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Calculating The Risk!
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Although I was gutted, there’s no question it was the right decision, even if some would say that an even more right decision would have to been to commit earlier to a field. Happily, fast forward a couple of years and I managed to finally complete the same flight to Membury and back in the same glider, but this time with a healthy margin of altitude coming back to the club. But in the intervening two years, I kept thinking about that image of Parham so agonisingly close in that earlier flight, and thought ‘What if I had pressed on?


So what would have happened if I had succumbed to ‘press-on-itis’? The numbers are interesting. At its best glide speed of between 50 and 57 knots, (max weight with and without ballast) an Astir has a claimed glide ratio of between 36 and 38 to 1. But that’s only what’s in the manual. With a pilot of 8 years experience but only 150 hours logged, with tunnel vision, getting low, and bugs on the glider’s wings, who knows what the true figure is?


A better way to crunch the numbers is to look at the SeeYou trace. At the time I decided to drop the gear and turn base for the field landing, I was at 700’ ASL, or 590’ above Parham elevation. I needed to cover almost exactly 3.5 km to squeak over the trees for a landing there. Looking at the barogram I could look at the actual descent rate during the whole 18 minute final glide and extrapolate it. Or perhaps more logically, look at the the last five minutes of my final glide as that was the most recent air I was in. So those two figures yield a descent rate of between 170 and 200 feet per minute.


At an average 58 knots ground speed, which is more or less what I was doing at the time, an Astir is covering about 30 metres per second, so 3.5 km would take almost exactly 2 minutes. So plugging in an average descent rate of 185 fpm yields a glide ratio of 31 to 1.


So if theoretically pressing on in theoretical air, I would have broken quite a few rules. I would have had to take a direct path to the field which would have taken me clean across Parham House at around 350’. Then I would have had to clear the trees at the field, using up 60 feet or so of my margin, giving me a 160ft cushion. I would likely have had to land on the tug strip between the private hangar and the flight line, with no margin to manoeuvre for traffic. Yes, I’m sure it’s happened before.


But all of this assumes no extra sink. And If I had hit extra sink of only one knot for those two minutes, I would have been picking bits of twig out of the bottom of the glider or even worse, decorating a tree. And what if I had hit that one knot of sink and bailed at the last moment into an unsuitable field? Would I have kept the speed and bank angle up in a final turn, or put in too much rudder and got dangerously slow as I pulled back subconsciously on the stick?


No, it doesn’t bear thinking about. I’ve made my fair share of stupid decisions in the past, and luckily, with a nod to my training and a dash of experience, this happened not be one of them. But the temptation certainly was there. Looking at the trace confirms to me that you can do all the calculations you like, but in the final analysis, if the main variable is dumb luck, it’s time to redo the maths. Either that or enter your first competition!