Check your GPS airspace alerts

[Genghis the Engineer]

Okay, first of all, hands up - I screwed up and infringed some class A recently. I've owned up to it, and have been doing a certain amount of introspection to see how I can avoid doing it again.


When covering all bases, I asked myself "hang on, I was using a Garmin GPS with current database and airspace alerts, why the hell didn't it warn me, it warned me about lots of other airspace".

I went into the menus (Tools > Setup > SUA Alarms) and discovered that my Garmin has separate on/off "switches" for:

Class B / TMA
Class C / TCA
Class D
Restricted
Military
Other
Euro Airway.

And whilst I had some of those turned on, I didn't have all of them turned on. I don't recall mucking with these settings since I bought it, so *think* this was the factory defaults.

Experimentally, I told it to restore defaults - and it turned ALL airspace warnings off. Eeek!


So, I've grovelled and apologised to NATS, and I've turned ALL my airspace warnings on.

Can I suggest anybody else who flies with an aviation GPS (which I'm guessing is most people reading this) checks their airspace warning settings, then only one of us has to apologise to NATS.

It would be interesting if anybody else who finds that their GPS wouldn't have warned them of CAS could (after turning all the warnings on!) post here what they found.

G

[Nipper2]

Standby for the "don't rely on GPS" ranters.

[Genghis the Engineer]

I'm usually one of them, and I admit to screwing up my conventional navigation in this case.

But the GPS is there, and it's supposed to be good at this sort of thing!

G

[Nipper2]

I have just checked on my old but excellent Garmin GPS 96c. The different alarms can be set independently and I have them all on.

The US origins of the device are evident in the Garmin Manual which has no mention of any Class D. The good news is that with everything switched on I do get alerts for it, presumably as the device flags CTAs (which is nearly the same).

[peterh337]

What model of GPS was it?

AFAIK most if not all of the Garmin aviation products do not show UK CAS correctly, especially Class A which historically had been depicted as just the airway lines (with no width showing).

I have a yoke mounted G496 which shows airspace of some sort, depicted as a mass of lines which bears little relation to the VFR chart, and keeps popping up spurious warnings which I ignore. I have it there as an "emergency DCT box with a 7hr battery life" for high altitude IFR flights It is also good for computing the ROD for continuous descents to a specific waypoint; continuously updated as you fly along, and when ATC give you a descent you can just load the figure into the autopilot.

IMHO the only GPS that's any good for navigating w.r.t. airspace is something that runs the real "printed" VFR chart. Nowadays you can get them in various forms, but not from Garmin.

But the GPS is there, and it's supposed to be good at this sort of thing!

It's not going to help you if you are under the base of CAS defined as a flight level and the QNH is 970 and the air is ISA-20 (I hope I got that the right way round... it is months since I did the IR Met exam )

[Genghis the Engineer]

An Aera 500 with a current Jeppesen database.

Clearly, if you don't update the airspace database regularly, it'll be wrong! But, I do.

The airspace is all there and corresponds to the chart correctly so far as I can see. The failings were in my navigation, and in the settings for airspace alerts - it was both of those being wrong that got me into trouble.

Going back to my original post, I'm blaming neither the airspace nor the equipment - I'm blaming the equipment settings (and my navigation) and suggesting that everybody else might like to check their equipment settings.

Regarding altitudes - you can also put altitude tolerances on some units: in your case 970hPa/mb/thingies versus QNE gives you an error of 1300ft: yeah, okay, that's going to catch you out, so set the altimeter (or an altimeter if you have two) appropriately as well. That is however another issue.

G

[maxred]

I had one a few years ago, Garmin 296, where I got the warning of airspace near and ahead. I thought I traveresed up the line, (it was tight and I used the GPS to keep me just out and to the right of the line, however, the controller said I had infringed his airspace, bollocked me, and asked me to speak with supervisor on my return. Due apology etc, however, I wondered about the accuracy at the time, I always assumed that it was 100%, and the GPS, did not lie, and conversley that radar would be 100% accurate.

Any comments on the accuracy of airspace lines on GPS maps??

[piperarcher]

I use a Garmin GNS430 in my plane (with an up to date database), and I am sure I have all the Class A + D alarms switched on, and I only see airspace alerts relating to the area around London City. I fly from Panshanger, and often fly through that gap between Stansted and Luton, and even flying close to either zone, or close to the upper limit below the CAS - I never see a warning.

It might be I just dont have my GNS430 set up properly, or as Peter says, it doesnt map the UK airspace correctly. I bought myself a NATS aware a few years ago. For airspace or airfield alerting, its much better than what I could hope to get out of the Garmin, and its a backup moving map in case I need it. The GNS430 is good for many things, but airspace alerts isnt one of them from what I can see.

[BackPacker]

Genghis, was your infringement horizontally or vertically?

Cause the remark by Peter could be relevant. A GPS (particularly a handheld one - I would not know about a panel mounted) will not be able to warn you accurately if you accidently ascend/descend through a certain flight level - which is after all a barometric pressure setting and not a fixed altitude above MSL.

[Genghis the Engineer]

Horizontal, but into airspace that wouldn't have been there if I'd been 1000ft lower.

G

[Dave Gittins]

Talking of vertical accuracy .... went to Prague and back last weekend on an easyJet with my Garmin 196 turned on. The altitude was a pretty moveable feasts through both flights and at one point on the way back when the flightdeck announced we were cruising at 380 the GPS said 36.5.

Wouldn't normally us it for vertical nav at low level VFR altitudes .. but the inaccuracy was a little surprising.

Any particular reason why ?

[BackPacker]

As I said, flight levels are defined as a barometric pressure, while the GPS measures actual feet above MSL (plus or minus a fairly big margin of error - significantly more than its horizontal error).

So depending on the actual QNH, the difference between FL380 (what the pilot sees on his altimeter, set to 1013.2) and 38.000 feet above MSL might be several hundreds of feet.

[Dave Gittins]

Take the point that it may well be a difference between the actual altitude vs. what 1013.2 shows (30ft / mB QNH 1000 = 396 ft) although 1,500 ft seemed a lot .... plus it was constantly drifting up and down by a few hundred feet.

Also when moving the device from the window to the table it was always the altitude that was lost first, rather than position and I wondered whether the vertical accuracy is far less, especially when only about 1/3 of the sky is visible to the antenna.

[RTN11]

Also when moving the device from the window to the table it was always the altitude that was lost first, rather than position and I wondered whether the vertical accuracy is far less, especially when only about 1/3 of the sky is visible to the antenna.

4 satellites are required for a 3D fix, only 3 are required for 2D. If you move the device in the cockpit, it may briefly lose one sat (escpecially older devices without 12 channel) and thus lose the Alt info. Note when you boot it up it ocassionaly says "need alt" meaning it has a 2D fix, and knowing it's alt would help to find the 4th sat to give the 3D fix.

Also, it does depend which 4 sats you get, and where they are in relation to you as to how accurate a fix you end up with. Think of getting a position fix from 3 VORs, it's not so good if they are all on one side of you.

[soaringhigh650]

I'm not surprised you guys keep busting airspace.

All those tiny little fillets do nothing but make your sectionals harder to read.

One good way to avoid airspace incursions in an unfamiliar complex area is to file and fly IFR. This lets ATC do all the work.

[SDB73]

4 satellites are required for a 3D fix, only 3 are required for 2D.

In a perfect world this is true (ish). To get anything close to an accurate fix you need more than four satellites - I would suggest at least 6 or 7.

If you could measure the doppler, distance, position (almanac), etc EXACTLY, then yes, all you need is 4, but we can't get close to that, so need more satellites in order to average out the errors.

-

I personally hate the airspace alerts on my Garmin 795, and 695 before that. They're easy to miss. Especially when you compare them to the SkyDemon, which bleeps away, and puts up a big banner across the screen which requires you to acknowledge it.

[LeeP-PA28]

SDB

My 795 also needs a touch acknowledgement for near / into controlled airspace.
It alerts me 10 mins and 5nm before entering - also tells me if its ahead or above?

I once almost burst controlled airspace (airway) in unfamiliar territory and as such, even for a ponder about, I always draw my route on a map, even if I deviate away from it and highlight controlled airspace.

One route for me is from the Sherburn overhead, to Pole Hill VOR (via Leeds Bradford) and over the IOM VOR. The class A around here is 3500, with an MSA of 2950ft. When one is in IMC, I usually cruise along at 3300ft but always very wary of pressure differences incase you do hit controlled airspace.

I've found the altitude reading of the GPS accurate enough - certainly within 2-300ft on the cruising pressure setting.

Lee

[cct]

My BK AV80 is pretty rubbish as well - despite trying to turn down the sensitivity, I get dozens of messages for airspace I am nowhere near, often repeated ad nauseam.

[RTN11]

I was given an airbox to try out, and it simply bombards you with airspace warnings, from something like 15 miles away. In south UK, you are rarely that far from any airspace.

Then, when you are in class D, it beeps like crazy, and there is no way to stop it. It doesn't take into account you actually have a clearance to be there.

[lordhedges]

If it was an Airbox device, you'd be able to turn on or off airspace warnings depending on their relevance.

If it was an Aware GPS then it was developed to a specification dictated by NATS.