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FlyingForFun
8th Nov 2006, 20:20
Myself and a colleague have been experimenting in our simulator with our GPS. Neither of us have very much prior experience of using a GPS, but with the CAA becoming more and more tolerant of its use on CPL and IR skills tests, it's becoming important to be conversant with it, and train students to use it.

Everything we do with it seems fairly logical - or at least it did until yesterday, when we tried using it for navigational aid on a SID for the first time.

The SID we were looking at was the ORTAC2W SID from Guernsey, which can be found here (http://www.ais.org.uk/aes/pubs/aip/pdf/aerodromes/32JB0602.PDF) (registration required). The text of the SID reads:
Straight ahead to 1836' (1500' QFE) or GUR D3, whichever is earlier. Turn right to track 025°M to intercept GUR VOR R005 to MANTA.....
We then brough up the SID on our GPS. We are using a Garmin 430 GPS. The database is a couple of months out of date (expired August 2006), but the SID plate hasn't been updated since 2001, so I don't expect that to be a problem. (I stress that this was in the simulator...!)

According to our GPS, the first leg of the SID is a 2.9nm leg on runway heading. This matches the plate (except it doesn't cover the possibility of turning at 1836', which I wouldn't expect it to).

The second leg, though, follows a track of 025° to MANTA. There are two interesting points here:

The second leg doesn't start at the same place as the first one finishes. When we tried flying the SID as per the GPS, we turned on command at the end of the first leg, and the GPS then immediately indicated we were several miles right of track on the second leg. Viewing the SID on the map display confirms a gap between the first and second leg.

According to the AIP, there are three legs from depature to MANTA: one on runway heading, one tracking 025°, and a third tracking the 005 radial from GUR. The third of these legs seems to be missing from the GPS SID.

After this, the GPS and the AIP versions of the SID are again the same, routing from MANTA to ORTAC.

So, the questions: is this normal? Is it related to the fact that the first turn might happen at an altitude, rather than a fixed position in space, and therefore the exact track to be followed will vary? (This was my best guess.) Is this feature common to all GPS systems, or just Garmin? Or just the Garmin 430? What would be the "recommended" technique for flying this SID, given that the route the GPS shows to MANTA does not comply with the SID in the AIP (i.e. it does not intercept the 005 radial from GUR at any point)? Is it acceptable to follow this route (which involves, after the initial turn, intercepting and then tracking the 025 QDM to MANTA), or is it mandatory to ignore the GPS and follow the VOR radial to MANTA?

Apologies to those of you who have been using GPS "properly" for years and think this is something that everyone ought to know, but I've spent relatively little time in an aircraft with a GPS except in circumstances where the CAA have told me I'm not allowed to use it anyway!

Thanks for any input - or, even better, written references.

FFF
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IO540
8th Nov 2006, 21:45
This is normal, FFF.

What you are flying are GPS "overlays" for the conventional procedures. These are often incomplete or slightly different.

They are useful for lateral guidance (GPS being much more accurate than VORs/NDBs) but they are usually not suitable for flying on their own i.e. just following the GPS track.

I don't know why this is. There are some technical reasons why some procedures are not fully coded, or not coded at all, into the Jeppesen overlay databases.

I have never found it a problem, but I am sometimes puzzled as to why only half the procedure is actually shown.

There are proper GPS SIDs and STARs in a few places in Europe and they are called RNAV (GPS).

FlyingForFun
8th Nov 2006, 22:18
Thanks, IO540. That's the kind of information I wanted.

I know that procedures like this are not to be used for sole navigation. But I had always thought it was acceptable (and even normal) to fly the procedure on the GPS, with conventional navaids tuned in to confirm that the correct track is being flown. Clearly not - at least not in this case.

As it happens, it's all academic from a training point of view, since the CAA still pretty much insist on using conventional navaids where possible. In the case of this particular procedure, what I was really looking to do was get GPS guidance to ORTAC. Since there is no conventional navaid to track for this leg, the CAA are happy to allow candidates to use GPS in this case (at the examiners' discretion). Anything before MANTA I don't really have any interest in, except for general interest. But this got my interest up!

FFF
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reynoldsno1
8th Nov 2006, 22:57
There are some technical reasons why some procedures are not fully coded, or not coded at all, into the Jeppesen overlay databases.

They are coded - the "problem" lies within the basic GPS receiver. TSO 129 stand alone receivers cannot read conditional waypoints i.e those that do not have a defined geographic position. So, climbs and turns at altitudes, intercepts, etc will be ignored. The database you use is the same as that supplied to an aircraft with a full FMS, which can read all the code.

It is a trap for the unwary. Additionally, if using a GPS to fly a conventional procedure, the database will tend to code any waypoint/fix/rep as "fly-by", by default. The procedure will have been designed for these waypoints/fixes/reps to be "fly-over".

Those countries that have allowed GPS in lieu of conventional aids usually require the underlying aids to be serviceable and actively monitored throughout any procedure.

englishal
9th Nov 2006, 01:45
Funny old chestnut the ORTAC 2W departure........

I have never figured out HOW you manage to follow a magnetic track across the ground without GPS or other navaids......or is it purec dead reckoning;)

tmmorris
9th Nov 2006, 12:13
Must be dead reckoning. That kind of thing crops up a lot in military departure plates - e.g. the Benson SID NW is essentially (without the plate to hand) straight ahead to XXXX ft then turn onto magnetic track YYY and contact Approach. So you need to have an idea of the drift (or a GPS with magnetic track readout :) ) to do that meaningfully.

Tim

FlightDetent
9th Nov 2006, 12:32
The overlay problem starts with the first turn definition right away. Such condition cannot be coded. :=

On all database coding you can have
waypoints - turn at 5 DME
waypoints with constraints - pass 5 DME at or above 2000' then turn right
pseudowaypoints - at 1500' turn right to intercept 320


The sequence whichever is earlier cannot be coded in such industry standard.
The sequence whichever is later cannot be coded in such industry standard.
The sequence at xxx' but not later than can be coded but incorrectly.

Too bad,

FD
(the un-real)

FlyingForFun
10th Nov 2006, 08:16
Ok, this is all fitting in with what I'd guessd myself anyway.

FD, I take on board what you say about not being able to code a "whichever earlier" condition. But, in the case of ORTAC2W, would it not make more sense to at least join up the first and second leg, and re-introduce the missing leg (tracking the 005 radial for GUR)?

Or is there some technical reason why it is not done this way?

FFF
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PS As for how to fly the leg from MANTA to ORTAC, we teach our students to ded reckon it, but as they get close they should be monitoring GUR VOR (probably on the RMI) and DME. If the DME is going to reach its target before the VOR, turn right, and vice versa. The sooner you start monitoring these the better, because you get a better idea of their relative rates of change.