Insights

Name the most important cockpit instrument. The bizarre answers I usually hear make my head spin. Many pilots and instructors place their primary emphasis on the regulations, but what about common sense? If we make pilots believe that the answers to all questions and all operational situations are contained in official publications, we will soon damage our worldwide reputation for piloting excellence.

Yes, the regulations do specify the instruments that must be installed and operating when the airplane is flown, but those rules say nothing about the practical aspects of the instruments or their relative importance.

With proper training, no instruments are needed to fly safely in single-engine training airplanes. Can you fly all the visual training maneuvers, from stalls and slow flight to chandelles and lazy eights, without instrument reference? I certainly hope so. The wing (attitude) and throttle positions, engine noise, and control pressures provide enough basic information for safe flight. The flight instruments are obviously required for additional preciseness, but safe flight has greater latitude.

If given an airplane with no instrumentation and the choice of one and only one instrument, I would choose the magnetic compass, which I call the master compass. Now I can fly a specific course and remain oriented in order to fly cross-country, which is the primary reason for having an airplane.

You say that I must have an altimeter. No, as long as I stay within 3,000 feet of the Earth's surface - which is easy to estimate - I can fly at any altitude I desire. I do need my wristwatch, however, so that I can determine groundspeed and time aloft for fuel consumption.

During preflight and initial taxi, I always check the magnetic compass. The top of the fluid level should not be visible, and when turning the compass should move freely. The compass contains non-acidic, white kerosene that dampens card movement, reduces friction, and prevents freezing.

I turn on all avionics equipment and make certain that no moveable metallic objects are located near the compass. The magnetic field in the cockpit must duplicate that which existed when the mechanics swung the compass (calibrated it). I then check the compass when taxiing parallel and perpendicular to the runway. For exact runway headings, refer to the Airport/Facility Directory.

Mechanics swing the compass periodically to ensure accuracy. To do this, they position the airplane over a compass rose - a reference circle located on the airport that is marked with known magnetic headings - and adjust the compensating magnets located behind the compass deviation card. The errors are then listed on the compass deviation card.

There are two compensating magnets, one for north-south headings and one for east-west headings. That's why I check the compass when parallel to and when perpendicular to the runway. Either magnet could be out of calibration.

Magnetic compass errors, explained in applicable textbooks, are easy to deal with. The acceleration/deceleration errors are nonexistent when flying a north or south heading, maximum when flying a west or east heading, and proportional when flying an intermediate heading. The mnemonic ANDS is used to remember that error: Accelerate, the card turns toward North; Decelerate, the card turns toward South.

The turning error occurs when you bank the airplane. If turning to a heading that's located on the northern half of the compass card, the card lags behind the actual heading; if turning to a heading that's on the southern half of the compass card, the card leads the actual heading.

The maximum bank limit for accurate magnetic compass turns is usually 18 degrees, and three rules are used to compensate for the error. When turning to north or south, the error is equal to your latitude; when turning to east or west, the error is nonexistent; and when turning to intermediate headings, the error is proportional. If at 30 degrees north latitude and in a right turn to 360 degrees, you roll out to wings-level flight at 330 degrees, the compass will stabilize at 360 degrees. If in a left turn to 180 degrees you roll out to wings-level flight at 150 degrees, the compass will stabilize at 180 degrees.

Your before-takeoff checklist should include the magnetic compass accuracy check. The fastest way to become totally confused during a cross-country flight and violate airspace restrictions is to fly with an inaccurate magnetic (master) compass.

Ralph Butcher, a retired United Airlines captain, is the chief flight instructor at a California flight school. He has been flying since 1959 and has 25,000 hours in fixed- and rotary-wing aircraft. Visit his Web site.