While GPS has revolutionized the way we navigate in two dimensions, the most common cause of pilot violations and CFIT accidents is the pilotís 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).
June 20, 1999
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:
- With one hand, grab the transponder mode switch and stop altitude squawk.
- With the other hand, perform a 2g pull-up or 0g push-over (as appropriate).
- 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
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.
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
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:
- The encoder output reports altitude only in 100-foot increments, so the AltAlert's
resolution is limited to the nearest 100 feet.
- 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.)
The 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
- 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.
Once 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.
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
Besides 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:
- APP: 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)
- 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
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
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
Not 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
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.
The $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
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.
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.
The 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
- 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
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",
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.
Like 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 horn...one 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?
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
- #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
- #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 it...it'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
CMM Altitude Tracking Module
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 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 a
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.
To 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.