PPL NAV cockpit calculations

[horizon flyer]

I use a windpotractor invented by a guy called Olof Bakker from Holland. With a 1/2 mil chart headings can be made instantly, just by laying on the track line, and flight time worked out as well. I have two of them and regard them as must have items. So easy to use only requires the wind speed of the day converted to a percentage of cruise speed in knots. The disque has percentage lines scribed on it so using a china graph the wind direction is pencilled in, then lay this point on the track line on the chart referenced to north and the crossing point on the edge of the protractor is the heading. Take the cruise speed and regard them as degrees and on the inner edge is a time in minutes so 130 knots = 6 minutes which is the time from the wind dot to the edge of the protractor, so just work along the track in these time intervals for total time. It is very clever, so simple and easy to do in flight. Why does anyone use the whizz wheel it's so ackward to use in flight. To find one google it, about £17 pounds from Holland.

[Jhieminga]

The 'Olof Bakker' protractor is now this one: https://www.pilotshop.nl/contents/en...rotractor.html

[hugh flung_dung]

Ditch the gadgets and learn the following:

1. maximum drift (MD) angle = windspeed x 60/TAS. MD is calculated during initial planning and written on chart or PLOG.
2. turn to track, use the DI to estimate the angle between the wind and track and turn towards the wind by 1/6th of MD for every 10 degrees (use full MD if angle 60 degrees or greater)
3. use the DI to estimate the head/tail wind by taking 1/6th of the windspeed for every 10 degrees from the beam (use full WS if angle 60 degrees or greater). Estimate groundspeed.
4. you will fly a distance equal to 1/10th of your groundspeed in 6 minutes so it's easy to estimate leg time

This completes all the necessary nav calcs. In the air it takes about 30 seconds to do and needs no distracting gadgets.
There's an explanation here of how to use the DI to estimate proportions.

To correct to track use either of these techniques to get you back close to track:

1. estimate the error angle, turn towards track by double the error and fly for the same time it took to get to the point where you started the correction.
2. Before flight calculate a Standard Closing Angle for your intended TAS: SCA= 3600/TAS. In flight, if you are off track by y miles then simply turn toards track by your SCA and fly for y minutes.

Now decide why you were off track and act accordingly.

These techniques really are very simple once they're in your head, but they sound complicated when reading from the page. They work.

HFD

[jan99]

I would like to second the Olof Bakker windprotractor recommendation.
Having preferred doing the approximations in my head I was sceptical but ended up using it for the practical exam. Simple, quick and with little chance of confusion. That was more than 10 years ago (ofcourse I don't know where I have it now).

[rarelyathome]

HFD.

Good advice. Only comment is that, strictly speaking, SCA should be 60/GS in miles/min (which makes the mental arithmetic easier) or 3600/GS. Either way, it is the GS that you should divide by not the TAS. In reality, using the TAS won’t be far off most of the time. So for most training aircraft, estimate distance off track and turn L or R 40 deg and fly for number of minutes you are miles off track (as you say).

Use HFD’s method - it really is easy and far better than getting distracted with gizmos.

[TelsBoy]

HFD's method is exactly as I was taught and what I still use to this day.

[hugh flung_dung]

Rarely...: you're correct, but I learned to do this before GPS could give you a groundspeed. Given all the other vagaries and inaccuracies TAS is "good enough".

HFD

[old,not bold]

I agree re HFD's rule. But I think I put it more simply. It's just the 1:60 rule at work.............let's not overdo making it more complicated.

[Gertrude the Wombat]

If people really want to use "1 in 60" rules and suchlike they might, or might not (I might just be peculiar), want to remember where it comes from, which is simply that

sin x = x

to a reasonable approximation (ie as accurately as makes any difference to your flying) for small enough x. Where x is in radians of course, and one radian is roughly 60 degrees, so there you are.

[hugh flung_dung]

Gertrude: unfortunately even basic trig frightens-off quite a few studes so, at the PPL level, I think it's best just to get people using the practical guidance. Those who want to derive the MD and SCA stuff for themselves will do so.

Old,not ...: in my groundschool sessions I have to teach the 1:60 stuff so people can pass the exam, but I hope nobody starts doing the calcs in the air to work out a closing angle. It's far better to avoid awkward sums and use the MDR stuff I described earlier for diversions, or even (heresy!) for all nav calcs, and to use SCA to get back close to the pre-planned track ASAP.
The link I gave earlier shows a simple visual way of using the DI (or any compass rose) to estimate proportions of MD and GS on any heading.

As an aside, the closing angle technique used to be widely taught, but this gets you to fly down a direct line to destination from your off-track position. This new line is one that you haven't planned or checked and may contain dragons, or worse. Clearly, the double track error technique only works to the half way point in its simplest form, using it beyond the half way point requires you to have a sensible serious of "opportunity" fixes noted on your chart so you can use double track error on part of the segment.
SCA is simple and works almost anywhere. The only time it's worth fiddling with it is at speeds of less than about 100kt, when it's better to use half the SCA for twice the time.

HFD

[old,not bold]

HFD I bow to your greater knowledge.

I should shut up, really; this is a person who, on 3 consecutive days a long time ago, landed at Le Touquet in the dark (SRA) with 90 hours and no Night Rating having set off from LGW much later than planned, followed that on the next day by landing at Orange military (very pretty red flares) instead of Orange civil which had just cleared me to land (there was an active thunderstorm over both) and then to complete the hat-trick on the next day crossing the threshold at Fuimicino in the belief that it was Ciampino, and then - realising my mistake - flying to Ciampino at about 500 ft AGL for 15 NM/10 minutes to turn onto final and land.

I seem to remember using the "double the error" correction a lot; I had a pretty little circular protractor with two arms which enabled a better estimate of track error than the eyeball. But on a 1:500,000 map in a slow aircraft, both of which I used all the way to Sharjah, you had to give it a bit of time to get a good position fix.

One of the most useful tips I used was to always draw the planned route on the map, and then mark off every 10-miles, with the total miles from departure shown. Provided you flew the planned course, this, plus the clock, provided an immediate position +/- a mile or two, and a check against gross error. (I did try doing it in minutes, but the wind usually made that unreliable.) Of course GPS has made all that stuff history, until it stops working. And I haven't seen a GPS that will tell you what heading to fly to get back on track and counter the drift. But I bet someone will tell me there is one.

[hugh flung_dung]

Old,not ...: I think we're in sync, I just didn't want people to get the idea that MDR was complicated, or that nasty sums were needed, or that they need to understand the 1:60 stuff to use MDR.

I prefer 6 minute marks (rather than distance marks) because the distance is just 1/10th of groundspeed and it's easy to look at the clock and then straight to the approximate position on the line. Of course, in reality they don't turn out to be 6 minute intervals but after the first couple you know they are 5'30", or whatever, and that's just as good. It's even reasonable to mark them at 1/10th of TAS and then to modify based on observed intervals with actual head/tailwind - but I wouldn't expect a pre-PPL stude to be able to do that.

(SE to Sharjah means you must be fairly bold, despite your tag ;-) )


OP: nav is an attitude of mind. Learn 1:60 to pass the exam, learn to visualise how things will look from the air so you can see them, and then use the simple MDR techniques for practical Nav. Do not buy gadgets. Use GPS as an aid and not as a master. Always do a gross error check after setting heading. Lookout!

Here endeth the sermon ;-)
HFD

[old,not bold]

SE to Sharjah means you must be fairly bold,

No, just young and very foolish at the time ("I've got 90 hours, what can possibly go wrong"), not least because it was a £700.00 Percival Prentice with an almost time-expired Gypsy Queen, a wartime 4-channel crystal-controlled VHF and a dodgy VOR bought for £25.00. My uncle Bob, who distinguished himself writing off one of the 3 Vickers Windsor prototypes in 1944, said it was the most stupid thing he had ever heard of.

[hugh flung_dung]

Excellent! I'm envious of what must have been a fun trip.

I did my fair share of dumb things, but it always strikes me as odd that the older we get the more risk averse we become, despite there being less lifetime ahead of us to loose if it all goes proverbials up. The (over) confidence of youth, I suppose.

HFD

[old,not bold]

must have been a fun trip

Indeed it was, with its moments, of course; discovering that in a Prentice the magnetos would short out in cloud with the moisture, as they did over the Italian hills in an ill-advised attempt to get to VFR on top without knowing where the top might be (the duty God was kind that day); later on an engine fire climbing away from Baghdad due to a missing exhaust gasket, leading to the discovery that although a firewire was fitted, it didn't work. There was no extinguisher in any case and never had been.

Flying from Damascus to Baghdad was a navigational challenge which I failed. 'Fly in controlled airspace or be shot down' was an incentive, but without an ADF and an absolutely featureless desert following the oil pipelines was the only way, reporting the NDBs visually, while pretending to be IFR-equipped and licensed. They tended be be sited at pumping stations, so the airway (base 8,000 ft, pretty much the aircraft's ceiling on a hot day) followed the pipelines. That worked until halfway when the dust haze removed any sight of the ground. Dead reckoning is easy crossing the Channel at 5Kts in my boat, less so in a Prentice when you have no reliable wind information, and I wandered inexorably off track, until I passed over a lake that I could recognise, about 50 Nm SW of Baghdad, with the fuel getting very low. By that time Baghdad was calling me, but I kept stumm until I could call "airfield in sight" as I didn't want to have to explain where I was. (When I reported in to ATC, I muttered about unexpected headwinds making me late, and then they gave me a carefully plotted radar map of exactly where I'd been since crossing the border, and we all had a jolly good laugh, as we did next day after I had left the aircraft, smoking gently, in the middle of the runway waiting for a firetruck and tow, just in their busy period.)