргосейш^ for generating valid test data. Thm vibro/acoustic/tenperature environment is too conplex, and the variety of equipment applications too great, for such detailed instructions to be given here. Instead, this method provides guidance for neriting a test procedure which will be more or less unique for the item under consideration.

1-3.1 Background. Experiosents have shown (ref a) that the only way tc reproduc-e the service failis distribution is to reproduce t-he service stress distribution. Stress distribution is a range of stresses, in proper proportion, of level and durations determined by mission profiles. The proportioning is applied to vibration, tenperature, thermal shock and electrical stress. Procedure I uses conbinations of tenperature, acoustic vibration, mechanical vibration, and store operating patterns to simulate in-aervice missions.

1-3.2 General. Military aircraft service ше nsy be characterized by a set ef specialized missions, with a duration and relative frequency assigned to each mission type. Each mission type is described by its mission profile and idealized mission history which specifies altitude, velocity and operating state as a fxjnction of time; and which locates the occurrence of stressful events such as special maneuvers, giSifire and landings, гхош such tnisaion profiles, corresponuing mission enV 1ГОПШёПь histories (e.g., vibration levels, skin tenperatures) can be constructed. Data from instrianented flights may be used in this construction. By treating the mission environment profiles probabilistically, sunning the durations of each stress level in each mission, and weighting by the relative frequency of each mission, a service distribution function for each stress may be obtained. A conposlte environaent profile may then be constructed for each stress of interest. Tnis cosposite envirorjBsntal profile is a sequence of stress levels constructed to sinulate the service environment profiles for the different missions taken together. Its total duration should be no longer than a few missions, it nust represent realistic flight conditions, and it must reflect the calculated combined service distribution function for the stress. A conposlte mission profile consists of the conbination of composite environmental profiles for each environment, so coordinated that the ffiixtio-e of stress levels at any point in time represents the typical service condition bsihg sinulatsd. Sissulation of typical (5th to SSth percentile) values is enphaslzed. If extreme yaluss were vsed siisilar t-o qualification teat levels, the results would not correlate with field experience.

S23.1-5

KETmOD 521.1

ICIMG./FREEZIMG RAIN SECTION II

II-l APPARATUS

II-l.l T0gt..f acuities

a. The required apparatus consists of a chairiber or cabinet with aijDciliary equipment which has the ability to establish and maintain the test conditions specified. The chamber must be equipped so that test conditions within the chanber can be stabilized within a reasonable time after the test item is loaded. Wbter delivery equipment (nozzles and drains) shall be arranged to preclude the collection of puddles N in the chasfcer. The chambep shall be equipped with iristrtsaentatlon capable of nalntaining and continuetsly monitoring the t-est eonditior.*. (See (Seneral Requirements, 5.1.2)

b. The thickness of the ice and the tenperature during eqmpmant operation are the inportant parameters. The precise methods for depositing the ice on the equipment

are not inportant. (See 1-3.2d)

II-l.2 Controls, Before each test, critical paramBters shall be verified. A spray

pattern wide enough to guarantee uniform inplngement for all test wind velocities

shall be assured. Suggested techniques for spray calibration (if specified or j

consldei essential) can be foxjnd in reference I-5j. Unless otherwise specified in

the equipraent specifications (or other docixnents), if any action other than test item

operation vsuch as opening the сЬвшЬег door) results in a Bignificaiit chmige in the

test item or chamber air tenperature (more than 2°C (3.6°F)), the tsst item will bs

restabilized at the required tenperat\s*e before continuatipn. If the operational

check Is not conpleted within 15 minutes, reestablish the test item tenperature

conditions before continuing.

11-1.3 Test interruption. (See General Bequii>ements, 5.2.4)

generate any adverge effects and normally the test shall be continued from the point of interruption once the test conditions have been reestablished.

b. Overtest interruption. Any interruption that results in тэге extreme exposure of the test item than required by the requirements doctment or equipment

specif ication should be followed by a eoiqplete operational and physical check. If no problens arc sncountsred, ths tsst itssn shall Ъе restored tc Its prstsst ccndltlcn and the teat reinitiated,

11-2 PREPARATION FOR TEST

II-2.1 Preliminary steps. Before initiating any testing: SSTHOD 521. 1

521.1-е /

Uo 60 60

Elapsed Hiesion Tlae In Minutes

FIGURE \523.1-1. TYPieal aircraft operational ш1аа1оп profile.

о Flight Tiae (Minutes)

PZaURC 523.1-2. Teaperature profile for a в1пк1ё BJealen tYDS

523.1-1 JffiTHOD 523.1