A Better Class of Compass

Thats what vertical cards are supposed to be. They deliver mixed results.

Like the proverbial bad penny, questions about vertical card compasses seem to resurface about once a year. We suspect owners most frequently consider them during annual when the whiskey model will no longer pass muster or needs fluid or other maintenance.

The vertical card is always an option, but were not sure we can recommend one unconditionally. We recently flew a Mooney with a vertical card and other than some screwy movements when turning to southerly headings, it worked like a charm and we have to say we liked it.

But in talking with other owners of vertical cards, our impression is that results are decidedly mixed. Some owners love them, others rip them out in short order, reverting to the conventional wet compass.

What Is It?
As far as we know, there are only two companys which manufacture vertical cards -Precision Avionics, Inc. (PAI) and Hamilton-suitable for light aircraft applications. Actually, PAIs PAI-700 is standard or optional equipment in some relatively heavy iron, including Lears, Regional Jets, and Challengers to name a few.

Costwise, these arent especially expensive as aviation parts go but they cost about three times as much as a typical Airpath conventional mag compass. A replacement Airpath typically retails between $76 and $134, depending on mounting, while the PAI-700 retails for $239.75, plus mounting brackets and compensators. Ballpark it at $300. The Hamilton HI-400, the equivalent of the PAI-700, sells for $235.95. From what weve seen, these units are very similar in construction and performance.

The idea and design behind the PAI-700 is simple and straightforward. The magnet that finds north is mounted on a shaft with jeweled bearings on each end. The compass itself is then mounted vertically, like a directional gyro face, on a shaft thats also on jeweled bearings.

To convert the magnets horizontal plane of movement to the cards vertical configuration, the compass has a little one-to-one gear set-up, sort of like a car differential. Unlike a wet compass, the mechanism doesnt ride in fluid but is mounted dry. Say goodbye to leaking seals, stinky kero and topping off the compass from time to time.

But wait. The fluid in a whiskey compass is supposed to dampen the nervous movement a suspended magnet would otherwise suffer. So how do they keep the vertical card from swinging back and forth like a useless yo-yo? Eddy currents. A small eddy current dampener is used to dampen quick movements and stabilize the card. (Owners say this works better in theory than in practice, at least in some applications.)

Then theres the old ANDS problem. (Accelerate-North; Decelerate-South) The magnetic pole of the earth is always over the horizon so in a conventional wet compass, the magnet tries to tilt downward, causing the magnets pole to dip. To counteract this, the opposite side, or like pole end of the magnet, is slightly heavier to tilt the magnet back towards level. This is also true of the PAI-700. In an effort to take out the ANDS error, Precision increased the strength of the magnet. The result of a stronger magnet decreases the error by increasing the alignment force of the magnet. They chose rare earth magnets that have a strength of 140 gauss or higher. That compares to a generic wet compass with a gauss of 40 to 70. So how do these improvements work? We asked some mechanics who have installed them, interviewed pilots who have flown them and tried a vertical card ourselves.

Right off the bat, its apparent that the installation of a vertical card is critical and can make what may otherwise be a great idea useless. The strengths of the PAI-700 also appear to be its downfall if not treated with kid gloves. The all important factor is location. The only location that seems to work well most of the time is the suspended mount from the center of the windshield and away from any magnetic tubes, screws or current carrying wires. The reason this is critical is due to that potent magnet. Besides sniffing out the magnetic pole aggressively, it also reacts more sensitively to any magnetic field, or any field distorting metal, to a greater degree than a weaker magnet would.

This means trouble if you have steel screws or current carrying wire too close to the unit and it makes a panel mount unit fairly useless. PAI and Hamilton discourage installation in the panel, for those thinking this thing is a natural as back-up DG.

One of the technicians we talked to had a difficult time with a Mooney, which has a steel tube cage with a tendency to magnetize itself over time. The level of magnetism isnt consistent, either, and can come and go for no apparent reason. Its no less of a problem for vertical card compasses. PAI sells what they call balancing balls to mount on the sides of the compass. These are positioned to counteract the magnetic anomalies in the aircraft and allow compensation adjustments within the range of the adjustable compensator.

They also recommend Mumetal (a high nickel content metal tape) to block undesirable interference from steel tubing, electrical currents and the like. Since all that stuff is extra dollars, youve probably deduced correctly that the labor bill can top out larger than anticipated. This is a trial-and-error process and can be frustrating. Or both. On the plus side, the owner of the compass installation which the tech had problems with reports no complaints.

Mounting Method
A second consideration is the mounting method. All mounts from PAI come with a foam isolator thats used to dampen out any vibration from affecting the card reading. One screw in the center of the foam mount area is all that solidly holds the compass to the mount.

This means that you have to install the mount in the position that gives a level attitude in flight. Otherwise the compass side of the mount will hit the airframe side and transmit vibration as well as tilt one way or the other. The foam can prove to be an Achilles heel over time, however. It will deteriorate, especially if the aircraft is stored outside, and this will allow the compass to move more freely than desired. Weve seen them flop around like dice balls from a rear view mirror if not maintained.

If the mount is the glareshield type, the deteriorating foam allows the unit to flop over to one side and the vibration begins to affect readings. We would like to see some additional noggin work on mounting methods, perhaps with some sort of silicon-gel filled bladder or the like.

Advice: Get an extra foam puck or two when ordering. Plan on changing it when it starts allowing the compass to tilt. We have flown such flawed installations and found them fairly useless for heading information. The compass ends up tilted and inaccurate. A poor mount like this always yields inaccurate compass headings, in our experience.

The compass must be level in flight attitude, which is not always exactly level with the leveling procedure for the aircraft. This seems to be the main difficulty in installations that mount on the windshield.

In Flight
Flying a vertical card is the same story as installing one. A good installation will work well in the air, barring any unforeseen oddities such as stray electrical currents. When the vertical card works right, it works as advertised and is a genuine improvement over the old slosher.

The ANDS error is less than youd expect but its still there. In general, however, you can turn to the heading you want without worrying about the wet compasss annoying reverse reading limitation.

Turbulence will still have an effect but the PAI-700 recovers significantly faster than the floating card type. At the same time, a bad installation will make the unit annoying and useless. It wont hang level and the card is inconsistent and off by well over 10 degrees. It will always act like its in turbulence if the mount isnt isolated properly with the foam. Based on our trials and interviews, the vertical card compass falls into the love-hate category. When installed carefully and with no oddball special circumstances, it works as claimed. If not, youll be better off with an old kerosene clunker.

To be realistic, plan on three or four hours of shop time to install one correctly-more or less depending on how much fussing is required to get it right. And be prepared for the fact that in some airplanes, it may never work entirely to your satisfaction and you may have to revert to the old-style compass.

-by Tom Ehresman
Tom Ehresman is a regular contributor to Aviation Consumer. Vertical card compasses are available from Aircraft Spruce and Specialty, Inc. at 800-824-1930 or log onto www.aircraft-spruce.com.