WATCH YOUR ALTITUDE

AirSport uses Mode C to keep you on target.

BY TOM BENENSON

REPRINTED FROM SEPTEMBER 1993 FLYING
Copyright ©1993 Hachette Filipacchi Magazines, Inc. All Rights Reserved.

Two lower-priced versions of AirSport's self-contained altitude alert system are now available for pilots who don't need all the options available on the PRO model. Both the AirSport IFR and AirSport VFR work the same way by receiving your transponder signals and displaying the code and altitude you're squawking so that you can see the same altitude the controller is seeing on his ground equipment. Since the alerters do not require any installation, they can work in any airplane.

In all three versions, the 5¼-by-5¼-by-2-inch box features a 32-character lighted display that provides six windows of information: altitude, squawk, baro, target, delta x 100, and function. Two concentric knobs on the right side of the box control up to 16 functions. Once the box senses.your transponder signal, it displays the code the transponder is squawking and, with the barometric setting entered, the altitude it is reporting. As you change the barometric setting with updates from controllers, the box continues to display the altitude the controllers are seeing on their scopes. The "target" window lets the pilot set the altitude to which he has been cleared or that he wants to maintain. As the altitude the encoder is transmitting comes within 900 feet of the target altitude, there is an audible tone and the "delta x 100" display changes from double arrows in the direction required to reach the target altitude to a single arrow and a digit representing hundreds of feet to go to reach the target altitude. When the target and actual altitude agree there is another tone. The "function" window displays the current mode in use. If turned to "information," the window will display "climb," "descend" or "level," depending on what is required to maintain the target altitude.

An "approach" function warns the pilot to check gear at 500 feet before the preset descent altitude is reached. Once level at the descent altitude, any farther descent causes an insistent warning tone. When the AirSport is connected to the pilot's headset with the interface cable, the warning tones are descriptive of the required action. A climb command is an ascending tone. If the pilot needs to descend to reach the target altitude, the warning is a descending tone. For each 100 feet above or below the target altitude a tone sounds. For example, if the airplane has climbed 300 feet above the target altitude there will be three descending tones repeated at 10-second intervals until the plane descends. Two other handy features are a density altitude mode that computes the actual density altitude and a Fahrenheit-to-Centigrade conversion that simultaneously shows both temperatures in two windows as you turn the knob to change the setting.

During some 30 hours of flying with the IFR model, I often noticed that the difference between the altitude indicated by my altimeter and that being transmitted by my transponder varied by as much as 200 feet-meaning my 300-foot ATC-dictated margin for error was really 100 feet, which explains why I'm occasionally being reminded by controllers of the correct altimeter setting, a subtle way they have of warning pilots to check their altitude.

Although the unit comes with heavy-duty Velcro to affix it to the top of the panel (or other cockpit location), I was most comfortable wedging it between the pilots' seats rather than obscuring my outside view.

The AirSport models will run for about nine hours on the internal rechargeable battery or they can be connected to onboard power through a cigarette lighter adapter. On one long day, flying from Panama City, Florida, to Denver, Colorado, the battery ran down and I couldn't get the system to work through the lighter adapter because the fuse in the adapter had blown. I was surprised at how much I missed the alerter's insistent, but strangely reassuring, tones.

Occasionally the almost-constant tones as I bumped along at the edge of the tolerance range became annoying, but a toggle switch on the left side allows the warning horn to be switched off. You can also reset the tolerance to make the unit more or less sensitive to altitude excursions. A second three-position switch turns the system on and off and activates the internal light.

One thing I particularly like about the AirSport is its ability to verify the performance of my transponder. It's hard to know how often a problem on the ground is mistakenly blamed on airborne equipment and results in a needless visit to the shop. When a controller complains, "I'm not getting your Mode C," it's satisfying to be able to respond, "I have transponder-monitoring equipment on board and I'm squawking the assigned code."

The IFR model, priced at $699, displays altitudes to 99,900 feet, has the approach function with gear warning, includes a rate-of-climb calculation, and displays the ratio of Mode A to C replies in the area. The IFR model also comes with the cigarette lighter cord adapter and a carrying case. The top-of-the-line PRO model ($899) can display altitudes as high as FL 999 (the target altitude can be set as high as FL 555), automatically switches between altitudes and flight levels, has the capability of setting the barometric pressure in either inches or millibars, and features a "sponder-scope" that can be used by technicians to analyze the transponder signals.

For additional information, contact AirSport Avionics, at 866/ 215-2295.

Back to AirSport Home Page