January 20, 2010

Wobbily (or the "self vector")

This is a VOR.

While most planes that fly around these days are equipped with fancy GPS receivers, there are still a few that don't; namely, some cargo 727s and the Northwest/Delta DC-9 fleet. We also work some smaller general aviation traffic that can't afford, or have no need, to upgrade beyond VOR (ground based) navigation. In addition, many of the no-longer-up-and-coming Very Light Jets never got their GPS's IFR certified.

Aircraft with GPS can be cleared direct to any point they wish, assuming such routing is safe and orderly within the context of the NAS. In my area's case, eastbound traffic can be cleared as far as the start of the North Atlantic route structure (where Canadian controllers transition to non-radar procedures). Westbound traffic can usually get whatever they want, as long as it keeps them north of the NY-Chicago line. Southbound traffic has to go through NY Center, so they don't get anything! Aircraft without GPS are confined to airways that are connected by operational VORs on the ground or by controller provided radar vectors. It should be noted that while GPS aircraft still fly the same routes as non-GPS aircraft, overflying the same VORs and waypoints, these GPS aircraft can still use the VORs in a virtual manner even if they are out of service. There are some conflicting rules about this when aircraft are below radar coverage (MSS departures that have GPS aren't allowed to fly over MSS VOR in their initial clearance because it is not receivable below 10000 to VOR only aircraft). The rules haven't caught up with our technology. We are still forced to clear aircraft short to a local NDB, as I've mentioned in previous posts.

Since VORs are slowly being phased out, less money has been delegated to their maintenance. They have become less reliable and their signals have become less powerful.

While VORs have never been as precise a navigating tool as the GPS is currently, it has become noticeably worse in recent years. Aircraft THINK they are navigating directly to a VOR on their radial, as indicated on their instruments, but their track tells a different story.

This DC-9 was cleared direct and was receiving SYR VOR at point D in the photo above. The aircraft stays on course according to the on-board instruments. Little does the crew realize how many extra miles they are flying by not having GPS. SYR VOR has always had a nasty tendency to make aircraft "dip" to the south, and then once the aircraft gets much closer, they turn back towards the actual VOR location and fly straighter.

I prefer my planes to fly direct to the next point in their flight plan in the straightest line possible, if not just because it looks better, but also, because it is the shortest route out of my sector. That is good for everyone involved. If an aircraft files non-GPS in their flight plan, I try to help them out and keep them close to their intended route as much as possible. This does add workload and radio transmissions, so they can't always be accommodated as I would hope. Also, if a DC-9 is first in line to DTW when there is in-trail spacing...well, I don't want them adding more miles to their route of flight. Every mile that non-GPS aircraft adds to its route, I have to delay the rest of the pack behind them.

So I try to watch the DC-9 as it heads west on J547 (straight line airway between CAM and SYR). They naturally wobble a little as they recapture the outbound CAM radial, adjusting for the wind and the weakened signal. When I get a few target hits in a row that line up well with J547, I jump into action. See point A below.

"Northwest 7xxx, fly present heading, vectors for SYR, expect direct SYR when you get a little closer."

Then once they are approaching SYR (point B, below):

"Northwest 7xxx, cleared direct SYR, contact Cleveland Center 119.37, good day."

These two pictures above are tracks from the same DC-9 flight from PVD-DTW a few days apart. The difference is that I assigned present heading to the second one and didn't let them fly all over the place. Of course, if the plane will need to be vectored for spacing to DTW then I just let the SYR VOR signal do the spacing for me!

This last picture is the GPS equipped Regional Jet that was cleared direct BUF south of CAM and flies a nice straight line to get there. Yes, they got a nice direct route because they had GPS. This RJ flight flies through a completely different sector (Rockdale instead of Utica like the DC-9s above) but only because the of the late night time frame. It flies through after the busy evening traffic but before the last big eastbound rush of the night, with lots of BOS, MHT, PVD, and ISP traffic descending in that airspace. The DC-9s come through in the late afternoon, and even if they had GPS, they wouldn't necessarily get a shortcut. That just wouldn't be safe or orderly for the Rockdale sector.

Now that we've established that GPS is great and planes can go anywhere they want, the next post will discuss why planes don't always WANT to go direct to far away places, regardless of all that NextGen hype about point to point VOR navigation killing the airlines with longer routes and wasted fuel and time...

Till then...


I updated the first picture to include what "direct SYR" should have looked like (the radial, not J547, that the DC9 was following to get to SYR) with a line and arrows.

As always, thanks for flightaware.com for their tracking maps!


Frank Van Haste said...

Dear Mike:

Wow!! I had NO idea that the variability of VOR signals was as extreme as you depict in your first image. I will typically keep the VOR dialed in on NAV2 when navigating /G along the airway and I've wondered why it seems a couple of degrees off sometimes when the VOT has told me my receiver is in primo shape. Now I know!

Q: Do you see the same effect at all altitudes or is it more extreme up in the flight levels where the service volume of the VOR may be a bit sketchy anyway.

This post is REALLY an eye-opener -- thanks so much!

Oh, and re: "Southbound traffic has to go through NY Center, so they don't get anything!" Yeah, right. Tell me about it. :-(

Best regards,


deltamike172 said...

It is definitely worse with the High Altitude VORs, like ALB, BUF, SYR...

DC-9s coming in on J82 which goes from JHW-ALB often end up 15 miles north of the airway (certainly NOT within the lateral confines of the actual airway) and then put themselves on a 30 degree right turn to get back to the radial when the ALB changeover happens just inside RKA sector. Its pretty sad. I've had to double check if they're actually going to BDL.....

ace said...

Although I'm in no position to quibble over whether maintenance or "advancing technology" is any part of it, I can report that it was common in West High at ZAU in the '70s to keep aircraft on a heading much longer than you needed to before issuing a 100 mile or so "direct" to avoid the sloppy navigation.

And it wasn't just IOW--it was also DSM, FOD, and PWE that did the same--the flights would all drift south several miles before heading back to the VOR. It was almost like flying a bearing to a NDB.

Sometimes I tried saying, "cleared direct IOW, but don't change your heading until you get close" trying to give them a clue. Didn't help.

Of course we saw that a lot, as the only RNAV airplanes were 747s (with INS), four legged DC-10s (also with INS), and the odd doppler equipped 707s. Everyone else was VOR point-to-point.

Of course every once in a while a flight would check on and tell us he was "radar vector equipped." That was worth a reward just for the entertainment.

ZJX, ORD, ZAU retired

Praxis Foundation said...

Really an excellent post. Before and after pictures of ground tracks, most excellent!

Anonymous said...

As a controller in the late 70s when Piedmont started service from CLT to DFW we (I) would often ask the crews of the early B737s what their on course heading was from KCLT direct KDFW. The answer was often 257. Clearances would be amended to CLT.direct.DFW to fly heading 257 until DFW suitable for navigation then proceed direct. I did not understand that this clearance amendment suppressed the preferred arrival route into DFW and this totally fouled up strip distribution for the arrival sectors at DFW.


The Piedmont flight crews loved the amendment but understood that they would be put back on a published arrival route to get into KDFW.