accordingly, but this is not done by guesswork, but by mental calculation – often using the 1 in 60 rule. For example, a two degree error at the halfway stage can be corrected by adjusting heading by four degrees the other way to arrive in position at the end of the leg. This is also a point to reassess the estimated time for the leg. A good pilot will become adept at applying a variety of techniques to stay on track. While the compass is the primary instrument used to determine one's heading, pilots will usually refer instead to the direction indicator (DI), a gyroscopically driven device which is much more stable than a compass. The compass reading will be used to correct for any drift (precession) of the DI periodically. The compass itself will only show a steady reading when the aircraft has been in straight and level flight long enough to allow it to settle. Should the pilot be unable to complete a leg – for example bad weather arises, or the visibility falls below the minima permitted by the pilot's license, the pilot must divert to another route. Since this is an unplanned leg, the pilot must be able to mentally calculate suitable headings to give the desired new track. Using the flight computer in flight is usually impractical, so mental techniques to give rough and ready results are used. The wind is usually allowed for by assuming that sine A = A, for angles less than 60° (when expressed in terms of a fraction of 60° – e.g. 30° is 1/2 of 60°, and sine 30° = 0.5), which is adequately accurate. A method for computing this mentally is the clock code. However the pilot must be extra vigilant when flying diversions to maintain awareness of position. Some diversions can be temporary – for example to skirt around a local storm cloud. In such cases, the pilot can turn 60 degrees away his desired heading for a given period of time. Once clear of the storm, he can then turn back in the opposite direction 120 degrees, and fly this heading for the same length of time. This is a 'wind-star' maneuver and, with no winds aloft, will place him back on his original track with his trip time increased by the length of one diversion leg. Another reason for not relying on the magnetic compass