IMC Question

[Juicy Lucy]

I have started an IMC course and apart from hating foggles I am finding it very interesting and incredibly baffling.

I have started doing NDB holds and the principle seems straight forward, but I cannot get my head around crosswind calculations. The Thom book 5 says to use 3x allowance and my instructor suggests 2x allowance and has a way of referring to the face of his watch to calculate this. I am completely confused and am a little embarrassed to ask him if he can make it any simpler.

I am begging to the collective wisdom of PPRuNe for assistance. Can anyone give me some easier ways of calculating crooswind values. I do not have a mathematical brain and am getting myself into a state over this.

Help

JL
x

[drauk]

The point here is that a cross wind doesn't have the magnitude of the actual wind unless it is at 90 degrees to your track (think of the whiz wheel from your PPL). However, because few people can do the necessary trig in their head nor do you want to use a calculator/whiz wheel, the watch face idea can be used to give a rough estimate. It's only rough, but it's good enough because (amongst other reasons) you don't accurately know what the wind is in the first place.

It works like this:

If the wind is 15 degrees off the nose, use a quarter of its strength as your cross wind figure (think 15 mins = quarter of an hour)

If the wind is 30 degrees off the nose, use half of its strength as your cross wind figure (think 30 mins = half an hour)

If the wind is 45 degrees off the nose, you guessed it, use three quarters of strength as the cross wind.

If the wind is 60 degrees or more, use it all.

If you can do the trig in your head you'd have got answers of 25%, 50%, 70% and 86% respectively, so you can see it's a pretty good approximation.

The other one you'll want to know is that (in the UK) the wind at 2,000' is approximately double the strength of the surface wind and it veers 30 degrees (i.e. add 30 degrees).

Now stand back and wait for a load of people to helpfully tell you that you shouldn't be embarrassed to ask your instructor things.

[tmmorris]

I'm afraid I was taught 3 x crosswind allowance outbound up to a max of 30deg, though I don't know of a clockface method. (The max 30deg is to make sure you don't get outside the 'protected' area for the hold, though in a light aircraft that's pretty unlikely!).

What method are you using to calculate the crosswind allowance?

Tim

(edited to add: my post crossed with drauk's but I was taught a 1/6ths rule which is basically the same as his: 10 deg off, 1/6 of the wind, 20deg off, 2/6, all the way up to 60deg off, 6/6, after that keep using it all. Obviously also applies to takeoff crosswind.)

[High Wing Drifter]

I have started doing NDB holds and the principle seems straight forward, but I cannot get my head around crosswind calculations. The Thom book 5 says to use 3x allowance and my instructor suggests 2x allowance and has a way of referring to the face of his watch to calculate this.
The proven method is 3xdrift if the wind is from the holding side of the beacon. 2xdrift if the wind is from the non-holding side. Finally 2xdrift if the resultant 3xdrift is within 30deg of the wind (head or tail).

The basic principle is that if you never want to overshoot the inbound track as it is difficult to get back on it, but if you are wide, holding off on the inbound turn is easy.

Re-clarify with your instructor for the conditions at the time and see if this is what he was referring to.

[Fuji Abound]

Firstly, the trouble with using 3x the cross wind component or twice the component is that these are quite course rules of thumb. Far better use 3x the cross wind component BUT reduce this to twice the cross wind component if the heading is within 30 degrees of the outbound track.

I find the best way of "working out" the cross wind component reasonably accurately is to remember for 20 degrees of cross wind it is about a third, 45 degrees about two thirds, and 70 degrees or more nearly all cross wind component. There are other "rules of thumb" but you will not be too far out.

There are a few other "tricks" he may have mentioned. If you look at the plate you will see that as you complete your outbound turn you should end up at right angles to the beacon. If you do not logically the bearing to the beacon will provide a give away as to whether you previously made the "corect" adjustment to the abound heading. Exactly the same is true of the base turn.

There are (I guess they are still around) one or two really simple downloads which simulate flying an NDB porcedure allowing you only to adjust the wind heading and component and the course you would fly outbound. They provide a brilliant way of enabling you to build up the mental picture of what is going on.

[IMC007]

Hi JL

The advice given by Drauk is correct and a good explanation, however I've always found it easier to get the student to use and visualise the 'clock' face using one of the VOR roses in the aircraft (as long as you aren't using it), that way you can dial up the track required and visually see where the wind is coming from and apply the 15, 30, 45, 60 degrees off method.

Furthermore you should not be embarrassed to ask your instrcutor to explain it again, that is what he/she is there for and I would imagine they would much rather you ask a few times and get it sorted out in your head on the ground then look as though you don't know what you are doing in the air, plus it's very costly to get it wrong in the air. I have no problem at all going over stuff with my students again and again until they get it, it's my job.

Good luck.

[IO540]

All the advice above is good advice but the following is based on my experience, through the IMC Rating and later the FAA IR.

The best thing is to spend £50 or so on FS2004, with a dirt cheap joystick, and fly some holds. There is a map feature which shows your track and it's very revealing...

All these rules of thumb get discarded as soon as one gets some situational awareness, and that comes only with doing a bit of flying.

Until one gets to that stage, the pilot shops will be doing a roaring trade in all sorts of daft gadgets for working out hold entries, drift, overhead joins, etc etc.

A flight sim is absolutely brilliant for instrument training. It will pay for itself on the first flight which, for a change, won't be substantially wasted. That's another thing about instrument training: the real learning one does on the ground (perhaps with a sim); in the air you just consolidate the stuff under extra pressure from having to do the radio as well. A lot of IMCR instructors put the student in a plane and fly some NDB holds; the student's head gets comprehensively done in by this.

A friend of mine has done the JAA ATPL and he was showing me his copious notes on absolutely countless little rules of thumb on instrument flying. Somebody at the CAA has worked out various obscure rules about "gates" and all sorts of ways of telling how the hold is working out while you are flying through different parts of it. The result is an absolute mental torture, which the FAA has comprehensively discarded, unsuprisingly. But I can see that if somebody is going to reach a very high standard of accuracy, without actually doing much real flying around, they need to learn all this stuff.

Get FS2004, open up something like a Mooney Bravo (nice instruments), position it near some NDB or VOR, and fly around.

[Fournicator]

Depends on the length of the outbound/inbound legs. As you can't correct for tracking errors in the turns, you must allow for the drift during these turns on the outbound leg. You don't need to correct for the inbound leg as you'll be tracking the beacon. Now for the maffs, bear with me.....

Both turns are Rate 1, hence you'll spend a total of 2 minutes in them. The aeroplane I usually fly flies around the hold at 180 knots TAS; a 3nm outbound leg therefore takes 1 minute, and I must therefore apply 3 times the drift, as I must do 3 minutes worth of correction in 1 minute. The basic drift is calculated from consideration of max drift (wind speed/groundspeed in nm/min) and the clock rule described above.

Similarly, a 4nm outbound leg at 120 knots TAS would take 2 minutes, and you'd therfore apply double the normal heading correction. Hope that all makes sense; feel free to point out if I've missed something or am at all unclear anywhere.

[Whopity]

The Thom book 5 says to use 3x allowance and my instructor suggests 2x allowance and has a way of referring to the face of his watch to calculate this. Then if he is an INSTRUCTOR he should explain how and why! The fact that he applies 2x drift implies he doesn't understand the reason for 3x i.e you are correcting for 3 minutes worth of flying in one minute.

Drift calculations are exactly the same as you use on VFR nav!

If you have a GPS look at the Track; the difference between your Track and your Heading is the Drift! Makes things much easier.

You should also be taught to look for the needle in the correct place when you start the inbound turn, then at 90 degreees to go and at 45 degrees to go, so that you know where you are in relation to the inbound radial.

[Fuji Abound]

"As you can't correct for tracking errors in the turns"

What about the turn early or late depending on where the beacon is mid way through base.

[High Wing Drifter]

As you can't correct for tracking errors in the turns
You can't if you are inside the hold, you can if you are outside the hold.

I think overall, everybody seems to have a different way of doing holds and this must be extremely confusing for Juicy Lucy! The Thom method is a simplified hold technique. The 2xdrift is valid under some circunstances depending on the conditions on the day. I suspect that is the case. Juicy just needs to re-clarify with the instructor to get that straight and take advice on how to do the hold from just the one person - her instructor :ok:

[drauk]

Perhaps the most important thing to remember Lucy is that you're not tested on holds as part of an IMC test. Of course, if you're using your IMC rating in anger you might one day have to hold, but if you're flying a slow plane in England almost any attempt at a hold will be okay. Maintaining your level is important, but otherwise you've got loads of space. Once you've been doing instrument flying for a while they'll get easier (and/or you'll use a moving map GPS).

[DRJAD]

If you can do the trig in your head you'd have got answers of 25%, 50%, 70% and 86% respectively, so you can see it's a pretty good approximation.

That is the kernel of it all. In other words think of the shape of a sine wave, that will tell you roughly how much value to apply to the actual crosswind according to its angle.

One of the problems with flying tuition, IMO, is the plethora of little rules of thumb and mnemonics. Its considerably easier I think to equip oneself with the basic mathematical thinking to produce reasonable estimates, rather than try to learn which of the rules of thumb, etc., (which might be alien to your personal way of thinking) is supposed to apply in the particular case.

[High Wing Drifter]

Perhaps the most important thing to remember Lucy is that you're not tested on holds as part of an IMC test.
You are tested if ATC put you in a hold. Unlikely, but possible.

[Morgo]

Perhaps the most important thing to remember Lucy is that you're not tested on holds as part of an IMC test.
My IMC examiner most certainly tested me on holds!

[IO540]

It's true that one may not get tested on holds but IMHO the IMCR gives you practically all the privileges of an IR (except no Class A, min vis 1800m, and IFR in UK only) and the pilot ought to be able to acceptably fly any instrument procedure that somebody stuff in front of his/her face.

Fortunately it isn't hard to do any of this stuff. It's hard to do it to perfection, with perfect wind corrections etc etc. but perfection is not actually needed, because the protected hold area is vastly bigger - it's big enough for a 747.

[drauk]

From LASORS:

The test includes full
and limited panel instrument flying, use of radio
navigation aids whilst flying by sole reference to
instruments, instrument approach procedures, bad
weather circuits and landings.

I've heard it said that if you get put in a hold it forms part of the test, but I've not seen it written down anywhere.

[mark147]

I've heard it said that if you get put in a hold it forms part of the test, but I've not seen it written down anywhere.
Have a look at CAA document SRG1176, the examiners record for the IMC test. You can find it here: http://www.caa.co.uk/docs/33/FORSRG1176.PDF

Note 3 says "Holding is not a required test item on the IMC Rating Test. However, if the applicant is required to hold by ATC then the hold should be assessed."

Mark

[Longbow55]

IO540 mentione dit, get a simulator program, MS flight sim 2004, ASA IP Trainer, I used both, but used the IP trainer the most. you can spend hours doing different manuvers and it doesn't cost you a dime. then you can hop in your trusty plane and practice. I was taught start with twice the correction.

[tmmorris]

You are tested if ATC put you in a hold. Unlikely, but possible.

Happened to me, the first 'real' hold I'd ever had (at Oxford). Also had to work out if at our rate of climb I would be high enough to enter the hold safely (answer: no, required an orbit to gain height before joining). Also one of my first experiences of real IMC. Altogether bracing, but I did pass!

Tim

[IO540]

I suspect some of the 2x v. 3x confusion may come from different ways the hold is taught.

Let's say you have a hold on which the inbound leg is 270, and the outbound leg is (obviously) 090 (to keep the picture simple).

If there was no wind I would fly the inbound at 270. Then I would turn onto a heading of 090 (not, as is often taught, fly a rate-1 turn for 1 minute) and, if there was any wind apparent on the previous (inbound) leg, apply the opposite wind correction.

Let's say the wind is from the south, 20kt. This will manifest itself through me having to fly a heading of about 260 to track the inbound (270) leg. The easy rule being "max drift is half the wind" tells me the wind is probably 20kt but I don't need to know this! All I need to know is the 20deg offset, so when I turn onto the outbound leg, instead of turning onto 090 I turn onto 100. Easy.

Done this way, one applies one wind correction on the inbound, and one wind correction on the outbound.

You can call it unconventional but it works extremely well. It also prevents you totally fu*ki*g up the whole thing if something goes wrong with the stopwatch... (which never happens ;) )

Critics will say I fly holds on headings rather than the stopwatch because I cheat and watch my actual position on the GPS. In fact I did all the stuff in the IR training and checkride this way and nobody said a thing. The only time I was timing turns was on partial panel, when the only instruments not covered up were the TC, the liquid compass, and the altimeter.

I don't really know why holds are taught to fly the curved legs by timing them for 1 minute. It yields inaccurate headings, unless your rate-1 is held precisely which is impossible to do in any turbulence. It also requires a 3x wind adjustment which is excessive at spamcan speeds - because the whole business of adding-up wind corrections relies on trigonometry of small angles, and 30+ degrees isn't a "small angle".

[Dr Eckener]

I don't really know why holds are taught to fly the curved legs by timing them for 1 minute

They aren't

The question should be moved to the instructors forum for some replies from people who actually teach this stuff.

Ask your instructor for clarification. That is all that is required. To fly a hold to the standards required for the IR takes several hours, time which does not exist during an IMC course, so things tend to get simplified. This is not to reduce the quality, it is just that the required outcome for the IR (inbound track +/- 5 degrees for 15 seconds) is not something anyone cares about in the real world.

[Fournicator]

Because I'm sure some of those hours building instructors know better than us mere mortals........

[DFC]

The following method for flying the hold is simple and will get you a pass on the CAA initial IR flight test. It is relatively easy and works in the real world as well

Basics;

1. You have to remain within the holding area and you have to establish on and track the inbound track for a "reasonable" amount of time during each hold.

2. While keeping it within the holding area it is good to be both slightly wide and slightly long at the end of the outbound leg.

3. The inbound turn from a slightly wide position should allow you to stop the turn with heading of about 60deg to go to the inbound track and lock the wings level so that :-

a) You remove the dip error; and

b) You can nicely judge the turn to exactly establish on the inbound track.

4. Have the gate and abeam point drawn on your chart for ease of reference.

To allow for wind - use 3* the inbound drift for the outbound unless that heading is within 30deg of the wind when you use 2*drift.

At the beacon start the stopwatch and turn onto the outbound heading that you have judged.

If you steady on the heading before passing abeam, check the time it takes you to reach the abeam point (you have a headwind outbound to allow for in the timing).

At the abeam point restart the stopwatch.

If you steady on the heading after the abeam, restart the stopwatch and remember you have a tailwind outbound to allow for in the timing.

If you reach the gate +/- the drift angle before time time is up then track the gate away from the beacon until the time is up.

If you are sightly wide and a few seconds long at the end of the outbound leg, you are guaranteed to make the hold.

Turn onto a heading 60 degrees from the inbound track (90 deg may be required if turning into a very strong wind across the hold). Level the wings and concentrate on keeping the wings level.

Now with no ADF dip to worry about, wait for the pointer to indicate about 10deg from the final track (depending on wind) and start the turn to establish inbound.

You are going to end up inbound +/- 5 degrees with some time to run to the beacon.

Repeat the process while refining the hold as you go!

-------

IO540,

Your system does not allow for the difference in radius of turn at each end in a crosswind i.e. turn into wind is small radius leaving you close to the inbound track, then the turn out of wind is large radius taking you screaming through the inbound track.

I love your comments when the only instruments not covered up were the TC, the liquid compass, and the altimeter.

Where was that then?.........A school that thought the ASI and VSI also failed when the gyro's failed? :D

Regards,

DFC :p

[MyData]

As someone going through IMC training at the moment it is threads like this one and the one on 'Descending through cloud..." that really demonstrate the value of PPRuNe. Thanks to everyone for their input.