The invention of lightning detection equipment was a significant advance for general aviation. Compared to the price ($25,000 and up — way up), weight (25 to 40 pounds), and display requirements of weather radars, lightning detection equipment is cheap ($4,000 to $15,000), light (about 5 to 10 pounds), and small enough to fit just about anywhere on the smallest airplane's instrument panel. These advantages make lightning detection devices the storm avoidance instrument of choice for most general aviation pilots.
Currently, BFGoodrich's Stormscopes and Insight's Strike Finder are the only games in town when it comes to dedicated lightning detection units. Stormscope imagery can also be imposed on radar and multifunction displays, and Honeywell's radars can be fitted out with that company's proprietary lightning detection hardware that superimposes lightning stike symbology on precipitation returns.
Operating principles
Stone simple. After powering up a Stormscope or Strike Finder, it goes through some diagnostics to make sure everything's working up to snuff. The units then detect any strokes of lightning and plot them on the display using dots or crosses. The range and relative bearing of the strike are relatively accurate. There is a "Clear" button that eliminates any depicted strokes, and a button to select various display ranges. The ranges run from 25 to 200 nautical miles.
If a lot of dots or crosses pop up quickly after clearing the display screen, that's a sign of a strong storm system or a very active thunderstorm cell. So, a densely packed area of dots or crosses is a place you want to give a wide berth.
Advantages
Perhaps the biggest advantage of lightning detection is its ease of interpretation. Dots mean thunderstorm-related electrical activity, period. With radar, you have to understand how to manage antenna tilt control to accurately view the shape and structure of precipitation returns, and attenuation of the radar signal is a big, big problem in lower-powered radars with smaller antennas. With attenuation, radar energy from the antenna is scattered so badly by nearby precipitation that you can't make out any precipitation contours. The display can go totally green from all the energy being reflected back at close range, and you can't see a true picture of what's really up ahead.
Lightning detection can show a 360-degree view of the situation around you, so you can see strikes behind as well as in front of you. Radars only give you a view of the 120-degree arc of airspace directly in front of you.
understand how to manage antenna tilt control to accurately view the shape and structure of precipitation returns, and attenuation of the radar signal is a big, big problem in lower-powered radars with smaller antennas. With attenuation, radar energy from the antenna is scattered so badly by nearby precipitation that you can't make out any precipitation contours. The display can go totally green from all the energy being reflected back at close range, and you can't see a true picture of what's really up ahead.
Lightning detection can show a 360-degree view of the situation around you, so you can see strikes behind as well as in front of you. Radars only give you a view of the 120-degree arc of airspace directly in front of you.
Newer lightning detection devices can be fitted out with heading stabilization. In aircraft with slaved gyro systems, this can provide a redundant means of showing heading information. In addition, the relative positions of any depicted strikes remain in their correct locations when the aircraft is turned. Non-stabilized systems require that you clear the display after a turn to obtain an accurate look at strikes.
Disadvantages
There are no free rides, so yes, lightning detection technology does have some peculiarities to keep in mind when interpreting lightning returns. Here are the main problems:
- Radial spread. Lightning detection devices are set up to listen for a "standard strength" lightning stroke. But all lightning is not alike. Strong strokes appear closer than they really are. Weak ones show up as being farther away. If a strong storm is at work, it can show up on a lightning detection display as a thin wedge radiating away from the airplane, and heading in the storm's direction. In essence, the lightning detection equipment is crying wolf, saying that the storm is nearer than its true range. BFG has developed a "cell mode" for its WX-950 Stormscope. This groups strikes in clusters, and the company says that this helps take care of the radial spread problem. But radial spread remains a factor in all lightning detection equipment. Some feel that the false ranging caused by radial spread is more good than bad, because it errs on the conservative side and gives plenty of advance thunderstorm warning.
- Correlative dissonance. Lightning strikes are an indication of thunderstorm activity, but severe turbulence can occur in areas unaffected by lightning. Same thing with hail, heavy downpours, and downbursts. So avoiding the dots or crosses is by no means a guarantee of a trouble-free ride.
- Ionospheric skip. Sometimes, lightning detection equipment will show strikes up ahead when there are none. This is caused by energy from very distant storms (say, up to 1,000 nm away) skipping off the ionosphere. This is most likely to happen at night.
- Cell smear. This looks like a growing group of dots or crosses that travels in the direction of your airplane. This happens because old strike signatures remain on the display, and as new ones appear they make an elongated shape that coincides with the airplane's direction of movement. Clearing the screen from time to time should take care of this type of false depiction.
- Installation problems. This can be a major headache. Improperly grounded strobe lights, magnetos, alternators, beacons, and so forth can cause false returns on a Stormscope or Strike Finder. It's vitally important that the airplane be examined for any random electrical "noise" that avionics or other electrical equipment may be making, and that the antenna be located well away from any of these sources of possible interference.
Flying strategies
The prime directive for using Stormscopes, Strike Finders, or any other type of storm avoidance equipment is to keep a safe distance from any active cells. This means staying at least 20 nm away. Staying visual is the best way of steering clear of any cumulus buildups.
A booklet published by Insight Avionics (Strike Finder Pilot's Guide, available for $10 from Insight Avionics, Box 194, Buffalo, New York 14205-0194; telephone 716/852-3217; fax 905/871-5460) gives perhaps the best strategies for using lightning detection equipment. It recommends using what Insight calls the "one-quarter rule" for avoiding storms along your flight path. This rule provides you with a storm avoidance angle for use when turning away from strike returns. The rule involves establishing an on-screen separation distance of one-quarter of the displayed range. Based on this range, the pilot then determines the angle of the evasive turn away from the strike returns.
At the 200-nm display range, for example, one-quarter of this distance is 50 nm. Dots appearing at 200 nm — the outer edge of the display — require no evasive action because they are so far away. At a 100-nm display range, however, a course correction of between 0 and 40 degrees will be needed to keep 25 nm (one-fourth of 100 nm) between you and the sparks. At 50 nm, your heading change will have to be between 30 and 90 degrees to keep a safe distance. And at 25 nm — the closest range either a Stormscope or Strike Finder will zoom in — the heading change should be between 90 and 180 degrees. The accompanying illustrations give good graphic views of how this rule works.
The assumption behind the one-quarter rule is that lightning detection devices can be used when flying around thunderstorms embedded within a much larger mass of clouds. The key word here is "around." Unless the strike returns are far, far apart — say, 100 nm, and staying that far apart — it's a bad idea to use lightning detection devices to wend your way between storm cells, whether you're on instruments or trying to navigate visually through the clear spaces between cell buildups. The cells/strike zones could merge. Also, remember the earlier warnings about non-lightning thunderstorm hazards. Just because you stay away from a bunch of dots doesn't mean you won't be slammed around in the soup.
Stormscopes and Strike Finders are great additions to any instrument panel. They can keep you away from killer cells, give you advance warning of young cells growing along your route, and give rough guidance for storm avoidance. But they should not be used as a sole means of storm information. They should be used in conjunction with flight watch, HIWAS, ATC weather advisories, pireps, and other sources of late-breaking weather information.
Some feel that radar offers more advantages when it comes to in-flight tactical decision making around thunderstorms. That subject's open for debate, and will be addressed in an upcoming issue.
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