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StudentHenry
28th Nov 2008, 09:21
Good Morning Pprune.

I'm a long time reader and user of Pprune but for my first exciting post I thought I would ask for some advice on the hold.

Let's say we're talking about Cranfield's hold: axis 214. Our wind is 245/40. Inbound drift at 135 IAS is about 9 degrees. Outbound heading with a quartering wind probably about 2x single drift, giving us an outbound heading of 016. Do these figures sound about right?

My second question is about the length of the outbound leg and the effect of a tailwind (in this case). Am I calculating the tailwind component as it will be on the drift corrected heading of 016 or on the original outbound heading of 034?

Thanks,

Henry

Mikehotel152
28th Nov 2008, 10:18
I tried it on Rant using those figures and you get one heck of an overshoot. I'm not sure what you've been taught but you need a bigger gate, so I suggest you try 3x the drift due to the strength of the wind.

Hope that helps.

MH152

Don Coyote
28th Nov 2008, 11:04
Simplest way is to apply one times the drift on the inbound leg and 3 times the drift on the outbound leg. Effectively you are using 1 times drift for the outbound leg itself and the other 2 to offset the effect of the wind in the turn.

Simple and works quite well on standard size holds.

pilotmike
28th Nov 2008, 16:35
Just a quick 'back of a fag packet' answer as I've not analysed it carefully, but...

1. Use 3x drift outbound, which should give you a heading which will be closer to North. This is because you're into what could be called 'dimishing returns'. By this, I mean the more you offset your drift outbound, the greater the crosswind component becomes. Your chosen 016 heading gave a crosswind component of 49 degrees (or about 30 kts), yet the Northerly heading increases that crosswind angle to 65 degrees (or just over 35 kts).

The Northerly heading is required because you need to end up aproximately where you would have been after your final inbound turn with nil wind, though probably slightly upwind of it (closer to the beacon). Because of the Eastward drift during the turn, it requires you to be at your gate significantly West of the position you would have started this turn from in still air, therefore with a much 'wider' gate than usual - just as Mikehotel152 suggests.

2. For the time outbound, I'd be using less than 1 minute, say around 40 seconds. This is not so much because of the outbound tailwind (now you are heading nearer to North, this will be far less significant at around 15 kts than it was when heading 016), but rather it has to do with the expected 40 knot wind inbound, which will have around 35kts 'on the nose' component. Therefore your inbound leg will take over 1 minute, probably around 1 min 20 seconds, so reducing the time outbound by a similar amount is required.

I hope this helps. It is difficult to give hard and fast figures in these circumstances where the wind is strong and the drift angles are significant. You are looking for 'ballpark' figures which give you a good inbound track without overshoot or undershoot, and a total hold time of close to 4 minutes.

The key skill is to be able to make corrections at various points around the hold to compensate for errors as they are identified during the hold, and also to be able to make a second hold which is better, based on the overall errors discovered on the first one.

Happy holding!

Lurking123
28th Nov 2008, 19:48
pilotmike, was that really a quick fag packet answer? :)

PS. 3 x drift outbound (up to a maximum of 30 degs) and about 15 seconds off the leg time.

PPS. Try drawing a triangle of velocities - that will give you the answer regarding the tailwind correction question.

StudentHenry
28th Nov 2008, 21:20
Hey guys,

Thanks for the answers. I think I understand it a bit better now, especially because of the fag packet answer. Maybe if I take up smoking I'll make a better pilot...maybe not! :p

IrishJetdriver
29th Nov 2008, 15:31
Seem to remember that the 3 gates arranged abeam, 30deg and 50deg from the upwind threshold are arranged so that it takes 30 seconds in calm air to fly in each sector. If you are too close then you'll do it in less than 30 seconds and obviously longer if wide. As others state, when adding a crosswind correction then triple the drift needed for the inbound leg when flying outbound. However, if you are extending the outbound leg in order to make an approach then make only a 1x correction for the extension otherwise you'll be miles too wide. Times for each sector are adjusted by 1 second per knot head or tail wind. The 50deg gate is where you start your turn. The question of how you are doing in the final turn is simply answered by looking at your needle in relation to the inbound QDM. With 90deg to go the needle should be 10deg above the inbound QDM and with 45deg to go it should be 5deg above. Easiest with an RMI but also works well with an RBI if you mentally place the needle on your DI. Surprisingly accurate by the way. Don't forget the dip for an NDB hold. Fly an accurate rate 1 turn.

Hope this makes sense as i've been simply selecting ENTER HOLD in the FMS for quite a while now.

Incidentally, my IR instructor's mantra of "slightly wide and slightly long - you won't go wrong" still rings in my ears and was very good advice. If you do want to make an unpublished outbound extension the ask ATC first. It proves that you know what is going on and helps prevent you infringing something you shouldn't.

Upslope
29th Nov 2008, 17:35
I know it sounds a bit complicated when you first start off but...... and here's the BIG BUT!!!! You can plan all you want on the ground with the forecast wind, but you can guarantee that it will be totally different when you get up in the hold. So what I suggest is this....

Fly the RMI and use it as your primary reference with the HSI as a secondary. i.e. if you need to go right or left a bit the RMI will terll you very quickly. If on the inbound you find that you have to offset 10 degrees to the right i.e. 221 to maintain 214 then on the outbound instead of 034 use 004 (3XDrift) and adjust accordingly but as a rule of thumb this will work.

As far as the speed is concerned.... well again judge it.
for example... When on the inbound leg you cross the beacon at 3:20 as opposed to 3:00, then try taking 10 to 20 seconds off your outbound leg. In the real world we don't have the luxury of planning holds if they happen they happen! Now in most jets the computer will fly a perfect hold but on occasion you will fly something like a King Air with a U/S Auto pilot and you will have to work it out on the fly so to speak. I have always used the above method and it has always worked out for me, and I passed many a check using it. Seriously guys, people get too involved in the theory and it all goes to crap.

Don't get too bogged down in the numbers, just fly the aeroplane.
Good luck

Tinstaafl
30th Nov 2008, 03:56
http://www.pprune.org/professional-pilot-training-includes-ground-studies/271232-how-do-you-fly-ifr-procedures.html has a fair few holding tips.

StudentHenry
30th Nov 2008, 11:46
Thanks for the help guys but I think there is something else you need to know. We use Cranfield in the UK for our IR test so we use an NDB for the hold and I don't think a technique based on the HSI and RMI will work!

IrishJetdriver
30th Nov 2008, 12:54
An RBI can be used just like an RMI. Forget there are any numbers on it and forget its got a bottom half. Just mentally transpose the NDB needle on to the DI and you'll be close enough. Remember that if you need to make a correction then it needs to be big enough to make the needle move in the direction you want. Inbound correction is fly same direction as pointer but slightly more otherwise it won't move (push the point), and when outbound then fly away from the tail (tug the tail) again by a big enough amount to make it move. Don't worry about the maths, just transpose the needle. Simple. I taught IMC in a knackered old C150 using the Cranfield NDB and it worked well enough. I've also done raw data NDB approached in a 737-800 admittedly with an autopilot and RMI but it still works !

Mikehotel152
30th Nov 2008, 14:25
I am using the system my FTO teaches and trying to keep things simple as the clever people on here are advising! So, I use 2-3 x single drift and a head or tail wind adjustment of 1 sec per knot. I fly the DA42 which has a nifty wind vector display, which seems to be pretty accurate, meaning you can update your pre-flight planning whilst on the hoof, as it were! :)

Take a 40 knot wind 30 degrees off your hold axis of 214. Max drift in a DA42 flying on one engine at 120 knots would be something like 20 degrees. Drift inbound would be about 10 degrees in this case. Given the wind is 30 or more degrees off the hold axis, we're taught to use 3 x single drift, giving 30 degrees into wind correction and an outbound heading of 004.

And for how long do we keep the outbound heading from abeam the beacon? Well, we're taught to start with a 60 second outbound leg and then deduct the headwind you experienced on the inbound track. In this example you would experience about 35 knots of headwind, so according to the 'system' you would have an outbound leg timed at 25 seconds!:eek:.

I just tried this on Rant XL and you get a gate of 50 degrees and a hold timing of just under 4 minutes despite a large overshoot requiring a big interception angle to the inbound track :confused:

Best I could manage on Rant before boredom near killed me was an outbound heading of 349 degrees - er, that's 4.5 x single drift :hmm: - with an outbound time of 27 seconds - okay, near enough the correct amount of headwind - which gave a gate of 59 degrees but a hold time of precisely 4 minutes :). Of course, this is using Rate 1 turns at 120 knots.

So, given these figures, what I want to know is how on earth you get yourself on to the inbound track without an overshoot, while keeping to a 4 minute hold, and remaining within the constraints of the 3 x single drift 'system'? Surely you can't!?




PS: Don't forget, Gents, that when you're on test you only do one hold unless our first one is brilliant - in which case the Examiner may think you were lucky - or rubbish - in which case the Examiner may feel sorry for you - and let you do a second one.

pilotmike
30th Nov 2008, 15:14
...an outbound heading of 004... and ... outbound leg timed at 25 seconds!.

I just tried this on Rant XL and you get ... a hold timing of just under 4 minutes despite a large overshoot requiring a big interception angle to the inbound track

So, given these figures, what I want to know is how on earth you get yourself on to the inbound track without an overshoot, while keeping to a 4 minute hold, and remaining within the constraints of the 3 x single drift 'system'?


In this example, flying outbound for longer than the 25 seconds you suggest would minimise both the problems you identified - less overshoot and a longer time for a hold nearer to 4 minutes. Using closer to the 40 seconds that I recommended would probably put you accurately onto the inbound track, whilst keeping the hold to little more than 4 minutes.

In my view, it is far better to have a slightly longer hold and be perfectly placed to intercept the inbound track than to use a greater outbound heading correction and risk messing up the inbound intercept just for the sake of a perfect 4 minute hold. It has the added advantage of keeping you within the protected area of the hold as well.

If however a perfect 4 minute hold was essential, then an outbound heading of nearer to 350 degrees for 30 seconds might well be close to optimum, though the outbound heading correction of almost 45 degrees is extreme, and risks putting you outside of the protected area. Much better, in my view, to accept a maximum 30 degree correction, and explain to whoever needs to know that the hold will be closer to 4 minuts and 20 seconds.

Mikehotel152
30th Nov 2008, 15:59
Pilotmike,

I take your point about keeping within the protected area.

On another tack, I gather larger aircraft such as 767s with their clever computers and autopilots alter the angle of bank, presumably to keep the hold as close to a 4 minute racetrack pattern as possible? We're not allowed to do this in the IR, so perhaps should expect holds to last over 4 minutes in strong winds? :confused:

MH152

pilotmike
30th Nov 2008, 17:19
I am not qualified to judge whether stronger winds tend to make for longer holds. I would bow the the greater knowledge of an IR instructor, and probably more to the point, to the (undoubtedly more important) requirements of an IR examiner.

As for the varying bank angle, holds are based on rate 1 turns, and are usually flown at around 210 knots. This conveniently requires a 30 degree bank angle. It is only when flying at other speeds that a different bank angle is needed. 18 degrees of bank is required at 120 kts, whereas a fraction under 35 degrees would be required if a rate 1 turn were to be flown at 250 kts.

ChrisLKKB
30th Nov 2008, 22:00
Just a quick 'back of a fag packet' answer as I've not analysed it carefully, but...

I take it you are a heavy smoker Mike ?

fernytickles
30th Nov 2008, 22:49
"Coping with wind in the hold "

I came on this thread thinking the poster was writing, in a coy way, about a problem with flatulence.....

Coy? On pprune? Silly old me.....