INTRODUCTION

Since 1977, when the MNPS rules were introduced, careful monitoring procedures have provided a good indication both of the frequency with which navigation errors occur and their causes. Their frequency is low: only one flight in around ten thousand flights commits a serious navigation error. However because of the accuracy and reliability of modern navigation systems, the errors which do occur are most often seen to be as a result of aircrew error.

Operational errors in the vertical plane also occur. Aircraft are sometimes flown at levels other than those for which ATC clearance has been issued. As in the horizontal plane, the frequency of vertical errors is low. However, the potential risk of even a single incidence of flying at an uncleared level can be significant.

It is therefore essential that crews do not take modern technology for granted. They should at all times, especially during periods of low workload, guard against complacency and over-confidence, by adhering rigidly to approved cockpit procedures which have been formulated over many years, in order to help stop operational errors from being an inevitability.
 

RARE CAUSES OF ERRORS

To illustrate the surprising nature of things which can go wrong, the following are examples of some extremely rare faults which have occurred:

• the lat/long co-ordinates displayed near the gate position at one international airport were wrong.

• because of a defective component in one of the INS systems on an aircraft, although the correct forward latitude was inserted by the crew (51°) it subsequently jumped by one degree (to 52°).

• the aircraft was equipped with an advanced system with all the co-ordinates of the waypoints of the intended route already in a database; the crew assumed that these co-ordinates were correct, but one was not.

• when crossing longitude 40°W westbound the Captain asked what co-ordinates he should insert for the 50°W waypoint and was told 48 50. He wrongly assumed this to mean 48°50'N at 50°00W (when it really meant 48°N 50°W ) and as a result deviated 50 nm from track.

• the flight crew had available to them the correct co-ordinates for their cleared track, but unfortunately the data which they inserted into the navigation computer was from the company flight plan, in which an error had been made.

• at least twice since 1989, longitude has been inserted with an error of magnitude of times 10. e.g. 100°W instead of 10°W, or 5°W instead of 50°W. Because of low angles of bank, the aircraft departed from track without the crews being aware, and both lateral and longitudinal separations with other aircraft were compromised.

• a crew based at and usually operating from London Heathrow was positioned at London Gatwick for a particular flight. One pilot inadvertently loaded the Heathrow co-ordinates into the INS, instead of those for Gatwick. This initialization error was only discovered when the aircraft had turned back within the NAT after experiencing a GNE.

• the pilot of a flight departing from the Caribbean area input the wrong departure airfield co-ordinates prior to departure. This error was discovered when deviation from cleared route seriously eroded separation with two other opposite direction aircraft.

MORE COMMON CAUSES OF ERRORS

The most common causes of GNEs, in approximate order of frequency, have been as follows:

• 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 reclearance in flight.

• the crew have been recleared by ATC, or have asked for and obtained a reclearance, but have omitted to re-programme the navigation system(s).

• the autopilot has been inadvertently left in the heading or de-coupled mode after avoiding clouds, 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.

• an error has arisen in the ATC Controller/Pilot communications loop, so that the controller and the crew have had different understandings of the clearance. In some cases, the pilot has heard not what was said, but what was expected to be heard.

Operational Height Errors

Most common height errors are caused by:

• not climbing or descending as cleared

e.g. a crew was cleared for a climb to cross 4030W at FL350. The crew mis-interpreted the clearance and took it to mean climb to cross 40°N 30°W (instead of 40° 30¢ W) at FL350.

While this was caused by a seemingly ambiguous clearance, crews must be on their guard and query the clearance if in any doubt.

• not following the correct contingency procedures

e.g. following an engine failure a crew descended the aircraft on track rather than carrying out the correct contingency procedures (see Chapter 12).
 

• An occasional error is to fly at one (uncleared) level and report at the cleared level!

e.g. the crew of a major airline reported at FL360 (the cleared level), all the way across the ocean but were in fact at FL350!! They had been cleared to cross 40°W at FL360 and correctly entered the cleared level into the FMC but did not execute the command prior to 40°W. During position reporting the aircraft level was reported by reference to the FMC altitude hold box.
 

LESSONS TO BE LEARNED

Never relax or be casual in respect of the cross-check procedure; this is especially important towards the end of a long night flight.

Avoid casual R/T procedures. A number of GNEs have been the result of a misunderstanding between pilot and controller as to the cleared route. Adhere strictly to proper R/T phraseology and do not be tempted to clip or abbreviate details of waypoint co-ordinates.

Make an independent check on the gate position. Do not assume that the gate co-ordinates are correct without cross-checking with an authoritative source. Normally one expects co-ordinates 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.

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.

Do not assume that the aircraft is at a waypoint merely because the alert annunciator indicates; cross-check by reading present position.

Flight deck drills. There are some tasks on the flight deck which can safely be delegated to one member of the crew, but navigation using automated systems is emphatically not one of them, and the Captain should participate in all navigation cross-check procedures.

Initialization errors. Always return to the ramp and re-initialize inertial systems if the aircraft is moved before the navigation mode is selected. If after getting airborne, it is found that during initialization a longitude insertion error has been made, unless the crew thoroughly understand what they are doing, and have also either had recent training on the method or carry written drills on how to achieve the objective, the aircraft should not proceed into MNPS Airspace, but should turn back or make an en-route stop.

Waypoint loading. Before departure, 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.

Use a flight progress chart on the flight deck. It has been found that 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.

Consider making a simple use of basic DR Navigation as a back-up. Outside polar regions, provided that the magnetic course (track) is available on the flight log, a check against the magnetic heading being flown, plus or minus drift, is likely to indicate any gross tracking error.

Always remember that something absurd may have happened in the last half-hour. There are often ways in which an overall awareness of directional progress can be maintained; the position of the sun or stars; disposition of contrails; islands or coast-lines which can be seen directly or by using radar; radio nav-aids, and so forth. This is obvious and basic, but some of the errors which have occurred could have been prevented if the crew had shown more of this type of awareness.

If the crew suspects that equipment failure may be leading to divergence from cleared track, it is better to advise ATC sooner rather than later.

In conclusion, navigation equipment installations vary greatly between operators; but lessons learned from past mistakes may help to prevent mistakes of a similar nature occurring to others in the future.

The Prevention of Deviations From Track as a Result of Waypoint Insertion Errors

THE PROBLEM

During the monitoring of navigation performance in the NAT MNPS Airspace, a number of GNEs are reported. (There were 24 in 1997 and 17 in 1998.) Such errors are normally detected by means of long range radars as aircraft leave oceanic airspace. Occasionally, potential errors are identified by ATC from routine aircraft position reports.

Investigations into the causes of all recent deviations show that about 75% are attributable to equipment control errors by crews and that almost all of these errors are the result of programming the navigation system(s) with incorrect waypoint data – otherwise known as waypoint insertion errors.
 

THE CURE

Waypoint insertion errors can be virtually eliminated if all operators/crews adhere at all times to approved operating procedures and cross checking drills. This Manual provides a considerable amount of guidance and advice based on experience gained the hard way, but it is quite impossible to provide specific advice for each of the many variations of navigation systems fit.

The following procedures are recommended as being a good basis for MNPS operating drills/checks:

• record the initialization position programmed into the navigation computer. This serves two purposes:

• it establishes the starting point for the navigation computations; and
• in the event of navigation difficulties it facilitates a diagnosis of the problem.

• ensure that your flight log has adequate space for the ATC cleared track co-ordinates, and always record them. This part of the flight log then becomes the flight deck Master Document for:

• read back of clearance;
• entering the route into the navigation system;
• plotting the route on your chart.

• plot the cleared route on a chart with a scale suitable for the purpose (e.g. Aerad, Jeppesen, NOAA en-route charts). This allows for a visual check on the reasonableness of the route profile and on its relationship to the OTS, other aircraft tracks/positions, diversion airfields, etc.
• plot your Present Position regularly on your chart.

• this may seem old-fashioned but, since the present position output cannot normally be interfered with and its calculation is independent of the waypoint data, it is the one output which can be relied upon to detect gross tracking errors.
• a position should be checked and preferably plotted approximately 10 minutes after passing each waypoint, and, if circumstances dictate, midway between waypoints. e.g. if one system has failed.

• check the present, next and next+1 waypoint co-ordinates against those in the steering CDU before transmitting position reports.

(C) HANKE-Aviation GmbH 2014