Most single-engine aircraft make poor platforms for airborne weather radar. Cost, panel space, and weight are the biggest impediments for owners considering such an installation. With these problems in mind, a lightning detector seems to be the easy solution to getting real-time weather into the cockpit of a light single.
In a nutshell, lightning detectors (or atmospheric devices as they are sometimes termed) work by detecting the electromagnetic output of a lightning strike. This detection is then displayed on a screen in the form of a dot or similar symbol, depending on the model. For a pilot, the job is simple; avoid the dots. With a lightning detector such as the BFGoodrich Stormscope or Insight Strikefinder, there is less operational learning required than that for a radar. With a lightning detector, attenuation is generally a nonissue. It can show thunderstorm information while you're still in your tiedown spot; it can view the sky in 360 degrees instead of the typical 90-degree view of a radar, and the maintenance and upkeep are relatively painless if the unit is installed correctly.
Owners of airplanes with little panel space and limited payloads get a lot more bang for the buck with a $5,000 lightning detector that weighs 2 or 3 pounds than they would by installing a $25,000 airborne weather radar that weighs 25 pounds. When IFR equipped, the instrument panel of many singles will not have much space left for a radar screen. Add in the cost of fairing a radome into the leading edge of a wing, the unit's useful detection range of 50 miles or less, and suddenly the all-seeing radar doesn't look like such a cost-effective idea. We're not trying to discount the advantage of radar, because it is an extremely useful and versatile tool. In a single, however, its usefulness can be limited (see " Radar Realities," page 60).
Weather radar and atmospherics, as well, cannot be solely trusted to navigate anyone through a line or other dense area of thunderstorms. In fact, don't plan on any device in a light airplane to get you through a dense area of thunderstorms — these devices are intended to provide guidance around areas of convective cells and possibly through areas with more widely scattered cells. Using lightning detectors doesn't mean that you will stay dry and out of bumps — you'll likely find plenty of those. Also, if you fly an airplane without a slaved compass system or a heading indicator with a bootstrapping capability (to provide heading stabilization to the display), you must clear the unit every time you complete a turn to get the most accurate storm location information. If you're flying in an area of numerous storms, this is not good. Unless it's really mature, it may take a few minutes for a cell to spew enough sparks to paint an accurate picture. In that amount of time it's entirely possible that you'll have turned again because of what your eyes saw. Eventually, an untrained user could stumble into an area of nasty weather, even when using a lightning detector. It's worth noting here that Insight has been working for a very long time on approving for its Strikefinder a self-contained stabilization unit that will be retrofittable to units in the field. If successful in the approval, owners of airplanes without slaved-compass systems can obtain a gyrostabilized lightning detector. Another option is obtaining from Sigma-Tek a new heading gyro that can bootstrap its heading information to a Strikefinder or Stormscope. Both the Insight and Sigma-Tek devices will cost around $1,500 — far less than the cost of installing an HSI.
Besides the problems with non-gyro-slaved units, atmospheric devices have other limitations that should be addressed. For example, the lightning detector won't keep you out of rain; detecting water is the job of a radar. A lightning detector can depict false returns, mostly at higher ranges (increased sensitivity). In the 200-mile range, for example, the sensitivity of the lightning detector is at its maximum. Through what's termed as signal bounce or radial spread, a monstrous storm 400 miles away could appear as a dot in the 50-mile area. The units have filters to reduce this error, but the incorrect ranging nevertheless seems to happen. As range (sensitivity) is stepped down, accuracy improves and circumnavigation of scattered cells is possible. Also, lightning detectors are best at plotting vertical strikes of lightning and can be confused by horizontal bolts of lightning that stretch for many miles. They're not sure where to plot the strike.
It is imperative to have the unit installed properly and tested on the ground. Once it has been installed, pick a day when there is thunderstorm activity forecast in your area. You'll want to familiarize yourself with the unit while safely on the ground. It's best to look at a local radar picture or lightning-strike data that some meteorologists track on the local news. Taxi the airplane to an area on the airport that is away from structures or trees and see what appears on screen. Lightning detectors are somewhat vulnerable to "noise" created by a nearby electrical disturbance such as an industrial area, or even the airplane's strobe light. If a dot is always at the same relative position regardless of heading, it is probably a disturbance coming from a strobe light or other onboard equipment. Turn off items one by one to isolate the disturbance. If cells are scattered and good VFR prevails, test the unit in the air. After becoming familiar with its capabilities and limitations you'll feel more comfortable about launching on a trip for which convective activity is forecast.
Of course, in a perfect world every airplane would have a radar and a lightning detector. The information from the two units can then be cross-referenced for the best thunderstorm and rain avoidance possible. But, in the real world, the light single owner will probably get the most bang for the buck with a lightning detector.
BFGoodrich Aerospace Avionics Systems, 5353 52nd Street, S.E., Grand Rapids, Michigan 49588-0873; telephone 800/253-9525 or 616/949-6600.
Insight Avionics, Box 194 Ellicott Station, Buffalo, New York 14205-0194; telephone 800/852-3217 or 905/871-0733.
Sigma-Tek Instruments, 1001 Industrial Road, Augusta, Kansas 67010; telephone 316/775-6373.
Lightning on the Fly
Weather avoidance with the WX-900 Stormscope
One way to become acquainted with the characteristics of your lightning detector is to position your airplane on an open ramp when thunderstorms are near the airport, and turn on the Stormscope or Strikefinder. Watch how it maps the activity and cycle it through the ranges to see how the weather depiction changes. Clear the display and observe how long it takes to remap.
After you scrutinize how it depicts the thunderstorms, try to correlate what you saw on the lightning detection device with a radar display of the same weather on cable television's The Weather Channel, one of the weather radar displays available in many FBOs or via the Internet. You'll sometimes be surprised how little the device will show when the radar looks full of weather. Keep in mind that the lightning detector depicts only the truly ugly parts of any thunderstorm system.
Flying the Stormscope
I first used this technique with a 3M WX-10 Stormscope, and employed it most recently with my BFGoodrich Avionics Systems WX-900 Stormscope. The key to using the WX-900 is to evaluate the clusters of crosses with the value shown on the discharge rate indicator, which is a numeric tally of lightning strikes displayed at the top of the screen. Rate-counter activity is the first indication that a thunderstorm is being detected. After the processor validates what it is receiving, crosses start to appear on the display.
There are no published guidelines from BFGoodrich on using the discharge rate indicator to evaluate the weather depicted. I have seen it display values from 0 to 298, with most thunderstorm areas displaying values between 20 and 140. The counter rises and falls with changes in thunderstorm activity; the higher the value displayed, the more active the thunderstorms. Jack McDaniel, a BFGoodrich field service engineer, said that the rate counter's intended use "is for the pilot to judge from past experience the relative intensity of what is displayed."
When you're ready to go flying in thunderstorm season, it's essential to obtain a big-picture view of the current and forecast thunderstorm situation. A visit to flight service or 20 minutes of watching The Weather Channel will provide an outlook of where thunderstorms are now, where they may be headed, and where they are expected to develop.
After departure and while en route, validate what is seen on the lightning detector display with periodic updates of the big picture through flight watch and ATC reports. When deviating for thunderstorms, it's imperative that you keep track of your fuel supply. It's not uncommon that conservative weather avoidance maneuvering will add 60 to 80 (or more) unexpected miles to your flight. I have had to land short of my destination for fuel after extensive weather deviations on a long-range flight.
Weather avoidance
How do I use the WX-900 to avoid thunderstorms? It's simple — unless I can see and avoid the thunderstorms, I maneuver the airplane so that no clusters of crosses penetrate the 25-nautical-mile ring on the 50-nm-range display. This may be conservative, but it should keep you from being tangled up in the violent bowels of a cumulonimbus cloud. I've been there once, and it's the reason there is a lightning detector in my panel.
Even when you adhere to the 25-nm rule, don't be surprised if you encounter some heavy rainfall, spirited turbulence, and even an isolated stroke of lightning. Thunderstorms are a dynamic event; if you insist on flying when they're active, you may find yourself in one that is forming or dissipating.
If you're in the terminal area of an approach control that has the new ASR-9 radar and its outstanding weather depiction capabilities, use a combination of the Stormscope's 25-nm range and the ASR-9 to avoid thunderstorms. If you're maneuvering to land, remember that the gust front of a thunderstorm and its wind shear can easily range 10 to 15 miles from the thunderstorm itself.
While you are flying en route and using the 100-nm display, sometimes clusters that never seem to move as you fly towards them will appear at the 75- to 90-mile range. They're actually strong storms beyond 100 nm, with stronger-than-normal discharges that fool the Stormscope into thinking they're closer than they really are. When they start to move down the display and begin to show on the 50-nm range, it is time to start figuring out which way to deviate.
Radial spread
Radial spread is a phenomenon causing a display of spurious discharges that will mask a strong thunderstorm's true position. Eventually a portion of the display will be filled with a triangular-shaped area of discharges which emanate from the display's center. Clearing the screen and observing where the storm is first mapped is the best way to determine the thunderstorm's actual location. Stormscopes using BFGoodrich's Series II technology appear to depict weather with much less radial spread than that seen on earlier models, but it is still present.
Which way to go?
When it's time to deviate, which way should you turn? Every weather situation is different, but here are some general tips:
- Considering your updated in-flight view of the big picture, fly the shortest thunderstorm-free route to your destination. Stay in visual conditions if at all possible.
- Use the movement of the weather to your advantage. If your course meets the middle of an area of thunderstorms that is moving northeast, deviate to the southwest. However, beware when deviating around the southern end of any line or group of thunderstorms, because that's where they tend to be the most severe.
- Try to stay on the upwind side of the thunderstorms to avoid the hail that sometimes falls from the anvil top.
- If you must deviate on the downwind side of a storm identified as severe, for every 1 knot of wind at your altitude, try to put 1 nm between your aircraft and the storm. For example, if the winds are 30 knots at your cruise altitude, try to stay 30 miles from the storm. This technique will help you to avoid not only the hail threat of a severe storm, but also its potential for clear air turbulence.
- Unless you're in visual conditions and are sure that you'll remain there, don't try to slip through a "hole" in a line of thunderstorms. If the line is severe enough to warrant the issuance of a convective sigmet, there is a strong possibility that the feature causing the thunderstorms may quickly fill any gaps in the line.
- While analyzing the display for deviation options, be sure to account for the skewed picture presented when strong winds aloft cause a large difference between your heading and ground track. This can sometimes be 25 degrees or more, depending on the speed of your airplane and strength of the wind.
- The old aviation adage "When in doubt, wait it out" certainly applies to thunderstorms. If you can't find a reasonable and safe way around thunderstorms, land and wait for conditions to improve. — Bill Kight
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