Three Low Cost Altitude Alerters

While GPS has revolutionized the way we navigate in two dimensions, the most common cause of pilot violations and CFIT accidents is the pilots failure to deal properly with the third dimension: altitude. In this regard, a good altitude alerter can be worth its weight in gold. Once the exclusive province of the turbine crowd, alerters have now come down in price to levels that most G.A. pilots (even renter pilots) can afford. We review three of them: the Icarus AltAlert ($995), the AirSport ($599 to $899), and the CMM Altitude Tracking Module ($249.95).

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Products“Sayyour altitude?”

No other three-word snippet of ATC phraseology strikes such fear into the heart of apilot. You know you’re in radar contact, that you’re squawking Mode C, that yourtransponder reply light is bright enough to read by, and that the controller knowsperfectly well what your altitude is. “Say your altitude?” is ATC rhetoric for”both you and I know that you busted your altitude, but you’d make the FAAenforcement lawyers ever so happy if you’d admit it on tape.” (Look it up in thePilot/Controller Glossary if you don’t believe me.)

During instrument training, all IFR pilots are taught the appropriate three-stepprocedure for responding to such a question:

  1. With one hand, grab the transponder mode switch and stop altitude squawk.
  2. With the other hand, perform a 2g pull-up or 0g push-over (as appropriate).
  3. Key the mic and, in your coolest, deepest low-seniority-number airline captain’s voice, say “Level at [assigned altitude],” while trying not to sound like you’re at 0g or 2g.

I hate to brag, but after decades of practice I’ve reached the point where I canexecute this procedure as one smooth unbroken maneuver in slightly under two seconds. Butabout ten years ago, as my fortieth birthday was becoming a distant memory, I startednoticing that I was getting to practice this procedure more and more frequently.Especially when flying single-pilot IFR (as I usually do), and especially while flyinglate in the day and a trifle fatigued.

I decided that it was time to do something about it, before I got crossways with theFAA, another aircraft, or a hilltop. I needed an altitude alterter.

An Altitude Alerter? What’s That?

Long required standard equipment in turbine-powered aircraft, an altitude alerter is aninvaluable cockpit gizmo that allows you to dial in your assigned altitude whenever ATCissues you one. It then monitors your actual altitude and compares it with the assignedaltitude you’ve dialed in.

Altitude alerter operation

As you approach that altitude while climbing or descending, the alterter gives you a”level-off alert” (generally 1,000 feet before you reach your assigned altitudein turbines…less in piston aircraft). Then, once you’ve leveled off, the alerter givesyou a “deviation alert” (climb or descend) any time you deviate from yourassigned altitude by more than a specified threshold (usually 200 or 300 feet). Thesealerts are generally provided both visually (lights) and aurally (tones), making themdifficult to ignore.

Up until about ten years ago, unfortunately, altitude alerters were almost unheard ofin piston aircraft – where they’re arguably needed the most, since these aircraft aregenerally flown without benefit of a copilot. The reason was that alerters required eitheran air data computer or at least a special-purpose altimeter with a special altitudepickoff. The result was that alerter systems were godawful expensive (generally $5,000 to$10,000).


Icarus AltAlert 3070Icarus AltAlert

All that changed in mid-1988, when a Baltimore-based companycalled Icarus Instruments introduced the first low-cost altitude alerter for generalaviation. Dubbed the AltAlert 3070 and priced at $995, this innovative product derived itsaltitude information from the aircraft’s existing encoding altimeter or blind encoder,instead of requiring an air data computer or special altimeter. This novel scheme had twomain disadvantages compared to traditional alerters:

  1. The encoder output reports altitude only in 100-foot increments, so the AltAlert’s resolution is limited to the nearest 100 feet.
  2. Since the encoder reports pressure altitude, the pilot is required to enter the barometric altimeter setting twice (in both the altimeter and the alerter).

On the other hand, the advantage of this scheme was between $4,000 and $9,000, and thatwas persuasive enough for me. I phoned Icarus’ president Steve Silverman in 1988, boughtone of the first AltAlert 3070s, and it has been keeping me honest altitude-wise eversince. It has always performed flawlessly, never hiccuped once, and until quite recently Iconsidered it the best money I’ve ever spent on avionics. (Today, I’d consider it a closecontest between the AltAlert and my ground-mapping handheld GPS.)

Basic Operation

Icarus AltAlert 3070The AltAlert 3070 fits in a standard 2 1/4″ panelhole (I strongly recommend mounting it as close to the altimeter as possible), and has adimmable 4-digit LED display and a pair of concentric control knobs. When the device isfirst powered up, it goes through an initialization sequence in which you’re prompted toenter three pieces of information in sequence:

  • BARO: the current altimeter setting.
  • DEST: the field elevation of the destination airport (to the nearest 100 feet).
  • TARG: your initial assigned (target) altitude.

You enter this information in the usual fashion with the concentric knobs, much asyou’d tune a radio.

Climb alertOnce initialized, the AltAlert spends most of its timein the default “target” mode, during which it displays your assigned (target)altitude and permits you to change that target at any time using the concentric knobs. Asyou approach the target altitude, the AltAlert provides a level-off alert by flashing”LEVL” in the display while generating a single audible “beep” in yourheadset or cabin speaker. Once established at your target altitude (by flying within thecruise deviation window for a few seconds), the AltAlert generates deviation alertsanytime you deviate outside that window by flashing “CLMB” or “DIVE”and generating a distinctive sequence of beeps. Whenever ATC assigns you a differentaltitude, simply dial it into the AltAlert and the process repeats. It’s that simple.

Other Alerts

Remember when you entered the destination airport elevation into the AltAlert atstartup? This is used to trigger a visual “GEAR” alert plus a distinctive sweptaudio tone as you descend through 500 feet above airport elevation, to remind you to checkthat your landing gear is down and locked. (If you fly a fixed-gear airplane, you canchange the alert to “GUMP” instead.)

Some other visual/aural alerts provided by the AltAlert include:

  • BAT: Displayed if AltAlert detects that your aircraft bus voltage has dropped to 12 or 24 volts, indicating that your alternator has gone off-line and the aircraft is running on battery power.
  • VAC: Displayed if you’ve installed the optional low-vacuum/pressure sensor and your instrument vacuum/pressure has dropped below 3.5 in. Hg.
  • EERR: Displayed if AltAlert has detected invalid data coming from your altitude encoder.

Other Modes

AltAlert: setting baroBesides the default “target” mode, the AltAlert has anumber of additional modes which can be selected by pushing in on the inner knob whileturning it. The most commonly-used of these is the “baro” mode used to informthe device of altimeter setting changes. When ATC issues you a new altimeter setting, youmust enter it into the AltAlert. To do this, simply press the inner knob and rotate it oneclick to the right. The display flashes “BARO” and then displays thelast-entered altimeter setting. Use the knobs to adjust it to the new setting. After a fewseconds, the AltAlert returns automatically to “target” mode.

If you fly at the Flight Levels, the AltAlert handles this automatically. If you dialin a target altitude above “17.9” the AltAlert display changes format to”F180″ and so forth. At or above pressure altitudes of FL180, the AltAlertdisregards the previously-entered altimeter setting and uses 29.92″ instead. Whencleared to descend from the Flight Levels, you can enter a new altimeter setting at anytime, and the AltAlert will automatically start using it as you descend below FL180.

The AltAlert supports a number of other useful modes, all selected by pushing andturning the small knob:

  • Three Low-Cost AlertersAPP: Approach mode is used when making a precision approach, and provides a distinctive visual (“DA”) and audible alert (triple beep) as you descend through a previously-specified decision altitude. This mode then automatically terminates, and the normal target altitude (normally, your missed-approach altitude) becomes active.
  • ALT: Altitude mode lets you view the actual altitude being generated by your encoding altimeter or blind encoder. It is useful for verifying proper encoder operation, and also may be used as a crude backup altimeter. It shows your altitude to the nearest 100 feet, corrected for the barometric altimeter setting that you’ve entered (and shows Flight Levels at or above FL180).
  • DTMR: Downtimer mode activates a short-term (up to 10 minutes) downtimer intended primarily to time non-precision approaches. You can pre-set the time, then start the timer at the FAF. When the time counts down to zero, the AltAlert issues a distinctive visual (“DTMR”) and audible alert (two pairs of beeps).
  • FUEL: Fuel timer mode activates a long-term (up to 10 hours) uptimer intended primarily to remind you when to switch tanks. When the timer counts up to a pre-set value, the AltAlert issues a distinctive visual (“FUEL”) and audible alert (three pairs of beeps).
  • CLK: Clock mode intended to provide total flight time or (if you set it before takeoff) time of day.
  • SET: Permits you to set various items of information into the AltAlert, including downtimer setting, decision altitude, fuel timer limit, destination airport elevation, and the time-of-day clock.
  • AUX: Permits you to set various parameters in the AltAlert’s permanent non-volatile memory. The parameters include display brightness, alert audio level, level-off alert threshold, cruise deviation alert threshold, home base (default) airport elevation, and selection of inches or millibars (hectopascals) for the barometric altimeter setting.

Recent Improvements

My original AltAlert 3070 has served me faithfully for nearly twelve years, and by nowI’d be lost flying without it. Fortunately, it has never needed repair so I’ve not had to.Since I purchased my early unit, Icarus has made a series of small refinements to it. Thelatest revision, the 3070G features easier baro entry, a time-of-day clock with a 10-yearbackup battery (so you don’t have to set it each time you fly). The gear-warning altitudeis now user-settable from 500 to 1,000 AGL. If you fly at the Flight Levels like I do,there is now a new BARO alert as you descent below FL180 to remind you to set youraltimeter. Finally, the aural alert tones have been “tuned” to really getyour attention when required.

There is also an optional GPS feature available for the 3070G priced at $295.There is a CDI switching mode with GPS/NAV annunciators for use with an external relaybox, and four GPS annunciators that also cause an audio tone when they come on.There is also a built-in altitude serializer for GPS receivers that need serial altitudedata.

Installation

The AltAlert is designed for permanent installation in the instrument panel by anFAA-approved avionics shop. Hookup includes running ten wires to from the AltAlert to thealtitude encoder, plus a wire to the aircraft audio panel and another to a source of DCpower (10 to 32 volts). Typical installation requires about four hours of labor – more ifyou have to shuffle your panel layout to accomodate the addition.

The AltAlert 3070G mounts in a standard 2 1/4″ round instrument hole, and costs$995 plus installation. You can buy it through your local avionics shop, or order it online at discount from one of AVweb’s sponsors.


AirSport

The only real shortcoming of the Icarus AltAlert is therequirement that it be permanently installed in the aircraft and hard-wired to theaircraft’s encoder, audio panel, and power. That’s fine for an aircraft owner like me, butit doesn’t help the renter-pilot, ferry pilot, or anyone else who flies a variety ofdifferent aircraft.

AirSport ProNot to worry, though. In 1991 – three years afterIcarus introduced its pioneering AltAlert – a pilot and aviation electronics expert namedDarryl Phillips from Sallisaw, Oklahoma, came up with an incredibly clever scheme fordoing just about everything that the AltAlert does – and a bunch of things that itdoesn’t – with a completely self-contained box that requires absolutely no installation,paperwork, or connection to the aircraft (electrical or otherwise) other than perhaps ahunk of Velcro to prevent it from floating around in turbulence. Basically, you just plopit on the glareshield, flip on the power switch, and it’s ready to boogie.

The secret of Phillips’ unit – which he dubbed the AirSport Pro and priced at $899 -is that it senses your altitude the same way ATC does: by monitoring the 1090 MHz outputof your Mode C transponder and decoding the transmitted altitude. The AirSport’s compact 51/4″ x 5 1/4″ x 2″ case houses a 1090 MHz receiver, antenna, rechargeablebattery, microprocessor, controls (two switches and two concentric knobs), and a32-character alphanumeric display.

There are some major advantages to this approach, as well as a few minor drawbacks. Onthe plus side, not only is the AirSport completely self-contained, but it’s guaranteed towork in any airplane with a functioning Mode C transponder. Furthermore, in addition tominding your altitude, it also monitors the proper functioning of your transponder andencoder. If ATC tells you they’re having trouble seeing you and asks you to recycle(which, incidentally, is a meaningless piece of ATC jargon that signifies nothing), onelook at your AirSport will tell you without question whether the problem is at your end ortheirs (it’s usually theirs, by the way).

On the minus side, the AirSport can’t function as an alerter unless your transponder ison and – since it speaks only when spoken to – receiving interrogations. In theory, ifyou’re flying at very low altitude and/or in a particularly radar-deprived area, yourtransponder might be out of range of ATC radar, and the AirSport might display its”NO XPONDER” error message. In real life, though, this is seldom a problem.First of all, at least in the 48 conterminous United States, there aren’t many places leftthat ATC radar can’t see. Second, even if ATC can’t paint your transponder, the chancesare good that it’s still receiving enough interrogations to make the AirSport happy. Andthird, in the very unlikely event that your transponder can’t receive interrogations fromany ATC radar facility, it may still get enough interrogations from high-flyingTCAS-equipped airliners and bizjets for the AirSport to work. Personally, I’ve never seenthe AirSport report “NO XPONDER” in flight unless I intentionally turned offMode C.

In the years following the 1991 introduction of the AirSport Pro, Phillips expanded hisproduct line to include four different AirSport models, ranging in price from $599 to$899. All four versions share a common design and operating philosophy, but thehigher-priced models have some additional features.

AirSport carrying caseThe $599 VFR model provides all basic altitude alerterfunctions and uses a backlighted LCD display, and comes complete with rechargeable batteryand wall charger. The $699 IFR model (AirSport’s biggest seller) adds an approach modethat provides DH, MDA, and check-gear-down warnings; it also comes with acigarette-lighter power cord and a very handy protective carrying case (pictured atright). The $799 IFR/VFD model is a premium unit with a gorgeous blue vacuum fluorescentdisplay instead of the LCD display. Finally, the top-of-the-line $899 Pro model addssupport for Flight Level flying, the ability to switch altimeter settings from inches tomillibars/hectopascals, and a unique “Sponder Scope” mode that displays rawtransponder output pulse trains and turns the unit into a miniature transponder/encodertest set.

All AirSport models come with a very generous 26-month warranty, plus what has to bethe best-written user’s manual I’ve ever seen for any avionics item, period.

Basic Operation

The basic altitude alerting logic of the AirSport (level-off alerts, deviation alerts)is essentially identical to the AltAlert, although the details of knob-twiddling are a bitdifferent. On the AirSport, the large outer knob sets the unit to one of 16 possiblefunctions, while the small inner knob is used to dial in data.

AirSport front panelThe AirSport’s large 32-character display (compared tothe 4-character display on the AltAlert) means that you can see almost everythinginteresting that the AirSport has to show you at once. The display is subdivided into sixinformation “windows” or fields:

  • ALTITUDE: The present Mode C altitude being reported by your transponder.
  • SQUAWK: The 4-digit Mode A code being transmitted by your transponder.
  • BARO: The altimeter setting you last entered.
  • TARGET: The target altitude you last entered.
  • DELTA: The difference between your present altitude and the target altitude, plus an up- or down-arrow that shows you which way to correct.
  • FUNCTION: Shows what function (mode) the AirSport is in, plus function-specific information.

Turning the large knob set to the 12 o’clock position puts the AirSport in”SetBaro” mode, in which the small knob is used to dial in the altimetersetting. Turning the large knob one click to the right puts the unit in”SetTarg” mode, in which the small knob is used to dial in the target altitude.One more click to the right selects “INFO” mode in which the function field ofthe display is used to display visual alerts (“LEVEL”, “CLIMB”,”DESCEND”, etc.).

Those three functions are actually about all you need to know about the AirSport totake it flying. The other 13 functions provide additional functionality that you canmaster later. We’ll get to them shortly.

AirSport back panelLike the AltAlert, the AirSport provides auralwarnings in addition to visual ones. It offers two different ways of accomplishing this.On the back panel of the AirSport is a warning horn…one that’s loud enough for you tohear quite well even in the noisiest cockput while wearing a headset.

Or for a truly hedonistic experience, disable the horn and use the supplied Y-adaptercord to hook the AirSport audio into your headset. Now, the AirSport will serenade youwith a virtual symphony of sounds: beeps (for level-off alerts), rising and fallingarpeggios (for deviation alerts), a “chirp-chirp” sound to remind you to checkwheels down (IFR and Pro), and an additional repertoire of aural alerts that would havemade Mozart or Leonard Bernstein proud. What’s next, Darryl…quadraphonic surround sound?

Other Functions

What about the other 13 positions of the large function knob? Well, here’s a quickoverview of what they do:

  • #4, Targ=Alt: Provides a shortcut method of setting the target altitude to the present reported altitude.
  • #5, SetDA: Enables you to set the DA or MDA for the approach mode (IFR/Pro only).
  • #6, Approach: Places the AirSport in approach mode. Provides Gear, DA, and Below DA warnings while on approach.
  • #7, Vertical Speed: Calculates and displays the aircraft’s rate of climb or descent.
  • #8, Pressure Altitude: Displays the raw Mode C pressure altitude output, without applying barometric pressure adjustments.
  • #9, Set OAT: Enables you to enter the OAT used for density altitude calculations.
  • #10, Density Altitude: Displays density altitude calculated from reported Mode C pressure altitude plus manually-entered OAT.
  • #11, Transponder Reply Count: This is a very interesting function that displays the frequency with which your transponder is replying to both Mode A and Mode C interrogations. Since ATC radar alternates Mode A and Mode C interrogations, but TCAS sends only Mode C interrogations, a higher-than-normal Mode C/Mode A ratio can signify that there is heavy iron nearby.
  • #12, Altitude Tolerance: Enables you to set the deviation alert threshold. Values from +/- 100 feet to +/- 400 feet are available.
  • #13, Level-Off Warning: Enables you to set the level-off warning threshold from 100 to 800 feet.
  • #14, Brightness: Varies the display brightness.
  • #15, Volume: varies the volume of the AirSport’s headset audio tones (not the horn).
  • #16, Rev: Displays the software revision, and performs certain other functions (including enabling the Demo and Sponder Scope modes).

Installation

None required! The AirSport is completely self-contained, and provides trulyconnection-free operation. It works in any aircraft with a functioning Mode Ctransponder…even works in an airliner cabin if you manage to smuggle it aboard. (Be sureto silence the horn!)

For a truly fulfilling audio experience, though, I recommend hooking up the headsetadapter cable so you can hear all the subtle aural alert tones in all their glory, insteadof just that brute-force warning horn. If the 8-hour operating life of the built-inrechargable battery isn’t adequate, you can use the cigarette lighter power cord to runthe AirSport off of ship’s power (14 or 28 volts, it doesn’t care).

Impressions

The AirSport blew me away the first time I flew with it…it’s that good. But nothing’sperfect, of course, and after ten years of flying with the Icarus AltAlert, there were afew nit-picky things about the AirSport that bugged me slightly. One is the fact that theAirSport’s gear-warning alert occurs only when the unit is switched to Approach mode. Lotsof flights don’t terminate in an instrument approach, and it seems to me that the gearwarning should occur on every flight, the way it does with the AltAlert.

Another nit-pick is the fact that the AirSport uses only one knob for data entry (thesmall one), instead of using both concentric knobs the way the AltAlert does. This meansthat to enter a one-inch altimeter setting change or a 10,000-foot target altitude changerequires more knob-twisting on the AirSport than it does on the AltAlert (where the outerknob changes the setting in one-inch or 1,000-foot increments). Like I said, it’s a nit.

But overall the AirSport is an absolutely superb unit. In fact, my biggest gripe isthat it doesn’t come in a panel-mount version for permanent installation. Presumably thebattery and horn would come out and the unit could be made a good deal smaller. I’d loveto be able to have the AirSport’s large display on my panel, plus its ability to monitorthe health of my transponder.

On the other hand, I can certainly understand why Darryl doesn’t want to develop apanel-mount version, given all the FAA certification hassles it would entail. I understandthat some aircraft owners have done permanent installations of the AirSport by means of aForm 337 Field Approval. I’d be tempted, but I’m not sure I could find enough free panelspace to accomodate the AirSport’s larger size. Oh well.

The AirSport may be purchased factory-direct from AirSport Corporation in Sallisaw,Oklahoma. The best way to order is online via the AirSportweb site.


CMM Altitude Tracking Module

ATM is small!Even AirSport’s low-end VFR model is – at$599 – too pricey for many pilots. For those folks, a company called CMM, Inc. hasjust introduced a tiny box that CMM calls an Altitude Tracking Module (ATM), priced atjust $199.95. Smaller than a pack of cigarettes, powered by a 9-volt battery andcompletely self-contained, the ATM employs a solid-state barometric pressure sensor and amicroprocessor to warn you if youve drifted above or below your desired altitude.The built-in baro sensor works up to 20,000 feet, but don’t try using the ATM in apressurized aircraft!

The front panel of the ATM includes two pushbutton switches and three color-coded LEDs.Operation couldn’t be simpler. Upon reaching your cruising altitude, just press the”set” button. The ATM will reward you with a green “on-altitude”light. If you should gain or lose 50 feet (or 100 feet, your option), a warning lightblinks (red for low, yellow for high). If your deviation reaches 100 feet (or 200 feet),the green light goes out. Pretty intuitive. For night flying, the ATM’s LEDs can be dimmedto a soft glow.

Velcro under glareshieldTo switch the ATM’s deviation alert threshold between100 and 200 feet, simply hold the “set” button depressed for five seconds. Allthree lights will blink in acknowledgement: once if the unit is in 100-foot mode, twicefor 200-foot mode. The same one- or two-blink mode reminder occurs whenever you power-upthe unit or set a new altitude. The threshold you set is remembered across power-downs.

Although the ATM is tiny, I don’t recommend mounting it on top of the glareshieldbecause bright sunlight can wash out the alert LEDs. I found it ideal to mount the unit onthe underside of the glareshield lip with a Velcro patch. A location right abovethe altimeter is ideal (as shown at right).

Impressions

I flew with the ATM for several hours, and found it to work as advertised. The 9-voltalkaline battery lasted about 20 hours, at which point the green light started flashing towarn of a low-battery condition. I was a bit concerned that the baro sensor might betemperature sensitive, so I ran some tests that involved putting the ATM in the freezerand then checking its stability as it thawed out. It proved to be surprisingly stable.

The ATM doesn’t really classify as an “altitude alerter” and CMM avoids usingthat term. The ATM has no provision for dialing in an assigned (target) altitude, so itcan’t be used to provide level-off warnings from a climb or descent. The ATM also has noaural alert capability – its alerts are strictly visual. And of course, there’s noapproach mode or gear-down warning. Consequently, I consider the ATM to be of little valueto instrument pilots whose objective is to avoid the classic “altitude bust”scenario: failing to stop a climb or descent upon reaching assigned altitude.

On the other hand, the ATM might be just the ticket for a VFR pilot who wants to beable to keep his eyes out of the cockpit and scanning for traffic, yet receive a timelywarning that he’d drifting above or below the desired cruise altitude.

The ATM may be purchased factory-direct from CMM, Inc. in Norcross, Georgia. It comescomplete with battery, two faceplates (for horizontal or vertical mounting), awell-written owner’s manual, and a two-year warranty. The ATM may be purchased online fromthe CMM web site or at discount fromone of AVweb’s sponsors.

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