This portable altitude alerter keeps you on the beam during IFR
and even VFR flight with a variety of sounds and functions

by Keith Connes
Reprint courtesy of PLANE & PILOT Magazine, January 1992

Oakland Center: "Grumman One Six Lima Charlie, you've just busted your assigned altitude!"

One Six Lima Charlie: "(GULP!)"

Actually, I presume that the controller's phraseology under these circumstances would be a little more formal, but equally forbidding. Fortunately, I've never been on the receiving end of this type of stern message, but the very possibility of an inadvertent deviation from an assigned altitude is something I think about on every IFR flight, and I imagine most other instrument pilots share this concern.

Bust your assigned altitude while under IFR, or even while negotiating through a TCA, and you're almost sure to have an unpleasant interlude with FAA. An even greater punishment could be inflicted by metal-to-metal contact resulting from your intrusion into another plane's airspace. This, of course, could happen if you dope off while flying VFR as well.

Enter the AirSport Pro, a device that warns you if you stray from your desired altitude, and also does a number of other things. Unlike other altitude alerters, the Pro is completely self-contained and portable. No installation, no FAA paperwork, no panel space needed. It comes in the form of a compact box that measures 2x5x5-inches. Power is supplied by a rechargeable lead-acid battery.

So how does it work? The Pro is a receiver with a computer, and what it receives is the Mode A output of your transponder and the Mode C output of your encoder. The set's display is a 32-character LCD (liquid crystal display). The controls consist of two concentric knobs plus switches for display backlighting and an alert horn. A headphone jack is on the back of the unit.

Six Windows

The display is divided into six sections, referred to as "windows" by the manufacturer: Altitude, Squawk, Baro, Target, Delta and Function.

The Altitude window shows the aircraft's altitude as reported by the onboard encoder, in 100-foot increments. Thus, there can be an error or lag of up to 50 feet or so before the displayed altitude changes from, say, 2900 to 3000 feet. What you're seeing in that window is what your local controller is seeing on his or her radar screen, so the Pro lets you know whether your encoder is operating properly.

Likewise, the Squawk window displays the Mode A squawk that your transponder is putting out. The User's Manual suggests that if there is a difference between the code you've dialed into the transponder and the code it is transmitting, it might be due to dirty switch contacts, a slipped knob or other problem in the transponder.

Bear in mind that your transponder does not transmit when it is not being interrogated, so in areas of poor or nonexistent radar coverage, you will get a NO XPONDER message, meaning no Mode A and no Mode C transmissions are being made. Under these conditions, it is possible that your Mode C will be interrogated by a TCAS-equipped airliner or corporate plane, in which case the Squawk window will show a series of dashes (Mode A, no; Mode C, yes). This will tell you that there's some heavy iron in the vicinity.

The Baro window shows the barometric setting you have entered. Since your encoder is permanently set at 29.92 Hg, it is necessary to update the display by entering a current barometric setting, as you do with your altimeter.

The Target window shows the altitude you have selected for alerting purposes. There is a cruise altitude setting and a descent altitude setting, the latter used during an approach.

The Delta window displays the difference, in hundreds of feet, between your present altitude and your target altitude. There is also a "fly up" or "fly down" arrow (two arrows when the difference is 1000 feet or more).

The Function window shows which function has been selected for entry or display. An Info function can be selected that will enhance the information in the Delta window with such annunciations as Climb, Level and Descend.

An Approach mode can be selected and a Descent Altitude entered into the computer. When you are in this mode and reach an altitude that is 500 feet above the Descent Altitude, the word Gear will appear in the Function window and an aural alert will sound. (Even if you have a fixed gear plane, this might be a good reminder to perform certain checklist functions.) The next annunciation will be Descent Altitude, when you have arrived there, accompanied by a tone. When you descend below your Descent Altitude, that's what you'll see: Warning-Below DA. And you'll hear another warning sound.

The Sounds Of Warning

When it comes to aural warnings, the Pro is a veritable orchestra. When you are 900 feet above or below your target altitude, a single tone will sound. Once you reach your cruising altitude, if you deviate, a number of ascending or descending notes will be heard, depending on whether you've wandered above or below the target altitude. For example, if you've deviated three hundred feet on the high side, you'll hear three descending tones.

You can program a variety of altitude buffers, within which no alert will sound.

In the Approach mode, the Gear alert is a chirping sound, meant to suggest the sound of tires kissing asphalt. There are yet other sounds when you reach and go below Descent Altitude, the latter being particularly urgent.

This means that you don't have to be watching the Pro's display to interpret its altitude-related messages. (But it helps to have a degree from the Juilliard School of Music.) You will get this panoply of sounds through your headset or aircraft speaker, if the unit is so connected. A less musical alternative is the Pro's built-in horn, which sounds only one note for any given type of alert.

The Pro will also calculate your rate of climb by measuring the time it takes to make a 100-foot altitude change.

I flew the Pro on a recent trip, and it did everything as advertised. Its main market would appear to be instrument pilots who don't fly with a 2- or 3-axis autopilot and want all the help they can get to avoid an unplanned altitude excursion. However, as I indicated earlier, the Pro certainly can be a safety aid for the VFR pilot as well. The pilot who rents wings, or flies more than one aircraft, can easily take the Pro from plane to plane.

The AirSport Pro lists for $899. Included are a carrying case, 110V AC charger, 12-24V DC cigarette lighter power cord, headset patch cord, and Velcro for mounting the unit.

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