SEARCH What's New

Thunderstorms need to be respected

Dale Nielsen

 

Thunderstorm season is back again, at least in BC. The cumulonimbus (CB) clouds that make thunderstorms carry a lot of power and fury and deserve a lot of respect. The following pilots did not show enough respect.

The pilot of a Piper PA-42 reported an engine failure on departure from Hamilton, Ont. And requested an immediate return for landing with emergency services standing by. The company later reported that lightning had struck the right propeller and a fire light had illuminated for the engine. The crew then shut the engine down.

The right propeller of a PA-31T Cheyenne was struck by lightning while in cruise flight at 17,000 feet. The aircraft landed safely at Winnipeg. The damage to the aircraft included minor damage to the propeller and the flap and elevator trailing edges.

A C-210 pilot set off on a crosscountry flight despite reports of severe thunderstorms along his route of flight.

Midway into his flight the pilot reported an updraft that was pushing him through 6,500 feet (he started at 6,000 feet). Two minutes later the aircraft disappeared from radar and the pilot did not respond to calls from ATC. The aircraft had broken up in flight. The right wing had failed first, then the left, which was torn from the fuselage taking the carry through spar and cabin roof with it. The elevators and rudder had separated from the tail, followed by the vertical stabilizer.

Just before he disappeared from radar, another C-210 pilot reported instrument problems.

A Mooney pilot was warned by a briefer that there were thunderstorms forecast for his route of flight. During descent into his destination the pilot reported a lot of turbulence and some rain. ATC recommended a 180-degree turn and the pilot eagerly agreed. A minute later the pilot did not respond to calls. Witness saw the Mooney crash into a barn while a severe thunderstorm raged overhead. Portions of the tail and wings had been ripped off in flight.

There were six more lightening strikes reported in 2009 and several pilots had close calls in turbulence and wind shear.

A developing thunderstorm is the result of a strong updraft causing the cumulus cloud to build rapidly (may exceed 3,000 feet/minute). Flying under cumulus clouds with thunderstorms forecast in the area is not wise.

Unless the cumulus clouds are widely scattered, we may not see one of them build into a CB. A 3,000 feet/minute updraft will cause a significant sheer and may suck an aircraft up onto the cloud.

A thunderstorm is mature when rain begins to fall. The cold rain falling sets up a strong downdraft, which spreads out as it hits the surface, creating strong, gusty winds, a sharp temperature drop, and a rapid rise in pressure. These downdrafts may exceed 2,500 feet per minute, while the updrafts during this stage may reach 6,000 feet per minute.

This strong vertical shear creates turbulence that may exceed the structural limits of an aircraft.

Thunderstorms can measure from less than five miles to more than 30 miles in diameter. Cloud bases range from 200 to 10,000 feet ASL and tops range from 25,000 to 65,000 feet. Thunderstorms may last from one hour to several hours.

The larger thunderstorms of 30 miles in diameter and tops to 65,000 feet are most likely to occur on the prairies in the summertime. The prairie terrain allows for the gathering of an immense amount of unstable air to fuel the updrafts.

Air mass thunderstorms result from surface heating. They occur in the middle and late afternoon and last 20 minutes to 1 1/2 hours. Steady state thunderstorms are associated with cold fronts and may form into line squalls (long rows of thunderstorms), which may last for several hours.

The primary hazard of thunderstorms is turbulence.

The vertical wind shears inside a cell where updrafts and downdrafts are side by side, can easily tear an aircraft apart. The downdrafts in, and under, a cell may exceed an aircraft’s ability to climb.

Downbursts and micro - bursts are sudden severe downdrafts that can slam a large airliner into the ground. A downburst may be larger than two nm in diameter with extreme winds that may last from five to 20 minutes. A microburst is smaller than two nm in diameter, with extreme winds lasting less than five minutes.

Microbursts may occur up to 10 nm from the thunderstorm. The winds from downbursts and microbursts may change direction suddenly causing wind shears and turbulence extending a considerable distance from the centre of the burst.

The outer edge of turbulence in advance of an approaching thunderstorm is called a gust front.

Lightning will not likely harm aircraft occupants directly, but there is no way of knowing what damage it may do to the aircraft. In the past, lightning strike damage has ranged from light to severe. Lightning strikes to aircraft may even occur in clear air a considerable distance from the cell.

Hail can cause considerable damage to an aircraft and has been known to fall in clear air well to the side a thunderstorm.

Heavy rain, reducing ceilings and visibility, may occur well in advance of thunderstorms.

A Squall line, a line of thunderstorms at a cold front, may extend for many miles, blocking the path of a desired flight. There is a tendency for us to want to pick our way between cells. Unless the cells are at least 20 nm (better yet 40 nm) apart, the associated turbulence from each may overlap. Many of the hazards mentioned above extend well behind the squall line.

Wind shifts near squall lines of up to 180 degrees can occur without warning.

Most publications suggest that thunderstorms be avoided by at least 10 miles and some suggest 20 miles.

Thunderstorms are an awesome force of nature. The light show can be incredible. They should be observed, however, from the ground or from a long way off if in the air. We must pay a lot of respect to thunderstorms; they have no respect for us.

Dale Nielsen is an ex-Armed Forces pilot and aerial photography pilot. He lives in Abbotsford, B.C., and currently flies medevacs from Victoria in a Lear 25. Nielsen is also the author of seven flight training manuals published by Canuck West Holdings.