Procedure turn accuracy

[drauk]

I recently got my GPS to record my track whilst carrying out some published holds and instrument approaches. I then overlaid this track on the approach map (on a computer - I didn't sit there plotting points on a paper chart!). I was a bit surprised to see how far off I was in respect of the radius of the turns (particularly in a holding pattern - base turns weren't so bad as I was then tracking something back to the airfield so didn't do a strict rate 1 turn for 1 minute).

I was doing this in about 30knts of wind, but let's assume nil wind. The turns on the map, for a standard hold, are approximately 1.6nm wide (by wide I mean the diameter of the circle of which the turn is a half of). The base turn for a category A aircraft is a similar size. However, a rate 1 turn at 100knts is only about 1nm wide. So clearly doing a rate 1 turn isn't going to follow the track on the map.

My question is, therefore, am I correct in assuming that a properly flown procedure is one which follows the rules (a rate 1 turn for 1 minute) rather than one which follows the track on the map?

[foxmoth]

If you are in 0 wind then you should follow the track on the map but in any wind the track will not fit which is why you use 3X drift outbound. What SHOULD match is the inbound track.

[drauk]

If you are in 0 wind then you should follow the track on the map So in zero wind how do you follow the track on the map for a 180deg turn in the hold? A rate 1 turn for 1 minute will not follow the track on the map if you're flying at 100knts. Are you really suggesting one either speeds up or does a less-than-rate-1 turn?

[foghorn]

Procedural approaches have a protected obstacle-free area that extend well beyond the line drawn on the charts. The line on the chart is for guidance only at certain assumed TAS/ nil wind situation. The protected area narrows down according to certain formulae depending on which portion of the approach you are in and how close to the threshold (on final) you are.

Pull the relevant ICAO docs on procedure design if you're really interested, however I can assure you they're a bit dry...

cheers!
foggy.

Having re-read the thread, i can see that it is about holds and not, as its title suggests, procedure turns, or procedural instrument approaches as assumed. So please ignore my random wibblings above. That's the danger of skim-reading

[foxmoth]

Sorry, had not read the post properly and assumed it was talking about shape not actual path on the map. As stated, radius does not matter, the main thing is that you are correct on the inbound track and the timming is right, rate1 should be used for the turns where this is appropriate for the a/c ( most light a/c).
n.b. an Airbus WILL pretty much follow the pattern rather than fly 3Xdrift outbound, but of course this is done by the computer.

[BEagle]

As far as I'm aware, inbound track to the holding fix is the sole mandatory requirement; 'due allowance' for the effect of wind shall be made to facilitate this.

Why do so many people make such a ludicrous art form of what is really only a fairly non-precision event? To hear some puddlejumper instrument instructors talk, you'd think that the NDB hold is more important than any other aspect of instrument flying. It really isn't!!

[englishal]

Published holds are not to scale...if you're flying a Falcon at 200 kts, then there is no way you'll fit into the nice race-track printed on the charts....However, if the hold is described, or you're told to hold with "5 mile legs" then timing becomes irrelevant.

Cheers
EA;)

[Dave Gittins]

I took my nice new Pilot III up with me to fly a few circuits at Fairoaks a few weeks ago and was amazed when I looked at it again at home, at how the crumbline wandered all over the sky only vaguely following the correct circuit. I was in the normal sorts of positions and following the other traffic in the pattern (about 4 of us trudging round in circles) occasionally extending downwind as folk were back tracking on the runway coz there was only a taxiway at one end, but it surprised me how far from the assumed nice rectangle we really were.

Now I don't see any problem or any danger in this and traffic and terrain separation is still the all important issue, I was just surprised how far from a true rectangle we really fly - It'll probably be a damn site worse with any real breeze.

[slim_slag]

Careful with this 'to scale' thing. I guess it depends on who publishes the plate, but round here the plan view is to scale. Definitely within the 10nm circle, obstacles etc have been placed on the ground where the plate says they are supposed to be :).

Having said that, as ea says, the graphic describing the holding pattern/procedure turn is not to scale. It's more of a pictoral aid to the pilot to let him know what side of the inbound track he should be on. And that's the other mandatory item, if it says left hand turns, you do left hand turns. The other side of the inbound track is not guaranteed to be protected airspace.

[Stan Evil]

I think Aerad plates are drawn at 170 kts (unless it's a Cat A or B only approach). But, as BEagle says, it's not something to get exercised about; holds are just a way of losing time in a reasonably controlled manner and of making sure you hit the beacon in a position where you can go outbound on the procedure safely. Procedure turns are the same - they're a means to an end - vis. getting into a position where you can pick up the inbound course or localiser easily. Obviously, on test, if you make a complete horlicks of it and hold in the wrong direction or get the timing so wrong on the procedure turn that you never see the centreline again then you'll probably be invited to have another go at a later date but, if you make sensible decisions, even if they don't work out quite right, you'll probably get away with it.

[Julian]

Well said BEagle!

Your turn radius will depend on a couple of things, wind being one - speed another. The faster you are moving the bigger the turn so even if the chart was to scale you would never 'overlay' the diagram exactly unless you were spot on the speed assumed on the chart (assuming 0 wind). Dont forget as well that you are speed limited dependant upon the holding altitude (<6'=200kts , <14'=230kts , 14'+=265Kts - Off the top of my head so someone correct me if I am wrong). If you have a xwind during the hold then although you can adjust your legs to meet timing requirements (assuming a 4min pattern) but you won't fly a perfect oval (more like a keyhole) as during the procedure turns you will pushed out on one turn and tightened in on the other due to the effects of the wind.

The main thing to worry about is making sure you stay on the protected side of the hold or procedure turn and dont stray into the unprotected side as it may be for terrain clearance and you wont get a second chance then!!!

Complete the procedure turn at the time required (or within the specified distance required if flying something such a backcourse localiser approach), stay on the protected side and get on the inbound track as Beagle says.

[FormationFlyer]

I agree with BEAgle...

The point of the hold is to waste time/fuel and has little other purposes.

The important bit is the hold entry which sets you up for whatever procedure you are going to do VOR/NDB/ILS etc...

As for accuracy for turns? The point of the procedure turn is to simply position you appropriately for interception of the inbound leg, with regard to obstacles in the local area (and a few other techy points not worth mentioning)...so therefore if you fly the outbound tracks accurately who cares about the turn? All this is important is that you capture the centreline...

With regard to turns in the hold - as long as BEAgle says - you capture the inbound track its really not that important....

I think a lot of pilots especially IR rated like to think there is some mystique & magic in all this procedure stuff - there isnt. Its a question of understanding needles and discipline both in terms of flying and cockpit management.

Hope this helps,
FF