Mag compasses live hard lives. They bake in the sun-splashed heat of the windscreen. They sustain endless amounts of airframe vibration, which not only makes them a challenge to read with accuracy but also contributes to ultimate failure.
You probably don’t think much about the compass in your aircraft until it spews its fluid all over the instrument panel and you realize FAR 91.205 requires you to fix it. Replacement options are slim and while a vertical card model is considered a step above the traditional whiskey design, installation technique is critical, and surprisingly expensive.
Not a Simple Upgrade
The whiskey compass got its name because it’s filled with alcohol. When the seals and gaskets in the compass housing become brittle and it leaks. Rebuilding them is possible, but that is becoming a thing of the past. Many shops simply replace the entire instrument.
Compasses can be mounted in a variety of configurations that can alter price and installation effort. Some hang from a mount on the windshield center post, some sit on top of the glareshield, and others mount in the instrument panel.
If you set out to replace one yourself, be sure to order the exact replacement because chances are, ordering a drop-in replacement from a catalogue based on looks will result in a model that doesn’t fit the existing mount. They also come in specific lighting voltages and in northern- and southern-hemisphere configurations.
The One and Only
Precision Aviation enjoys sole proprietorship of the TSO’d vertical card compass with the $280 PAI-700. Company principal John Coskey proudly told us there are over 77,000 PAI-700 units in the field stemming from over 30 years of manufacturing. The PAI-700 is offered in serious iron the likes of Canadair Challengers, Lear 45s and a variety of helicopters.
Much of this success is from design appeal. The PAI-700 mimics the presentation of a heading indicator, which takes the guesswork out of compass turns. Precision doesn’t solve all the problems inherent with old-school mag compasses because the magnet drive is subject to lead and lag effect, but Precision maintains that the beauty of vertical card design is the absence of pendulosity.
Further, the design is inherently stable due to eddy-current damping (no kerosene required). Precision’s design is unique with a high-powered, north-sniffing magnet mounted to a bearing and shaft assembly. Unlike a floating wet compass design, the magnet’s horizontal axis of movement is converted to the vertical using a set of transmission-like gears. The face of the compass is then mounted vertically on a shaft and bearing assembly. While turbulence will still have an effect on a vertical card model, you can expect it to recover itself significantly faster than the floating card type—if it’s mounted properly.
As high-quality as the PAI-700 is, the snag is getting it mounted in a way to keep it moving freely with as little interference as possible. The install kit includes a foam isolator that’s used to smooth out vibration. From there, it’s up to the technician to install the mount in the position that keeps the compass level in flight. It’s more difficult using the deck or glareshield mount and if they don’t get it right, it’s easy for the compass card to hang up or stick. It can also make calibration the proverbial bear.
Our experience has proven that a hanging mount yields better results than a deck mount. Also, that foam isolator is known to dry out over time and good preventive maintenance is having your shop replace it.
The unit comes with a choice of 28-, 14- or 5-volt lighting and is available in clear, blue or red. There’s also a variety of mounting brackets to accommodate the unit in nearly all aircraft.
SIRS Navigator compass
In our view, a worthy alternative to a vertical card design is the SIRS Navigator. The Navigator is made in the UK but has TSO certification and sells for $272 through Aircraft Spruce. The navigator modernizes the traditional whiskey compass, building in some modern designs including blue LED backlighting and fluorescent yellow legend.
The silicone damping design seems to eliminate the classic bobbling motion of the floating compass card that’s inherent with other wet compasses. Our experience flying with and calibrating this compass has been favorable. If you buy one, don’t lose the special adjustment tool that fits the adjustment points; it’s unique to the unit.
Falcon Gauge markets the MCVC-2L-A, which is a vertical card compass. This is a non-TSO’d unit aimed at the experimental market. We have no experience with the quality or performance, other than it appears to mimic Precision’s compass. If you have any experience with this unit, we’d like to hear about it.
We’re fond of the easy-to-steer presentation of the vertical card design of the PAI-700. However, the finicky installation often yields disappointing performance, which is no fault of the unit. Take it to the pros for installation and brace for a hefty invoice.
If you are convinced that your only means of navigating a main system failure will be with a mag compass, the PAI-700 vertical card unit is probably for you. Otherwise, we would go with the modern SIRS Navigator or a proven Airpath traditional compass. Let’s face reality: You’re probably only using the real compass to set your DG on the ground or double check the groundtrack you get from your GPS.