GPS altitude and circuits

[Hildy]

I have now flown 3 hours, 2.5 of which have been flying circuits. See http://url-is.com/flighttrack for my log of flight hour 2.

 

If you look at the track side-on in google earth, there is something noticeable about the final circuit. By looking at the speeds, you can see where touchdown happens because the speed drops down quickly at that point (although it only logs every second). Anyhow, the altitudes after that point continue to drop - about another 50 feet, so that the final approach looks more like a spiral.

 

Is there any way to improve the altitude accuracy of the GPS? I think that there's an inertial term in my GPS's firmware that thinks that because I am descending, I will keep descending - until there is a solid lock that proves that I am moving level again.

 

Otherwise, can someone suggest a better GPS unit?

 

Next lesson: touch-and-goes.

 

 

[farri]

Hi Hildy, You say you`ve only flown 3 hrs! I`m interested to know if the use of a GPS, while doing circuits, is your choice or the way it`s being taught these days?

 

Frank.

 

 

[fly_tornado]

I think your GPS is a bit slow. look how far apart the tracks are on the runway. on my GPS they are all on top of each other.

 

 

[Guest davidh10]

Frank;

 

I cannot answer for Hildy, but there was a GPS in the aircraft all during my training. It was used after landing to review the accuracy of circuit patterns, accuracy of turning about a point, figure eights and approach line-up with runway. Although the instructor would be able to accurately evaluate these things while flying, I found it useful feedback from a student perspective. Once I started Navs, it was used as a compass only. It wasn't until I completed my Pilot Certificate that I learned how to operate the GPS.

 

In my own aircraft, I still use it largely as a compass, but also have a physical compass as a back-up in case of GPS failure. I still review nav routes for accuracy after the flight.

 

 

[facthunter]

i'm flat out wondering why you are even involved with a GPS in the circuit at low hours. You should be concentrating on attitude used to maintain altitude, and your airspeed, tracking accurately, as well as keeping a GOOD lookout outside. leave the GPS alone at this stage and learn to fly the thing first. Nev

 

 

[Spin]

I doubt Hildy is having anything to do with the GPS whilst in the circuit, it more than likely gets turned on with the avionics during the preflight and from then acts as a data logger until turned off again.

 

I must say that although they are undoubtedly clever and useful bits of kit, I find it more useful to have an instructor point out via visual cues where I might be going wrong, rather than rehashing the flight on screen afterward, when I don't have any opportunity to correct things. Dunno, maybe it's a generational thing:big_grin:

 

 

[farri]

Nev, you`ve said it for me!!!

 

Though I`m aware of the advantages of a GPS in the circuit,I`m also aware of the disadvantages and there are more of the latter.

 

Frank.

 

 

[farri]

If dependant on the GPS, what happens when a new airport or landing site has to be used?

 

There are no waypoints in the GPS and memorised reference points won`t be there either???

 

Frank.

 

 

[slartibartfast]

Just for those who are new to this (I know you know David) - remember a GPS can only act as a compass if there is no crosswind. A GPS shows you track made good, not where you are pointing.

 

 

[Hildy]

I doubt Hildy is having anything to do with the GPS whilst in the circuit, it more than likely gets turned on with the avionics during the preflight and from then acts as a data logger until turned off again. I must say that although they are undoubtedly clever and useful bits of kit, I find it more useful to have an instructor point out via visual cues where I might be going wrong, rather than rehashing the flight on screen afterward, when I don't have any opportunity to correct things. Dunno, maybe it's a generational thing:big_grin:

This is my own personal GPS. My instructor has nothing to do with it.

 

What I am using it for is to give a number for how well I am doing. So, I fly using visual cues, and afterwards, I measure the average track error in both lateral and vertical directions. This, obviously, doesn't work if it's not accurate - or rather, I can't get a "score" better than the error in the GPS.

 

The width of my downwind leg is about 180m. So I am concentrating on trying to fly the same track every single time (not plotted on the GPS, but by picking the visual cue points and banking the same amount for the same amount of time) to see if I can get that down.

 

I've found that downwind is the hardest part to fly accurately because of the lack of visual cues.

 

 

[Hildy]

I think your GPS is a bit slow. look how far apart the tracks are on the runway. on my GPS they are all on top of each other.

No, that's my awful flying.

 

 

[facthunter]

Primarily refer to the runway. Other aerodromes will not have visual cues, as tracking or turning points. (that you know about). Make all your turns and tracking in relation to the runway. Allow for known ( or encountered ) winds. Nev

 

 

[Hildy]

Primarily refer to the runway. Other aerodromes will not have visual cues, as tracking or turning points. (that you know about). Make all your turns and tracking in relation to the runway. Allow for known ( or encountered ) winds. Nev

How do you pick

 

- how long before you turn crosswind after climb

 

- how long the crosswind leg is

 

and how do you accurately maintain a certain distance from the runway on downwind?

 

 

[facthunter]

It is a judgement that you will acquire, ( IF you are taught properly, and absorb it). Give yourself time. Normally turn crosswind on height achieved. The distance out for downwind turn is done by looking back at the strip. Nev

 

 

[Thirsty]

I find in the aircraft I fly that turning downwind once at cct height is about right ie climb on upwind to 500' then turn crosswind and fly to 1000' then turn downwind. Of course that can change depending on the climb performance of the aircraft you're flying and the wind on the day but as a rough guide it works for me. You'll get to know your aircraft and you'll also start to get a good feel for ccts - you'll do enough of them :)

 

 

[Hildy]

It is a judgement that you will acquire, ( IF you are taught properly, and absorb it). Give yourself time. Normally turn crosswind on height achieved. The distance out for downwind turn is done by looking back at the strip. Nev

Aha! I think that's the bit that I'm having problems with. I can't estimate land distance in the air yet.

 

 

[Guest davidh10]

Just for those who are new to this (I know you know David) - remember a GPS can only act as a compass if there is no crosswind. A GPS shows you track made good, not where you are pointing.

If you have a track (ie destination or waypoint) programmed, then you get a "Bug Indicator", that shows "Track Made Good", but it also shows the compass direction you are actually flying (not heading), so yes, cross wind will cause track and heading to be different.

 

 

[fly_tornado]

GPS tracking is a good tool for checking your accuracy in circuits, doesn't really help you much with the round out and flare as its a bit too slow/inaccurate

 

 

[Guest Pioneer200]

My advice is get rid of the GPS if you are only flying circuits.

 

I can't understand why you would want to and need to look at the data of your flights when doing circuits.

 

Its all about PRACTICE, you should get a good feeling of when to turn crosswind and downwind etc after a few hours.

 

Its a busy part of flying without being distracted by a GPS.

 

 

[Yenn]

If you wantt to end up not knowing what you are doing, by all means keep watching the GPS, but remember even if you do perfect circuits, all exactly the same track, you will still not be doing it correctly. Just think about it, do yoy fly the same track if the wind is 5kts down the runway, as you do when it is 20kts. Surely higher headwinds entail earlier turns onto base leg and much shorter final legs. How about coming in with a slight downwind component to an uphill landing.

 

I really cannot see any useful purpose in using GPS except if the conditions never vary. Worrying about the GPS record is going to slow down your progress.

 

 

[Guest basscheffers]

Is there any way to improve the altitude accuracy of the GPS?

Yes, it's called WAAS and could have been launched relatively inexpensively on any of the Optus communications satellites that went up over the past decade.

However, in their commercial wisdom, Airservices decided they were going to create a ground based augmentation system instead and charge people for using it. A stupid idea that was never implemented.

 

So now we have neither. Great.

 

 

[Guest davidh10]

I've found that downwind is the hardest part to fly accurately because of the lack of visual cues.

It shouldn't be as you are flying parallel to the runway. As you turn onto downwind, look to see that you are flying parallel and then pick a target point ahead to fly toward. As you have only a couple of hours, I wouldn't worry, as you will get the hang of it with practice. Talk with your instructor about it. You wll learn about following roads, compensating for cross wind and the rest in good time. All these skills will help with the accuracy of your circuits.

 

Don't score yourself. Just note where you are deviating from a rectangular circuit pattern and discuss with your instructor. If he hasn't mentioned it, that is because you are doing fine along the learning curve :-)

 

 

[foxy]

Hildy,

 

as has been said above- id ditch the gps...at least until you have finished your flight, then you can review it, but i wouldnt use it in flight.

 

The constant viewing of the runway is what will determine your circuit performance. The gps is going to add too much to your mind at this stage, and can be a hinderance.

 

With regard to how long before turning, etc in the circuit, this is the approach i use.....continue on upwind until 500ft has been reached.....turn onto crosswind, looking underneath my wing, ( i fly a Jab), until i can see the runway appear....level the wings...this concept usually gives a pretty good 90 degree line from the upwind point.

 

when i have reached 45 degrees from the end of the runway, is when ill turn downwind....roll the wings level, to track parallel to the runway....always looking at that runway throughout downwind to determine any drift you may encounter.......at 45 degrees from your threshold...turn onto base, again giving you a nice 90 degree line to the runway. the turn onto final is pretty simple, and you will pick up the turning points eventually, as the wind will always play games with you. Usually, in the Jab with an approach speed of 65-70knts, i aim to turn final at 650ft, which should allow for an established, wings level line on final, by 500ft.

 

i know its a bit long, but i hope it helps.

 

Liz.

 

 

[sfGnome]

Folks, have a look at what Hildy said in his first post. "If you look at the track side-on in google earth...". He isn't using GPS to as a navigation tool in the circuit, he's using it to review his flight performance back home in the comfort of his computer chair.

 

I did (and still do) exactly the same thing with a small portable GPS recorder that sits tucked out of the way, silently remembering what I've done. When I was learning circuits, it taught me that those nice 90deg turns were anything but, and these days I still review every flight later that night for track and altitude accuracy.

 

Hildy, the most likely reason that your altitude track doesn't look as expected is that the GPS will be 'smoothing' the recorded path - it assumes that a sudden change in direction (ie no longer descending) must be wrong, and so it continues your downward path for a little while. You may be able to remove the smoothing if you dig around in the configuration of the GPS or in the program that you use to upload the data, but I wouldn't worry too much. You know with much more accuracy whether you hit the ground hard or rounded out too high than any GPS recorder ever will.

 

(p.s. I just looked at your track. Given that I can see where you wandered around in the instructors office and when you did the pre-flight check, I'm guessing you were using a sports gps watch. Right? :thumb_up:)

 

 

[Ferris]

Hildy, Firstly I would like to welcome you to the aviation fraternity. It is true that when you discover flight that your eyes are forever cast skywards (with apologies to the original author). I fear that by using a GPS in the circuit you are developing a very bad habit, that arguably could be quite dangerous. Whilst in the circuit, the pilot workload is at its highest and also with the highest risk of collision, without being distracted by a GPS. If you must have it going , then put it away where it will not be a distraction. Also at some stage your instructor is going to want to see you competently fly a circuit without any instruments at all, so you need to get a feel for the aircraft and learn the visual clues. Once you go solo and start your navs you have to be able to navigate without a GPS, otherwise the CFI won't sign you off. GPS are great units and really useful, but they make us lazy - and this from a bloke that has one running on every x country. Hildy in short you are better off putting the GPS away until you've learnt the basics. Regards Ferris.