AC 90-79 DATE 7/14/80
ADVISORY CIRCULAR DEPARTMENT OF TI~ANSPOIITATION Federal Aviation Administration Washington, D.C.
FAR GUIDANCE MATERIAL Subject: RECOMMENDED PRACTICES AND PROCEDURES FOR THE USE
OF ELECTRONIC LONG-RANGE NAVIGATION EQUIPMENT
1. PURPOSE. This advisory circular presents recommended operational practices and procedures for the use of electronic long-range navigation equipment in oceanic or remote land areas.
2. RELATED FAR SECTIONS. FAR Ol.l(b), 91.20, 91.75, 91.123, Appendix C of 91, !21.103, 121.355, 121.389, Appendix G of 121, 123.27, and 135.2.
3. RELATED READING MATERIAL. Appendix 1 contains a list of related publications and inf,*rmatinn-on how they may be obtained.
4. BACKGROUND.
a. Recently, many domestic operators have received Civil Aeronautics Board (CAB) Ecoilomlc Authority to operate over international routes.
Additionally, the standards for navigation have been upgraded in a large portion of the airspace over the Sorth Atlantic to accommodate the increasing air traffic. Similar standards are under consideration for some airspace over the Pacific. Seu navigation equipment is presently being introduced into the aviation community to meet these requirements and to serve dS a replacement for Loran A which is scheduled for a phased shutdown through 1980. These factors, as well as others, indicate that effective navigation practices and procedures must be applied by all operetors to ensure that a high level of safety is maintained. The recommended operational practices and procedures presented in this advisory circular have proven to be an effective means of attaining successful navigation..
Initiated hy: AFO-2b0
AC 90-79 7114180
b. It is recognized that many methods of navigation exist, and that various types of electronic navigation equipment are available. However, the general trend of equipment development and user choice has been toward automated equipment. This circular will address three of the most commonly used types of long-range navigation equipment: Doppler, with update means, Inertial Navigation System (INS) and Omega. Some procedures or practices are applicable to all three types of equipment. These common recommended procedures are presented first, followed by recommended practices and procedures unique to each of the three types of equipmdnt. Recommended practices and procedures are also presented for the resolution of various navigational difficulties.
5. DEFINITIONS. For the purposes of this advisory circular, the following definitions apply:
a. Gateway. A specific navigation fix where a flight transitions from short-range (airways) navigation to long-range (oceanic) navigation or vice versa. Such fixes ordinarily are airways navigation facilities (VOR, NDB, etc.) located at or near oceanic airspace entry/exit points.
b. Long-Range Navigation. Flight operations outside (beyond) the published service volume (range) of short-range (airways) navigation facilities (i.e., VOR, VOR/DME, NDB).
C. L+‘:*id Reckoning (DR) . A method of directing an aircraft and estimating Its position by the application of time, direction and speed data to a previously determined position.
d. Cross-Check. This is a term for methods used to carefully monitor and verify navigation information so as to ensure compliance with a currently effective Air Traffic Control (ATC) clearance. The keystone of successful navigation is effective cross checking procedures. In order to minimize the exposure to human error, each manual insertion of input data into the navigation system should be carried out in its entirety by one crewmember and then recalled and verified by another. Cross checking procedures should include:
(1) Verification that all data inserted into the system is correct;
(2) Verification that thp systems are operating properly;
(3) Confirmation that system controls and switches are properly configured to provide the desired information; and
(4) Frequent confirmation of navigation performance to ensure that the ATC assigned track is maintained and that the information presented by the systems remains reasonable.
e. Waypoint. A geographic point specified for navigation, flight planning or air traffic control purposes. For automated navigation systems Par 4
(i.e., INS, Omega), a waypoint is usually inserted as latitude and longitude coordinates. For semi-automated navigation systems (i.e., Doppler), a waypoint is usually inserted as a bearing and distance fron a previous position.
6. GENERAL NAVIGATION PRACTICES AND PROCEDURES. Experience indicates that the increased accuracy and reliability of modern automatic navigation system:: can induce a degree of complacency in the opet-rotor which may result in the failure to routinely cross check system performance. Under these circumstances, human errors may remain undetected for excessive periods.
Although navigation errors are infrequent occurrences, human errors have accounted for a majority of the errors attributed to aircraft equipped with automated systems. Most inadvertent navigation errors have occurred when the equipment was functioning normally. The operating procedures prescribed were either inadequate or were not followed. A common error associated with automated systems is incorrect programing of the oceanic waypoint latitudes by multiples of one degree (60 r,autical miles {llm)). In an organized track system, this can result in the flight maintaining a wrong track with high precision and thereby constituting a serious threat to other aircraft properly occupying that track and flight level. Vigilance and diligence in pi-operly applying established procedures are essential ingredients of safe oceanic navigation. Although operational procedures (checklists) may differ slightly between specific navigation systems, many good 1 lctices and procedures av:e basic to 111 automated and semiautomated systems. These basic practices and procedures are presented in this paragraph.
a. Preflight Planning. One of the basic fundamentals of good navigation is prior planning. Aside from planning to ensure successful navigation under normal circumst;lnces, the successful resolution of many in-flight navigation difficulties depends to a large degree on thorough preflight planning. Some of the more important factors are addressed below:
(1) The Navigation Flight Plan. Since many op?rntors use a computerized navigation flight plan, this planning task has been greatly simplified. Powever, care should be taken to verify that all en route waypoints are correctly and legibly shown on the flight plan. Also, it is desirable to select a waypoint loading sequence and number each waypoint accordingly. If more than one copy of the flight plan is to be used by various crewmembers, designate one as the official copy. To eliminate possible confusion, ensure that all necessary information (i.e., routing changes, estimated times of arrival, waypoint loading sequence) is recorded in this flight plan and this copy is used for all reports to ATC.
Additionally, if the flight is within the North Atlantic Organized Track Structure (OTS) , obtain, a copy of the current track message (ATC expects the flightcrew to have a copy) and be alert for a conflict between the flight plan aqd the track message. Track messages are issued periodically and describe the North Atlantic routes, gateways and flight levels available ior eastbound and westbound flights during the period indicated in the message (usual.ly 12 hours).
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(2) Weather. In addition to the normal review of weather information concerning terminals, be alert for hazardous weather en route which may require a flight plan change or in-flight rerouting. It is important to obtain a copy of the wind flow chart (constant pressure chart or equivalent) for your flight level and route since this information may prove valuable when evaluating wind forecasting “errors” and if DR operation should be necessary due to equipment failure. It is desirable to plot the route of flight on the chart to increase its usefulness. Also, as the flight progresses, consideration should be given to plotting actual wind information on the chart a5 a means of evaluatlng the accuracy of the forecast.
(3) NOTAhS. Besides checking I!lbtlces to Airmen (NOTAMS) fur facilities at departure, destination and alternate airports, NOTAMS concerning navigation aids or special airspace restrictions along the planned route of flight should be checked. Omega users should obtain NOTAMS concerning Omega station operational status to ensure that the required stations are in service. The Airman’s Information Manual (AIM) contains additional instructions for obtatning infonn&tion concerning Omega.
(4) Equipment Preflight.
(1) In addition to operating procedures (checklists) to confirm proper system operation, care should be taken to ensure that the navigation equipent is properly propramed. This is a very important procedure which should nat be rushed. All navigation information (coordinates or courses ar.d distances) should be programed by one flight crewmember and verified for correctness by another. Also, verify that the same waypoint loading sequence is used for each system. At this time, it is desirable to indicate on the flight plan that the present position (if applicable) and waypoints have been entered and crosschecked. If time becomes a factor, it is more important to verify that the first 2 or 3 waypolnts are correct than to rush through the procedures to “insert” as much information as possible. Consideration should be gil:en to using another cross-check which compares the flight plan or charted distance between waypoints and the distance computed by the navigation system to detect progr;r,ing or f 1 ight planning errors. This serves as a double check on waypoint verification and will also reveal an error in the fl iaht plan should one exist. A difference in distance of more than + 2 nm may indicate - a programing or flight planning error.
(ii.) It Is advisable not to transcribe waypolnt coordinates from source documents (track message, en route charts, etc.) onto the flight plan for insertion into the navj.gation computers. In order to avoid errors in transcription, waypoint coordinates should be inserted into the computers directly from the source docllmenrs.
(iii) Since the initial stages of the flight can be very busy, consideration should be given to ensuring the navigation system waypoint transfer switches are placed in “auto” to facilitate outbound tracking and waypoint changeover during this period.
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7114180 AC 90-79
(iv) With systems such as INS or Omega, which navigate during ground operation, it is advisable to cross check present position, taxi distance or groundspeed, as appropriate, prior to takeoff to confirm proper system operation and to ensure that the present position remains reasonable.
b. En Route - Within Range of the Olltbound Gateway. Flights should not cc..Llnue beyond the outbound gateway unless the required long-range navigation equipment is functioning properly. To confirm proper operation, certain cross-checks should be performed while within range of the gateway navigation aid. Since this may be the last positive positton cross-check until the inbound gntewsy, Lhe following practices may also provide valuable information for resolving any later navigation difficulties:
(1) ATC Clearance. All ATC ocennic clearances should be cross checked by two crcwmembere to ensure the clearance is copied correctly. Any flight plan waypoints w!lich may have been rcviced in an ATC clearance should be crossed out and the revised coordinates entered in a legible manner.
Prior to proceeding outbound from the gateway, the currently effective ATC clearance should bc coppared tcJ the flight plan and the information in the navigation computers for the gateway and the subsequent waypoints should be vertfied.
(2) Cross Error Cheek. A Itross (xt-ror check is ;I position accuracy cross-check, using normal ai rwny foci li tics such as VOR, VOK/I)ME or NIIR.
The ‘gross error check is 11sual1y accomplished by flying directly over the’ gateway (if possible) nnd s~lhsc~qucnt ly cstahl ishing the a1 rcrnft on the outbiound course rosin): the entcway n;~vi~:;lt 1 on aid. ‘I’lli s (.heck sr rves the Ion which c;ln bc rlscd to dc~terminr which stecrfng reference.
followln~ purposes:
(i) Dctccts cat-rors wh information since takeoff.
(ii) Provides informat system is most ilCCllriltt2 for llse <as a ( i i I ) Provitics an opport\lnity to correct posltlon informntion, 1 f ncccsr;ilry.
(iv) Can bc used LO collflrm that the aircraft Is cst;tbllshc~d on the outbound course ;Ind Is t rackink: toward ttlc next waypoint, (v> Cilrl hrb used to confirm that the nlrcraft is proceeding vln the cclrrcntly cfiectivc A’I’C cltnrnnrr.
(3) “RndiolNav” Switches. In cases where flight instruments are used for the display of cithcr airways (VOR) information or information from the long-range navigation system, the “K;tdio/Nav” swf tcltes should be left in Par 6 5
.
AC go-79 7114180 the “Radio” position after passing overhead the gateway navigation aid until the radio information begins to become degraded. The switches should then be placed in the “NAV” position.
(4) Compass Ceviation Check. Consideration should be given to performing a ‘compass deviation check on systems, such as INS, which use true heading information from sources independent of the aircraft compass system.
The compass deviations can be determined by comparing the INS derived magnetic heading to each compass heading. This information may bc used later in the flight to determine the most accurate system should il divergence between systems occur. The compass deviations can bc appli(ad LO the respective compasses to determine ttlc actual magnetic head1 ng. I,ocal variation can be applied to the true heading of each INS to obtain tII(b derived magnetic headings. The most accurate INS strould be t.lle ()I:(’ wi ttr ;I magnetic heading which compares favorably with the actunl magnrt i(. h~~ildi~~~.
C. En Route - After Passing tl~c Cntcway. The fol lowin); l)r;lct I (‘es should be used to monf tor flight progress and confi rm tlint tllc assi ~;nc~tl route of fligh; is maintained:
(1) The system dc~tcrmincd to bc most acc~lrat(~ duri 112 tI;(h gross drror check should usually bz sctl acted as ttlc il\ltopi lot stccari I-I~ rr~fc~t-c~n(~t~.
(2) Whenever posst t-11(,, ttle sys I tarn d(~si~;na t cd as ii st c!e r I nq reference should rout i ncly display
(3) AnotI.er system stloillti crosst rack (XTK) i nf or*1 ilti OII.
(4) Crosstrack (XTK), dist, (TKE), if ~~~ililbl~, should be period!c;~ 1 ly cross checked to conf i t-m ttliit trdck centerline is maintained.
d. En Route - Approaching Each Waypoint. Wtjcn wi tilt ri approxl 1112 t cal y t WC, minutes of each waypoint, ttre following practices should b(l tlsc!d:
(1) Both pilots should verify that the subscquc’nt waypoint in tI1(3 navigation displays agrees with the currently effectivcs A7’1: clra;lt-;inc-(b.
(2) Position information on the navigation displays st,ould be recorded and compared with the currently effective ATC clenran(*c in ordc!r to ensure that the aircraft is where it is supposed to be.
e. En Route - After Passing Each Wa,, Int . Within sevcrn after passing each waypoint, the following practices should be
(1) Confirm that the navigation systems have swirchcd flight segment (leg change).
1 mi n..t t’s used :
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7114180 AC ‘:0-73
(2) Confirm that the aircraft is tracking along the next flight segment (tracking outbound).
(3) Approximately 10 minutes after passing C?il(Yh waypoint, the present position informatior on the navigation displays should he plotted on n nnvtgntton chart to confl rm that the ATC clc~nrance is satisfied (not appl icnble to most Doppler systclmns).
f. En Rolrte - Approaching rhc Inbound Cntcwny. Ccartaiii preparations -- should he mndc for the transition from long-range nnvigatton to nirwnys n;iviKat ton. The following practices arc rccommcndc~d:
(I ) AS SOOI~ ;Is feasj hlc, set \I,> tll(b Ililvi)<iltl(J~~ ratiios to rcccbivc t IIcb t IlboIrIlti g:llc~Way IlilVt&J,;lt ion ilid.
(2) WII<~II tl~c gat.~w;~y navigaticjn ;iid is ])rOVidinE r-cl iahlcl i nfot-mat Ion, IJ~;ICC tl~c. “R;~dio/Nav” swl t(:tl in thl, “Radio” posit ion and steer t Ilc* aircr;lft so i1S to ;ic’clllirc! ;~nd ma I nt;c i ii t. Iich prol)c’ r i nhcjund radia I/hcnring.
(4) WIlcn ovc~rll(~;rd t 11~1 ):;~t cw;ry, rc%c~cJrtl thts posit I on i nformilt i on f ram t tIc’ llilVi)~iltiO11 diSplilY>;. ‘I‘hl s i nform;it i on c;111 b(b viscid to c.onf I rm system a(-c~lr;lcy. it Ls r-c%<-ommentlc~d t II;I~ :;y:;~t*~ *~(‘(.llr;r(~y (~cJmp~lt:~t ions hc m;tde after ,~rriVii1 I0 ;~void (.~nflicts wittl ottlcat- (,(J(.klJi~ tl\~t it’s tltlt-i~l>: ~IIP critical pc~riods of clc~scc~rlt , ;i1)1Jro;i~‘tI .III~ IantI I~);.
I: * After Arrival. ‘II I C’ i1idivid11;11 n;~vi);;it Ion sysLc>m c’rrors iIlld error -___---- r;i t (2s , i f appl icnhlc*, stlcJill(1 h(b <.ornp\~~c’d ;1n(1 rc~corclc~d lfor fllttlrc’ rcafcrcncc.
It is clc~sl r;lhl(b to rcs(,or-d ttlis informat io11 in ;I doc~~lrn(~llt wh 1 c 11 rcxrna i ns ;I ho;] t-d t tl(a ;ii rc.r;il L Lo I)t-“vi (I(, :;\lhschcllicrit 1’1 I>:lltcrc~ws with il recent history of systcam pear1 orin;lnc(~. ‘l‘h i s i II f orma t i on m,~y hex used wit Ii most systems to prc~d 1 c t i ntl i V id11;l 1 sysl.c~Iii p(‘r fOrlllilllC(l for flltlrrc f 1 ights under similar (‘i rcilmstancc~s. Atldition;~lly, ttlis informiltion may prove valuable tr suhsequc~nt f 1 i);tlt.crc!ws for rtbsolving navii:at ion ahnormalitit~s, such <,s a d i vr>rl;c!nc(’ hcacw(bcan syst elms.
7. SPECIAL I’RAC’I’I (:E;S ANI) I’t~OCISI)lIHES - I)f~I’PI,I:R. In addition to tllc general pract tress and proccd=cas (:(Jntil incd in paragrzh 6 , thca following information ;lppl Le.5 to Doppler llilVi~~~ltlOll systems. Since a Doppler system (sensor plus computer) is ;I semi-a~~t(J:nilt ic. dcnad reckoning dcavicc which is 1~‘~s accurate tll;ln ;II~ INS or Omcag:;I sysLcm, ;I me;ins of llpdat ing tllc Doppler is usually required if ncccptahlc [Josition ;Iccur;lr:y is to bcx achieved on long-range flights. INS, Omcagn or I,ORAN (: nl;ly bc I~scd as the updating reference for the Doppler system. l’tlcb followil~g f;ictors should he considered when using Doppler- navif;ation systems:
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AC 90- 79 7/14/80
a. Compass Accuracy. Most Doppler systems measure groundspeed to an accuracy of about one percent and drift angle to a fraction of a degree.
Its directional reference, however, is the aircraft’s compass system. If the overall Doppler/compass system is to be usefully accurate, the compass should be swung and compensated so that its error does not exceed one degree on any heading.
b. Preflight.
(1) During preflight, the flight plan courses and distances for those flight segments where Doppler navigation is required should be verified. Normally, the courses should be determined to the nearest one tenth of a degree and the distances to the nearest nautical mile. This is routinely accomplished by using course and distance tables designed for this purpose. Care and accuracy are important factors during this cross-check.
(2) If the Doppler system is to be used for navigation from takeoff, both “A” and “B” stages should be programed and the “auto/manual” switch should be placed in “auto.” Also, the proper position for the “land/sea” switch should be determined since this affects the accuracy of the groundspeed information.
C. When Approaching the Outbound Gateway.
(1) The Doppler system performance records for recent flights over similar routes should be reviewed to determine If a system deviation correction should be applied. If the records indicate that a deviation correction may be necessary, apply the corrections to the Doppler systems used.
(2) Both pilots should verify that the outbound course and distance programed in the active stage conforms to the currently effective ATC clearance.
(3) Unless required otherwise by ATC, the aircraft should be flown directly overhead the gateway fix to obtain the most accurate starting position practicable. When directly overhead the gateway, both pilots should ensure that the Doppler computers have been activated and that the proper stage is selected.
(4) The aircraft should be established on the outbound track by using the gateway navigation aid. Once this is accomplished, the gross error cross-checks discussed in paragraph 6 should be accomplished.
(5) Consideration should be given to using an additional cross-check. This is accomplished by applying drift angle to compass heading and comparing the result (actual track) to the flight planned magnetic course.
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.
d. Updating the Doppler Computer.
(1) Since Doppler systems (in the magnetically slaved mode) fly a ‘I rhnmb line” (curved) track and most navigation charts commonly used reflect “great circle” (straight) tracks, certain precautions should be observed when updating Doppler systems. Although a grea’ circle course and a rhumb line course begin and end at common points, a divergence between the two courses exists between the waypoints. This divergence normally reaches a maximum near the midpoint of the leg and the magnitude of the divergence increases as the latitude and the distance between waypoints increases.
(2) Under normal circumstances, position fixes for Doppler update purposes should be obtained within 75 nm of a waypoint to minimize the possibility of inducing an error into the Doppler system due to the “rhumb line” effect. This practice should be applied to both manually obtained and automatically obtained position fixes.
(3) When Doppler systems are used in the grid (free gyro) mode, the Doppler track will approximate a “great circle” and “rhumb line” effect is not a factor. Under these conditions, the updating restrictions previously detailed are not normally applicable.
8. SPECIAL PRACTICES AND PROCEDURES - INS. In addition to the general practices and procedures in paragraph 6, the following recommendations apply specifically to INS:
a. Preflight.
(1) Since INS is a dead reckoning device and not a position fixing device, any error induced during alignment will be retained and possibly incremented throughout the flight unless removed through updating procedures. Tnerefore, during preflight, care should be exercised to ensure that accurate present position information is “inserted” into the INS. Although most INS will automatically detect large errors in preserl position latitude during alignment, large errors in present position longitude may exist without activating a warning indication. Therefore, when cross checking present position coordinates, be alert for the correct hemispheric indicator (i.e., N, S, E, W) as well as the correct numerical values. Since most INS cannot be renligned in flight, special procedures, such as ground realignment, may be required to correct a significnt error in present posit ion. If the INS in use have the capability of “gang-loading” (simultaneous loading) by use of a remote feature, care should be taken so that any data entered by this method is cross checked separately on each individual INS to detect data insertion errors.
(2) The INS software identification and modification status codes should be verified to ensure that the proper equipment is installed and the appropriate operating checklist is used.
AC 90- 79 7/14/80
(3) The operating checklists should include a means of ensuring that the INS are ready to navigate and that the navigation mode is activated prior to moving the aircraft. Any movement of the aircraft prior to activating the no ‘sation mode may induce very large errors which can only be corrected by ground realignment.
(4) After the system i6 p!aced in the navigation mode, the INS groundspeed should be checked when the aircraft is stationary. An erroneous reading of more than a few knots may indicate a faulty or less reliable unit. If this occurs, a check should be made of the malfunction codes.
b,. In-Flight Updating. Since INS are essentially accurate and reliable, there is the possibility that in an attempt to obtain a slight improvement in accuracy by in-flight updating, a human mistake may result in a more serious error. On the other hand, INS errors generally increase wi ttr time and are not self-correcting. Therefore, if large tracking errors arc‘ permitted to occur, aircraft safety and separation criteria may be qlgnificantly degraded. The aforementioned factors should be considered in an ’ decision relative to in-flight updating. As P guide to flightcrews, some operators consider that unless the ground facility provides a prccisc (~ll~~ck, and unless the error is fairly significant (e.g., more than 6 nm or 2 nm/tlr), it is preferable to retain the error rather than update.
9. SPECIAL PRACTICES AND PROCEDURES - OMEGA.
a. Since the control/display units (CDUs) of most Orncga systrbms at-c similar in appearance to those used for INS, persons familiar with INS may have a tendency to assume that Omega hils slmi Iar performanc(a c.tl;rr;lcIcrist I(‘:;.
This assumption could create significant problems. INS is a prec I si on tic:;~d reckoning (DR) device which is wholly self-contained wlthln tlje al rc.t;lft and has a nominal position degradation of about one mile per hour of fliK:ht.
Omega, in contrast, con?inuously resolves aircraft position by proccbsbi 11~ radio signals received from a global network of transmittcirs. It is therefore possible for Omega to be affected by signal propagation disturbances and abnormally high local radio noise levels.
b. In normal operation, Omega provides a position accuracy of I to 3 miles which, unlike INS, does not degrade with incrcnsing flight timr,.
However, most Omega systems compute position in signal “lanes” wh!ch arc.‘ ;I function of the signal wavelength. A disturbance of sufflcic,. I magni tudc may force the computed position into an adjacent “lane” and thereLy (:ilklsc <III error which is measured in Ilultiples of the basic “lane” width. T!ti s occurrence is termed a “l.ane slip.” Most Omega systems possess an auxiliary operating mode termed “Lane Ambiguity Resolution” (LAR). The purpof;e of this mode is to correct the “lane slip” by returning the present posit I on to the correct “lane.” A more detailed description of OMEGA lane ambiguity IS provided in ICAO Circular 139 - AN/95, Aviation Use of OMEGA.
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c. Whereas INS position errors normally accrue gradually with elapsed flight time, most Omega errors occur suddenly and are usually multiples of the basic lane width. Effective cross checking procedures should be accomplished at regular intervals and LAR or inflight updating should be initiated when the position accuracy is in doubt. In addition to the general practices and procedures contained in paragraph 6, the following recommendations apply to Omega systems:
(1) Preflight.
(11 He alert for any NOTAMS affecting the operational st;1111.s of the indivi.iual Omega transmitters; particularly for possible iIbnOrln;il operation. Deselection of any station reported to be in ahnormal opcriltion should be considered at the onset of the flight. Also he alert for any NO’I‘AMS relating to propagation disturbances, such as Sudden Ionospheric Di stllrhances, Sudden Phase Anomalies, or Polar Cap Anomalies, whf ch m;ly affect Omtlga positioning accuracy. Scheduled Omega status brondc,asts on st;l t i on WWV stlo\lld be monitored as a means of obtaining current Omega inform;ition.
(ii> The OMEGA software and modification starl~s c,odes shollld be verified to ensure that the proper equipment is illstdl!cd ilnd ttl;lt ttj(a appropriate operating checklist is used.
(iii) At certain ground locations, partfcul;lrly at c:onp,:ested terminals, abnorma 1 ly high radio noi SC levels may adversely affect th(a Omega. For example, synchronization may take longer than norma 1 or ttrc.
inserted ramp coordina:es may drift after insertion. Synchronization or I)K warning lights usually indicate thi s situation. This prohlcm normally disappears, if the Omega equipment is scrvlccnhle, SilrJrtly after the switcn to aircraft power or after the aircraft is movrad from the gate. (:a rc’ stlould he cxorclscd during taxi, since abrupt turns may cil\lse a momcnt;iry loss <If signals which could affect system accuracy. It is good pL<dctice t-o cross check present position coordinates or taxi distance before t;lkcsof f to detect any errors which may tlave occurred sinc.c initializ;ltion.
(2) Tn-Flight llpdnting. The snmc consider-at ions b;lsic to upd;lt i 11):
an INS also apply to Omega d\lc to the normal accuracy iind t-c21 iahi 1 i ty of t hesc systems. flowever, in addition to the capability to update ovcrtlcad ;I navigation aid, most Omega systems are capable of performing ;r I,hR if certain signal strength and stiltion geometry requirements are met. linlr~ss n n apparent Omega error exceeds approximately h nm, ;i Inns slip mny not necessarily have occurred and LAR or \lpdat ing is not normally recommended.
If an LAR apnears to he necessary, ttle I.AK should he tni tiated on only one’ system at a time so that the other system rc*mains unaffected for use as a cross-check. The LAR should he attempted first on the system believed to be the least accurate.
10. NAVIGATION DIFF’CLJI.TIES. Although the accuracy and reliability of the newer automated naviGtion systems are excellent, malfunctions and failures occasionally occur. When a malfunction occurs, crews should guard against jumping to conclusions since hasty actions are seldom necessary and may Par 9 11
AC 90- 79 7114180 further complicate the situation. Experience has shown that successful resolution of navigation difficulties in oceanic areas usually requires a thorough, tlioughtful process which normally begins during preflight planning. The following guidance is presented for consideration when navigation difficulties are encountered or suspected:
a. Detection of System Failures. In general, system failure is usua considered to have occurred when one or more of the following situations develop:
(1) A warning indicator is activated which cannot be reset.
(2) Self-diagnostic or built-in test equipment (BITE) indicates that the system is unreliable.
(3) The position error overhead a known geographic position exceeds the maximum permissible tolerance established for a particular navigation system.
(4) The system operation is warning or malfunction indications, longer useful for navigation.
so abnormn 1, despite the absence of that the crew considers the system no
b. Detection of System Degradations or Malfunctions. While system failures are usually straight forward, malfunctions or gradual system degradations usually are more difficult to resolve. This is particularly true when only two systems are on board. Navigation difficulties of this type are usually detected by a divergence between the navigation systems which often occurs gradually. This factor may reduce the possibility of identifying the faulty system unless periodic cross-checking practices are diligent Ly used. The following f?.ctors should be considered when attempting to identify a faulty system:
(1) Check the malfunction (BITE) codes for indications of system fault.
(2) For Omega, the system receiving the most stations and the best quality signals should generally be the most accurate.
(3) Review the gateway gross error check for indications of the most accurate system.
(4) If a regular record of system performance has been maintained and is available, a review of the record may give a clue as to which system is faulty.
(5) If possible, use VOR, ADF, DR, airborne radar, or other navigation aids to obtain a position fix.
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