Elbert D. Botts decided that white lines painted on the surface of the road werent effective at night or in the rain, so he invented Botts Dots: Raised pavement marker dots that separate highway lanes. The rumble of tires they create is supposed to alert you if you wander out of your chosen lane.
Thats the principle behind altitude alerting systems. You dial a target altitude into the device and it beeps or chimes if you wander off your assigned altitude. Big-dollar jets and sophisticated autopilots have this feature as standard equipment and its usually is part of an altitude pre-select system. (See sidebar, page 20.)
For the rest of us, theres the Icarus AltAlert 3070G, AirSport IFR and the Shadin AMS 200, three aftermarket boxes that can easily be added as accessories.
We picked these units to compare because theyre the most popular choices for modestly priced ($800 to $1200) systems in GA airplanes and each is designed for a different application and customer.
What They Do
There are three basic features to an altitude alerter. The first alerts when you arrive at a selected altitude, the second alerts when you deviate from the selected altitude and the third warns if the gears not down as you descend to the destination airport and pass through a specified altitude.You may think youre not a customer for this sort of electronic helper technology and that altitude alert systems are more crutches for sissies who cant hold altitude than they are a basic safety item. Perhaps so. But consider this: Whats the deductible on your hull coverage? If the gear alert function saves you one time, it will have paid for itself.
The arrival-at-altitude feature may be more suited to pressurized twins or turbod singles that typically cruise in the teens or higher, since they spend a lot of time climbing and descending through the mid-levels. But even pilots of normally aspirated airplanes that cruise in the single-digit thousands can benefit from a tap on the shoulder once in a while.
The deviate-from-altitude feature may not be such a luxury when you consider the potential consequences. Whats the penalty for wandering 10 degrees off a heading? If no loss of separation occurs, probably just a wake-up call from the controller, if that. An altitude bust is a more serious offense with more serious consequences.
Three hundred feet either way triggers Centers automatic snitch patch. If the deviation results in loss of IFR separation, now comes paperwork and maybe enforcement action or worse, controlled flight into terrain. Thats the bad news. The good news is that for under a thousand bucks you can have virtually the same system that the high-flying kerosene burners use.
We found another handy feature in these devices, especially for single-pilot IFR folks. Weve got heading bugs to help us keep track of newly assigned headings. Weve got flip-flop radios to help us keep track of new comm frequencies. Whats your system for keeping track of a newly assigned altitude? Write it down? Hope you remember it? Dial it into the card of your ADF?
Weve used them all. The task of actually dialing in the target altitude into one of these gizmos not only acts as a scratch pad to store it, but the active process of choosing a target altitude is another way for you to mentally review the new clearance.
Just like the heading bug or the flip-flop radio, you start by dialing your target altitude into the display. As youre climbing, the alerter gives you a heads-up beep as you approach your desired cruise altitude and another beep as you reach it.
The Icarus allows you to set the level-off alert from 100 to 1000 feet, the AirSport alert is fixed at 900 feet while the Shadin unit has a pilot-settable level-off alert between 100 and 1000 feet from the target altitude, when it beeps and flashes LEVEL. (If connected to a serial encoder, the Shadins level off alarm is fixed at 15 seconds from the target altitude.)
In cruise, the units compare your actual altitude with the target altitude you dialed in and will chirp if you exceed the user-defined range. How do they know your actual altitude? From the altitude encoder output of the transponder. Its at this juncture that the units vary most significantly. Heres how:
When youre flying, your transponder is busy replying to a variety of interrogations. It gets Mode-A interrogations (request for four-digit squawk code) from ground-based radar beacon systems. It gets Mode-C interrogations (request for encoded altitude) from the same ground based system.
Those two responses are combined into a target that shows up on ATC scopes. Your transponder also replies to Mode-C interrogations from TCAS-equipped airplanes and any military AWACS aircraft that happen to be nearby. Transponders reply to the interrogation by sending one coded pulse stream for Mode-A and a different coded pulse stream for Mode-C.
One more thing about transponders: They always send out pressure altitude, which ATC corrects for the local barometric pressure before your altitude is displayed on the controllers scope. Pressure gradients are assumed to be more or less linear up to 18,000 feet. Above that, theyre a little more unpredictable, which is why we always set the Kollsman for 29.92 inches in Class A airspace, above 18,000 feet.
The Icarus AltAlert displays encoded altitude information that it picks off from a hardwire to the transponders altitude output, which is a trick used by such devices as EIs Superclock and other similar units that have a temperature probe so they can display density altitude and do TAS calculations. Since you have to manual-ly insert the local altimeter setting, the Icarus corrects pressure altitude to your local true altitude. The Shad-in AMS 2000 works similarly. It gets altitude info from the encoder and displays it in 100-foot increments. But it can do better than that. When used in conjunction with a serial encoder, it can display altitude in 10-foot in-crements and even features a sepa-rate page showing instantaneous vertical speed based on that output.
The AirSport, on the other hand, gets its information through a built-in 1090 MHz antenna that grabs the coded pulse streams as they leave the transponder antenna on the bottom of the airplane. Since theres no transponder hardwire and the unit runs on an internal battery, its ideal for pilots who find themselves in a variety of airplanes or who rent.
You can actually use the AirSport to monitor the squawk and altitude of the airliner youre flying on, assuming it doesnt scare a white-knuckled seatmate into buzzing the flight attendant. On the ground, the Icarus and Shadin show field elevation when the transponder is on ALT but you wont get a display on the AirSport until get a radar hit.
The Icarus lets you choose between cruise deviation alerts of 100 and 200 feet while the Shadin is settable between 100 and 300 feet in 100-foot increments or 10-foot increments if you have a serial encoder. The AirSport is just as flexible, with seven different alert settings between 100 and 900 feet.
As you descend, the alerter gives you a heads-up beep as you descend to your target altitude and another beep as you reach it. In approach mode, the alerter warns when youve reached decision height. All three units give you a blinking visual alert and an option for an audio alert piped through the audio panel.
Whether portable or permanent mount, you need to install the alerter where its easy to get at because these things require a lot of attention. All altitude alert systems require that you reset the barometric pressure each time you change the Kollsman window of your altimeter. And youll need to dial in a new target altitude every time you climb or descend.
Fortunately, the units are physically small and not so deep that you cant find a spot for one even in a tight panel. The Shadin measures 3 1/8 by 1 1/2 inches and is 5 3/4 inches deep. The Icarus fits into a standard 2 1/4-inch round hole and the 2 1/4-inch square chassis extends 6 inches behind the panel. The AirSport, primarily intended portable use, is rectangular, measuring 5 1/4 x 2 inches by about 5 inches deep.
Shape, input, power and sound. Thats how the units vary. First the shape. The Icarus is 2 1/4 inches round. Its designed to fit permanently into the instrument panel. It requires hardwiring to the transponder output, plus connections to the intercom/audio panel and DC power. The Shadin has similar mounting and wiring requirements.
The AirSport is designed so that you can move it from airplane to airplane. Although it can be panel mounted, the AirSport is 7/8 inch shy of a nice fit into your 6 1/8-inch wide radio stack. Thats intentional to avoid TSO approval hurdles and to give owners installation options.
The Icarus will accept 11 to 30 volts from ships power; the Shadin requires 14 to 28 volts. TheAirSport operates for about eight hours on battery power, which can be recharged either from ships power inflight or an AC wall adapter.
All three display a visual alert when you deviate from your target altitude, but they also offer audio alerts that you can turn up, down or off. The Icarus offers one, three or 10 chimes, which are the same sound for up and down, while the Shadin has eight levels in eight different tones, all settable by the pilot.
The AirSports alert is more musical. When youre below your chosen altitude, the AirSport plays a distinctive up arpeggio and when youre above your chosen altitude, it plays a down arpeggio. If you climb 300 feet above your target, the AirSport plays the down phrase three times.
The Icarus, on the other hand, tells you youve wandered, but not which way. Ditto the Shadin, although both flash an altitude deviation alert thats plainly visible. With the AirSport, you know before you look. And the guy that designed AirSports sound alerts has a sense of humor. When you put the unit into demo mode to set the volume, it plays the four-note theme from the Twilight Zone.
All three allow you to store a destination field elevation altitude independent of your cruise altitude and will display the word GEAR 1000 feet above it, along with an audio tone. The Shadin and Icarus beep the gear warning but the AirSports alert is chirp, chirp, chirp the sound of three wheels greasing on. (You hope.)
The Icarus display uses four-letter abbreviations (LEVL, TARG, CLMB and DIVE) and two-digit altitudes (1.1, 5.3) for level, target, climb and dive and only displays one item at a time. The Shadin has an eight-character display and to alarm a deviation, it shows the actual altitude, plus up or down arrows pointing the way to resolve the deviation.
The AirSport always displays six things: your Mode-C output (in four digits, rounded to the hundred), your Mode-A output, the barometric pressure youve selected, the target altitude, the delta window (how many feet to target altitude) and the current selected function.
Each of the units has other useful features. The Icarus has a built-in clock and downtimer that keeps track of fuel tank switches and timed approaches. It also offers a GPS option for $295 that relays waypoint arrivals and airspace alerts from a panel-mount GPS. In other words, it adds audio annunciation to the GPS.
The Shadin is unique for having serial encoder capability to read altitude in 10-foot increments. So when setting oddball DA/DHs or MDAs, it does it better than the other two. It also calculates density altitude and something called performance ratio, which tracks engine performance as a function of density altitude.
The AirSport doesnt have a clock or a timer but it will calculate density altitude once youve set the temperature. It will also indicate rate of climb measured two ways and it will show the ratio of Mode-C replies to total replies. Why is this useful? If Mode-C hits far exceed the total replies, the queries are coming from nearby TCAS-equipped aircraft, since only ground stations query Mode-A.
In addition to the ordinary alerts, the AirSport offers a variety of custom deviation settings. Theres a no-lower-than setting youd use if youre flying over some airspace and dont want to descend into it but dont mind how high you go. Theres a no-higher-than setting youd use if youre flying under some airspace and dont want to climb into it, but dont mind how low you go.
The Icarus 3070G is the latest model in the line and sells for $995. The GPS option adds $295. Add whatever the avionics shop will charge you to wire it and fill out the 337 form and thats it. Installation should run between $300 and $500. The Shadin AMS 2000 base price is $1200 and should require similar installation costs.
The AirSport altitude alert is available in four models. We picked the $699 IFR model to compare but for $799, you get the same features as the IFR model with a better display and for $899, the Pro model provides easier tuning for the flight levels, millibar/hectopascal settings and a test mode that displays the raw pulse streams to help troubleshoot transponder output problems.
A Toss Up
In our view, theres no clear winner here. For panel mounts, the Icarus and Shadin have very similar capabilities and both handle altitude alerts and offer better timing functions than the AirSport does. The Shadin is marginally more customizable than the Icarus and we like the 10-foot altitude display option.
The AirSport provides similar alerts but offers better transponder data monitoring. For permanent mounting, we think the Icarus is a slightly better value, but only because of its lower cost and because its specifically designed for unobtrusive panel mounting. For portable use, the AirSport is the walk-away winner. The Icarus and Shadin just arent intended for that purpose.
Rumor is that nexgen GPS will have onboard altitude alerting. But if you cant wait for that or you like your current panel-mount, any of these alerters are a worthy addition to your IFR toolbox and a modest price. Nontheless, we wouldnt consider them as must have equipment.
by Joe Godfrey
Joe Godfrey is a musician, freelance writer and Bellanca Viking owner. He lives in Leucadia, California.