View Full Version : Dull, Geeky Question

Bob Viking
13th Mar 2018, 18:10
Picture yourself flying an arc round a VOR or TACAN (not on autopilot). Letís say, for arguments sake, the arc is 12 miles and youíre flying at 230 knots (which just so happens to be what I was doing earlier with my student).

We all know the Ďthrupenny bití technique. We have also probably, on a very rare occasion, managed to get bang on the arc and find a constant angle of bank that works a treat.

I remember someone, much better at maths than me, trying to explain a formula for working out what angle of bank youíd need but Iím damned if I can remember it.

So my question is does anyone know a rule of thumb for working out angle of bank to fly an arc that can be factored for speed and arc size?

I told you it was dull.


13th Mar 2018, 18:23
Google is your friend!!
Bank Angle for an Arc Approach (http://code7700.com/rot_bank_angle_arc_approach.htm)

Bob Viking
13th Mar 2018, 18:50
I normally google everything but didnít think the answer would be out there.

I must add though that itís still a clunky rule of thumb. It also is only calculated for 150 knots.

Is there a way to factor speed into it as well?

In my experience, at 230 knots itís closer to 10-12 degrees AoB for a 12NM arc.

Maybe Iíll just fly a few laps of the base and do some trial and error. On a still wind day of course.


13th Mar 2018, 19:24
You just substitute your actual speed in nm/min for the 2.5 in the formula for turn radius. ie 230kts is close enough to 4nm/min. The final answer is close to 5 deg AOB in still air - see if it works for real!!.

13th Mar 2018, 19:25
...am I still on Jet Blast?

13th Mar 2018, 23:33
I need to lie down.

13th Mar 2018, 23:45
From what I remember when I was taught my single engine IR with the late Dick Fox the wind was never the same every time we flew an Arc. Additionally if the Arcs were descending the wind was constantly changing.
So all the theory about angle of bank was a load of B...ux at 140 Kts if manually flown.
I seem to remember reading loads of theory on this and none of it actually working in practice.
What seemed to work for me was that it was all based on ‘rate of change’ on the DME distance and if I could keep the rate of change minimal and if I could keep a 10 mile arc between 9.7 and 10.3 then he didn’t moan so much and was less grumpy. Bless his soul.
This is the same sort of question that Dave ‘Duke’ Webb at PPSC used to give out about how wind socks fly.
ps Yes thanks GtW (see next post) seem to remember that trying to keep the to/from speed at zero also had a lot to do with it as well.

Gertrude the Wombat
13th Mar 2018, 23:52
My rather more modest attempt at an arc, at 50 miles and 100 knots, simply meant flying at right angles to the VOR radial and making occasional fine adjustments until the DME to/from speed read zero. Much easier if you slow down a bit :)

Ascend Charlie
14th Mar 2018, 00:04
Step 1.
Get to 90 deg from the radial, set the CDI for 10 degrees ahead of your position. Fly straight line.
Step 2
Pass through the CDI central indication, still in a straight line, until it is 10 degrees behind.
Step 3.
Turn 20 degrees towards the station, repeat step 1.

Step 4.
Take this question off Jet Blast, and put it somewhere more appropriate.

Loose rivets
14th Mar 2018, 03:39
I had a long break from flying but went back for a 'retirement job'. During that period someone had invented flying arcs. WTF!!??

I have never seen so many people using 90% of their brainpower, fluffing about trying to make a shape in the sky while they should be aviating. DAFT, I tell you.

Oh, since this is JB, I'll boast my socks off and reveal I found I could do it without any significant thought, but then, I have a positronic brain.

What did happen that was fun was after worrying how my flying might be after so many years, my pal bustled into my house with MS Flight Sim and a joystick. He's a great pal. I couldn't be arsed to keep setting up different scenarios so I found a place I'd been to lots, LBA, and set the wind to 25 degrees off Rny 15 at something knots. I flew it many, many times.

To my astonishment, I was sent to LBA for training and found myself on that runway. Wind, 25 off from the right, at that exact knottage at 2000'.

Anyway, the traning captain was charming and soon we were chatting away as I flew holds and ILS etc. After entering the hold for the several'th time yer man suddenly said, 'You're freaking me out! You're arriving over the beacon within 2 seconds every time'.

I recalled my early days of struggling to get to the beacon in the correct season, let alone minute, so the maths were working out as per MS's program. So, I thought I'd string him along for a while and just smiled wryly without giving the game away. Come to think of it, I'm not sure I ever told him why those shapes in the sky seemed so natural.

Incredible you know, a few quid's worth of kit in a bedroom was being played out for real with pretty well spot on simulation. I suppose I was flying nothing by my memory of the very low power PC. Streams of binary code had taken the place of Doug Brown's air-driven, totally analogue D4 Link trainer in a wooden shed at Southend. Strange though, we had to get through a CAFU instrument rating having flown nothing but that wheezing machine.

Kids of today . . .

Bob Viking
14th Mar 2018, 05:07
Please donít confuse my post with me actually giving a sh1t!

Itís just that, after ATC made us fly round the hold four times yesterday because there was a bl00dy helicopter doing an ILS ahead of us, my mind started to wander.

After all weíve said about techniques my aeroplane very helpfully has a green circle on the map that I can fly my jet along anyway. The autopilot isnít quite clever enough to do it for me though. No thought required.


14th Mar 2018, 05:25
Loose rivets does write well, doesn't he?

14th Mar 2018, 11:40
Buy his novel, then you will really see how well he writes.

14th Mar 2018, 14:32
bl00dy helicopter doing an ILS ahead of us

I've been told to slow down because I was catching up with some twin jet ahead.

Bob Viking
14th Mar 2018, 15:29
For the jet driver that is.


14th Mar 2018, 17:52
Google Radius of turn nomograph.......

14th Mar 2018, 20:14
Buy his novel, then you will really see how well he writes.

I did, and he does and I hope more will.

Loose rivets
15th Mar 2018, 04:22
Please don’t confuse my post with me actually giving a sh1t!

Bob, this is Jet Blast. Giving a shit is expressly forbidden . . . unless it's about how to navigate a roundabout. Oh, and boilers. One can give a shit about boilers.

Talking of which, it's nearly 3 AM and my boiler's still on. Shit.

Buy my book? Coo, I wish someone would. I need to get the sales out of single figure this year. No, figure, singular, is correct in one country last year :{

I've been thinking about writing a biography, though leaving out the dull, or anger-making bits. Just the insanely improbable happenings, relying on not being believed so that I won't be prosecuted.

There's a tale or two about G-AXGE coming back from work in the still of night. Thread? Well, CLN VOR is involved. And a flarepath, and the Rivetess' legs hanging out of a car with the flarepath-lighing Gaz blowlamp still blazing away and flailing about near the MkII Zepher's head lining.

I don't know what the two of them had been doing, but my aircraft was on short, short finals before she jumped out of the car, let alone back in. They only ever did get two tins alight - which I didn't need as I was following a big back windscreen with illuminated entertainment to take my mind off the Oak trees on my left, and the pylons on rising ground ahead.

It was the blobs of mud from his spinning tyres hitting my windshield that I didn't like.

Somewhere in such a book I could reveal how I used to do a full procedural VDF Approach in a DC3. Came off a nice shiny jet to get my first command and some of the navigation was a bit, erm, absorbing. I countered the mean examiner's change from QDM's to QDR's by imagining a huge compass rose over the earth around the airfield. Get a new reading and imagine that opposite needle going the other rotational direction.

It was the long lonely trips to Stavanger, earphones pressed hard on ears to try to hear the Consul count; the dashes, nul area, count the dots, and look on a chart that filled the flighdeck. God only knows how we found our way there.

This is the kind of thing I like. Kind of Noir-disturbing-because it could be an everyday thing and the public wouldn't know.


The post script 17 I made myself giggle. It was an exercise int public perception-killing.