Any cloud connected to a severe thunderstorm carries the threat of violence. – AC 00-6A
One of the dangerous myths of aviation is that of the all-weather general aviation airplane. And while its true that modern technology has done wonders in making flying more reliable and safer, its important to remember that Mother Nature always has the last word. Heres an important case in point.
This mishap involved a turbocharged Piper Saratoga that broke up in flight in the vicious winds found in and around severe thunderstorms. The pilot was attempting to find his way through a band of thunderstorms, which extended from northeast to southwest over central New Mexico. Two cells near the accident site were reported as very strong and developing rapidly.
The closest cell reportedly grew from 35,000 feet to 45,000 feet in about 10 minutes. NTSB meteorology experts concluded the pilot probably was avoiding the cells visually until shortly before the mishap, whereupon he entered instrument conditions.
This should not have been a problem since the aircraft was equipped with a B.F. Goodrich WX-1000+ Stormscope. Following any heading changes this system continually aligns displayed electrical discharges with the airplanes course. These discharges remain displayed for approximately five minutes unless cleared manually.
In a flight from a refueling stop at Lubbock, Texas, to Las Vegas, Nev., the pilot was cruising at 14,500 feet. At 13:27 he requested an IFR clearance. Shortly thereafter he was cleared direct to Las Vegas at Flight Level 180.
At 14:02 the pilot requested a turn south to avoid weather. At 14:09 the controller asked how much farther south he planned to go, whereupon the pilot said he would turn back northwest in five to 10 miles. That was his last transmission.
At 14:18 the Saratoga was at 17,800 feet and entered instrument conditions. Only 48 seconds later it was at 15,500 feet – a descent rate of 2,875 fpm. Then the aircraft made a 180-degree turn back to the east.
In desperation he had violated one of the AIMs long-standing rules of accidental thunderstorm penetration: Dont turn back once you are in the thunderstorm. At the time of the mishap a rancher near the accident site reported a sharp report. Then at 14:33 a forest service fire watch tower reported smoke.
According to the FAA, the pilot had about 500 hours, with 100 hours in the last six months, and had earned an instrument rating about two years earlier. He had progressed from a Piper Archer to a 1997 Saratoga and then, about five months before the accident, he traded for the new 1999 Saratoga. At the time of the accident the airplane and engine had only 74 hours.
Before purchasing the 1997 Saratoga the pilot/owner had received about 20 hours of model specific training. After the purchase, the manufacturer provided three days of free training. When he acquired the new 1999 Saratoga he was once again entitled to free training. In fact the training was scheduled, but the pilot later canceled it. The only training investigators could document in the new aircraft was a three to four hour biennial flight review four months prior to the accident.
The instructor reported the pilot was very new with the equipment displays, electronics operation, and airplane systems. In addition there was no documentation showing the pilot had been trained in the use of supplemental oxygen or the use of the W1000+ Stormscope.
ATC radar data showed the aircraft entered IMC at a ground speed of approximately 170 knots – far above the max gross weight maneuvering speed (Va) of 119 knots. The wreckage showed the left wing with an upward bend 65 inches inboard from the tip. The main spar broke at the wing root. The right wing was found parallel to the main fuselage and attached by control cables.
The stabilators main spar box showed downward bending on both ends. Both leading edges showed downward bending at the outboard ends. Separated pieces of the stabilator showed compression bucking of the lower surfaces, with tension tearing on the top surfaces.
In short, there was ample evidence of in flight over-stress and structural failure.
The Saratogas WX-1000+ is a top-of-the-line lightning detector. Where theres lightning, theres likely to be turbulence. While the device isnt perfect in keeping airplanes separate from thunderstorms, it does give the pilot the opportunity to circumnavigate storms while in IMC. Interpreting the display, however, takes training and practice, with conservative, progressive experience and an understanding of its limitations. You dont just get a quick briefing then go boiling into a line of Level 6 thunderstorms – unless of course youre in a hurry or have an active death wish.
Because it measures only the electrical discharge associated with lightning, the Stormscope cant tell you about areas of heavy rain and teeth-rattling turbulence if theres no lightning present. And the more potent the storm the more likely it will throw off rain and turbulence farther from its core.
The AIM has recommended for years that pilots give a wide berth to thunderstorms – 20 miles at least, and 40 miles for large cells. Once you get into a thunderstorms backyard, anything can happen.
If you happen to fly under the anvil of a large cumulonimbus, for example, windshear turbulence with up and down drafts exceeding 6,000 fpm may be felt 15 to 30 miles away. The Saratoga pilot was attempting to penetrate a squall line, which is described by Advisory Circular 00-6A as a line of storms containing severe and steady-state thunderstorms that present the single most intense weather hazard to aircraft.
In addition, you may fly into hail spewed out of a thunderstorm even if youre flying in the clear. This is especially true near or beneath the anvil top.
Finally, any area of strong or very strong radar echoes separated by less than 20 or 30 miles should be assumed to contain severe turbulence throughout the system.
Thunderstorm winds can be divided into four categories: gust fronts, microbursts, shear and tornadoes.
Gust front winds extend up to 20 miles from the leading edge of a storm, with winds increasing in velocity by more than 100 knots. Changes in wind direction of from 40 to 180 degrees are also characteristic, along with the possibility of a roll cloud. The variation in direction and velocity combine to create wind shear. As we know, this can be disastrous during takeoff and landing.
The so-called mamma clouds found in the gust front, while they sometimes appear benign, can have very severe winds and turbulence. In fact they are often the source of tornadoes.
Microbursts will be approximately 6,000 feet in diameter above the ground and 12,000 feet in diameter at surface level. Anything larger is classified as a downburst. Downdraft winds will be as strong as 6,000 fpm, with a velocity greater than 80 knots. Maximum horizontal winds occur about 75 feet above the ground. Their life cycle is about two minutes, with the event completely over in five minutes.
While some think this phenomenon is characteristic of a very large storm, research has shown that just the opposite is true. Ted Fujita and Fernando Caracena, both recognized authorities in the field, have emphasized that microbursts are frequently generated from benign-appearing cells. A common occurrence is that a new, building storm cell will generate a catastrophic microburst while a nearby older storm cell dissipates. This has been the case with several high visibility airline accidents.
The new cell builds from the outflow of the mature cell, and has very strong updrafts. These updrafts carry moisture higher than could be supported by continuous flow updrafts. Then the large mass of moisture-laden air begins to fall.
Caracena identified a vortex ring that concentrates the wind at ground level. This ring expands rapidly after touchdown while restricting air outflow and accelerating wind velocity. The destructive effect resembles a tornado, and aircraft control is problematic.
The insidious danger of microbursts is that their effects can be encountered in clear air, well in front of or behind a thunderstorm. In fact, a microburst can be dry, with its effects being shown by a telltale ring of dust on the ground. Because they most often occur from small, rapidly building cells that lack significant moisture, they are unlikely to be detectable by radar.
The third category of thunderstorm winds consist of the often violent up- and down-drafts required for the storm to mature. These rising and falling currents result from orographic lifting and heat exchange inside the cloud. Flying through these wind shears places major stress on both the airframe and the instrument flying skill of the pilot.
The final category of thunderstorm winds is the tornado. Air is often drawn into a thunderstorm with tremendous force. In the process the air rotates and accelerates, then forms a funnel-shaped low-pressure vortex.
If the funnel touches the ground it is classified as a tornado. If it touches water it is called a waterspout. If it remains airborne it is a funnel cloud. Getting involved with any form of funnel-shaped low pressure vortex is normally catastrophic.
During the initial stages of thunderstorm development, moisture is carried aloft by rising air currents The moisture condenses as it rises and the water droplets can freeze and combine to form hail. The size of the hailstones varies with the amount of moisture and the vertical development of the storm.
Once the droplet size exceeds what the updrafts can support, they fall. Anytime you get within 20 miles of a thunderstorm – even in clear air -you become vulnerable, especially if you are under the storms anvil top. Weather radar does not always paint hail, as it has poor dipole characteristics compared to rain. Its probably invisible if it is thrown outside the cloud into clear air.
All forms of ice are found in thunderstorms. It is most prevalent at intermediate altitudes, where you will find super-cooled water droplets well above the freezing level. Fly into this zone and the ice accumulation will be instantaneous and all-inclusive. In the blink of an eye the entire airplane can be encased in a variety of ice forms – deadly glaze ice in particular. This is why turbine-powered airplanes must have engine inlet heat already activated. Adding engine inlet heat after the accumulation occurs causes the ice to slough off and damage the compressor and fan blades.
Often this results in a double engine flame-out. With reciprocating engines, blocking the air inlets means alternate air or carburetor heat must be used. Tests show that the most severe airframe icing occurs at intermediate altitudes when super-cooled water is encountered well above the freezing level.
Though modern fuel systems are designed to prevent an explosion from a lightning strike, it is still possible. Other hazards include temporary blindness, damage to the electrical system, loss of radio or instruments and skin damage. While most lightning-induced airframe damage is superficial, a couple years ago, a Sabreliner 40 was downed by it. And the loss of navigation, communications or flight instruments while in turbulent IMC can set the stage for disaster, too. The hazard is most prevalent during prolonged flight through precipitation when the OAT is between 5C and -5C. Sometimes flight through the upper level of a thunderstorm will trigger lightning that otherwise would not have occurred. However it is most frequent at low and intermediate altitudes.
A lightning flash is a very long electrical spark that extends between one center of electrical charge in a cloud and another center of opposite polarity charge on the ground, in another cloud, or even in the same cloud.
The mechanism that produces lightning comes from warm air rising upward into a developing cloud. As the air cools, water droplets condense and form the cloud. When rising air reaches a temperature of approximately -40 degrees C the water droplets freeze and some of the ice crystals form hail stones, which fall through the cloud. As they fall, small positively charged splinters separate, leaving the hail-stones negatively charged.
Vertical currents within the cell carry these positively charged ice splinters upward, making the top of the cloud positively charged. When the potential near one of the charged areas exceeds the threshold for atmospheric breakdown, lightning results.
The bottom line is that, no matter how well your airplane is equipped, you still must avoid thunderstorms by 20 miles. If there are two cells in a squall line, you may be able to go between them if theyre 40 miles apart. Anything less is Russian roulette. Dont let that wonderful radar and lightning detector fool you. Theyre for weather avoidance, not thunderstorm penetration.
Aircraft Strength Envelope
Piston-powered airplanes have different strength criteria than those with turbine power. In the recips, the top of the green arc on the airspeed indicator is Vno, or maximum structural cruising speed. At or below Vno, an aircraft certified under FAR 23 is able to handle gusts of 50 fps (3,000 fpm) up to 20,000 feet. At speeds greater than Vno – the yellow arc – the strength requirements diminish. Virtually every light airplanes POH states something to the effect of, Do not exceed the Vno except in smooth air, and then only with caution.
The redline, or never exceed speed (Vne), is defined by FAR 23 as being no higher than 90 percent of the airplanes maximum demonstrated dive speed (Vd). At Vd, the airplane must handle gust loads of 25 fps (1,500 fpm) up to 20,000 feet, decreasing linearly to half that at 50,000 feet.
Unlike Transport Category airplanes, GA aircraft have no published Vb, or turbulence penetration speed. However, if you can stand the bumps, any speed up to the end of the green arc is your turbulence penetration speed. In many airplanes, the top of the green arc is Va at maximum gross weight, but Va goes down with lighter loads. Its more comfortable to give a larger margin between your speed and the maximum allowable.
Turbine powered airplanes use Vmo, maximum operating speed, and Mmo, the limiting Mach number. Because turbine powered airplanes routinely cruise at or near their Vmo/Mmo and because they decelerate more slowly, their limitations are based on 80 percent of Vd.
However, turbine aircraft have an established turbulence penetration speed, Vb. In older airplanes certified under CAR 4B Vb guaranteed the aircraft to withstand gusts of 66 fps (3,960 fpm) to 20,000 feet, then 38 fps at 50,000 feet. Today with Amendment 86, new jet transports are certified to a Vb of 56 fps (3,360 fpm) at sea level, 44 fps (2,640 fpm) at 15,0000 feet, and 26 fps (1,560 fpm) at 50,0000 feet. In some aircraft Vb is the same as Va. In the Sabre 80 for example, Va is 225 knots IAS while Vb is 225 knots IAS or .7 Mach, whichever is slower.
In severe turbulence most of us instinctively favor slower speeds because we are more afraid of structural failure than inadvertent stall. Yet the record shows that because of built-in strength margins you are much more likely to encounter serious trouble due to inadvertent stall rather than structural failure. In the Saratoga accident, the pilot was caught at a cruise speed of 170 knots, which should have been within the aircrafts structural capability. Then, while reversing course in the severe thunderstorm, as the vicious gusts took the airplane out of its design envelope he lost control and it broke apart.
The best defense against this kind of accident is good judgment. Remember that if it looks dangerous it probably is. Careful attention to reports and forecasts of severe turbulence and thunderstorms is obvious. Professional training in use of the weather radar and lightning detectors is an absolute requirement.
Dont be too proud to delay a trip if extensive thunderstorms or severe turbulence are reported or forecast along your route of flight. If you are airborne and flying anywhere near a thunderstorm, cruise at a slower speed, because you must be ready for turbulence when it hits. Get inside 20 miles of the big ones and you risk ending up like the Saratoga pilot and his passenger.
Most important, remember that weather radar and a spark detector are there to help you avoid severe weather. They do not allow you to penetrate it. No matter how much equipment you have, there is still no such thing as an all-weather airplane. Mother Nature always has a surprise waiting.
-by John Lowery
John Lowery is a former Air Force pilot, accident investigator and corporate pilot.