deviation outside the assigned airspace creates a flight safety hazard. In such cases, the aircraft has failed to navigate to the degree of accuracy required for ATC and has failed to comply with Title 14 of the Code of Federal Regulations (14 CFR) and International Civil Aviation Organization (ICAO) requirements. In a non-radar environment, ATC has no independent knowledge of the aircraft’s actual position or its relationship to other aircraft. Therefore, when a deviation from an agreed upon clearance occurs, it seriously downgrades ATC’s ability to detect a navigational error and resolve collision hazards.
e. Concept of Navigation Performance. Maintain the concept of navigation performance that involves the precision for both the assigned route and altitude by an aircraft operating within a particular area. Navigation performance is measured by the deviation (for any cause) from the exact centerline (CL) of the route and altitude specified in the ATC clearance.
(1) Errors and Flightcrew Competency. This includes errors due to degraded accuracy and reliability of the airborne and ground-based navigational equipment and the flightcrew’s competence in using the equipment. Flightcrew competence involves both Flight Technical Errors (FTE) and navigational errors. FTE is the accuracy with which the pilot controls the aircraft as measured by success in causing the indicated aircraft position to match the desired position.
(2) Impact of Traffic Density on Navigation Performance. Standards of navigational performance vary depending on traffic density and the complexity of the routes flown. Different separation minimums applied by ATC in these two areas reflect the variation in traffic density.
For example, the minimum lateral distance permitted between co-altitude aircraft in Chicago Center’s airspace is 8 nautical miles (NM) (3 NM when using radar), while in North Atlantic (NAT)/MNPS airspace it is 60 NM. The airspace assigned by ATC has lateral dimensions on both sides of the exact CL of the route of flight specified in the ATC clearance equal to one-half of the lateral separation standard (minimum). For example, the overall level of lateral navigation performance necessary for flight safety must be within 4 NM of the airway CL in Chicago Center’s airspace, and within 30 NM in NAT/MNPS airspace. Title 14 CFR part 121, §§ 121.103 and 121.121 require that each aircraft must be navigated to the degree of accuracy required for ATC. Part 91, § 91.123’s requirements related to compliance with ATC clearances and instructions also reflect this fundamental concept.
(3) ICAO Standards and Recommended Practices (SARP). The concept of navigational performance is also inherent in the ICAO SARPs. For example, Annex 2 states that the aircraft “shall adhere to its current flight plan” and “when on an established ATS route, operate along the defined CL of that route.” This is true unless the Strategic Lateral Offset Procedures (SLOP) are approved.
f. Degree of Accuracy Required.
(1) Control of Air Traffic. The fundamental concept for all IFR navigation standards, practices, and procedures is that you must navigate all IFR aircraft to the degree of accuracy required for control of air traffic. When a flight remains within the assigned three-dimensional block of airspace at all times, the IFR navigation standards, practices, and procedures will consider that the operator navigated that aircraft to the degree of accuracy required for the Page 112 Par 10-1
control of air traffic. If an aircraft deviates outside its assigned block of airspace (except during a declared emergency), the operator did not navigate that aircraft to the required degree of accuracy.
(2) Separation Minimums. ATC separation minimums represent the minimum dimensions of a three-dimensional block of airspace that ATC can assign to control flight. These separation minimums have been established for IFR operations in controlled airspace. These standards are usually established through international agreement and implemented through national regulations. These minimums are established for particular categories of navigational operation and specified areas. Examples include navigation on airways in the national airspace of ICAO member states and long-range navigation in oceanic or remote land areas. Separation minimums establish the minimum lateral, vertical, and longitudinal distances that can be used to safely separate aircraft operating within a specified area. Separation minimums also represent the minimum level of overall navigation performance which you can accommodate at any time without jeopardizing flight safety.
(a) Any aircraft deviating greater than one-half the separation minimums established for that operation has failed to meet the required level of navigational performance to the degree of accuracy required for control of air traffic. For example, the vertical separation minimum for airplanes operating above FL 290 in the United States is 1,000 feet. Each aircraft’s actual altitude must remain within +/- 240 feet of the assigned altitude even when considering factors such as atmospheric pressure variations and instrument or pilot errors. This is the Reduced Vertical Separation Minimum (RVSM) requirement. Where the United States provide Air Traffic Services (ATS), 14 CFR and ATC directives establish separation minimums.
Where contracting ICAO member states provide ATSs, those state’s national regulations and ICAO documents establish separation minimums. Operations in uncontrolled airspace do not receive ATS, and separation minimums are not normally established for uncontrolled airspace.
(b) U.S. national airspace separation minimums can be found in the current edition of Federal Aviation Administration (FAA) Order 7110.65, Air Traffic Control. FAA Order 7110.83, Oceanic Air Traffic Control, current edition, prescribes separation minimums in international oceanic airspace delegated to the United States by ICAO. ICAO Document 7030, Regional Supplementary Procedures, current edition, prescribes separation minimums in international airspace.
g. Concept of Operational Service Volume. The concept of operational service volume is critical to understanding and applying the principles of air navigation. Operational service volume is the volume of airspace surrounding an ICAO standard airways navigation facility that is available for operational use. Within that volume of airspace, a signal of usable strength exists as co-channel interference does not operationally limit it. Within this volume of airspace, a Navigational Aid (NAVAID) facility’s signal in space conforms to flight inspection signal strength and course quality standards, including frequency protection. ICAO standard NAVAIDs are VHF Omnidirectional Range (VOR), VOR/distance measuring equipment (DME), and non-directional radio beacon (NDB). Global Navigation Satellite System (GNSS) is also an approved ICAO NAVAID and is applicable in both Class I and Class II navigation areas. The national airspace systems of ICAO contracting member states are based on the operational Par 10-1 Page 113
service volume of these facilities. You can predicate navigational performance within the operational service volume and ATC separation minimums on the use of these facilities.
h. Categories of Navigational Operations. A thorough comprehension of the categories of navigational operations is essential to understanding air navigation concepts and requirements, and in evaluating an operator’s ability to navigate to the required degree of accuracy. In the broad concept of air navigation, the following paragraphs identify two major categories of navigational operations.
i. Class I Navigation. Class I navigation is any en route flight operation conducted in controlled or uncontrolled airspace that is entirely within operational service volumes of ICAO standard NAVAIDs (GNSS, VOR, VOR/DME, and NDB). The operational service volume describes a three-dimensional volume of airspace which categorizes any type of en route navigation as Class I navigation. Within this volume of airspace, IFR navigational performance must be at least as precise as IFR navigation and must use GNSS, VOR, and VOR/DME (or NDB in some countries). The definition of Class I navigation is not dependent upon the equipment installed in the aircraft. En route a VFR flight navigated by pilotage is conducting Class I navigation when operating entirely within the operational service volume. However, the VFR navigational performance in this example must be as precise as VFR pilotage operations are required to be. The operational service volumes of ICAO standard NAVAIDs solely determine the lateral and vertical extent of airspace where you conduct Class I navigation. You cannot conduct Class I navigation outside of this airspace.
(1) VFR or IFR Navigation Operations. Class I navigation also includes VFR or IFR navigation operations on the following:
• Federal airways.
• Published IFR direct routes in the United States.
• Published IFR off-airway routes in the United States.
• Airways, Advisory Routes (ADR), direct routes, and off-airway routes published or approved by a foreign government provided that these routings are continuously within the operational service volume (or foreign equivalent) of ICAO standard NAVAIDs.
(2) Separation Minimums. Class I navigation requirements are directly related to separation minimums used by ATC. IFR separation minimums applied in the U.S. National Airspace System (NAS) and most other countries are based on the use of ICAO standard NAVAIDs. ATC, however, can only apply these separation minimums within areas where the NAVAIDs signal in space meets flight inspection signal strength and course quality standards.
An ICAO standard NAVAID’s signal in space conforms to flight inspection signal strength and course quality standards (including frequency protection) within its designated operational service volume. Therefore, you can predicate air navigation and the safe separation of aircraft within that service volume on the use of these facilities.
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(3) Qualifications for Class I Navigation. Within areas where the safe separation of aircraft is based on the use of ICAO standard NAVAIDs, navigate any IFR operation with at least the same precision specified by the appropriate national separation minimums. Any operation or portion of an operation (VFR or IFR) in controlled or uncontrolled airspace with any navigation system (VOR, VOR/DME, NDB, inertial navigation system (INS) and GNSS) is Class I navigation for that portion of the route that is entirely within the operational service volume of ICAO standard en route NAVAIDs.
j. Class II Navigation. Class II navigation is any en route operation not categorized as Class I navigation and includes any operation, or portion of an operation, that takes place outside the operational service volumes of ICAO standard NAVAIDs. For example, an aircraft equipped with only VOR conducts Class II navigation when the flight operates in an area outside the operational service volumes of federal VORs/DMEs.
(1) Class II Navigation: Operating Outside the Operational Service Volume.
Class II navigation involves operations conducted in areas where the signals in space from ICAO standard NAVAIDs do not meet flight inspection signal strength, course quality, and frequency protection standards. Therefore, ATC cannot predicate aircraft separation on the use of these facilities alone and must apply larger separation criteria. When operating outside the operational service volume of ICAO standard NAVAIDs, you cannot rely upon signals from these stations as the sole means of conducting long-range operations to the degree of accuracy required for the control of air traffic or as the sole means of obstacle avoidance. Therefore, when operating outside the designated operational service volumes of ICAO standard NAVAIDs, operators must use long-range navigation systems (LRNS) (GNSS, LORAN-C, INS/inertial reference system (IRS)) or other approved procedures which may include dead reckoning (DR), pilotage or flight navigator. These systems and/or techniques are necessary to navigate to the degree of accuracy required for the control of air traffic and to avoid obstacles. DR is a contingency procedure that you cannot plan.
(2) Class II Navigation Definition. The definition of Class II navigation is not dependent upon the equipment installed in the aircraft. All airspace outside the operational service volume of ICAO standard NAVAIDs is a three-dimensional volume of airspace within which any type of en route navigation is Class II navigation. For any type of navigation within this volume of airspace, the IFR navigational performance must be as precise as the navigational performance assumed during establishment of the ATC separation minimums for that volume of airspace. The navigational performance for VFR operations in a Class II navigation volume of airspace must be only as precise as VFR navigation operations are required to be.
(3) Class II Requirements. It is possible in turbojet airplanes, with proper procedures and training, to fly many routes between the southeastern United States, Caribbean Islands, and South America with VOR/DME and NDB equipment. In these situations, you can meet Class II navigation requirements even though significant portions of these routes, less than 1 hour, are outside the operational service volumes of ICAO standard NAVAIDs. In the domestic United States, it is not uncommon for low altitude VFR flights in aircraft such as helicopters to conduct Class II navigation while outside the operational service volumes of ICAO standard NAVAIDs when operating over routes of less than 100 NM in length. Class II navigation includes transoceanic operations and operations in desolate/remote land areas such as the Arctic.
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(4) Class II Navigation NAVAIDs. Class II navigation does not automatically require the use of LRNS. In many instances, you can conduct Class II navigation with conventional NAVAIDs if special navigational techniques supplement these NAVAIDs. Any portion of an en route operation in controlled or uncontrolled airspace, with any navigation system or any navigation technique, is Class II navigation for that portion of the route that is outside the operational service volumes of ICAO standard en route NAVAIDs.
10-2. IRS/INS AND LONG-RANGE NAVIGATION PROCEDURES.
a. Background. Recently, an aircraft deviated approximately 60 miles from an assigned NAT track and came within a few feet of colliding with an aircraft assigned to an adjacent track.
Following the near miss, the aircraft that had deviated from its track did not follow established contingency procedures for aircraft experiencing navigational uncertainty, thus creating the potential for further conflict with other aircraft as it returned to its assigned track. The crew’s failure to operate the navigation equipment in a disciplined systematic manner during all phases of flight caused the incident. The crew’s failure to comprehend the relationship between navigation performance, contingency procedure, and collision avoidance further complicated the incident.
(1) Navigation and Human Errors. Although navigation errors are infrequent, human errors account 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, but the operating procedures prescribed were either inadequate or the operator did not follow them. Experience indicates that the increased accuracy and reliability of modern automatic navigation systems can induce a degree of complacency on the part of flightcrews, and may result in failure to routinely cross-check system performance. Under these circumstances, human errors may remain undetected for excessive periods. A common error associated with automated systems is incorrect programming of the oceanic waypoint latitudes by multiples of 1° (60 NM).
In an organized track system (OTS), 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 FL. Vigilance and diligence in properly applying established procedures are essential to safe oceanic navigation. Although operational procedures may differ among navigation systems, many good practices and procedures are basic to all automated and semi-automated systems.
(2) Practices and Procedures for IFR Long-Range Navigation Operations. IFR long-range navigation operations using pilot-operated electronic long-range navigation equipment should use the practices and procedures recommended in this document. Prior to issuing operations specifications (OpSpecs) authorizing operations requiring long-range navigation equipment, the FAA principal operations inspector (POI) should ensure that the operator’s training program, manuals, and check airman program include and emphasize these practices and procedures. For operations currently authorized by OpSpecs or a letter of authorization (LOA), review the operator’s navigation program to ensure that it follows the guidance contained in this AC. The Flight Technologies and Procedures Division, AFS-400, must approve any deviation from these requirements.
b. Weather. In addition to the normal review of weather information concerning terminals, crews should be alert for hazardous weather that may require a flight plan change or in-flight re-routing. It is important to obtain a copy of the wind flow chart (constant pressure chart or the equivalent) for the FL and the route flown. This information may be valuable when evaluating wind forecasting errors or if DR operations become necessary due to equipment failure. As the flight progresses, give consideration to plotting actual wind information on the plotting chart as a means of evaluating the accuracy of the forecast.
c. Notices to Airmen (NOTAM). Besides checking NOTAMs for departure, destination and alternate airports, check NOTAMs concerning NAVAIDs or special airspace restrictions along the planned route.
d. Waypoint Management. The navigation program should include a standard system for indicating waypoint status as detailed below. The specific procedures recommended are in the list below. Variations in specific symbols may be necessary to accommodate the individual operator’s program.
• The crew stores the waypoint coordinates in the computer. (Enter the waypoint number next to the relevant waypoint coordinates.)
• A second crewmember independently cross-checks the coordinates and zone distances. (Circle the waypoint number.)
• The second crewmember cross-checks the coordinates and zone distances during the approaching waypoint check. (Draw a diagonal line through the waypoint number.)
• Waypoint passage has occurred. (Draw a second diagonal line through the waypoint number.)
• Cross-checking during all phases of flight. (Flight planning, preflight, en route.)
• Official (master) document.
• Plotting.
e. Plotting Procedures. Use of plotting procedures has had a significant impact on the reduction of gross navigation errors (GNE). All flights using long-range navigation equipment as the sole means of navigation require the use of this technique to plot the flight route on a plotting chart and to plot the computer position approximately 10 minutes after waypoint passage. Routes of shorter duration that transit airspace where special conditions exist, such as reduced lateral separation standards, high density traffic, or proximity to potentially hostile border areas may require the use of plotting procedures. All turbojet operations where the route segment between the operational service volume of ICAO standard NAVAIDs, VOR, VOR/DME, and NDB exceed 725 NM and all turboprop operations where the route segment between the operational service volume of ICAO standard NAVAIDs exceeds 450 NM require plotting procedures. The operational service volume is that volume of airspace surrounding a NAVAID which is available Par 10-2 Page 117
for operational use, within which a signal of usable strength exists, and where co-channel interference does not operationally limit that signal. You can determine the operational service volume for a specific NAVAID by contacting the Frequency Management Section within each regional Airway Facilities Division. Operational service volume includes the following:
• The officially-designated standard service volume excluding any portion of restricted standard service volume.
• The extended service volume.
• Within the United States (including offshore control areas (CTA)) by published instrument flight procedures (Victor or jet airways, standard instrument departures (SID), Standard Terminal Arrival Routes (STAR), Standard Instrument Approach Procedures (SIAP) or instrument departure).
• Outside the United States, any designated signal coverage or published instrument flight procedure equivalent to U.S. standards.
f. Flight Planning. Many operators use a computerized navigation flight plan. Take care to verify that all en route waypoints are correct and legibly shown on the flight plan. It is good practice to select a waypoint loading sequence and number each waypoint accordingly. If using more than one copy of the flight plan, designate one copy as the official copy. To eliminate possible confusion, ensure that all necessary information, routing changes, estimated time of arrival (ETA), and waypoint loading sequence is on this flight plan, and use the official copy for all reports to ATC. Additionally, if the flight is within the North Atlantic Organized Track System (NAT OTS), obtain a copy of the current track message and be alert for conflict between the flight plan and the track message. Track messages are issued approximately every 12 hours and describe the NAT routes, gateways, and FLs available for eastbound and westbound flights during the period indicated. While planning an over-water flight, pilots should review NOTAMs for any condition that may affect the operation and accuracy of LRNSs. Approach the use of heading information for cross-checking with caution. In steering a given route segment with a navigation computer, the true heading required to maintain a Great Circle course will change.
For example, the true heading to maintain the Great Circle course from 50°N. 30°W. to 50°N. 40°W. will be 274° at 30°W., 270° at 35°W., and 265° at 40°W. Differences in variation along the route will further change the magnetic heading required to maintain course. The flightcrew must have a thorough understanding of the flight plan heading information and DR technique in order to use this check with any degree of certainty.
g. Navigation Preflight (at Aircraft). Verify navigation system software identification and modification status codes. Cross-check inputs to navigation computers. One crewmember should carry each insertion out in its entirety and another should recall and verify it. Cross-check computer flight plan zone distances with zone distance displayed in navigational computers.
Perform the cross-check of coordinates and zone distances on all computer systems individually when using the remote loading feature. For INS, after placing the systems in the navigation mode, check the groundspeed while the aircraft is stationary. A reading of more than a few knots may indicate an unreliable system.
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• Cross-check computer flight plan, waypoint coordinates and identifiers with source documents (airfield diagrams, en route charts, and NAT track messages, if applicable).
• Plot the flight route on a chart of appropriate scale. Operational experience has demonstrated that a scale of 1 inch to 120 NM provides the most benefit for plotting purposes.
• Compare routing information on ATC flight plans, computer flight plans, NAT track messages, plotting charts, aircraft observations and Aircraft Report (AIREP) forms.
• It is advisable not to copy waypoint coordinates from source documents (track message, en route charts, etc.) to the flight plan for subsequent insertion into the navigation computers. To avoid errors in transcription, insert waypoint coordinates into the computers directly from the source documents.
• Since the initial stage of the flight can be very busy, give consideration to ensuring the navigation system waypoint transfer switches are in the “auto” position to facilitate outbound tracking and waypoint changeover during this period.
• With systems such as INS that navigate during ground operations, 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 accurate.
h. Equipment Preflight. In addition to operating procedures such as checklists for confirming proper system operation, take care to ensure the proper programming of the navigation equipment. This is a very important procedure and you should not rush it. All navigation information, coordinates, or courses and distances should be programmed by one crewmember and verified by another. In addition, crews should verify that each system uses the same waypoint loading sequence and indicate on the flight plan that the crew entered cross-checked the present position, if applicable, and waypoints. If time becomes a factor, it is more important to verify that the first two or three waypoints are correct than to rush through the procedure to insert as much information as possible. Give consideration to using another cross-check that compares the flight plan or charted distance between waypoints and the distance computed by the navigation system to detect programming or flight planning errors. This serves as a double check on waypoint verification and will also reveal any error in the flight plan. A difference of more than 2 NM may indicate a programming or flight planning error.
i. Pre-Takeoff and Coast Out. Before takeoff, cross-check the computer present position to confirm proper system operation. At least two crewmembers should copy and confirm the oceanic clearance. Perform accuracy check to compare navigation computer position with VOR, VOR/DME or NDB. Direct overflight of a NAVAID and for cases when you do not directly overfly the NAVAID apply the NAV accuracy check. Record the gross error in the flight log.
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j. Within Range of the Outbound Gateway. Flights should not continue 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 NAVAID. Since this may be the last positive position cross-check until the inbound gateway, the following practices may also provide valuable information for resolving any later navigation difficulties.
(1) ATC Clearances. Two crewmembers should cross-check all ATC oceanic clearances to ensure the clearance is copied correctly. Cross out any flight plan waypoints that were revised in an ATC clearance and enter the revised coordinates in a legible manner. Prior to proceeding outbound from the gateway, compare the current ATC clearance to the flight plan and the information in the navigation computers for the gateway and verify the subsequent waypoints. Verify and change (if necessary) the clearance track on plotting chart.
(2) Gross Error Check. A gross error check is a position accuracy cross-check using normal airway facilities such as VOR, VOR/DME or NDB. The operator usually accomplishes the gross error check by flying directly over the gateway (if possible) and subsequently establishing the aircraft on the outbound course using the gateway NAVAID. This check detects errors that may have accrued in position information since takeoff, provides information used to determine the most accurate system for use as a steering reference, and provides an opportunity to correct position information. The gateway check also confirms that the operator establishes the aircraft on the outbound course and is tracking toward the next waypoint.
(3) Flight Instruments Used for Display. When operators use flight instruments for the display of either airways (VOR) information or information from the LRNS, they should leave the “radio/nav” switches in “radio” position after passing over the gateway NAVAID until the radio information becomes degraded. Operators should then place switches in the “nav” position.
(4) Compass Deviations. Give consideration to performing a compass deviation check on systems such as INS that use true heading information from sources independent of the aircraft compass system. You can determine the compass deviation by comparing the INS derived data later in the flight to determine the most accurate system should a divergence between systems occur. You can apply the compass deviations to the respective compasses to determine the actual magnetic heading. Operators can apply local variation to the true heading of each INS to obtain the derived magnetic headings. The most accurate INS should be the one with a magnetic heading that compares the most favorably with the actual magnetic heading.
k. After Passing the Gateway. Select the system determined to be the most accurate during the gross error check as the autopilot (AP) steering reference. When not used for other purposes, this system should display present position. Routinely check cross-track, track angle error (TKE) and distance-to-go. Display computer position coordinates and compare with ATC clearance to confirm that you maintained track CL.
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l. Approaching Waypoint. Within 2 minutes of each waypoint, both pilots should verify that the subsequent waypoint in the ND agrees with the current ATC clearance. Cross-check coordinates of the approaching waypoint and subsequent waypoints. Compare zone distance on the flight plan to that displayed on the navigation computer for the next leg. Compare computer flight plan ETA with ETA information displayed in navigation computers. (On some systems, you may accomplish this cross-check more easily during waypoint passage.)
m. After Passing Each Waypoint. Approximately 10 minutes after passing each waypoint, plot the present position information on the NDs on a navigation chart to confirm that the ATC clearance is satisfied. Confirm that the navigation systems have switched to the next flight segment (leg change). Verify that the aircraft is tracking along the next flight segment (tracking outbound).
n. Approaching the Inbound Gateway. Make certain preparations for the transition from long-range navigation to airways navigation. The FAA recommends the following practices:
• As soon as feasible, set up the navigation radios to receive the inbound gateway NAVAID.
• When the gateway NAVAID is providing reliable information, place the “radio/nav” switch in the “radio” position and steer the aircraft to acquire and maintain the proper inbound radial/bearing.
• Unless otherwise directed by ATC, fly the aircraft directly over the gateway.
• When over the gateway, record the position information from the navigation displays (ND). You can use this information to confirm system accuracy. Compare VOR, VOR/DME, NDB course and distance information with that displayed in navigation computers. The FAA recommends that you make system accuracy computations after arrival to avoid conflicts with other cockpit duties during the critical periods of descent, approach and landing.
o. After Arrival. Compute and record the individual navigation system errors and error rates (if applicable) for future reference. It is desirable to record this information in a document that remains aboard the aircraft to provide subsequent flightcrews with a recent history of system performance. This information may be used with most systems to predict individual system performance for future flights under similar circumstances. Additionally, this information may prove valuable to subsequent flightcrews for resolving navigation abnormalities, such as divergence between systems.
10-3. LRNS PROBLEMS AND RECOMMENDED ACTIONS. Although the accuracy and reliability of the newer navigation systems are excellent, malfunctions and failures occasionally occur. When a malfunction occurs, flightcrews should guard against jumping to conclusions since hasty actions are seldom necessary and may further complicate the situation. Experience has shown that successful resolution of navigation difficulties in oceanic areas usually requires a thorough, thoughtful process that normally begins during preflight planning.
The training program manuals and check airman program for air carrier operations should emphasize procedures followed in the event of partial and total instrument failure. Non-air carrier operators should be able to demonstrate this emphasis in their training programs if requesting an LOA for oceanic operations in special airspace. The following guidance is for consideration when encountering or suspecting navigation difficulties.
a. Navigation Errors. Monitoring procedures used during oceanic operations indicate the frequency and course of navigation errors. Considering that operators make thousands of flights, errors are actually rather infrequent. Navigation systems are generally so reliable that there is some concern about overconfidence. Therefore, crews should guard against complacency.
(1) Frequent Causes of Errors. Frequent causes of errors include the following:
• Making a mistake of 1° of latitude when inserting a forward waypoint.
• The crew received re-clearance by ATC but forgot to re-program the navigational system.
• The operator left the AP in the heading or de-coupled position after avoiding severe weather or they left it in the VOR position after departing the last domestic airspace VOR. In some cases, this occurred after distractions by selective calling (SELCAL) or flight deck warning indications.
• The controller and crew had different understandings of the clearance because the pilot heard what he/she wanted to hear rather than what they actually said.
(2) Rare Causes of Errors. Rare causes of errors include the following:
• The latitude/longitude coordinates displayed at the gate position were incorrect.
• Because of a defective chip in an aircraft system, and although the crew inserted the correct forward latitude, it “jumped” 1°. (INS only.)
• The aircraft has an advanced system that includes all waypoint coordinates already in the database equipped. The crew assumed the coordinates were correct, but one was not.
• Although the crew had the correct coordinates available, the information inserted into the system was from an incorrect company flight plan.
b. Detection of System Failure. In general, we usually consider system failure to have occurred when one of the following situations develop:
• Activation of a warning indicator that cannot be reset;
• Self-diagnostic or built-in test equipment (BITE) indicates that the system is unreliable;
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• The position error over a known geographic location exceeds the maximum permissible tolerance established for a particular navigation system; or
• The system’s operation is so abnormal that, despite the absence of warning or malfunction indications, the flightcrew considers the system no longer useful for navigation.
c. Detection of System Degradation or Malfunction.
(1) Identifying a Faulty System. While system failures are usually straight forward, malfunctions or gradual system degradations are usually more difficult to detect. This is particularly true when only two systems are onboard. A divergence between the navigation systems, a situation that often occurs gradually, usually detects navigation difficulties of this type. This factor may reduce the possibility of identifying the faulty system unless operators diligently use periodic cross-checking practices. Consider the following factors when attempting to identify a faulty system:
• Check the BITE codes for indications of system fault.
• Review the gateway gross error check for indications of the most accurate system.
• If you maintain a regular record of system performance and it is available, a review of the record may give a clue as to which system is faulty.
• If possible, use VOR, automatic direction finder (ADF), DR, airborne radar, or other NAVAIDs to obtain a position fix.
• Cross-check heading, groundspeed, track, and wind information between systems and compare this information with the best known positive information such as position over a fix.
• Attempt to contact nearby aircraft to obtain wind or groundspeed and drift correction information that may identify the malfunctioning system.
• The compass deviation check may provide a clue as to which system is faulty for systems such as INS.
(2) Determining a Faulty System. Even though you take these steps, a divergence between systems may occur, but the flightcrew may be unable to determine which system is at fault. When this occurs, use the practices described in the following paragraph.
d. Recommended Actions Following System Failure. After detecting a system malfunction or failure, inform ATC that the flight is experiencing navigation difficulties so that you can adjust the separation criteria, if necessary. Reporting malfunctions to ATC is an ICAO requirement and 14 CFR part 91 requires compliance. If you can identify the failed system with a high degree of confidence and the other system appears normal, the best course of action may be Par 10-3 Page 123