Three Low Cost Altitude Alerters

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While GPS has revolutionized the way we navigate in two dimensions, the most common cause of pilot violations and CFIT accidents is the pilot’s 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).

Products "Say your altitude?"

No other three-word snippet of ATC phraseology strikes such fear into the heart of a pilot. You know you're in radar contact, that you're squawking Mode C, that your transponder reply light is bright enough to read by, and that the controller knows perfectly 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 FAA enforcement lawyers ever so happy if you'd admit it on tape." (Look it up in the Pilot/Controller Glossary if you don't believe me.)

During instrument training, all IFR pilots are taught the appropriate three-step procedure 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 can execute this procedure as one smooth unbroken maneuver in slightly under two seconds. But about ten years ago, as my fortieth birthday was becoming a distant memory, I started noticing 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 flying late in the day and a trifle fatigued.

I decided that it was time to do something about it, before I got crossways with the FAA, 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 an invaluable cockpit gizmo that allows you to dial in your assigned altitude whenever ATC issues you one. It then monitors your actual altitude and compares it with the assigned altitude 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 altitude in turbines...less in piston aircraft). Then, once you've leveled off, the alerter gives you a "deviation alert" (climb or descend) any time you deviate from your assigned altitude by more than a specified threshold (usually 200 or 300 feet). These alerts are generally provided both visually (lights) and aurally (tones), making them difficult to ignore.

Up until about ten years ago, unfortunately, altitude alerters were almost unheard of in piston aircraft — where they're arguably needed the most, since these aircraft are generally flown without benefit of a copilot. The reason was that alerters required either an air data computer or at least a special-purpose altimeter with a special altitude pickoff. 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 company called Icarus Instruments introduced the first low-cost altitude alerter for general aviation. Dubbed the AltAlert 3070 and priced at $995, this innovative product derived its altitude 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 two main 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 that was persuasive enough for me. I phoned Icarus' president Steve Silverman in 1988, bought one of the first AltAlert 3070s, and it has been keeping me honest altitude-wise ever since. It has always performed flawlessly, never hiccuped once, and until quite recently I considered it the best money I've ever spent on avionics. (Today, I'd consider it a close contest between the AltAlert and my ground-mapping handheld GPS.)

Basic Operation

Icarus AltAlert 3070The AltAlert 3070 fits in a standard 2 1/4" panel hole (I strongly recommend mounting it as close to the altimeter as possible), and has a dimmable 4-digit LED display and a pair of concentric control knobs. When the device is first powered up, it goes through an initialization sequence in which you're prompted to enter 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 as you'd tune a radio.

Climb alertOnce initialized, the AltAlert spends most of its time in 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. As you approach the target altitude, the AltAlert provides a level-off alert by flashing "LEVL" in the display while generating a single audible "beep" in your headset or cabin speaker. Once established at your target altitude (by flying within the cruise deviation window for a few seconds), the AltAlert generates deviation alerts anytime you deviate outside that window by flashing "CLMB" or "DIVE" and generating a distinctive sequence of beeps. Whenever ATC assigns you a different altitude, 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 at startup? This is used to trigger a visual "GEAR" alert plus a distinctive swept audio tone as you descend through 500 feet above airport elevation, to remind you to check that your landing gear is down and locked. (If you fly a fixed-gear airplane, you can change 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 a number of additional modes which can be selected by pushing in on the inner knob while turning it. The most commonly-used of these is the "baro" mode used to inform the device of altimeter setting changes. When ATC issues you a new altimeter setting, you must enter it into the AltAlert. To do this, simply press the inner knob and rotate it one click to the right. The display flashes "BARO" and then displays the last-entered altimeter setting. Use the knobs to adjust it to the new setting. After a few seconds, the AltAlert returns automatically to "target" mode.

If you fly at the Flight Levels, the AltAlert handles this automatically. If you dial in a target altitude above "17.9" the AltAlert display changes format to "F180" and so forth. At or above pressure altitudes of FL180, the AltAlert disregards the previously-entered altimeter setting and uses 29.92" instead. When cleared to descend from the Flight Levels, you can enter a new altimeter setting at any time, 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 and turning 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 now I'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. The latest revision, the 3070G features easier baro entry, a time-of-day clock with a 10-year backup battery (so you don't have to set it each time you fly). The gear-warning altitude is 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 your altimeter.  Finally, the aural alert tones have been "tuned" to really get your 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 relay box, 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 altitude data.


The AltAlert is designed for permanent installation in the instrument panel by an FAA-approved avionics shop. Hookup includes running ten wires to from the AltAlert to the altitude encoder, plus a wire to the aircraft audio panel and another to a source of DC power (10 to 32 volts). Typical installation requires about four hours of labor — more if you 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.


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

AirSport ProNot to worry, though. In 1991 — three years after Icarus introduced its pioneering AltAlert — a pilot and aviation electronics expert named Darryl Phillips from Sallisaw, Oklahoma, came up with an incredibly clever scheme for doing just about everything that the AltAlert does — and a bunch of things that it doesn't — with a completely self-contained box that requires absolutely no installation, paperwork, or connection to the aircraft (electrical or otherwise) other than perhaps a hunk of Velcro to prevent it from floating around in turbulence. Basically, you just plop it 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 output of your Mode C transponder and decoding the transmitted altitude. The AirSport's compact 5 1/4" x 5 1/4" x 2" case houses a 1090 MHz receiver, antenna, rechargeable battery, microprocessor, controls (two switches and two concentric knobs), and a 32-character alphanumeric display.

There are some major advantages to this approach, as well as a few minor drawbacks. On the plus side, not only is the AirSport completely self-contained, but it's guaranteed to work in any airplane with a functioning Mode C transponder. Furthermore, in addition to minding your altitude, it also monitors the proper functioning of your transponder and encoder. 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), one look at your AirSport will tell you without question whether the problem is at your end or theirs (it's usually theirs, by the way).

On the minus side, the AirSport can't function as an alerter unless your transponder is on and — since it speaks only when spoken to — receiving interrogations. In theory, if you're flying at very low altitude and/or in a particularly radar-deprived area, your transponder 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 left that ATC radar can't see. Second, even if ATC can't paint your transponder, the chances are good that it's still receiving enough interrogations to make the AirSport happy. And third, in the very unlikely event that your transponder can't receive interrogations from any ATC radar facility, it may still get enough interrogations from high-flying TCAS-equipped airliners and bizjets for the AirSport to work. Personally, I've never seen the AirSport report "NO XPONDER" in flight unless I intentionally turned off Mode C.

In the years following the 1991 introduction of the AirSport Pro, Phillips expanded his product 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 the higher-priced models have some additional features.

AirSport carrying caseThe $599 VFR model provides all basic altitude alerter functions and uses a backlighted LCD display, and comes complete with rechargeable battery and wall charger. The $699 IFR model (AirSport's biggest seller) adds an approach mode that provides DH, MDA, and check-gear-down warnings; it also comes with a cigarette-lighter power cord and a very handy protective carrying case (pictured at right). The $799 IFR/VFD model is a premium unit with a gorgeous blue vacuum fluorescent display instead of the LCD display. Finally, the top-of-the-line $899 Pro model adds support for Flight Level flying, the ability to switch altimeter settings from inches to millibars/hectopascals, and a unique "Sponder Scope" mode that displays raw transponder output pulse trains and turns the unit into a miniature transponder/encoder test set.

All AirSport models come with a very generous 26-month warranty, plus what has to be the 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 bit different. On the AirSport, the large outer knob sets the unit to one of 16 possible functions, while the small inner knob is used to dial in data.

AirSport front panelThe AirSport's large 32-character display (compared to the 4-character display on the AltAlert) means that you can see almost everything interesting that the AirSport has to show you at once. The display is subdivided into six information "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 altimeter setting. 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 of the 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 to take it flying. The other 13 functions provide additional functionality that you can master later. We'll get to them shortly.

AirSport back panelLike the AltAlert, the AirSport provides aural warnings in addition to visual ones. It offers two different ways of accomplishing this. On the back panel of the AirSport is a warning that's loud enough for you to hear 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-adapter cord to hook the AirSport audio into your headset. Now, the AirSport will serenade you with a virtual symphony of sounds: beeps (for level-off alerts), rising and falling arpeggios (for deviation alerts), a "chirp-chirp" sound to remind you to check wheels down (IFR and Pro), and an additional repertoire of aural alerts that would have made 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 quick overview 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).


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

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


The AirSport blew me away the first time I flew with's that good. But nothing's perfect, of course, and after ten years of flying with the Icarus AltAlert, there were a few nit-picky things about the AirSport that bugged me slightly. One is the fact that the AirSport's gear-warning alert occurs only when the unit is switched to Approach mode. Lots of flights don't terminate in an instrument approach, and it seems to me that the gear warning 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 (the small one), instead of using both concentric knobs the way the AltAlert does. This means that to enter a one-inch altimeter setting change or a 10,000-foot target altitude change requires more knob-twisting on the AirSport than it does on the AltAlert (where the outer knob 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 is that it doesn't come in a panel-mount version for permanent installation. Presumably the battery and horn would come out and the unit could be made a good deal smaller. I'd love to be able to have the AirSport's large display on my panel, plus its ability to monitor the health of my transponder.

On the other hand, I can certainly understand why Darryl doesn't want to develop a panel-mount version, given all the FAA certification hassles it would entail. I understand that some aircraft owners have done permanent installations of the AirSport by means of a Form 337 Field Approval. I'd be tempted, but I'm not sure I could find enough free panel space 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 AirSport web 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. has just introduced a tiny box that CMM calls an Altitude Tracking Module (ATM), priced at just $199.95. Smaller than a pack of cigarettes, powered by a 9-volt battery and completely self-contained, the ATM employs a solid-state barometric pressure sensor and a microprocessor to warn you if you’ve 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 a pressurized 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 light blinks (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 dimmed to a soft glow.

Velcro under glareshieldTo switch the ATM's deviation alert threshold between 100 and 200 feet, simply hold the "set" button depressed for five seconds. All three lights will blink in acknowledgement: once if the unit is in 100-foot mode, twice for 200-foot mode. The same one- or two-blink mode reminder occurs whenever you power-up the 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 glareshield because bright sunlight can wash out the alert LEDs. I found it ideal to mount the unit on the underside of the glareshield lip with a Velcro patch. A location right above the altimeter is ideal (as shown at right).


I flew with the ATM for several hours, and found it to work as advertised. The 9-volt alkaline battery lasted about 20 hours, at which point the green light started flashing to warn of a low-battery condition. I was a bit concerned that the baro sensor might be temperature sensitive, so I ran some tests that involved putting the ATM in the freezer and 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 using that term. The ATM has no provision for dialing in an assigned (target) altitude, so it can't be used to provide level-off warnings from a climb or descent. The ATM also has no aural alert capability — its alerts are strictly visual. And of course, there's no approach mode or gear-down warning. Consequently, I consider the ATM to be of little value to 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 be able to keep his eyes out of the cockpit and scanning for traffic, yet receive a timely warning 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 comes complete with battery, two faceplates (for horizontal or vertical mounting), a well-written owner's manual, and a two-year warranty. The ATM may be purchased online from the CMM web site or at discount from one of AVweb's sponsors.