Calculating actual wind

[ACACIACREEK]

If you are flying an aircraft not equipped with an air data computer, is there a formula that allows you to work out exactly what direction the wind is coming from and its strength by seeing the single drift on the RMI and the groundspeed from a DME unit? Given a wind direction and strength from the MetOffice, I have no problem calculating the theoretical single drift and groundspeed to be expected, but I am more interested in the real world case where you have to work backwards from the information presented to you on your instruments. Am I correct in thinking that there is such a formla/formulae, and, if so, what is it/are they and in what order do you go about solving the wind? Perhaps I am wrong and you need information not available to you to solve the exact wind direction and speed.

Any ideas?

Thanks

[galaxy flyer]

If you know TAS, crab angle (difference between track and heading, a.k.a. drift angle depending) you can find crosswind by:

TAS (in nm per minute i.e. 420 kts is 7 nm per min.) X crab angle = crosswind component.

TAS + or - ground speed = headwind or tailwind component.

E.G. 480 KTAS with 5 degrees of right crab and 420 KGS =

40 kts of cross wind and 60 kts of headwind.

Next step is to scale out those two components on the RMI, as in TACAN point to point navigation

GF

[ACACIACREEK]

I don't think an IR examiner would allow me to whip out a CRP-5 or an electronic version. Taking a 330/30 wind and tracking 360 degrees. Max drift would be 15 degrees, single drift 7 degrees and ground speed about 93 knots from a TAS of 120 knots, giving you a heading of 353 degrees. From my RMI and DME, how would I be able to see that the headwind of 27 knots comes from a wind from 330 degrees and 30 knots strength, and not from a wind further to the left, say 310 degrees, but of greater strength. This wind would also give you a ground speed of 93 knots, albeit the single drift heading would also change to 346 degrees.

Clearly I don't want to tie myself in knots up there but there is only one wind from a certain direction and of a certain strength that will give you a certain single drift and a certain ground speed. If someone can say to me......"okay first look at your single drift......blah blah.....then take your ground speed and do this....blah blah".......which will allow me to tie this wind down to an exact strength or direction, or say " you cannot work this out without a CRP-5", or even " you cannot work it out exactly at all", then, fair enough, I'll guess I'll either have to fly what I planned based on the forecast or "wing it".

What is everyone else outwith the air data computer (and the magenta line chasers) world doing?

[ACACIACREEK]

galaxy flyer,

I tried using your method and it works brilliantly. I think this is based upon the Cosine and Ratio stuff I've been reading about recently. Thanks very much!!!

[ACACIACREEK]

galaxy flyer,

Sorry, I got a bit ahead of myself there. Yes, if you imagine the line from 330 degrees on the RMI outer edge to the centre of the RMI as being equal to 30 knots in strength then, after using your calculations, place the headwind and crosswind visually on the RMI it does indeed show you the wind is from 330 degrees. But this is from a starting position of knowing the wind is 30 knots in strength, and when I'm up there I wouldn't know this. If I were to imagine this line as being equal to 40 or 50 knots, putting in your calculated headwind and crosswind would give an inaccurate direction for the wind. So excuse me if I'm being completely stupid here but how do I prove that the radius of the RMI equates to the strength of the wind? Or rather, how do I prove this is equal to the value of 30 knots?

[ACACIACREEK]

Rainboe,

I'm asking myself the question so that if the wind does happen to be different from the forecasted wind which I based my planning on, then I can ensure I can do some quick recalculation to the heading and timing for the ndb hold. To not do so would be foolish, but knowing which quadrant the wind is coming from isn't good enough to be able to work out suitable heading and timing, for depending on what the inbound to the hold is, a wind from a certain quadrant could give either a head or a tail wind, and the single drift could vary considerably. So, I would like to know how to calculate the exact wind strength and direction.

[galaxy flyer]

AC

It has been a looong time since I did this trick, but I think you need to scale the head/tail wind opposite what you would think. If it is a headwind scale DOWN from the center of th RMI needle, scale the cross wind into the wind, then "tie" the two points together.

GF

[Slasher]

Rainboe, := The "theoretical" stuff has it uses - e.g. some Asia ports are notorius for passing winds that "fit" your aircraft limits. So if yr cross wind limit is 40 kt and yr speed is 140kt, 40kt/2.3min = 17* drift expected at begining of the flare. If I find I have 19* or more and I do a GA.

[misd-agin]

30 degrees off the nose = .9 (90%) headwind and .5 crosswind component
45 degrees off the nose = .7 HW and .7 XW
60 degrees off the nose = .5 HW and .9 CW

The .9 (90%) is actually .86. I doubt anyone's doing the math to that extent(.86) when the tower reports the winds on short final.

[DFC]

Why make life difficult for yourself.

If you want to check the wind then ask ATC for a wind check and simply adjust the surface wind up to what is will roughly be at the hold level.

With that wind, you can use your head to calculate the single drift for the inbound and thus what you are going to do on the outbound. You can also quickly know the head or tailwind so that you can adjust the timing.

If you sit down and use the wizz wheel to plan headings and timings in the hold prior to departure or even for that matter doany form of notes regarding the hold headings and timings, you really need to try and stop relying on these because some day soon ATC will direct you to a hold you have not planned for and what will you do then?...please refer to the above advice!

Regards,

DFC

PS, remember that even on the IR initial test, the first time round the hold is simply feeling your way....the second (and subsequent? )time(s) should be better

[ACACIACREEK]

Rainboe,

The only reason I am striving for accuracy is because I have seen so many people fail the NDB section of the IR. I believe the reason for this is that most students fly the hold and approach based on forecasts from the Met Office, and that when the forecasts aren't accurate enough, the students don't recognise this and cock things up. Students may have been fortunate/unfortunate enough to have gone through their training with forecasted winds being very close to actual, which would lull them into over-relying on these forecasts. An actual wind considerably different from the forecast wind on IR day causes these students to fail this section. I want to make sure that enroute to the hold I am in a position to look at my instruments and calculate the wind direction and strength, to compare it against my planned wind on the ground and, if different, to get some calculations done rapidly. I am entirely in agreement with you on keeping it simple, but I don't think I am overly complicating it. I guess the only way of assessing the wind is to pull out the CRP-5 or electronic equivalent, so I'll have to investigate whether this is allowed.

DFC,

So for calculating the hold wind at 3500', are you saying that veering the spotwind from ATC by 30 degrees and doubling the strength would give an accurate enough wind?

misd-agin,

Your input is only useful if you know what the wind strength and direction is...... and that is exactly what this thread is about.

Thanks to all for your opinion!

[DFC]

Some students do indeed fail the NDB section because they are using inaccurate forecast winds and pre-calculated headings.

They do not fail because the wind is wrong.

They do not fail because their planned heading is wrong.

They fail because they either do not recognise that it is not going the way it should or even if they do recognise the divergence, they are unable to correct the error or worst case - a fail point when tracking to or from the beacon - turn the wrong way trying to correct an error.

What you need to be able to do is orientate yourself so that at any moment in the hold you know where you are and can decide if thet is OK or if a correction needs to be made.

If you fly the hold at 120Kt then here are some rules of thumb;

For every 2Kt of crosswind on the inbound, the drift is 1 degree. Or crosswind/2 is drift.

Calculate the crosswind in the same way (in your head) you do for take-off and landing.

For timing, add 1 second per knot of headwind outbound (subtract for tailwind).

Start watch overhead the beacon and check and reset at the abeam QDM.

Assuming you were on the inbound track at the beacon then - the outbound timing is the same as the time indicated whan passing the abeam QDM.

Always use the 30degree "Gate" to check progress near the end of the outbound.

Finally remember the requirements are to enter the hold in the prescribed manner (remember the entry procedure is based on heading and not track), remain in the holding area and establish on and track the inbound track for a reasonable amount of time. No more and no less.

Use RANT for practice until you can enter and fly any hold using any wind without having to make any pre-calculations.

Aviate, Navigate, Communicate!!

I usually veer - northern hemisphere (back in the upside down half) the surface wind by 20degrees and add on 50%. But your method will be close enough.

Just remember that if the average hold leg is 4nm long or less then a 10 degree error will put you some 0.7 of a nm away from your ideal position. Not too hard to correct for that provided you can use the pointer to a) recognise that there is an error and b) can recognise what correction is required.

Finally remember that you are training to use the IR practically after the test. Be it as a PPL or as a professional pilot. Either case you will never be able to carry enough paper to plan all the possible holds and will not have enough time to plan them pre-flight as a professional.

KISS principle at all times.

Regards,

DFC

[ACACIACREEK]

DFC and Rainboe,

Thanks for your input, I'll take it onboard.

[gimmesumvalium]

Had a colleague recently who did all these calculations at 100 ft based on his onboard equipment - followed by a heavy landing! Needed 2 days to get engineers there to do a heavy landing inspection. Ironically, Tower reported winds were within limits.

[low n' slow]

Rainboe: I completely agree!
Whilst flying any NDB based procedure, the first round is just a check. Calculating corrections on less then 5 degrees is of no use to anyone with a normal directional gyro.
I did it like this and I passed my skilltest...
Coming into the hold, you want to be tracking and not homing in to the beacon. This will give you a good guidance as to where the wind is coming from. Then fly the first orbit with minor corrections to what you believe is correct. You want to be making rate on turns in all heading changes and this might force you to deflect not just your heading going outbound, but also the track made good (the protected area should cover this). The aim is to then make a rate one turn and end up on a perfecktly spaced inbound track. The inbound track should be on track and on time. If after the first outbound track, you produce a 50 second inbound track before passing the beacon, you know you have a tailwind coming in and a headwind going out. Add at least 10 seconds to the next outbound and this will give you a good correction. If you overshot the inbound course during the turn, turn the outbound track into the wind a little and this will correct for drift during the inbound turn.

Having said this, it can be very useful to draw up a couple of holds with different winds and do the exact math in the comfort of your own home. Do a couple of examples and then you'll get a feel for how to correct for different winds. But doing this in the plane is utter nonsense. I had a teacher that tried to force us to manouver the plane simply by calculating our way around the procedures. Needless to say, it never worked. Fly the plane instead and see what happens. If you know the mechanics behind drift and windcorrections, you'll be able to pick up on any changes you have to do and from these, get a picture of what you need to do.

Last tip is to try it on MS Flight Sim or similar. It really helps.

/LnS