Archive for October, 2014

Farm Strip Skills: Part #2

Tuesday, October 21st, 2014 | Permalink

PLOG on the Chart to Marshland

PLOG on the Chart to Marshland

On arrival this looked like it was going to be a good day, but the longer you looked up, the worse the clouds became.

Obviously then the answer was to stop looking up and go get on with it!

All the planning on the Chart :  Wiz Wheel, forget it.

I’d been told not to plan this trip out to Marshland, we’d do it on the fly.

Now to learn a few more notations & symbols for drawing a PLOG on to a chart.   If it’s good enough for the military to have all their planning & radio frequencies on their chart, it’s good enough for me.

Normally you’d work out the wind corrections on a wiz wheel (Flight Computer), then work out heading corrections from that.  Very accurate, so long as the wind forecast is accurate.   However, you can’t do this easily if you’re diverting, so there’s a good case for learning to fly using Maximum Drift and some rules of thumb.

Maximum Drift

A quick calculation will give you a pretty accurate Maximum Drift (due to wind) you might experience on a flight:

Step 1 :  Calculate the max drift if the wind was perpendicular to the route being flown.

Max Drift = (60 / True Indicated Airspeed) * Wind Velocity Forecast for the Altitude being flown

Math in an aeroplane is actually harder then it sounds, even for math you’d find easy on the ground.   This is because you’re now traveling at ~100+mph, have people talking on the radio to you or have to listen in case they start, are trying to maintain a heading and altitude (and don’t forget to keep looking out the window!) – the constant ‘buuuuur’ of the engine also doesn’t help concentration.

For a Cessna 172, with a typical indicated airspeed cruise of ~100 knots, you can optimise step 1 further by saying 60 is close to 50 and IAS (Indicated Airspeed) and TAS (True Airspeed) will be close enough at cruise speed:  so 50 / 100 equals 0.5.

Now our equation becomes:

Max Drift =   0.5 * Wind Velocity Forecast for the Altitude being flown

Sure you now have a bit more error in the system, but here’s an example, lets say the Track we want to fly is 080 Degrees and the Wind is forecast to be 165/25  for our flight altitude.

If we do the equations above, with TAS = 100 knots.

Accurately  :      (60 / 100)  *  25   =   15  degrees

Estimate     :      25 / 2  =         12.5  degrees.

So with the wind at 85 degrees to our intended track and a reasonably high wind of 25 knots, rounding up, the quick estimate is out by 2 degrees.

Over a 10-20nm leg of journey, this error is going to be very negligible (i.e. With 2 degrees error, after 60nm, you’ll be 2 nm away from where you thought, so over a typical 10-20nm leg the crude method would put you, out by 0.3 – 0.6 nm).    If you still don’t like that small amount of error, remember that TAS here has been approximated to the Indicated Airspeed cruise of 100 knots.   TAS will actually be higher by a few knots, at 2,000ft AMSL in a Cessna 172 with IAS of 100 knots, flaps up and in level flight TAS will be very close to 104 knots.   Do the accurate math above using a calculated TAS, but use only the IAS 100 knots for the crude method and you’ll find that now the error is under 2 degrees.  It’s pretty good and probably more accurate then the error and danger of going eyes down while you get a flight computer out and start remembering equations and how to do division on the thing!

Step 2 :  Using the clock rule system, adjust as applicable for the angle you’re flying relative to the above wind (you might be flying straight into/with the wind).

Take the difference between the track on the chart and the forecast wind direction, multiply the number from step 1.  by

  • 1/4  if the difference is up to 15 degrees,
  • 1/2  > 15 and <= 30 degrees.
  • 3/4 if up to 45 degrees.
  • For anything else assume Max. Drift (e.g. multiply by 1).   This will give you a slightly pessimistic outcome.


If Step 1 gives us a Maximum Drift of 15 Degrees, then using the details from the above (Track we want to fly is 080 Degrees and the Wind is forecast to be 165/25 ).

165  –  80  =  85  Degrees difference, so we can just assume that we’re going to get the full 15 degrees of drift.

If we changed our track later on in the flight to say 140 degrees, then

165 – 140  =  25  Degrees Difference, so   15 * 0.5   (Multiply by a half as it’s greater than 15 Degrees, less than 30)   = 7.5    Degrees of drift.

If you’re still feeling bitter about my approximation method on step 1 above and have a desire for accuracy, it might be worth noting now that in the above condition with 25 degrees of difference, the drift error between the approximation and the precise calculation is less than 1 degree!


Lets go flying ………even if those clouds do look a lot darker.

The briefing was far shorter then your typical PPL training briefing, but even so by the time we got out to the aircraft, dark clouds on arrival, were looking lower and darker then they were before.   We discussed going again, was it worth it, should we go have a look anyway or not?    I elected we just go have a look.   In my heart I knew this was being optimistic, but the CFI wasn’t going to say yes if it was entirely pointless or unsafe and even if we just went up, turned round and came back, it would still be valuable experience of the weather with the proper instruction available etc.

A wall of darkness

As we set heading over Point Alpha, it didn’t take very long to know we could scrap this 🙁     Already at 1,700ft and being pushed down with every minute flown, the north of Cambridge looked like a wall of darkness and cloud – maybe, just maybe, if we dropped to ~1,000ft and pushed on past Waterbeach, maybe it’d clear.

The instructor asked what I wanted to do?   While on the ground I was of the view that there’d still be some things I could learn by going up in less then ideal starting conditions with experienced instruction.   Now I just felt this was more a question of “but should you carry on” rather than “Do you want to”.

I saw no point, it was a lot of maybe’s – that might not come true and if they didn’t how bad would it be when we gave up and tried turning round?   Even if they did come true, we’re aiming for a grass strip in the middle of nowhere, when we get there we’ll have no idea of the surface winds and even if we can get a rough idea of them, probably it’ll make for a hell of a challenging landing and I’ll come away from it feeling rubbish and like I’d forgotten how to land.   That would add no value to this objective.

I elected to call it a day and turn back for Cambridge.

Air Traffic heard our call to return and asked if there was a problem, this is a pretty normal response if you call to return only ~5nm from take-off.   I reassured them that it was just a return due to weather and they let us come straight back in on a Right Base join, which was helpful both for the deteriorating weather and to keep my costs down 🙂

Epic Wind shear:   Falling at ~3,000ft/Min

As I lined up on final approach there was a crosswind and you could feel it gusting hard on the controls.   I’d had worse on the way out to Calais, where almost full opposite aileron control was required to keep a Cessna straight, but now we were trying to land and there’s a big difference between keeping it straight at 2,000ft and keeping it straight at 500ft.

Still I was happy I was on it for this landing, the instructor wasn’t asking for control so they still had faith too 🙂

Then as we passed through 250ft, we just fell!    In ~2 seconds we dropped from 200ft to 100ft, 50 feet a second (or 3,000ft per minute!!)

Throttle Fully open, the Cessna managed to get lift again.

The window outside went from a happy picture, to the runway filling the screen in milliseconds – we’d just dropped out of the sky, for that second or two there was no flying involved, we’d just fallen straight down.    It’s a moment like this that makes you appreciate being in a C172 and not something with less power.  However, Cambridge is a mile long runway, so once recovered and flying again, the CFI said I could just re-land it a bit further down the runway – it meant we had to back track after landing but, still better than a full go around.

Back on the Ground Safely

Once safely back on the ground with time to reflect, the CFI said that the natural reaction of trying to pull back on the controls with only a little power would have been absolutely no good and we’d probably have just slammed the tail of the plane into the runway – he also said it was probably the worst wind shear he’d been in, I take that as meaning it was about as bad as it can get.

Still, it was recovered, we landed safely and I felt I took some good things away from it all – even if we didn’t reach our destination, the real skill in aviation is to know when to quit, rather than just press on regardless 🙂