World projection used in ND map

[Dan Winterland]

I did ask the question of someone involved in the development of a navigation and display system. Obviously, he could only talk about his system - and it's quite an old one. He stressed that other systems were almost certainly different - but probably not radically so. And his answer is that we should not think too deeply about it! The navigation plan in the Flight Management Guidance Computer uses a spherical model and how they resolved to represent this on the ND wasn't the foremost of priorites at the design stage and wasn't considered too deeply. It's a visual guide only with no real thought to projection as they assumed no pilot would be getting his/her plotter out of their nav bag and placing it against the screen. The plotted waypoints, fixes, beacons and airfields are practically considered being a bearing and distance from the present position.

This system was designed to be used up to 320nm with later developements up to a range of 640nm. At 320nm, there wouldn't be a great deal of distortion. More so at the longer range. He mentioned that in practice, there would be few probelms associated with this as the 'to' waypoint would be on track and any subsequent waypoints would appear as such when they become the 'to' waypoint. I asked about a case where at the 640nm range, you wanted to fly direct to a distant waypoint off track using a HDG mode. He mentioned then yes, there would be distortion initially, but as the heading got close to the required track, then this would reduce and the pilot would naturally correct it. But it would be assumed that the pilot would engage a NAV mode anyway and the kit will be navigating on the spherical guidance computer model. He also mentioned that a pilot probably wouln't be aware of it as there were other factors such as drift to consider which would make any distortion virtually undetectable.

I then put it to him that once on track, the projection on the ND would be a transverse mercator with the origin being the airacrft track. He said that essentially that would be true, although he hadn't really though about it that way. He did stress that in this system, the aircraft was navigation on the most perfect map projection available - a globe - albeit an electronic one. The graphic display provided for the pilot was just a representation of that and should only be taken at face value as the aircraft will be navigating from the guidance computer and not from the display. He did also point out that when this system was developed, computers were considerably less powerful than they are now - and also that the displays of the time were all CRTs which had slightly curved screens anyway - which would not aid any notion of projection! Although there are more modern systems in use, this system is still installed in one of the todays best selling airliners and hasn't changed a great deal since introduction because of the costs of recertification.

So in answer to the original question regarding the 737NG, ''if you would enter 1 waypoint that is, say, 500 NM ahead, would you get a straight or a curved line in the ND?'' the answer according to my contact is straight. And having flown both Airbus and Boeing aircraft, I would tend to agree.