Max drift calculation [Archive]

glenb

31st Jan 2006, 08:17

learnt this one off my cfi and it works a treat

windspeed divided by tas in miles per minute

tas 120 knots i.e 2 miles per minute wind 360/20

20/2 = 10degrees max drift

now this is a bit confusing but great if i can explain it well enough without a picture

you takeoff off on a flight with a pre calculated max drift of 10 degrees in the above example

you turn onto a track of 330 for example on the dg. look to where the wind is coming from and drop the line straight up or down as applicable. if it falls half way along the horizontal from the centre of the dg you have half of 10 as your drift, if it falls 2/3 the way along you have 2/3 of the 10 as your drift etc. try it, i guarantee it will be accurate to within 1 degree of what you get off your whizz wheel. cheers glen

hugh flung_dung

31st Jan 2006, 12:27

glenb.
I agree, it's an excellent technique but (unfortunately) not widely taught. If you go horizontally from the wind vector until you hit the vertical centre line you can do the same thing for head/tail wind component as a fraction of windspeed.
Then divide the groundspeed by 10 and you have the distance flown in 6 minutes.
Sooooooo easy. All solved and not a wizz wheel in sight.

HFD

oldfella

3rd Feb 2006, 22:07

I've posted a reply to this question somewhere else on a forum but cannot think where.

I have an idiots guide, that has proven useful to new PPL guys, to simplified navigation using max drift and clock codes. Email me and I'll look it out and forward it to you.

BEagle

4th Feb 2006, 08:21

See: http://www.pprune.org/forums/showthread.php?t=208745

Max drift = Wind velocity / (TAS in miles per minute)

Then apply the relevant 'clock factor' - i.e. if the angle between the wind and track is 15 deg use 1/4 max drift, 30 deg use 1/2 Max drift, if it's 45 deg, use 3/4, 60 or more use all of it.

Unfortunately you will read nonsense such as '3 deg per 5 kt' - please ignore that and use the correct method as described above.

oldfella

4th Feb 2006, 14:09

Surely drift is a factor of groundspeed rather than TAS

BEagle

4th Feb 2006, 14:20

Surely drift and groundspeed are the results of W/V applied to TAS?

P.Pilcher

4th Feb 2006, 14:53

Correct. We are touching on vector "arithmetic" here but because such a mathematical title will scare the majority of people away unless they have a grounding in Mathematics, Physics or Engineering, all sorts of devices have been invented to prevent people from realising what they are really doing but enabling them to get the answer that they require.
Basically, for those who don't already know there are two types of quantity: scalar quantities which only have magnitude, like mass or volume or time and so on and vector quantities which have both quantity and direction. The most important one for pilots is velocity, because we need to know in what direction our aircraft will travel and at what speed over the ground - a vector quantity. The answer is the sum of two other vector quantities: these are the direction that the aircraft is pointing plus the speed at which it is travelling through the air and the speed of the wind and the direction that the air is travelling. When two vector quantities are added we need to take into account the directions of these vectors as well as their magnitudes.

Think of it another way: the aircraft is moving through the air in a certain direction at a certain speed. The air itself is also moving at a different speed and in a different direction, and it is the combination of these quantities (the result when we add them) which gives us our heading and groundspeed.

Now pity our poor groundborne bretherin, the boating fraternity. When they do similar sums they have to take into account the speed and direction of their boat, the speed and direction of the wind and the speed and direction of the tide! No wonder our forebears got frequently "uncertain of their position" before Mr. Harrisson invented his seagoing clock to enable them to work out their longitude!

P.P.

oldfella

4th Feb 2006, 23:23

BEagle is correct in that nav solutions start with TAS. The confuser goes from plotting TAS against W/V and course. It then resolves groundspeed and drift.

If using max drift - actual drift using clock method for MDR you have to divide W/V by groundspeed (nm/minute) to get Max Drift.

BEagle

5th Feb 2006, 06:20

Not so.

Max drift occurs when the wind direction is at 90 deg to the track and is calculated from the ratio of TAS and wind speed - all the 'clock factor' for the proportion of max drift to use for the actual difference between wind direction and track is doing is approximating the front face of the computer by using 1in60 values rather than exact trigonometrical values.

Perhaps you're thinking of SCA correction?

homeguard

5th Feb 2006, 10:32

Surely to avoid a circular arguement we need to agree the purpose of 'max drift'.
Based on W/V forcasts we calculate the heading to steer and groundspeed (time). We therefore minimise off track error. A major concern is that we do not infringe Controlled airspace, Danger/prohibited areas and other places nor unwittingly hit anything. We therefore consider an assumed off track worst case scenario in deciding on our intended track.
Radians devised by the ancient Babylonians (the marsh people of Iraq) worked this out for us thousands of years ago. In aviation we call it the 1:60 rule. BEagles 'clock code' as he saids is simply that, Wv v TAS, The change in heading by the pilot that then changes ground speed and will prevent drift has nothing to do with worse case scenario calculations.
You must decide the worst case scenario to use, be it; 15 degrees (1/4), 30 degrees (1/2), 45 degrees (3/4) and 60 degrees (the lot) but applied simply. Max drift needs to be no more complicated than that.

UAV689

5th Feb 2006, 19:34

This is how I was taught by the RAF on my EFT flying course,
before you take off calculate your max drift, by your miles per minute divided by wind speed, eg 120 knots 2 miles per min/20 knot wind.
Then for your heading look at your wristwatch. wind is up to 15 degrees off your heading take a 1/4 of max drift. if it is up to 30 degrees off take 1/2 max drift. if it is 30-45 degrees of use 3/4 of max drift, and any thing more use all max drift.
but remember to apply it in the corretion in the correct direction!
If the aircraft is equipped with a HSI you can set the heading bug on your course and turn the bar to the wind direction and at a glance it maked the above method so easy.
Wizz wheels are not needed, I was never taught the use of one in the RAF and was never off a target/waypoint by more than 20 seconds, passed my nav test by being 4 seconds fast..oh hapy days!:ok:

BEagle

5th Feb 2006, 20:21

What? The RAF doesn't even teach knowledge which even a 17 year old PPL student is expected to know?

Ridiculous. Of course you should know how to use a navigation computer!

Perhaps it's time to re-examine the credit given to EFT graduates for the PPL navigation exam.....

UAV689

6th Feb 2006, 17:32

seriously they do not, I completed EFt about 2 years ago and there is not computer at all on it, the nav is much more indepth at PPL level, on EFt a lot more is taught so that you can react quicker to changes in flight and spend less time with your head down with a wheel. But then again it is geared to put pilots in the cockpits of a fast jet and how cares about drift at 450 knots!

You do not use a wheel at all, not for fuel/time/headings/it is all done in your head. which i feel is enough personally. everyone develops there own nav methods and i never use a wheel now.

BEagle

6th Feb 2006, 17:37

In flight - of course head-out mental work is needed at low level.

But for pre-flight planning, you MUST know how to use a nav computer!

I understand that the Royal Navy and Army do still teach such basic aviaition skills - why doesn't the RAF?

No wonder some service pilots have had such difficulty learning how to navigate a PA28 accurately at 2500ft.....

UAV689

6th Feb 2006, 21:22

but why? when we was taught we start off doing medium level navs, about 3000 ft, then work down to 500ft (used to be 250 in the days of the dog) we use regular check points spaced no more than 5-8 mins apart, use HAT checks before and after each checkpoint (Heading, Altitude, Time), you know the wind before you take off and the clock method of max drift is accurate enough. When I completed my PPL the wheel was the part of the course I was dreading most, when I used the wheel on the ground I then re worked using the clock method and the results were nearly always the same. I can honestly say that when I was on the squadron I know of not one pilot getting lost on any nav, on my ppl completion it was a regular having a stude lost!! You don't even do the route with the instructor then fly it yourself you just plan and go on your own.

In my flight test for my ppl i did the nav just like i was taught in the RAF and at the end the instructor told me " I can see you have flown before because none of my instructors know how to do what you did in the air, and you did it better than them!" proof of the pie!:ok:

BEagle

7th Feb 2006, 09:10

What pre-flight method (using MDR) are you taught in order to calculate your groundspeed of at an IAS of 120 kts at 3000 ft with an OAT of, say +5 deg C and a QNH of 1013?

Track 245 deg M, W/V 285/20 - what GS would you use?

UAV689

8th Feb 2006, 16:43

like i said you use the same method for drift calcualation, but on your legs if you have a head wind and are slower you just recaluate your ETA. This is why for example on a 15 min leg you have a chx point 1/2 way (7min 30) and you then simply work out your new ETA. You also put away your map for the whole flight, only getting it out 30 secs or so before your next ETA and then refresh for what you are looking for. (as you memorise your route before t/o)

The whole system is geared towards low level navs where you cannot afford to be blundering around with a wheel/staring at a map every 30 secs.

Clock and compass are the most important instruments when doing a nav.

G-SPOTs Lost

8th Feb 2006, 17:22

for nearly every training single windspeed divided by two will work, its this anal attention to detail that will mean somebody flying into a hill or landing with the gear up.
It only really going to be useful in the case of an in flight diversion (as in the PPL skill test) at which point you will be using a hand drawn line to track to and an ETA based on either 1, 1.5 or 2 miles per minute so what if the max drift is out by 2 or 3 degrees IT DOESNT MATTER!!!!!!
You would do the student a better service by getting his head outside and emphasising that a QDM call (if available) would better serve his purpose.
I would be delighted if one of my students simply added or subtracted 5 or 10 or 15 degrees instead of confidently announcing 13 degrees left drift required which unfortunately didn't take into account the 10 degrees that he has been flying off heading whilst he/she worked it out!!!

BEagle

8th Feb 2006, 17:27

No - what G/S do you plan with?

You surely don't just assume still air and then cuff it on observed times at fixes? In the example I gave, what G/S did you use to work out your ETA at the turning point prior to take-off?

And yes, I have done some low level flying in my time. At 420 KIAS, for example, the correction to make good time was based on 'increase KIAS by same numerical value as seconds early or late and maintain for the numerical value of miles per minute'. Thus 10 sec early at fix, fly at 420-10=410 for next 7 min. It worked very well.

But medium level requires proper pre-flight computer work in low speed (<180 KIAS) aircraft as errors are induced to a far greater extent if the wind differs from the planned W/V than in fast aircraft at low level.

Having (presumably) worked out an MDR G/S in your pre-flight planning, what technique do you apply to your track distances to obtain an accurate ETA?

blagger

8th Feb 2006, 20:56

Do 90 - acute wind angle then use clock analogy to get headwind/tailwind component.

E.g. Track is 180

Wind is 240/30. So 90 - 60 = 30

30, so take half of the windspeed, gets headwind component of 15kts.

Hence, groundspeed for planning on this leg is IAS - 15kts (say 75kts for average GA ac).

To work out fix times, do simple leg distance / groundspeed calc.

E.g 42nm leg = 42/75 x 60 = 36 mins

Then work out proportional distances/times to fixes on the leg, ideally at some good fraction like 1/4 or 1/3 way along.

I don't think you have to bother about temp. differences etc.. on the IAS

BEagle

8th Feb 2006, 21:10

Some of the posts on this thread will make very useful discussion points at the next instructor seminar....

No wonder so many airspace busts happen if people are so glib about accurate pre-flight planning.

UAV689

9th Feb 2006, 16:22

i agree, there is no excuse for airspace busts, that is down to poor airmanship. if you are planning to fly near airspace always us ground points in relation to where the airspace is, there is no excuse.

BEagle

9th Feb 2006, 17:00

Worked out the G/S yet?

Minimise sources of error by applying the well known principles (or at least they were when I was taught navigation in the RAF):

PLAN ACCURATELY
FLY ACCURATELY
THINK AHEAD

blagger

9th Feb 2006, 17:14

Not sure if BEagles comment about glib planning refers to anything I said in my post..... anyway, if we are talking about Nav planning I think you also need to consider the context. For example, if I was on a PPL or CPL training course, overseas landaway etc... I would plan with whizz wheel and do a full PLOG etc... If I was just doing a localish landaway, say Leicester to Sheffield City I would draw on the route, work out wind corrected headings, ground speeds, times at fixes etc.. with MDR and back that up with a GPS route to same points. Flying it I would use my fix times backed up with the 'time to' type function on my GPS as a cross check. I don't think that's gash?

funfly

9th Feb 2006, 17:43

Surely it's important in this type of discussion to remember that mental mathmatical gymnastics requires a person with the mental ability and agility more related to a selected and young RAF trainee, flying and 'living' aviation on a daily basis.
Most of us in general aviation may be as bright as them but are not necessarily as young or as regular in our practice of in-flight mathematics.
Nothing can replace the experience of our more mature years but there is little doubt that, for the hobby flyer, nothing can better detailed pre planning for any flight - anything which will reduce mental effort during the actual flight must contribute to safety.
In G.A. we have the luxury of a) time to pre plan and b) aircraft with lower speeds and less demands on the pilot than military aircraft, e.g. we do not have weapon systems to manage or low heights to negociate.

BEagle

9th Feb 2006, 17:45

Neither do I, blagger. What you describe is a blend of pragmatism and experience.

UAV689's method has yet to be explained, but it does not seem particularly sound....

UAV689

9th Feb 2006, 18:53

you are taught to get round a nav route safely and in the easiest manner possible, you work dont work out your g/s just your heading correction using MDR. because your using well defined points, memorising your route exactly (after leg 1 I turn right to 220 should cross a power lines and see a lake to my right etc) and your using these HAT chxs pre and post turning point. Simple, easy, safe fail proof. If your running late or fast you just recaluate your new ETA based on the what the time difference is , eg 30 secs late after first half of leg your gonna take an extra minute for the whole leg. As you have now excess power to catch up on lateness whats the point of working out GS. Never bust airspace in my life and don't plan to, and knowing how fast you are going will not stop you busting airspace, but more likely make you do it if you have your head down, working out a complicated formuale.

I am sure the big boys in the RAF using all those techniques you speak of, but for EFT its not needed.

BEagle

9th Feb 2006, 19:01

How can quantify any observed errors unless you have an accurate original assumption against which to assess them?

You need to work out your G/S pre-flight in order to be able to quantify your observed errors. Pre-HAAT and post-HAAT are not relevant in this context.

Do you actually know how to do so? Seemingly not.

Still waiting for your solution to the problem I set you.

blagger

10th Feb 2006, 07:00

In response to BEagles problem, think the GS with whizz wheel is 100kts and with the MDR method is 103kts. On a 40nm leg would be 30s difference in timing for example.

Of course, all this is based on forecast winds - think that is another thread in itself! It'd be interesting to see what people use for planning winds - I tend to use the spot forecast, but also have a look at METARS of airfields along the way and approximate the 2000ft wind to just check there is no wacky differences around.

BEagle

10th Feb 2006, 07:12

I came up with 104.5 knots using a web-based planning tool, and 105 using MDR:

Wind is 40 deg off, so about 3/4 to apply to the headwind, that's 15 knots, G/S is 105.

Which is 7/4 miles per minute and makes ETA estimates very simple.

The point which blagger and I have both made is that use of MDR for pre-flight planning is insufficiently accurate unless you also work out the planned G/S and corresponding fix point ETAs before flight. But it would be just as quick to use the whizz-wheel - particularly for time v distance estimates.

Using still-air IAS for low level high speed flying is one thing, ignoring W/V for pre-flight planning in a plastic puddlejumper at 3000 ft quite another. In the example I quoted, you would start your navigation exercise with an immediate 14% error which is wholly unacceptable. Try that on a PPL Skill Test and you will certainly fail.

UAV689

10th Feb 2006, 09:50

these errors are so small, no one will fly at exactly the correct speed for exactly the whole leg, at precisley the correct heading. which i why the dead reckoning method works, and if from 30 secs/1 min away from your waypoint or further if you see it simply fly to it! would be interesting to see how many UAS studes have bust airspace in comparsion to ppl studes..i agree the proper planning is needed but in real life it is over the top.

Glider pilots regularly fly in excess of 500 kms without calculating any of this before they fly, just drawing a route on the map.

BEagle

10th Feb 2006, 10:10

PLAN ACCURATELY
FLY ACCURATELY
THINK AHEAD

UAV689, what you describe is thoroughly gash and unprofessional. Neither is it 'Dead Reckoning'.

Glider pilots, incidentally, are unlikely to maintain a precise track due to the availability of lift, so your comparison is invalid.

All my UAS students were able to plan accurately - are things really so dumbed-down nowadays?

Failure to complete proper planning is not 'over the top' - it is pure laziness.

G-SPOTs Lost

10th Feb 2006, 20:12

Beagle
2 straightforward reply's please.
Do you have your nav students take a whiz wheel and ruler in the air with them?

What is the procedure you advocate for diverting to an unplanned alternate airfield lets say due to a blocked single runway?

And with all due respect, all this talk of low level nav at 420knots and vulcans has absolutley no relevance here, lets base the rest of this thread on a Cessna 152/172 shall we.
head down nav planning in flight or trying to do complex mental arithmetic in flight is bollox imho

BEagle

10th Feb 2006, 20:48

Pre-flight - Dalton or its electronic sibling.

In-flight - MDR. Never a whizz-wheel!

In flight diversion:

Draw line from nearby landmark to diversion aerodrome using chinagraph and edge of checklist.

Measure distance using miles scale on back of checklist (which ours have as I designed them that way!) - write on laminated back of checklist.

Use checklist like parallel rule to nearest VOR rose - draw line through rose and read off track.

You now have track and distance.

Apply MDR wind to establish heading and groundspeed - you should have worked out max drift prior to flight and drawn a wind arrow on your chart.

My checklists have nice clear laminated backs upon which to scribble your sums. But don't forget to l:sad:kout!

Set off when ready; fly heading and time backed up by visual confirmation.

I agree that high speed low level techniques are not relevant to low speed medium level navigation.

In the Vulcan one had a navigator to work such things out. Two actually...

G-SPOTs Lost

10th Feb 2006, 22:22

So....
to quote you "Worked out the G/S yet?"
Whats the ground speed on the diversion because you need an ETA becuse you may stumble into the CAS beyond your destination. :rolleyes:

Whats the Ground speed, No Whizz wheel, no internet application you have 25 hours on a C150 and probably 2 to three hours ground instruction on Nav???

You are using a transposed radial from a VOR that might be 60 miles away on the other side of your map, on your knee , in a cramped cockpit whilst flying the airplane and trying to remember what your instructor said.

I'll say again - complex mental arithmetic in the above situation is prone to error and worse still gross error, he/she might work out the drift to the degree and apply it incorrectly.

Simple concepts: -
Wheres the wind? is it pushing or pulling? therefore am I doing 1, 1.5 or 2 miles per minute.

Where do I want to go? if i point straight at my destination will I be left or right of it therefore will I employ 5 or 10 or 15 degrees of heading change to allow for the drift.

Why complicate matters significantly to get a very small increase in accuracy at the speeds we are discussing.

Accurate well tutored preflight planning is essential but at this level and at these speeds lets just use the principal of kiss

homeguard

10th Feb 2006, 23:36

I've not listened to such a lot of rubbish for a long time!
The basics of navigation training is not about blagging your way around the home counties or wherever else is just down the road. GPS is a fantastic tool and so at one time were VOR's thus considered. However, the RNAV computations and the software of the modern GPS exploit the very same traditional navigation techniques for which so many of you in your ignorance treat with such contempt.
I wonder how many of you reinventors of the wheel have been any great distance away from the comfort of home flying to the range of your aeroplane over unkown country with few known features WITHOUT the aid of our modern kit. If you have you will not be one of those who choose to ignore the very well defined principles of PLAN, PLAN, PLAN. Do the work on the ground in comfort with a cup of tea and a bacon butty or work your nuts off shouting for MUM! when your completely lost - the choice is yours.
Whatever your choice please do your students a favour and teach them properly for they may need those basic skills sometime in the future which will save them from becoming a fool to circumstance.
Should the worse happen they won't be shouting mum, they'll be cursing YOU!

BEagle

11th Feb 2006, 04:00

G-SPOTs lost - I'm not sure whether you're being deliberately perverse, or cannot understand English?

Let's say your cruising at an IAS of 90 kts. The wind, as before, is 40 deg off at 20 kts. Which you assessed from the angle between your track and the wind arrow...

40 deg off at 20 kts equals, as we've seen earlier, roughly 3/4 of 20, which is 15 kts headwind - so your G/S is now 90-15= 75 kts. Or 1 1/4 miles a minute. 5/4 miles a minute. Thus if the distance is 47 miles, that'll take 47 x 4/5 = 47 x 8/10 = 37.6, call it 37 1/2 minutes....

Apart from measuring track and distance as I explained, you can do the rest in your head - or, if your mental arithmetic is too weak, on the back of the checklist....

Think in terms of simple fractions and easy mental arithmetic. If you are a lazy digi-yoof, that may take a bit of practice...

Anyone with only 25 hours on a C152, of which only 2 or 3 are navigation should declare an emergency in such a situation as they will probably lack sufficient experience to be able to cope on their own.

I do not allow people to fly a solo navigation exercise until they have passed the navigation exam and the communications exam. They must also be able to read their preent position from the GPS and be able to use the transponder.