"I may be way off....VOR", on this this?

[speedbirdzerozeroone]

Just a few (hypothetical) questions,

Let’s say I’m flying a 747 along an airway to over-fly a said VOR, (before flying on a new airway away from that VOR) and the aircraft is being flown using the autopilot, with the NAV engaged. Whilst I’m (lets say) 60 miles out then the autopilot should easily stick to the centreline of the given airway but as I approach the VOR it becomes more sensitive (and in high speed aircraft, they may begin a series of ever more dramatic S turns… not that I would let it get to that stage).

My questions are:

1) If you are assigned to fly down an airway (with no Rader Heading from the controller) what is an acceptable tolerance (in miles) to fly along either side of the corridor centreline without getting spotted by the controller! (Before he susses you shouldn’t be flying a kite let alone a big 400!)

2) And also, flying away from the VOR, would remote sensing the VOR beacon on the AWAY radial provide exactly the same centreline as that given by any standard onboard GPS equipment.

…just trying to fly like the Pro’s. (Aren’t we all?)

[CAT3C AUTOLAND]

I am not sure about the technical side of the autopilot on a B747-400, but you mention that the aircraft would start to make S turns the closer it came to the VOR in order to track a radial, or I assume this is what you mean?

I would imagine the aircraft will be flying with reference to the route that was punched into the FMC therefore it would be navigating using the IRS, which would be backed up from the VOR/GPS. As the aircraft approached the cone of confusion it would fly perfectly straight and then automatically track outbound, once again using the IRS etc.

With respect to the tolerances of the airway, I would make the assumption that this would be the width of the airway itself i.e. 5 nm either side of the centreline. If you were to drift out of the airway ATC would no longer be able to offer you a radar control service and would therefore let you know about it. This has happened to me when cutting corners in the airway. Whilst flying to Exeter down L9 from CPT VOR, ATC offered us the option to cut of the corner rather than going all the way to BCN VOR in Wales to save time. Therefore we were outside the airway for a few minutes and our service was down graded to a RAS while flying outside controlled airspace, then upgraded once again to radar control when entering the southbound airway to Exeter.

Not sure about GPS tolerances, aren't VOR +/- 5 degrees?

[Piltdown Man]

Answers, I think:

1. You know it gets more sensitive, so does the designer. Therefore the reactions of the autopilot are "slugged" as it gets closer. But don't forget, the drift will have been calculated a long while back, so you wouldn't expect large heading alterations once this has been determined by the system. As for the appropriate tracking tolerances, these are published all over the place (JAR/OPS etc...) and they all depend what sort of airspace you are in. Generally though, you wouldn't use a VOR "raw data" unless forced to do so. The FMS is a damn site easier!
2. It should do, but...

[speedbirdzerozeroone]

Thanks guys,

I figured that the FMC would kind of make transitions TO and FROM all nice and smooth but I just wanted to know what the proper operating conditions would be like if just NAV was engaged without the FMC prompting the autopilots. However, you guys have pretty much solved any doubts on this.

Cheers.

001

[Pub User]

Speedbird

I'm not sure what you mean by "just NAV engaged". Nor am I familiar with the 747 FMC/autopilot setup, however I expect it's similar to that on the 757/767.

The autopilot does not have a mode, which tracks navaids without FMC 'prompting'. Its lateral modes are such that you are able to either select a heading, hold a heading or follow the FMC (called LNAV - Lateral Navigation).

The FMC uses any known navaids within range to constantly update its known position. Even if you manually restricted it to a particular VOR as you approached it, it still would not slavishly follow the signals, but would merely use them as position confirmation.

I suspect you have been misled by MS Flightsim, which has a realistic-looking autopilot panel, which functions very differently from the real thing.