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Gross Navigational Error (GNE)

International Operations

Why all the fuss about plotting procedures and checking Flight Management System programming over and over again when flying oceanic? If you download the FMS waypoints from the same people who filed the flight plan, what can go wrong? What indeed.


 

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GNE Defined

[ICAO NAT Doc 007, ¶8.1.5] Regardless of how sophisticated or mature a system is, it is still essential that stringent navigation and cross checking procedures are maintained if Gross Navigation Errors (GNEs) are to be avoided. A GNE within NAT airspace is defined as a deviation from cleared track of 10 NM or more.

[ICAO NAT Doc 007, ¶8.1.6] All reported navigation errors in North Atlantic airspace are thoroughly investigated. Records show that navigation equipment or system technical failures are now fortunately rare. However, when they do occur they can sometimes be subtle or progressive, resulting in a gradual and perhaps not immediately discernible degradation of performance. Chapter 11 of this Manual provides guidance on detection and recovery when such problems are encountered.

[ICAO NAT Doc 007, ¶8.1.7] About half of NAT flights route via an OTS track and a large portion of the remaining random flights follow routes that at some point approach within one or two degrees of the outermost OTS tracks. One consequence of this is that a single digit error in the latitude of one significant point of an aircraft’s route definition will very likely lead to a conflict with another aircraft which is routing correctly via the resulting common significant point. The risk of an actual collision between two aircraft routing via a common point, as is the case when such errors are made, is further exacerbated by the improved technical accuracy of the modern navigation and height keeping equipment employed.

[ICAO NAT Doc 007, ¶8.1.8] The importance of employing strict navigation system operating procedures designed to avoid the insertion of wrong waypoints or misunderstandings between the flight crew and ATC over cleared routes cannot be over-emphasised.

Causes

[ICAO NAT Doc 007, ¶14.3.1] The most common causes of lateral navigation errors, in approximate order of frequency, have been as follows:

  1. having already inserted the filed flight plan route coordinates into the navigation computers, the flight crew have been re-cleared by ATC, or have asked for and obtained a re-clearance, but have then omitted to re-program the navigation system(s), amend the Master Document or update the plotting chart accordingly.
  2. a mistake of one degree of latitude has been made in inserting a forward waypoint. There seems to be a greater tendency for this error to be made when a track, after passing through the same latitude at several waypoints (e.g. 57°N 50°W, 57°N 40°W, 57°N 30°W) then changes by one degree of latitude (e.g. 56°N 20°W). Other circumstances which can lead to this mistake being made include receiving a re-clearance in flight.
  3. the autopilot has been inadvertently left in the heading or de-coupled mode after avoiding weather, or left in the VOR position after leaving the last domestic airspace VOR. In some cases, the mistake has arisen during distraction caused by SELCAL or by some flight deck warning indication.
  4. an error has arisen in the ATC Controller/Pilot communications loop, so that the controller and the flight crew have had different understandings of the clearance. In some cases, the flight crew has heard not what was said, but what they were expecting to hear.

I was once flying from Anchorage, Alaska to Tokyo, Japan when our navigator made the classic east versus west error in our longitude. When the INS got to that waypoint the airplane made a sharp right turn. That was a good thing, it got all of our attentions. We were lucky it wasn't a more subtle error.

I was once asked to audit a company GNE where the crew simply downlinked the Europe to White Plains flight plan from satellite and saw 99 waypoints loaded but for some reason KHPN was missing from the last leg. They typed in KHPN and called it good. At 30°W the airplane cycled from the 99th waypoint, which was at 30°W, directly to KHPN. The crew wasn't plotting or even monitoring headings at each waypoint. I recommended the crew be required to face the consequences.

Do GNE's Occur Frequently?

A Three Day Snapshot in 2016

[Flight Service Bureau]

  • April 22nd (Friday) — Democratic Republic of the Congo Boeing 727 100 (9QCDC/DRC001) from Santa Maria Island, Azores (LPAZ) to St. John’s NL (CYYT). At 1235Z, Observed on radar to be over position 4720N 4745W, which was approximately 60 miles north of the cleared route 45N 45W – 47N 50W. The crew reported correctly while in oceanic airspace. The flight was cleared direct to YYT and landed without incident at CYYT. There was no traffic, and no other impact to operations.
  • April 24th (Sunday) — Neos Airline Boeing 767-300 (INDDL/NOS730) from Ferno, Italy (LIMC) to Havana, Cuba (MUHA). Cleared via 49N030W 48N040W 45N050W. At 30W, the flight reported 48N040W 44N050W. The aircraft recleared to 45N050W prior to proceeding off course.
  • Apr 25th (Monday) — Transportes Aereos Portugueses Airbus A330-202 (CSTOO/TAP203) from Lisbon, Portugal (LPPT) to Newark, NJ (KEWR). Cleared 46N030W 46N040W 45N050W. The aircraft reported proceeding via 46N030W 46N040W 44N050W, as per the original flight plan. The aircraft was recleared via 45N050W prior to proceeding off course.

What Happens after a GNE?

[ICAO NAT Doc 001, ¶5.5]

  • 5.5.1 Radar stations capable of monitoring the boundaries of the NAT Region collect data on flights within MNPS Airspace, together with that on non-MNPS Airspace flights. The former data provides a direct input into the risk modelling of MNPS Airspace, whilst the latter provides a wider appreciation of navigation in the NAT Region and allows follow-up action to be taken on a larger sample of flights believed to have experienced navigation errors.
  • 5.5.2 The data collection process comprises two parts:
    1. continuous collection of all deviations of 25 NM or more (i.e., GNEs); and
    2. collection of data on deviations of between 15 and 25 NM as required.
  • 5.5.3 When a GNE has been detected by the ATS Provider State or has been reported to ATC by the pilot, that ATS Provider unit will, in cooperation with the Operator, investigate its cause. It is important that all agencies react promptly to reports of GNEs. Investigations should be made at once so that consideration can be given to the need for swift remedial action. In order that deviation reports can receive prompt attention, each airline/Operator should nominate a person to be responsible for receiving reports and to initiate investigations; the name and full address of this individual should be notified to each relevant ATS administration.

[ICAO NAT Doc 001, ¶5.10] Follow-up Action on Observed and Reported GNEs

  • 5.10.5 Different administrative arrangements exist within those States participating in monitoring programmes although follow-up action on GNEs should, in general terms, be as indicated in the following paragraphs.
  • 5.10.6 For aircraft operating within MNPS Airspace:
    1. the observing ATC unit should, if at all possible, inform the pilot of the aircraft concerned of the observed error and also that an error report will be processed; any comment made by the pilot at the time of notification should be recorded;
    2. the Operators (including military) and any other relevant ATC units should be notified of the observed deviation, either directly by the observing ATC unit or by an agency designated by the State concerned, using the speediest means available (facsimile, AFTN, etc.) and with the least possible delay. This should be followed as soon as possible by a written confirmation. (For message and letter formats, see Appendix C). All notifications should be copied to the CMA [Central Monitoring Agency]; and
    3. the appropriate State of Registry or the State of the Operator will be sent a copy of the written confirmation along with a covering letter by the CMA.
  • 5.10.7 For aircraft operating outside MNPS Airspace:
    1. the observing ATC unit should, if at all possible, inform the pilot of the aircraft concerned of the observed error and also that an error report may be processed; any comment made by the pilot at the time of notification should be recorded;
    2. where the observed deviation from track is 50 NM or more, the procedure detailed in the previous paragraph (covering aircraft operating within MNPS Airspace) will be followed (see Appendix C); and
    3. where the observed deviation from track is 25 NM or more but less than 50 NM, the observing ATC unit, or other agency designated by the State, should notify the CMA of the deviation with the least possible delay (facsimile, AFTN etc.) using the appropriate message format shown at Appendix C. This should be followed as soon as possible by a written confirmation where this is deemed necessary. The CMA will then advise the State of Registry.

    In other words, they will be contacting the FAA. But what happens after that? I've asked and have never heard of certificate action as a result. The worst thing will be on behalf of the operator (or your owner). So, given that, what is the motivation? The answer is the obvious one: you don't want to hit anything or become lost.

How to Avoid a GNE?

Cross Check!

[ICAO NAT Doc 007, ¶14.4] Perform navigation cross-check procedures throughout the ocean crossing. Do not relax or otherwise skip steps when it comes to following those procedures.

Cross-checks begin approaching each waypoint.

See: Oceanic En Route / Approaching Each Waypoint.

Radio Procedures!

[ICAO NAT Doc 007, ¶14.4] Avoid casual R/T procedures. A number of GNEs have been the result of a misunderstanding between flight crew and controller as to the cleared route and/or flight level. Adhere strictly to proper R/T phraseology and do not be tempted to clip or abbreviate details of waypoint coordinates.

Initialization!

[ICAO NAT Doc 007, ¶14.4] Make an independent check on the gate position. Do not assume that the gate coordinates are correct without cross-checking with an authoritative source. Normally one expects coordinates to be to the nearest tenth of a minute. Therefore, ensure that the display is not to the hundredth, or in minutes and seconds. If the aircraft is near to the Zero Degree E/W (Greenwich) Meridian, remember the risk of confusing east and west.

Systems that use GPS for initialization are far less likely to have this problem. Make sure you follow your aircraft manufacturer's latest procedures. I once flew a Challenger 604 that allowed the initialization be updated at the end of the runway but the procedure discouraged this practice if GPS was installed. Some of our pilots would always update the initialization with runway position because that's what they did before GPS. They were, of course, decreasing the accuracy of the system.

Proper Waypoint Loading!

[ICAO NAT Doc 007, ¶14.4] Before departure, at least two pilots should independently check that the following agree: computer flight plan, ICAO flight plan, track plotted on chart, and if appropriate, the track message. In flight, involve two different sources in the cross- checking, if possible. Do not be so hurried in loading waypoints that mistakes become likely, and always check waypoints against the current ATC clearance. Always be aware that the cleared route may differ from that contained in the filed flight plan. Prior to entering the NAT HLA ensure that the waypoints programmed into the navigation computer reflect the Oceanic Clearance received and not any different previously entered planned or requested route.

See: Oceanic Departure / Flight Plan Entry.

Coast-Out Navigation Accuracy Check!

[ICAO NAT Doc 007, ¶14.4] Before entering Oceanic Airspace make a careful check of LRNS positions at or near to the last navigation facility – or perhaps the last but one.

There are many ways to do this, some more accurate than others.

See: Navigation Accuracy Check.

Stay on Altitude, Especially When on Course!

[ICAO NAT Doc 007, ¶14.4] Do not initiate an on-track un-cleared level change. If a change of level is essential and prior ATC clearance cannot be obtained, treat this situation as a contingency and execute the appropriate contingency offset procedure, when possible before leaving the last cleared flight level. Inform ATC as soon as practicable.

This may seem almost nonsensical but it does happen.

See: Loss of RVSM Capability in Oceanic Airspace and Weather Deviation in Oceanic Airspace.

Cross-Check Present Position at Each Waypoint

[ICAO NAT Doc 007, ¶14.4] Do not assume that the aircraft is at a waypoint merely because the alert annunciator so indicates. Cross-check by reading present position.

Navigation Cross Checks!

[ICAO NAT Doc 007, ¶14.4] Complete navigation cross checks with more than one flight crew member. There are some tasks on the flight deck which can safely be delegated to one member of the flight crew, but navigation using automated systems is emphatically not one of them. All such cross-checks should be performed independently by at least two flight crew members.

Plot!

[ICAO NAT Doc 007, ¶14.4] Complete flight progress charts periodically. Making periodic plots of position on a suitable chart and comparing with current cleared track, greatly helps in the identification of errors before getting too far from track.

Pilot message boards are filled with self-assured oceanic pilots claiming plotting is unnecessary. "I've been flying oceanic for thirty years," some of these will say, "and I've never plotted!" What you don't see are comments from all those sorry saps who got caught by personal error or just plain circumstance who could have been saved by a plotting chart. I've met several seasoned veterans who simply didn't know how and were afraid to admit that.

See: Plotting.

References

ICAO NAT Doc 001, Guidance and Information Material Concerning Air Navigation in the North Atlantic Region, Seventh Edition, January 2002

ICAO Nat Doc 007, North Atlantic Operations and Airspace Manual, v. 2019-2, applicable from 28 March 2019

Revision: 20190224
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