43 SA Flyer Magazinecolumns
waving in his face and screaming at him to
be very careful because he was trespassing
in the territory of a severe weather factory.
I’ll come to those three points in a
moment, but let’s look at two conflicting
statements. Met said there was a “cut-off
low”, and that one should expect “heavy
rain with hail and gusty winds, with visibility
reduced to 1 km or less.” However, the
pilot claims the weather was fine with good
visibility – but he also said he was diverting
because of thunderstorms. Hmmmm ... I
think I back Met on this one.
We should also note that while he was
dodging weather, he just happened to be
over the intersection of the Sneeuberg and
Winterberg mountain ranges.
So, does anyone still want to be
a passenger in this aircraft – dodging
thunderstorms and mountain peaks?
Apart from the mountains forming
geographical barriers, they also happen
to perform a key role in triggering the
formation of thunderstorms.
This crash happened because the
pilot didn’t understand three things about
thunderstorm development. Let’s discuss
Met 101 for a moment, to help others avoid
suffering a similar, or worse, fate?
Like everyone else, when I first
started my pilot training, I hated Met. It’s
an intangible, nebulous, subject made
ridiculous by the inclusion of Latin, and
incomprehensible terms like ‘Coriolis effect’
and ‘saturated adiabatic lapse rate’.
It took me a while to realise that this is
actually a simple, enjoyable subject. Once
you’ve got the basics, it opens up a whole
new fascinating world. Even on the days
when you are not flying, you can look up
at that vast hemisphere above you, and
marvel.
Anyhow, let’s look at just four simple
facts that could have saved this poor guy a
lot of grief.
First, you must expect turbulence and
thunderstorms if the temperature drops
quickly during your climb after takeoff.
Read that again. It’s dead simple, and
very important. But how many pilots watch
the OAT (outside air temperature) during
the climb?
Here’s how it works. In the ‘Standard’
atmosphere, the temp will decrease by 2°C
for every thousand feet you climb. So, if it is
20°C on the ground, then is should be 18°C
at 1,000 ft AGL, and 16°C at 2,000 ft AGL,
and so on.
Now if, by some quirk of nature, which
you don’t need to understand now, the
temperature drops more quickly than that –
say by 3°C or 4°C for every thousand feet
you climb – then turbulence is guaranteed.
And you are entering the breeding ground
of thunderstorms.
The more rapidly the temperature drops
during your climb – the bigger the storms.
Couldn’t be easier.
The next thing that should have been a
red flag was that Met had muttered about
a “cut-off low”. These things are really bad
news for pilots – and most of us haven’t a
clue what they are.
Again it’s dead simple. You know the
ITCZ (the Inter Tropical Convergence
Zone)? Don’t panic, it just means a band of
wandering gypsies. They are actually a line
of thunderstorms that all hold hands and
circle the globe all the time.
They move north and south with
the seasons. In our summer, they come
towards us. Every now and then a bunch of
them (each with its own little low pressure
area) decide to branch off and head all the
way down Africa.
They are exactly what they are called- cut-off lows – and they bring serious rain
and thunderstorms. The 1968 flood in PE
which dumped 430 mm (17 inches) of rain
on the city in one day, was caused by a cut-
off low. They are not to be trifled with.
Next, a point I mentioned earlier.
Thunderstorms need a trigger, consisting
of rising air, to get them started. And
mountains do this admirably. Even a light
wind that is forced to rise up the slope is
enough to get the whole weather factory
operating at full blast.
Finally, thunderstorms often hide above
a layer of cloud. So you simply don’t know
they are there, which makes it all the more
important to get the very latest info from
Met, and to know the signs that they may
be lurking.WHAT CAN WE LEARN?
Okay, so here’s a brief recap:- Pay attention to the warnings from
the Outside Air Temp gauge. - Always have the latest Met, and
understand the terms they are
using – like cut-off low. - Mountains often trigger
thunderstorm development. - Thunderstorms don’t always look
like thunderstorms from below.j
ZU-DRI was written off due to hail damage.How many pilots
watch the OAT
after takeoff?