A Perceived Horizon (ground lights) Actual Horizon A O M CLUTCH MR TEMP MR CHIP STARTER ON TR CHIP lOW FUEL LOW RPM N E W 3 33 30 6 GS 15 20 I0 0 I0 15 VERTICAL SPEED 100 FEET PER MINUTE UP DOWN 5 5 33 30 24 2I I5 I2 6 3 2 MIN TURN DC ELEC L R 3$_{0.0}$ 29.9 29.8 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT 20 2 0 I0   I0 I0  I0 20 2 0 TEST STBY PWR IN Hg ALg. MANFOLD PRESS 25 5 15 30 20 10 35 20 30 40 50 60 70 80 90 100 0$_{10}$ MPH KNOTS 120 110 100 90 80 70 60 50 40 30 20 110 100 90 80 70 60 50 110 100 90 80 70 60 50 E R %RPM 2 0 2 0 I 0 I 0 I 0 I 0 2 0 2 0 TEST STBY PWR B Perceived Horizon (shore line) Actual Horizon (overcast sky) A O M CLUTCH MR TEMP MR CHIP STARTER ON TR CHIP lOW FUEL LOW RPM N E W 3 33 30 6 GS 15 20 I0 0 I0 15 VERTICAL SPEED 100 FEET PER MINUTE UP DOWN 5 5 33 30 24 2I I5 I2 6 3 2 MIN TURN DC ELEC L R 3$_{0.0}$ 29.9 29.8 I 0 0 F E T I 4 5 6 7 9 2 0 8 3 CALIBRATED TO  20,000 FEET ALT 20 2 0 I0   I0 I0  I0 20 2 0 TEST STBY PWR IN Hg ALg. MANFOLD PRESS 25 5 15 30 20 10 35 20 30 40 50 60 70 80 90 100 0$_{10}$ MPH KNOTS 120 110 100 90 80 70 60 50 40 30 20 110 100 90 80 70 60 50 110 100 90 80 70 60 50 E R %RPM 2 0 2 0 I 0 I 0 I 0 I 0 2 0 2 0 TEST STBY PWR Figure 12-8.  At night, the horizon may be hard to discern due to dark terrain and misleading light patterns on the ground.  Preflight Flicker Vertigo Night Flight or helicopte flying an a pproaching, parallel course. To  determine the direction of flight, the pilot should observe the  other aircraft’s position light s. Remember the following: red  on right returning; that is, if an aircraft is seen with the red  position light on the right and the green position light on the  left, the observed aircraft is traveling in the opposite direction. flight until c ondi tions im prove, unless proper instrument  flight training  has been  received  and the helicopter has the  appropriate instrumentation and equipment. Aircraft preflight inspection is a critical aspect of flight  safety. It must comply with the appropriate rotorcraft flight  manual (RFM). Preflight should be scheduled as early as  possible in the flight planning sequence, preferably during  daylight hours, allowing time for maintenance assistance and  correction. If a night preflight is necessary, a flashlight with  an unfiltered lens (white light) should be used to supplement  lighting. Oil and hydraulic fluid levels and leaks are difficult  to detect with a blue-green or red lens. Windscreens should  be checked to ensure they are clean and relatively free of  scratches. Slight scratches are acceptable for day flight but  may not be for night flight. The search light or landing light  should be positioned for the best possible illumination during  an emergency descent. Flicker vertigo is technically not an illusion; however, as  most people are aware from personal experience, viewing  a flickering light can be both distracting and annoying.  Flicker vertigo may be created by helicopter rotor blades or  airplane propellers interrupting direct sunlight at a rate of 4  to 20 cycles per second. Flashing anticollision strobe lights,  especially while the aircraft is in the clouds, can also produce  this effect. One should also be aware that photic stimuli at  certain frequencies could produce seizures in those rare  individuals who are susceptible to flicker-induced epilepsy. Careful attention must be paid to the aircraft electrical system.  In helicopters equipped with fuses, a spare set is required  by regulation, and by common sense, so make sure they are  on board. If the helicopter is equipped with circuit breakers,  check to see that they are not tripped. A tripped circuit breaker  may be an indication of an equipment malfunction and should  be left for maintenance to troubleshoot before flying.  All aircraft operating between sunset and sunrise are required  to have operable navigation (position) lights. Turn these lights  on during the preflight to inspect them visually for proper  operation. Between sunset and sunrise, these lights must be  on any time the helicopter is operating. The night flying environment and the techniques used when  flying at night depend on outside conditions. Flying on a  bright, clear, moonlit evening when the visibility is good, and  the wind is calm is not much different from flying during the  day. However, if flying on an overcast night over a sparsely  populated area, with few or no outside lights on the ground,  the situation is quite different. Visibility is restricted, so be  more alert in steering clear of obstructions and low clouds.  Options are also limited in the event of an emergency, as it  is more difficult to find a place to land and determine wind  direction and speed. At night, rely more heavily on the aircraft  systems, such as lights, flight instruments, and navigation  equipment. As a precaution, if visibility is limited or outside  references are inadequate, strongly consider delaying the  12-7