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2 I-R-E TRANSACTIONS-AERONAUTICAL AND NAVIGATIONAL ELECTRONICS September

A Maintenance Plan for Airborne RadioEquipmentT. R. W. BUSHBY*

Summary-This paper briefly outlines th e Australian system of One of the first requirements for a maintenance plan

maintenance for airborne radio equipment, and gives in more detail is foreknowledge of the reasons for, and incidence of,th e results of a recent investigation into th e reasons for, and inci- unscheduled removals. Having discarded the ideal of nodence of , unscheduled removals of units.

The probability of unscheduled removal, derived from the in- operational failures as impracticable, an upper limitvestigation data, was used as a criterion for the assessment of the must be set to the number of failures that can beoptimum period between scheduled overhauls. It was found that, in tolerated in an equipment over a prescribed period; amany instances, the probability of failure is greatest immediately satisfactory maintenance service must plan for a prob-after overhaul and decreases with time. Comparisons of the per- ability of failure that is acceptable.formance of various types of units are given.

A knowledge of the probability of a specific failure

OBJECTIVES assists in determining the relative importance of theaction to be taken to avoid it. It is economically un-

A; \ MAINTENANCEservice forairborne radioequip- desirable to take action to avoid an event that is ex-ment has two objectives: to prevent, as far as tremely unlikely. When setting permissible limits for

possible, unscheduled removal of the equipment probability of failure, consideration must also be givenfrom the aircraft, and to test and, if necessary, repair to the effect on operation of the failure. For example, aunits that have been removed for suspected faults. six-decibel loss in communication-receiver sensitivityIdeally unscheduled removals should not occur. In the may be regarded as less serious than the total loss of anpresent state of our knowledge and application, the important navigational aid. If the probabilities for theideal is unattainable, so that the target of an irreducible two events are equal, preventive action might be re-minimum of such removals must be substituted. The garded as economically unwarranted fo r the receiver,emphasis is therefore on prevention rather than cure. but essential for the navigational aid.The first objective becomes the important one, the sec- Summarizing briefly, it may be said that the formula-ond one being an unavoidable evil. tion of a plan requires answers to the following ques-

tions. First, why do unscheduled removals occur? Sec-THE BASIS FOR A PLAN ond, when do they occur? These two questions can be

Certain features of airborne units (e.g. structural combined into a third as follows. What is the probabilityparts) confidently may be expected to perform their at any time of (1 ) any unscheduled removal, (2 ) a

functions, without attention, throughout the whole of specific type of unscheduled removal, or (3 ) unscheduledthe required life of the unit; others (e.g. tubes) ordi- removal of a particular unit?

narily will not, so that failures will occur unless some Although it is appreciated that the continuous satis-action be taken to prevent them. It therefore may be factory functioning of the whole of the radio equipmentassumed that a program of scheduled overhauls is an is the optimum target, costs must be considered. Itessential requirement of preventive maintenance. In might be found, for example, that for certain types orthis preliminary discussion, the word "overhaul" is used failure, the probability can be reduced almost withoutin a broad sense, to indicate action, not necessarily an limit, by increasing the frequency of overhauls, but cost

overhaul in the more restricted sense. Under the broad of maintenance may be expected to be approximately in-definition of overhaul may be included tests, inspections, versely proportional to maximum acceptable probabil-and checks. ity. This leads to the last question. What is the maxi-A plan for such a program must include consideration mum acceptable probability of (1 ) any unscheduled

of the following factors: removal, (2 ) a specific type of unscheduled removal, or

(1) The times at which overhauls are to be done. (3) unscheduled removal of a particular unit?

(2) The action to be taken at such overhauls. The maintenance engineer can assist operations per-

(3) The place at which such overhauls are to be done sonnel in an appreciation of this last question, but hemust look to them for the answer. A balance must be(eg. in th aicat'tayrpi-ae ra struck between the cost of unscheduled removals and

central repair-base or bases).(4)Meas fralutin efectvenss f te pan,in the cost of the maintenance necessary to avoid them.

the form of adequate records of performance.TERAOSFRUCHDLDEMVS(5) The cost of the plan.

Why do unscheduled removals occur? Past experience..vato Det,AagmtdWrls.Asrlsa t. is probably the most useful guide in determining the

Sydney, N.S.W., Australia. *'reasons fo r unscheduled removals. There ar e compara-

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1954 Bushby: A Maintenance Plan for Airborne Radio Equipment 3

tively few cases in which such information can be had The first cause points to the necessity for training andfrom any other source. In the case of new equipments, experience in recognizing the symptoms of malfunctioningunless they contain novel or unusual features, past ex - of the equipment, as distinct from malfunctioning fromperience of the performance of devices of a similar na- external causes. The second and th ird a re also mattersture in existing equipments will still be helpful. Manu- for training and experience in diagnosis and correctfacturers' instruction-books often give recommenda- procedures. The fourth calls for changes in tests.tions regarding possible faults, but unless they are Item 2 (unpredictable tube failures) includes total

based on adequate life-testing data, they must be re- failures of tubes that can fairly be classed as unpredict-garded as arbitrary in nature. able by any pre-testing in the field, and points to theIn Aust.ralia, the records of the performance of equip- need for reliable tubes.

ments for practically all Australian airlines are cen- Item 3 (tube deterioration) includes those cases oftralized. ANn investigation was recently made into the malfunctioning caused by sub-standard performance ofperformailce of fifteen different types of equipment used tubes, excluding total failure. This item comprises in-in approximately 200 aircraft flown by 19 Australian stances in which the tube performed satisfactorily at theoperators. The sample used consisted of 2,440 units, in last overhaul, but deteriorated in service sufficiently toproportions approximating the whole. The investigation cause noticeable malfunctioning.was made over a period of 90 days, and 384 reports of The failures under Items 2 and 3 are already undermalfunctioning, resulting in unscheduled removal from investigation by the tube manufacturers, in co-operationthe aircraft, were analyzed. A 90-day period was used with airlines. The requirement is for a tube that is im-

because it was,at the

time,the

maximum permissible mune from sudden failure under airborne operating con-service life between scheduled overhauls of most of the ditions, and that remains within specified performanceunits. A few units having a period of less than 90 days tolerances over a predictable period.were discarded, in the interests of uniformity; a few Item 4 (mechanical breakages and wear) includes in-having a )eriod of over 90 days were used which period stances of breakage, excessive wear, distortion, andis disregarded for the purpose of this analysis. Abridged similar mechanical failures. They were found to beresults are shown in Table 1. mainly of random type. In a few instances, a specific

TABLE 1 type of failure constituted a significant proportion ofth e total. These instances were investigated, and cor-

Cumulative Number of rective action was taken.Unscheduled Removals Item 5 (electrical breakdowns other than tubes) in-

Reason fo r Removal At 30 days At 60 days At 90 days cludes breakdowns in components (such as resistors and

Pe r Per per capacitors) other than tubes, provided that the break-|No. cent No . cent No. cent down was judged to be the primary cause of failure. In

1. No fault found 64 2.62 100 4.10 11 8 4.84 this case also, it was possible to take corrective action in2. Unpredictable tube cases of a significant number of failures of one type.

failures 35 1.43 66 2.71 79 3.24 Item 6 (dirty mechanisms) includes instances of3. Tube deterioration 21 0.86 33 1.35 34 1.40 fiueatiual oeg iti ea-otcs4. Mechanical breakages failure attributable to e.g. dirt in relay-contacts.

and wear 42 1.72 70 2.87 97 3.985. Electrical breakdowns THE INCIDENCE OF UNSCHEDULED REMOVALS

other than tubes 22 0.90 33 1.35 47 1.93TH INDECOFUSEULDRMVS6. Dirty mechanisms 3 0.12 6 0.24 9 0.36 When do unscheduled removals occur? An analysis

Cumulative totals 18 7 7.65 30 8 12.62 38 4 15.75 of the reasons fo r unscheduled removals provides_______________- -____ __- - -valuable information as to the action necessary to

2,056 units were either removed at 90 days for scheduled overhaul,or ha d not failed at the end of this period. minimize these events. No less important is an analysis

of the times at which they occur with respect to the timeItem 1 (n o fault found) includes instances in which at which the unit was overhauled. The purpose of a

malfunctioning was reported by aircrew or maintenance scheduled overhaul is to remove any existing faults frompersonnel, but could not be confirmed on test. There are the unit, and to take action to avoid future faults. Iffour possible causes for such removals: (1) erroneous re- this purpose could always be reasonably achieved, thereporting (e.g. no fault, in fact, existed), (2) wrong diag- should be slight probability of failure of a unit of equip-nosis (e.g. the fault was in a unit other than that ment shortly after overhaul, and the probability shouldremoved), (3) incorrect or wrongly applied test proce- increase with time. Such a condition is demonstrated ifdures (e.g. the wrong tests were applied, or the right the number of failures in unit time is either constant ortests were incorrectly applied), or (4) invalid tests (e.g. increasing with time. This has been found to be the casethe prescribed tests do not disclose the fault). In this with the simpler types of units.analysis no attempt was made to determine relative pro- With the more complex units, there is an apparentportions of unscheduled removals attributable to vani- paradox, in which the number o f f ai lu re s in unit time

ous causes, but previous experience indicates that the shows a marked decrease with time, that is to say, mostgreat majority fall into the first two classes, of the failures occur soon after overhaul.




4 I-R-E TRANSACTIONS--AERONAUTICAL AND NAVIGATIONAL ELECTRONICS September

THE PROBABILITY OF FAILURE

Th e failure-rate itself does not give a clear indicationTOTAL

of the probability of failure. A decreasing failure-rate _____does not necessarily mean a decreasing probability of Q.

failure. As units fail, less survive; the failure-rate has tobe related to the units surviving at the time. The most _1_-useful single figure of merit is the probability of failure,

which is a function of the failure-rate divided by the I><0units surviving. This figure of merit has therefore been_ ~~~~~~~wadopted for this analysis. To achieve standardization in _____|__presentation, the figure used is the probable percentageof units failing per day (denoted by P). 0

Equations were fitted to th e curves for the cumulative Inumber of failures, the failure-rate wa s determined by Z

differentiating the equation, after which the probabilitywa s found by dividing dyldx by the number of units _ ;

surviving. For a comprehensive analysis, this method is intime-consuming. In order to save time, more rapidlmethods are now being evolved, which give results ____ \

comparable with those obtained by the more lengthy _ _\ones. Extreme accuracy is economically unwarranted;the trends are more important than the absolute values.

It has also been found that the various types of unit \fall into logical groups, having similar performance DAYS AFTER OVERHAULcharacteristics, so that it has been unnecessary to make

sepaatedetrmintios'fr ech seciic ypeof unit. Fig. 1-Trends of probability of unscheduled removalseparate determinations for each specific type o f u nl t. by reason for removal-all units.

rate of decrease becoming greater with lapse of time.The same question may be asked as for Type 2.

What is the probability at any time of any unsched- Type 4 (mechanical breakages and wear) shows a

uled removal? Fig. 1 shows the trends of probability for decrease throughout the first half of the period, andall units, based on the figures in Table 1. The top curve then settles down to a constant value. It could possiblyshows the probability for unscheduled removal from rise if the period between overhauls were extended.

any cause. It will be observed that the probability is Type 5 (electrical breakdowns other than tubes) de-

least when units are due for scheduled overhaul. For creases to a minimum at 50 days and then rises.some time, there has been a school of thought holding Type 6 (dirty mechanisms) is the only curve havingthe view that scheduled overhauls are too frequent, characteristics that would be normally expected, that isand that the consequent disturbance to the equipment to say, probability increasing with time.does more harm than the overhaul does good. In short, Fig. 2 (opposite) shows the trends of probability forthat overhauls provoke failures. Is it safe to say that unscheduled removal for the various types of unit. Thethese results confirm that view? curves for dynamotors and indicators, control-units,The other six curves in the figure show the trends loops and antenna-coupling units, and selector-boxes

for the six items indicated in Table 1. The curve for (Group D passive networks) have the least probabil-Type 1 (n o fault found) shows a marked decrease with ity, and exhibit normal characteristics, in that the

time. Can reasons be assigned fo r this decrease? It is probability rises with time. The curve fo r main unitspossible that disturbance of the equipment by removal (Group B) also has normal characteristics, but as ex-of units may aggravate the tendency for faults to appear pected, the probability is somewhat higher than for thein e.g. interconnections. Secondly, it is known that there less complicated units. Included in this group are thoseis a psychological reaction by aircrew to disturbance of main units of a simple nature, such as single-frequencynot only the radio equipment, but other features of the transmitters and single-range tunable receivers. Theaircraft. Many pilots ar e apprehensive of performance curve for selector-boxes (Group C), at first, shows aof changed units, whether radio equipment or engines, marked decrease in probability, and then an upwardType 2 (unpredictable tube-failures) tends to rise at trend. These selector-boxes ar e those in which the single-

first, and then shows a marked decrease. Is it possible tube amplifier is part of the selector-box. The top curvethat these faults also are aggravated by disturbance of applies to main units (Group A-the more complicatedthe equipment? types of unit, for example, automatic direction finders,Type

3(tube

deterioration)shows a decrease

inthe and multi-channel

transceivershaving mechanical

fre-probability of failure immediately after overhaul, the quency-changing devices).




1954 Bushby: A Maintenance Plan for Airborne Radio Equipment 5

LO MAIN UNITS -GROUP A w

m1 ;X-i3I lm_____ SELECTOR BO ES-GOUPD___\_-J _ \

4

0 30

X MAIN UNITS-ROULP B

0

s eTOR BOXES GROUP C

z )YN~~IAMOTORS & INDICATORS,.,

downf 1hes unit byLtype of Slr,tecrvsbigDYATROEHUidnife Cincordnc twit Tal 1.CreN n

D

0~ ~~~~~~~~~~~~ NN .OPFQgN Trend ofpoal fushdue eoa

4m ____ EETOR BO ES-GROUP D_____~.0 30 60 90ci, ~DAYS AFTER OVERHAUL

c

Fig. 2-Trends of probability of unscheduled removal by type of unit. byre

As these units (Group A) give the worst performance,a further chart was prepared (Fig. 3) giving a break- 03 09down of these units by type of failure, the curves being DAYS AFTER OVERHAULidentified in accordance with Table 1. Curves 1, 2, and 3 Fig. 3-Trends of probability of unscheduled removalshow characteristics similar to those observed in Fig. 1. by reason for removal-main units Group A.Curve 4 for these units shows a rise similar to that forCurve 5 in Fig. 1. On the other hand, Curve 5 in Fig. 3 (3) P decreasing with time.-The two remaininghas not yet passed the minimum value. There is no sig- curves of Fig. 2 have this characteristic. Although itnificant difference in Curve 6. may be difficult to assign reasons for such a perform-

ance, the data strongly suggest that a worthwhile im-THE l)ETERMINATION OF THE OPTIMUM SERVICE provement can be had by extension of the overhaul-

period by about 50 or 100 per cent.In a foregoing section of this paper, th e reasons for (4) P first increasing and subsequently decreasing.-

unscheduled removals were classified into types, and Although no over-all curve for any group of units hadbrief notes of the corrective action necessary were given. this characteristic, it is clearly seen in Curve 2 (unpre-The time at which corrective action is needed must also dictable tube failures) in Figs. 1 and 3, and indicatesbe determined. It is suggested that the trends of prob- that the probability for this type of failure may be ex -ability for unscheduled removal, shown in Figs. 1-3, pected to decrease with extensions of time.provide useful data for this determination. The aim is to (5 ) P first decreasing and subsequently increasing.-achieve a minimum average probability of unscheduled This characteristic is seen in Curve 5 (electrical break-removal. Five curve shapes are possible. downs other than tubes) in Fig. 1 (all units), and in

(1) P constant with time. In this case, the period be - Curve 4 (mechanical breakages and wear) in Fig. 3tween scheduled overhauls has no effect on the average (Main units-Group A). It would appear that the exist-probability. A substantial increase in the existing period in g overhaul-period of 90 days is a little below optimumfor units hiaving this characteristic is justified. None of for these instances.the units used in the analysis had such a characteristic. Thus it is seen that these data were useful in justifying

(2) P increasing with time.-Five of the curves in extensions of overhaul-periods, and provide a logicalFig. 2 have this characteristic. In such cases, it is neces- basis fo r such extensions. Previous practice in this re -sary to know the maximum permissible probability. If spect was somewhat arbitrary, and had little factualthe probability at the end of the period reviewed is in backing. It had been felt that a large number of failuresexcess of this value, the reduction in overhaul period was sufficient justification for a reduction of service-necessary will be obvious from the curve, time between overhauls, and vice versa. It was assumed

Although no quantitative figure has yet been se t for that the average probability would be reduced by moremaximum permissible probability, extensions of over- frequent overhauls. It should now be clear that this ishaul-periods of two to three times seem justified fo r not necessarily true. In no case could a reduction in

units comprising the lower four curves. For main units overhaul-period be justified by the data gathered in this(Group B), extensions of about 50 per cent were made. investigation. However, it was realized that the exten-




6 I-R-E TRANSACTIONS-AERONAUTICAL AND NAVIGATIONAL ELECTRONICS September

sions should be regarded as exploratory, and subjected (5) Aircraft checks-these are identified by num-to review as soon as sufficient data became available on bering from 1 to 14. They cover all requirements forperformance throughout the extended overhaul periods. nonremovable parts of the equipment. This work was

Australian practice is to base al l unit overhaul-periods previously done at one time, and involved groundingon elapsed time. As the aircraft involved differ widely the aircraft for some days. Dividing the work into 14in flying-hours per unit time, and nature and region of separate checks largely avoids this necessity. All butoperations, it was found undesirable to use flying-hours two of th e checks can be done at overnight stops.

as a basis. Trouble was experienced through equipmentbeing left too long in aircraft having comparatively little BENCH MAINTENANCE

use, particularly in tropical areas. It has been fairly well No overhauls, in the restricted sense of the word, ar eestablished that, in such areas, idle equipment de - done in the aircraft. Units faulty or due for scheduledteriorates more rapidly than that in frequent use. There overhaul are replaced, and all subsequent work is donewere other reasons of a nontechnical nature f or t hi s de - at overhaul bases. Overhauls are divided into fourcision. Some radio inspections and checks done in the classes.aircraft are still based on flying-hours, for the reason (1 ) Intermediate overhaul-the intention of thisthat checks other than radio use this basis. The aircraft overhaul is to check that the unit is within requiredis therefore available at such times. performance tolerances, and if so, to disregard the pos-

Permissible periods of shelf-life have been recently sibility that it might be made to do better. It consistsintroduced. Future analyses will include investigation of a general physical inspection and tests on the more

of possible deterioration of unitsheld as

replacements important characteristics of the unit, e.g. sensitivity ofunder conditions of normal storage. receivers, power-output of transmitters, and calls for no

MAINTENANCE PROCEDURES further action if the requirements are met. There areexceptions to this principle in th e case of mechanical de -vices, where additional requirements such as lubrication

It is well established practice to keep maintenance in and checking of brush-wear may be necessary.the aircraft to a minimum. As far as possible, units are (2) Complete overhaul-the intention of this over-

readily removable from the aircraft, and are replaced haul is to restore the unit to "as new" condition as farwhen faulty. There remains a need for inspections and as possible. Practically all characteristics of a unit are

qualitative tests on the equipment at comparatively checked, adjustments such as circuit-alignment are

frequent intervals, and for less frequent checking of called up , and certain parts of the unit are more closelythose parts of the equipment that are not readily re- inspected and re-conditioned if necessary.

movable. As the recent investigation did not cover these (3) Overhaul after repair-the intention of t hi s over-procedures, they will be briefly outlined, without at - haul is to check any repair work done on faulty units.tempt to justify them, other than to say that they seem The requirements for repair work will vary so muchto satisfy requirements. with circumstances that the procedure is usually left

(1) Routine inspection-a physical inspection and to the discretion of the technician, although guidancequalitative check of all readily accessible radio equip- can be given in some cases. The subsequent tests shouldment. It is normally done daily on aircraft in the public- be called up in detail, and it is not unduly difficult to

transport category. do so.(2) Periodic inspection-a physical inspection of less (4) Special overhauls-requirements arise from time

accessible units, e.g. those under floor-boards, and a test to time for overhauls outside the normal maintenancefor noise-interference. This inspection is done at each plan. Changes in frequency-assignments may have to

100 flying-hours or so, to coincide with an aircraft be made within certain time-limits, and cannot wait for

inspection when e.g. floor-boards are normally removed. the next scheduled overhaul. In such cases, special over-(3) Pre-departure inspection-this is required if the hauls are called up.routine inspection has been done some hours before de - The provision of a relatively infrequent completeparture. This inspection is distinct from the pre- overhaul, with intervening performance checks (inter-departure inspection by aircrew, and is intended only to mediate overhauls), meets requirements for scheduledensure that there has been no unauthorized or inad- overhauls. The need for repairs occurs at random times,vertent interference with the radio equipment during and although specific procedures are not usually pre-the period between routine inspection and departure. scribed, the subsequent tests should be. If provision is

(4) En-route inspection-this is made at airports of then made for special requirements, the skeleton plancall enroute, and consists of a check with aircrew on for bench overhauls is complete.previous performance of the radio equipment, and for The results of the analysis given in preceding sectionspossible damage, e.g. during refueling, to units external of this paper can be put to good use in drafting the re-to the aircraft. quirements for the different types of overhaul.




1954 Greenslit: A Thunderstorm Avoidance Radar for Civil Aircraft 7

RECORDS with information from the field. Field staff are inclined

Having devised a plan, and set it in motion, the job to the view that there is "too much paper-work." Theycan by no means be regarded as finished. It must be re- feel that they would be better occupied by "getting on

viewed from time to time: dead wood must be cut out, with the job," than in spending so much time in keepingadditions and changes may be necessary. A system of records. As in other spheres, a balance must be struck

recording equipment performance must be devised in between two apparently conflicting requirements. It isorder to give the information necessary for such re- agreed that paper-work should be minimized, but no t at the

views. It must be centralized, therefore it must be fe d expense of adequate records.

A Thunderstorm Avoidance Radar forrI *1A *Civil Aircraft

C. L. GREENSLIT*

INTRODUCTION terrain such as that found in the midwestern part of the

D ESIGN OF A commercial airborne radar system country, where few distinctive radar targets are found.

poses many unique and interesting problems. Fortunately, other solutions to the navigation problem

Many of these stem from the extreme versatility have been found which recognize no such limitations andof radar equipment which effectively forms a third such systems as the LF ranges, VOR and DME are now

supereye for the pilot. That this new eye can "see" great in use. Where available, these aids should certainly form

distances through fog or rain is indeed marvelous, but the primary navigational system. To operators in areas

the popular aura which surrounds radar recognizes little where such aids are not available, radar may well form

or no limit to its abilities. The military, by necessity, an important if not primary navigational tool.must optimize the radar design for specific application, In addition to its ground-mapping function, a rela-although th e resulting complications may be many. The tively simple ground station and the airborne radar may

commercial aircraft operator, on the other hand, cannot combine to form a Racon or Beacon system which, inafford such luxury, and a radar designed for this use effect gives VOR-DME information. When interro-must havse characteristics weighted by the operator's gated by the radar, the ground station replies in an

problems in order and magnitude. identifying code that appears on the radar scope at therange and azimuth of the ground station.

AIRBORNE RADAR AS A SOLUTION TO FLIGHT PROBLEMS A second problem in aircraft travel is that of obstacles

In order to assess the value of radar as a flight tool it in the course of flight. In this category we find such

is first necessary to examine some of the problems asso- things as mountains, other aircraft, and storms. The

ciated with this modern form of transportation. Navi- avoidance of high terrain may be easily accomplishedgation is, of course, a major factor. In the early days of with radar and a so-called safety-circle technique. To

flight, the pilot navigated by visual contact with the utilize the technique, the scanner is tilted down a pre-

ground and identified landmarks. Commercial aviation determined number of degrees and, with relatively

of today, with its requirement of providing all-weather smooth ground below, radar, reflections from the lowerservice on schedule, must, of necessity, turn to other edge of the beam are shown as the inner circle on the

means of navigation. At first g la nc e, r ad ar , with it s Indicator. Should points of high terrain be encountered

ability to penetrate the night or intervening clouds, in the flight path, reflections from these will move inmight seem to be a solution. Under certain conditions it toward the sweep origin on the Indicator as the plane

may even give the pilot a picture of usefulness ap- approaches. By assigning a minimum clearance altitude

proaching that of visual contact. For aircraft flying over and the necessary range for executing evasive maneuverswater, coastlines and offshore islands in general present in case clearance cannot be attained, and knowing thea di.stinctive radar dislSp ay. antenna beamwidth and tilt, a range circle on the indi-The major diffculty is using radar as a substitute for cator may be calculated by elementary trigonometry

the eye in contact flying occurs when flying over level which is the safety circle. If the display target circle, orany individual targets come within this safety-circle

* Bendix Radio, Baltimore, Md..ra n g e mark, less than minimum clearance is indicated.

a maintenance plan for airborne radio equipment-f4w

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