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The Magnetic Compass’ errors
Under some conditions of flight, this instrument can react in wild motion.
The mechanical and other reasons for the eccentric behaviour of this instrument will not be discussed here. What is important to know at this time is that the magnetic compass can be relied on only when the helicopter is flying straight, and at a constant airspeed.
When a helicopter on a northerly heading executes a turn, the compass card remains stationary on its pivot or momentarily turns in the opposite direction. This falsely indicates less than the real amount of turn. The compass may be said to lag in turns from a northerly heading.
When a helicopter on a southerly heading executes a turn, the compass card turns faster than the helicopter, instead of remaining stationary on its pivot. This falsely indicates more than the real amount of turn. The compass may be said to lead in turns from a southerly heading.
When a helicopter is maintaining an easterly or westerly heading, changes of speed will cause the compass to falsely indicate a turn, deceleration will indicate a turn to the south and acceleration will indicate a turn to the north.

 

 

EXERCISE 21 - RAPID DECELERATIONS
“A helicopter is operating low level in restricted visibility; the pilot has reduced speed to 60 mph because of the weather. While flying in a narrow valley, high tension cables appear rapidly out of the gloom.” This situation is an all too-often occurrence, so what can you do? Correct! — a rapid deceleration, or quick stop, as it is sometimes called. The above example is one use for a quick stop; another is as a means for developing co-ordination and accuracy.
The purpose of a rapid deceleration is to stop the aircraft rapidly. There are two main variations: the into-wind and the downwind stop, obviously if you are crosswind a partial turn into wind is utilized. The downwind or 180° emergency turn entails turning the helicopter back into the wind during the stop, avoiding the dangers of a low airspeed while facing downwind.
INTO WIND 
Fly the helicopter straight and level, at 70mph and 50 feet AGL, as you run in choose a heading reference in the distance (Fig 21-1). Give a verbal warning “Practice quick stop, go”, on the executive word “go” proceed as follows:
1. commence a gentle flare by easing back on cyclic, at the same time lower collective to maintain height, apply right pedal to maintain heading;
2. as the flare starts to lose effect gently raise the collective to maintain height, as the groundspeed approaches zero select the hover attitude with cyclic, then continue to apply power to establish the hover at 50 feet AGL;
3. keep your heading throughout and avoid over-controlling with the pedals;
4. once you have established the hover, move the helicopter forward and down to normal hover height; and
5. maintain RPM throughout with throttle if required by type.
The gentle flare is to help you to become proficient at the exercise and smooth coordination  of the controls. Once you have mastered it, the manoeuvre can be speeded up to achieve the necessary stopping distance. The larger the flare, the greater will be the tendency to climb, leading to larger control movements overall and greater difficulties with control coordination.

 
Figure 21-1: Quick stop into wind 
 
Your instructor will show you that if you flare too harshly the needles could split, and how to recover from this situation. Be sure to anticipate with collective before losing translational lift, to stop any tendency to sink, otherwise you could be setting up the conditions necessary for vortex ring.
DOWNWIND (180° EMERGENCY TURN)
Fly the helicopter straight and level, at 70 mph airspeed, 50 feet AGL.
1. Commence a level co-ordinated turn using approximately 30 degrees of bank, ease back on cyclic to initiate a gentle flare, maintain height with collective and balance with pedals;
　
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