GUARDING AGAINST COMPLACENCY. Es-pecially since 1977, when the MNPS rules were introduced, careful monitoring procedures have enabled the NAT Provider States to obtain a good indication, both of the frequency with which navigation errors occur, and their causes. Their frequency is low, and only one aircraft in many thousands is found to have a serious navigation error. Navigation systems are now so reliable that a typical crew member could theoretically spend his complete career flying across the At-lantic without ever being more than five miles from cleared track. Not surprisingly, this may eventually lead in some cases to over-confidence.

RARE CAUSES OF ERRORS. To begin with, and to illustrate the surprising nature of things which can go wrong, here 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 chip in one of the INS systems on an aircraft, although the correct forward latitude was inserted by the crew (51º) it subsequently “jumped” by one degree to (52º).
· The aircraft was equipped with an advanced system with all the co-ordinates of the way-points of the intended route already on tape; the crew assumed that these co-ordinates were correct, but one was not.
· When crossing 40ºW westbound the Captain asked what co-ordinates he should insert for the 50ºW waypoint and was told “48 50”. He wrongly assumed this to mean 48º 50’N at 50º 00W and as a result deviated 50’ from track.
· The flight crew had available to them the cor-rect co-ordinates for their cleared track, but unfortunately the data which they inserted into the navigation computer was from the com-pany flight plan, in which an error had been made.

COMMONER CAUSES OF ERRORS. However, the most common causes of gross errors, in ap-proximate order of frequency, have been as fol-lows:

· 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 locating through the same latitude at several way-points (eg 57ºN 50ºW 57ºN 40ºN 57ºN 30ºW) changes by one degree (eg 56ºN 20ºW). Other circumstances which can lead to this mistake being made include getting a reclear-ance in flight.
· The crew have been recleared by ATC, or have asked for and obtained a reclearance. But they have forgotten to re-programme the INS or ONS.
· The autopilot has been inadvertently left in the heading or de-coupled position after avoiding clouds, or left in the VOR position after leav-ing the last domestic airspace VOR. In some cases the mistake has arisen during distrac-tion caused by SELCAL or by some flight deck warning indication.
· An error has arisen in the ATC-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 he wanted to hear said.

LESSONS TO BE LEARNT.

· Never relax or be casual in respect of the cross-check procedure, this is especially im-portant towards the end of a long night flight.
 
· Avoid casual RT procedures. A number of gross errors have been the result of a misun-derstanding between pilot and controller as to the cleared route. Adhere strictly to proper 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, of course, you expect co-ordinates to be to the nearest tenth of a minute, so make sure that the display is not to the nearest hundredth, or in minutes and seconds. And if you are near to the prime meridian, remember the risk of confusing east and west.
 
· Before entering Oceanic Airspace make a careful check of INS/ONS position at or near to the last radio facility - or perhaps the last but one.
 
· Do not assume that you are at a waypoint merely because the alert annunciator indi-cates; cross-check by reading present posi-tion.
 
· 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 partici-pate in all navigation cross-check procedures.
 
· Initialisation errors: Always return to the ramp and re-initialise INS NAV mode is se-lected. If after getting airborne it is found that during initialisation a longitude insertion error has been made, unless you thoroughly un-derstand what you are doing, and have also either had recent training on the method or carry written drills on how to achieve the ob-jective, you should not proceed into MNPS airspace, but should turn back or make an en-route stop.
 
· Waypoint loading. Before departure, make a check that the following agree: computer flight plan, ICAO flight plan, track plotted on chart, and if appropriate, 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 Pilot-Chart on the flight deck. It has been found that making periodic plots of posi-tion on a suitable chart and comparing with current cleared track, greatly helps to pick up 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 stuff, but some of the errors which have oc-curred could have been prevented if the crew had shown more of this type of alertness.
 
· If you suspect that equipment failure may be leading to divergence from cleared track, it is better to advise ATC early rather than late.
 
· In conclusion, navigation fits vary greatly between operators. If any of the above does not apply in your case, remember that it may help to prevent someone else making a mis-take, and may stimulate you to avoid mistakes of a similar nature.