Big, bad and beautiful

These were sizeable airliners crewed by experienced pilots who paid the ultimate price at the hands of mother nature.

The energy within thunderstorms is only one aspect of their violence. There is lightning, windshear, hail, reduced visibility in heavy rain, microbursts and all manner of phenomena waiting for pilots across the globe. So what makes a thunderstorm tick?

Anatomy of a thunderstorm
Man has always been in awe of the giant cloud mass that constitutes a thunderstorm. The sheer mass of water held aloft by these impressive clouds is measured in millions of litres. Whether its far-reaching tops, the amazing light show or its rumbling war cry, the various dimensions of a storm hold their own impressive qualities.

Yet the requirements for a storm to develop are relatively straightforward — a parcel of unstable air that once set in motion wants to keep rising, a trigger to “lift” that unstable air in the first instance and an abundance of moisture in the atmosphere. The ‘trigger’ may be in the form of a frontal system, heating from below, the convergence of air masses or ‘orographic lifting’ which sees rising terrain force the air upwards.

Whatever the trigger, once the unstable moist air rises on vigorous updraughts of about 3,000ft aminute, it cools and condenses as it climbs. Ultimately, the moisture in the bubbling cumuliform cloud will form droplets and ice crystals too large to be supported by the updraughts.

At this point they will begin a journey downwards, creating strong downdraughts next to the updraughts, with tremendous ‘shear’ resulting between the two. This is referred to as the ‘mature’ stage of the storm and sees the first precipitation falling to the ground while turbulence is at its peak. Logically, this is a very dangerous stage for aircraft nearby.

Finally, the downdraughts become dominant and the precipitation begins to subside in the absence of any further condensation from the now absent updraughts. The top of the cloud will extend further horizontally in a shape commonly referred to as an ‘anvil’. And relatively quickly, the once-angry thunderstorm will break up and dissipate. Unfortunately, while this particular ‘cell’ has subsided, thunderstorms often hunt in packs.

Storms may be individual towering pillars with defined edges,or a line of storms and ‘embedded cells’ within a greater cloud mass. The individual storms tend to result from rising air masses lifted by devices such as heat and terrain. Conversely, the line of thunder may be frontal in nature along the edge of two colliding air masses.

Such systems often are characterised by numerous fast-moving cells with low cloud bases —and in their most severe form are announced by the arrival of treacherous squall lines.

Whatever the mechanism of formation or the nature or number of thunderstorms, they are an area of the sky that is unfriendly to aviation. Beyond their towering tops they are associated with heavy rain, hail, turbulence and all manner of dangerous phenomena. It is not the place for a pilot to wander into unawares.

By-products with bite

There is no doubt that when viewed from the bar of the local aero club, a thunderstorm can be seen as an example of nature’s power and beauty. Unfortunately, from the cockpit of an aircraft, no matter its size, it’s more a sign of foreboding. And if its dark, brooding anger unleashes into full fury it brings almost every possible adverse effect a pilot’s imagination can conjure.

The major issue for aircraft is turbulence and not just the occasional bump or two — this is turbulence that can be severe enough to potentially cause an aircraft to break up in mid-air.

Much of the turbulence can be traced to the rapidly rising and descending ‘draughts’ and the windshear that lies between them. The updraughts can suck a light aircraft up into the cloud or punch an airliner trying to overfly the cloud top too closely.

But while updraughts generally are stronger, downdraughts can be equally devastating for aircraft attempting to dart beneath a storm cell. Additionally, the sheer aggression and turbulence of a squall line is very sobering. These fast-moving squalls often lie in advance of the storm line but their fury is just as savage.

Much of the danger associated with the turbulence is associated with ‘gusts’, where short, sharp bursts of up-and-down draughts really rattle the airframe. The main dangers here stem from the severe loadings imposed on the airframe as well as the violent changes in aircraft attitude and airspeed that can induce a stall or push an aircraft beyond its ‘velocity-never exceed’ (Vne) and other operational limitations. It’s very ugly stuff!

But turbulence is only one of the dangerous elements of a thunderstorm. Strong surface winds emanating from the base of the storm, and often in advance of it, pose a hazard. Parked aircraft as well as those taking off and landing are particularly vulnerable to these winds and their strong associated wind shears.
Hail is another damaging by-product. At its worst between 10,000-30,000ft in the heart of the storm, these little balls of ice also can be found outside the cloud beneath the anvil. I encountered hail over 20 miles away from a developed thunderstorm when isolated hail-stones shattered a wingtip and a landing light cover. The business jet that flew just downwind of the anvil was written off due to hail damage.

The king of the turbulence court is the microburst. A microburst is a very localised downdraught emanating beneath a storm cell and producing very damaging winds. The low-level wind shear caused by its gust front makes it particularly lethal to aircraft and ‘dry’ microbursts are void of precipitation, further concealing their deadly nature and presence.

There have been several notable accidents involving microbursts bringing down airliners — further reminder that no one is immune from the brutality of a thunderstorm.

Icing also can also prove an issue. Airframe, intake and carburettor icing are all very real possibilities in the moist air mass, particularly in the region where the temperature falls to between 0 and 10 degrees, just above freezing level.

And if the icing, turbulence and hail don’t get you, the lightning may ruin your day. While the Hollywood concept of an exploding aircraft may not be a real danger, the fact is that a direct strike quite possibly will perforate the aircraft skin with a hole the size of a coin.

Significantly, the more frequent outcome is that some electrical equipment and radios can be fried by the electrical discharge of a strike. And be wary of your magnetic compass — it should be treated with a degree of suspicion after a lightning strike.

Less severe but more picturesque is the build-up of static electricity. While ‘wicks’ on the trailing edge of the aircraft’s wings and tail can reduce the effect, the static often can still create a great deal of interference through the HF and VHF radios.

On the positive side, the harmless light show of St Elmo’s fire creeping up the windscreen and along the airframe is a sight of which aviators are consistently in awe.

Fight or flight

Given the daunting evidence so far, you won’t be surprised to hear that evasion undoubtedly is the best policy when dealing with thunderstorms. They’re areas of aggressive unstable weather that have the potential to lead to tragedy by so many means. However, if confronted with one, there are some key points to bear in mind.

Firstly, there is no such thing as a safe thunderstorm. Even a small developing cumulonimbus cloud contains severe updraughts, turbulence and shear. Flying beneath thunderstorms should also be avoided as the draughts can suck the aircraft into the storm or force it down towards the terrain. This is particularly the case in storms that have developed orographically near high terrain.

The strength of the draughts generally will far exceed the ability of a piston-engined aircraft to out-climb them and in instances of severe windshear, even a high-performance jet is in peril.

When avoiding a thunderstorm, it’s best given a wide berth and on the upwind side. Hopefully, the bulk of the turbulence, hail and other precipitation will blow downwind.

Additionally, trying to climb over a developing cell is not wise given the rapid rate of updraught and development. Similarly, jet airliners should try not to fly directly over a storm in clear air as the rapid convection and even hail can continue for many thousands of feet above its top.

If forced to fly in cloud in the area of thunderstorms, weather radar is a must. Even so, put as many miles upwind as possible between the aircraft and the cell. Even clear of the eye of the storm, turbulence, icing and lightning may still be major issues.

All passengers and crew must be seated and firmly strapped into their seats. Always fly the aircraft at its reduced turbulence penetration speed but avoid getting too slow as this will only reduce the margin over the stall speed. On this note, the airspeed indicator can fluctuate rapidly if subjected to severe gusts so it may call for increased attentiveness to the aircraft’s attitude and a constant power setting.

Ensure all anti-icing protection is used and continuous engine ignition may be prudent on turbine types. If there’s lightning, turn all of the cockpit lighting up to ‘full bright’ to minimise the impact of the flash on the eyes.

Lastly, don’t try to out-run a thunderstorm — you’ll generally lose and become vulnerable attempting to land in the shadow of an approaching storm with its associated precipitation and windshear. If possible, a 180-degree turn in good time to live and fight another day will provide the safest option. When storms are in the air, always have an alternative plan.

Safety first

The thunderstorm is a dominant force in the air above our earth and beside it our aircraft are little more than gnats. When cumulonimbus clouds fill the skies the only safe place is on the ground and even then the storms anger can uproot tie-downs and flip aeroplanes like coins.

That being said, should we find ourselves in the presence of these giants only distance and respect will offer any degree of comfort.

The aircraft must be flown well within its limits and away from the danger as quickly as possible. Anyone who has been mercilessly pounded by a thunderstorm, cracked by its lightning and thrashed by its rain will attest to the severity of its nature.

Aircraft size is no protection against a storm and the biggest airliners can be equally vulnerable. It is a phenomenon of raw beauty and we are indeed guests in its domain. For this reason alone it is best viewed at a distance from the safety of an armchair at the airport lounge or the aero club.