Max drift calculation

[expedite08]

Hi all,

Just a quick question for you, does anyone have a tried and tested method or rule of thumb for working out the max drift of a wind velocity when working out crosswind components. say W/V 280/24 What would be the max drift? and how did you work it out?

Many thanks

;)

[FlyingForFun]

Max drift = 60/TAS*windspeed.

This comes from the 1-in-60 rule - an approximation which says that if a right-angled triangle has a long side of around 60 units, the length of the short side will be approximately the same as the angle oposite that short side. This approximation works very well for smallish angles.

Unfortunately, I can't think of an easy way of explaining how to get from the 1-in-60 rule to the max drift forumula without using diagrams.

Since you generally tend to fly at one particular TAS in your particular aircraft, you can simplify the equation even further for your favourite aircraft. So, for example, if your aircraft generally cruises at 90kt:

Max drift = 60/90*windspeed = 2/3 windspeed

Next - remember that this max drift will only apply when the wind is between 60 and 120 degrees from your side. For smaller angles, use the clock code (that's the same rule you use for working out crosswinds on a runway for takeoff and landing) applied to the max drift to work out the drift for your heading.

FFF
--------------

[High Wing Drifter]

The classic version is: Wind / (TAS/60). My top tip is that obviously TAS/60 = nms/min. So I just round that to 1, 1.5, 2, 2.5, etc for cardinal TAS speeds of 60, 90, 120, 150, etc. Much easier to to the mental stuff when the pressure is on like that. The nms/min number pops up in many rules of thumbs. Although in this case you would normally figure it out on the ground.

say W/V 280/24 What would be the max drift? and how did you work it out?
As mentioned above, you need the TAS to do the sum.

[IO540]

For a normal GA spamcan (say 100kt) max drift is half the wind.

So if the crosswind component is 20kt, the required heading correction will be 10 degrees.

For 150kt is it a third of the wind.

The above are as close to the precise slide rule ( :yuk: ) figure as any human can hold a heading :O

If you go much slower then it becomes pretty critical, but then you probably won't be going very far.

If you are going much faster, you will have proper methods of navigation, like a GPS.

[BEagle]

IO540, almost - but not quite - complete nonsense.

'Crosswind component' is not considered using max drift; the idea is first to work out the maximum possible drift from the wind considered; that is equal to Wind speed/ (TAS in mi/min). Thus at a TAS of 90kt it would be 2/3 of the wind, at 100kt, it would be 3/5, at 120kt it would be 1/2 and at 150kt it would be 2/5. Fractions being much simpler than decimals in such circumstances.

Work out max drift pre-flight, then analyse the approximate angle between track and wind and use the 'clock factor'. If it's 15 deg off, use 1/4 of max drift, if it's 20 deg off use 1/3, 30 off use 1/2, 45 off use 3/4 and if 60 or more use all of it.

Simple enough - unless unfamiliar with analogue instruments such as wrist watches.

A useful and simple method of heading estimation sadly lost on button-pushing digi-yoof.

[IO540]

What you are describing is some old tradition, I have no problem with that. But check my figures on your slide rule. Not far off at all.

Of course there is a little problem with all this, and that is that the pilot won't know the crosswind component to start with, not accurately enough. Even the F214 figures are usually highly inaccurate.

Garbage in = Garbage out.

[BEagle]

Hardly 'an old tradition'......

Your figures are generally about 17% in error.

Perhaps good enough for the playstation generation of button pushers; others set themselves higher standards by correct application of the max drift technique.

[IO540]

17% of 10 degrees (if that's what it is) is 1.7 degrees.

How long can you fly a heading of 11.7 degrees for?

On a plane with a non-slaved DI that drifts a few degrees every few minutes?

With a bit of turbulence which chucks the compass around by plus or minus 10 degrees, so you can't read it properly and thus you can't adjust the DI from it.

This is about rules of thumb. Usually, all you have is the surface wind when you departed, say it's 270/10. You are at 3000ft, and a quick stab gives you 290/30. You have to fly a leg, or enter a holding pattern on which the fix is on a northerly track, which means you will have just about all of that 30kt as crosswind.

15 degree offset on the heading.

If you want more accuracy, you get yourself an air data computer, or you very carefully read the speedo, the DI, the OAT, the altitude, and work it all out. But then, with all that kit, you won't be asking these questions in the first place :O

I have never used a playstation.

If you are ex RAF (I suspect you are) they would have taught you to do things more precisely. But then you would have been selected from the cream of the cream, at the age of about 18, and put into an intensive training regime where you don't have to worry about job, life, wife, ex wife, kids, etc. This is a far cry from today's PPL intake. Fortunately you didn't have to fight too many wars; the other side won't be doing dead reckoning if they've got something better. I had exactly this discussion recently with a serving RAF instructor; he tried hard to justify dead reckoning but really couldn't explain how they could perform in any real action.

[J.A.F.O.]

It's aeroplane; never, ever call it a plane.

Sorry to wander off track, as it were, but it annoys me.

[chriscook]

In response to IO540, BEagle wrote:

Your figures are generally about 17% in error.
Perhaps good enough for the playstation generation of button pushers; others set themselves higher standards by correct application of the max drift technique.

Hmm ...

Applying the rule of thumb figures you advocate, BEagle, for a TAS of 90 kts and a Wind Velocity of 30kt at 15 deg off, the drift correction to be applied is 2/3 x 1/4 of the wind speed = 5 deg.

But the true drift correction is 7.3 deg.

So your rule of thumb would be in error by 31.5%, BEagle, nearly twice the shock horror error you observed in IO540's figures. Perhaps in future you should fly with a set of trigonometry tables!

It isn't, however, your rule of thumb I'm quibbling with. That's the one I was taught, and it works perfectly satisfactorily.

[vetflyer]

Chris .... can i fly with you whilst you fly 7.3 degrees drift .. that should be awesome.

I05... DR Nav works ..... recent trip RAF QFI worked fine .. found Turning points .... no problems

GPS works also and very nicely.

Whizz wheel useless in airborne C152 and not many airdata computers about.

BEagle please go back to black font as I enjoy your posts and need my glasses for the blue writing.

sorry about spelling and lack of joined up sentences but have had 2 or 3 cold beers ..( in different time zone )

Cheers

[High Wing Drifter]

Even the F214 figures are usually highly inaccurate.
Sometimes, but I've not found this to be the case very often. Whatever I normally plan on the ground works in the air. Any usual variences in wind speed and/or direction don't tend put you far off course anyway, especially if you are attempting to work to a reasonable level of accuracy.

Fractions being much simpler than decimals in such circumstances.
Horses for courses methinks. I like decimals because the (TAS/60) element plugs into other rules of thumb that I will find useful during the IR such as radical interception anticpation, descents etc. Also, if you round to whole numbers or to ?.5 decimals only then the mental math is a sinch. I find it easier if I have say 32 / 2.5 (e.g. 32kts/(150kts/60)) to just mult both by two and divide 64/5 = 13. Personally I seem to find that much easier than thinking 32*2/5 or 32*0.4!

That's the one I was taught, and it works perfectly satisfactorily.
It does indeed, faultlessly I would say, a simple and very accurate method. However, I have some sympathy for IO540s view. If you simply assess the cross wind as being none, small, medium or big, then you simply offset by 0, 5, 10 or 15 deg and assess it from there. If you have drifted 'a lot' change heading by 10 deg, if you have drifted 'a bit' alter heading by 5 dgrees. I was taught this during my CPL not as technique for the test but as a demonstration if how simple it is to end up where you want to be without much effort. I am in no way saying this is the way a PPL student should navigate (or anybody for that matter), but from my experience the results are more than good enough.

[chriscook]

Chris .... can i fly with you whilst you fly 7.3 degrees drift .. that should be awesome ........ sorry about spelling and lack of joined up sentences but have had 2 or 3 cold beers ..( in different time zone )
Cheers
Ah well, irony is lost on some ... or maybe it was those beers. :)

[Sleeve Wing]

"How long can you fly a heading of 11.7 degrees for?"

"If you are ex RAF (I suspect you are) ---- selected from the cream of the cream, at the age of about 18, and put into an intensive training regime where you don't have to worry about job, life, wife, ex wife, kids, etc. -------This is a far cry from today's PPL intake.--------- Fortunately you didn't have to fight too many wars; "

1. Quite right, lO54O, virtually impossible, isn't it ?, --- except in a fast
aeroplane.

2. Quite a chip we have on our shoulder as well, unfortunately; not uncommon, but totally unfounded. We all had the same opportunities. It's how we chose to use them.
You really ought to listen to BEagle you know; he talks more BASIC sense on these threads than most.( And I don't even know him !)

Rgds, Sleeve.

[dublinpilot]

Hang on a second guys.

IO540's method works fine, and it involves much less mental arithmetic than Beagles.

Assess the cross wind component using the 1 in 6 rule.

Wind is 10 degrees off, then 1/6 of wind speed = cross wind.
20 degrees off, then 2/6 of wind speed = cross wind
.....
60 degrees off, then full wind speed = cross wind component.

Then if you're aircraft cruises at 100kt to 135kt take half of your cross wind, and adjust your heading by that.

17% out? :p That's irrelevant! If it was 30% out it still wouldn't matter that much.

If your correction was a massive 15 degrees, and you were a massive 30% out, you'd be correcting by a massive 5 degrees in error! :hmm:

Haven't we all, who have learnt dead reckoning, learnt to fly with the correct heading, identify a fix at our waypoint, and then adjust that heading to A) fix our track heading, and B) close back on to our track by the next waypoint?

The drift correction, is effectively only used between our first two waypoint. After that we should have adjusted our heading, by reference to the results of the first leg. So even the massive 5 degree error is not going to be too important.

dp

[vetflyer]

Oops.. sorry ChrisCook.. missed the irony ..... my fault as you kindly suggested... too many beers.............best leave IO54O & BEagle to it then.

Cheers

:) now where is that :cool: beer ............

[BEagle]

dublinpilot, there's not much point in learning an inaccurate method when the correct MDR method is pretty straightforward, I would suggest.

I agree with IO540 about questioning the practical value of MDR and would certainly prefer that pre-flight computations were completed with a planned wind applied to track. Whether you use a whizz-wheel or elctronic device doesn't really matter, but I rail against current RAF methods of using high speed low level techniques for pre-flight planning in low speed aircraft flying at medium level - I've had quite a few ex-RAF basic students say that they've never been taught to use a Dalton computer; they use MDR for heading and 2 miles per minute for timing. Sorry - not accurate enough for a PPL Skill Test, chaps. But the RN and Army do still teach such things, I understand.

When I use a GPS, I do a quick MDR assessment of heading and groundspeed, so that I can fly the correct track without having to 'wind find' empirically when airborne; a quick look-see at the CDI bar will show how accurate my estimate was. But for basic beginners, I really do think that the old-fashioned whizz-wheel (or electronic equivalent) should be used and then heading and time flown once airborne. To start with. Then later on, once they've got the basics sorted out, by all means use GPS.

Some youngsters' mental arithmetic really is very weak indeed. But they must learn how to sort out an unplanned diversion and that is where MDR really helps. Draw line using edge of checklist and chinagraph, measure track and distance. If you've already worked out max drift prior to flight and drawn a wind arrow on the map, it's then pretty easy to 'do the maths' on the back of the checklist. Of course, if you know how to use a GPS, you can then enter the destination into the box and press the buttons - but if the diversion isn't in the database that becomes a bit of a faff!

I mean the 'generic' digital 'playstation generation' who can't think things out in their head, by the way IO540, not you individually.

[IO540]

The reason DR works is because at typical piston GA speeds, and typical winds in which UK PPL-level GA flies, the correction isn't very much. If you correct by say 10 degrees, and it should be 13, and your legs are say 10 miles long, the resulting lateral error in the ground position is tiny.

In a 60kt aircraft and with a 50kt wind aloft, the situation is much much worse. You can easily be looking at wind corrections of 45+ degrees. But there aren't many 60kt planes, very few people fly when the 2000ft wind is 60kt (that's 30-40kt on the ground), and almost nobody with a 60kt plane will be trying to fly it with that sort of wind on the ground.

So............ one gets away with it, due to the arithmetic of small angles.

All the rules of thumb mentioned will work just fine. Some are easier to work out than others, that's all.

The RAF gets away with it even better because the wind correction at 500kt when flying at relatively low levels is miniscule.

I have no chip on my shoulder, by the way. I just refuse to believe someone telling me, no matter how experinced they are, that I can fly a heading of 11.7 degrees versus 10 degrees. I've got 550 hours, do 150 hours a year, and while I make no claims for being able to fly I do happen to know that represents a reasonable currency in this business.

I have a £30,000 autopilot and even that could not hold a heading to that accuracy. I can probably just about read the HSI to that accuracy, without a magnifying glass. The only way to hold a heading within say 2 degrees is by following a GPS track, or by tracking a succession of navaids.

[Aussie Andy]

Three degress per 5kts = simple!!

Andy