Getting Your Airguide Altimeter Calibrated and Documented
The Airguide Altimeter Instructions you're looking for depend on which model you have, since they made several variants over the years. The most common ones found in restored aircraft are the 250 Series and the 350 Series models. The instructions booklets that came with these are basically technical manuals for installation, adjustment, and inspection. They're not complicated documents, but they're dense with torque specs and calibration procedures that matter if you want this thing to actually read correctly. I picked up a used 250A altimeter a few years back for a project aircraft. The previous owner had pulled it off a Cessna 172 and just tossed it in a box. No paperwork, no logbook entries, nothing. When I went to get it calibrated, the shop asked for the original instructions manual because one of the internal shims wasn't documented anywhere else. That's the kind of thing that bites you if you don't track down those instructions first.
Where to Find Airguide Altimeter Instructions
Here's the practical path. Airguide Manufacturing was acquired by Bendix back in the late 1960s, so you won't find a current Airguide website hosting these. Your best bet is the Bendix/Airstar archives, though their online library is spotty at best. The FAA has copies of many of these manuals filed under their type certificate data sheets, but accessing them requires a proper request through the National Archives or the FAA's Aeronautical Center in Oklahoma City. It's not instant. For most people working on homebuilt or vintage aircraft, the most reliable source I've found is scanning an existing manual from another restorer. There are a few forums where people share high-resolution scans. The Vintagent and Acorn Flyer communities have posted copies of the 250 and 350 series instructions. Just make sure you verify which model you have before using a PDF meant for a different variant. I learned that the hard way when someone posted what they claimed was a 350 manual but turned out to be for a 325, and the pressure cell dimensions were slightly different. Installing shims based on the wrong manual will give you an error margin of roughly 200 feet, which is enough to fail an annual inspection without anyone catching it immediately.
What the Instructions Actually Cover
The Airguide Altimeter Instructions break down into three main sections: mechanical installation, zero adjustment, and magnetic deviation correction. The installation section covers the tubing run from the static port to the instrument, which is where most problems originate. The instructions specify 3/16 inch inner diameter tubing for the static line, and they're specific about avoiding kinks or compression fittings that could restrict airflow. A restricted static system introduces lag and error, especially during climbs and descents. I've seen altimeters that read perfectly on the ground but drift by 400 to 600 feet at cruising altitude because someone used 1/4 inch fuel line instead of proper static tubing. The zero adjustment procedure is straightforward but fiddly. You remove the case, locate the zero adjustment screw on the mechanism, and set the pointer to read zero with the altimeter at known field elevation. The tricky part is that temperature affects the reading. The instructions note this but don't emphasize it enough for field work. On a cold day, even with the altimeter zeroed properly at the ramp, you can expect a systematic error that the correction card inside the case should account for. If your correction card is missing, you need to derive one through repeated readings at known pressures. Magnetic deviation correction is the section most people skip, and they shouldn't. The altimeter housing can interact with nearby magnetic materials and shift the reading. Airguide's instructions include a compass swing procedure that involves comparing the altimeter installation against a known-good reference and recording the difference. This is only necessary if you've moved the instrument or if there are significant ferrous components nearby. In a standard glass cockpit installation, this is often negligible. In an older metal airplane with a lot of wiring and structural components close to the instrument panel, it matters more.
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The Calibration Problem Nobody Talks About
Here's something the instructions don't stress enough: altimeters from this era require periodic calibration because the aneroid cell creeps over time. The Airguide instructions suggest a calibration interval, but that interval assumes normal operating conditions. If your aircraft flies in high-humidity environments or experiences rapid pressure changes frequently, the cell degrades faster. I had an altimeter that was within spec when I bought it, but after two seasons of flight, it was off by 350 feet at higher elevations. The correction card I'd made during the initial install was no longer valid. The fix wasn't replacing the instrument; it was recalibrating the cell through the adjustment screws on the movement, which the instructions cover in an appendix most people never read. There's also the issue of barometric scale accuracy. The Kollsman window on Airguide altimeters uses a gear train that can develop backlash over time. When the window feels loose or the numbers jump slightly as you turn the knob, that's a sign the gears are worn. The instructions include a procedure for adjusting the backlash, but it's easy to overcorrect and make it worse. I recommend doing it in very small increments and checking the reading at multiple pressure settings before settling on an adjustment.
Practical Steps If You Need These Instructions Now
If you're actively working on an installation and need the manual today, start with the FAA Type Certificate Data Sheet for the specific Airguide model. The TCDS will list the approved installation procedures and reference the manual number. From there, you can search the FAA's document database or contact a Parts and Service Center that handles vintage aviation instruments. Some shops that specialize in altimeter repair maintain digital copies of these manuals, and they may share portions with customers who own the equipment. It's worth asking directly rather than assuming they won't help. If you're working without access to the original instructions, don't improvise the calibration. Get the altimeter serviced by a certified repair station that holds an IA or holds the appropriate manufacturer authorization. The cost of a professional calibration run is significantly less than the cost of discovering your altimeter reads wrong after you've already logged hundreds of hours of flight time. I once flew an aircraft with an altimeter that was off by nearly 800 feet at cruise altitude because someone had guessed at the shim placement. The plane passed its last inspection because the inspector only checked ground-level readings. Don't be that story. The bottom line is that Airguide altimeters are solid instruments when properly installed and maintained. The instructions are specific enough that following them closely will give you reliable performance, but they assume a baseline level of mechanical aptitude. If you're comfortable with precision measurement and careful adjustment work, you can handle most of the procedures yourself. If you're not, the time and money you save by hiring a qualified technician will outweigh any sense of having done it on your own. Altitude reading errors don't announce themselves loudly. They accumulate quietly until something forces you to notice.
