Instructors Column

Mark Wilson, CFI of Wellington Gliding Club is a firm believer in bar-room tales as a valid form of training. It is far, far better, he says, to learn from the mistakes of others, rather than make your own. So grab your beer as you read his story and learn about the mistakes he made.

Approaching 1000 hours in gliders, with many competitions and many more field landings under my belt, I made a classic.

Phynns strip, near Kawhatau, Mangaweka is a fairly big, slightly sloping ‘farm strip’ which is an ideal site to explore the Eastern side of the Ruahine Ranges. The strip is in the form of a bench halfway up the northern side of a small hill and slopes down to the West. The Fielding and Wellington clubs tend to meet up there once a year. This was my second visit to the site.

It started as these things often do, at about 2000 feet AGL working a gentle, weak evening with a very small amount of north ward drift. I was flying my Libelle, watching the cloud starting to hug t he downwind slopes of the ranges to the East. There had been a bit of weak south-easterly wave ‘organising’ (more sort of abusing) the thermals earlier. Time for a beer.

I joined a high, wide left-hand circuit, for an uphill landing towards the east. A K6 had stopped close to the bottom end of the airfield but was being cleared. Great, I could land short and stop by my trailer. I chose to approach at 55kts. The air was smooth with no perceptible drift from the cross wind so I elected to use the bound ary fence as the reference point to land as short as possible.

T he next bit happened in rather less time than it takes to read it.

I was overshooting slightly so a few seconds of full brake sorted that out – I was nicely set up, 55kts, half brake. It looked good. Bit of sink, I closed the brakes. More sink and starting to drop below glide slope - 50kts, brakes firmly shut. The vario was indicating 7-8 down. I realised with horror that even just clearing the wire fence

was rapidly becoming a serious problem! Keeping the nose down, with my speed creeping back up to 55 knots I was now lower than the strip. Rather than spud into the side of the hill I banked hard left & tried for the paddock below the airstrip.

With not enough time or space to turn along the length of the paddock and with a only a short patch of grass ahead I pulled full air brake, jammed the wheel brake on hard and forced the glider onto the ground smack in the middle of a herd of surprised sheep and a big clump of thistles. The sheep did what sheep do, bunched up and

tried to out run the glider. With a wire fence & sheep approaching rapidly I ground-looped the glider. I was now travelling sideways at speed, with the trailing wing and nose pushed onto the ground and waiting for the moment when my Libelle would snap in two. The glider lurched to a stop and the horrible noises ceased. The fence was about a wingspan away.

I later discovered that other than the resulting mess from decapitating a swathe of thistles whilst travelling in all possible directions, my Libelle survived totally unscathed, as did the sheep.

What went wrong?

My easterly approach was flown alongside and into the lee of a slope. With the south-easterly airflow smooth and hugging the ground contours (remember I mentioned the clouds hugging the ranges) even a moderate 10 knots wind down a 30 degree slope is 5 knots of sink. A textbook half airbrake approach gives typically 5-7 knots sink rate, i.e. fully closing the brakes can just about compen sate for 3-5 knots of sinking air. It’s fairly obvious that a normal half b rake approach angle was going to be inadequate, even if it looked good to start with. I’m now painfully aware that in hilly terrain even a gentle wind can generate enough vertical sink to force you into undershoot. Of course, also using the boundary fence as a refer ence with no undershoot area and a ‘minimum’ approach speed left m e with no further margins (other than sheer luck).

Why did the pilot of the K6 also land short? Because he had to! He later told me he was familiar with the sink on the approach in those conditions and had originally set up his approach to land rather further up the field. By sensibly selecting an into field refer ence point, he had just enough margin to cope with the sink.

The rule is “It’s always better to hit the far hedge slowly than the near fence at speed.”

When choosing a reference point for your approach, always keep a margin to allow for a potential undershoot.

Never fly an approach with less than half airbrake. Close the brakes until you really do need at least half.

Increase your margins if over-flying undulating ground or in the vicinity of tall obstacles like trees or buildings.

Take note of bar tales and learn from other’s experiences. I don’t mind sharing my misfortune if it saves others from doing the same or worse.